Overview of current atmospheric reanalyses

Submitted by Cathy.Smith@noaa.gov on Thu, 10/07/2010 - 12:47

 

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Current / State-of-the-art: 

ASR | ERA-20C | ERA-20CM | ERA-Interim | JRA-55, JRA-55C, JRA-55AMIP | MERRA-2 |  NCEP CFSR | NOAA-CIRES 20CRv2c
 

Possible issues: Consider other datasets for use in new research projects: NCEP/DOE II  | NCEP/NCAR NCEP NARR 

Superseded / Caution use for new research projects: ERA-40 | ERA-15 JRA-25
 

Model Change: NCEP CFSR (2011 and after)
 

Updated in real-time for public use (days behind): NCEP/DOE II  | NCEP/NCAR NCEP NARR  | NCEP CFSR? |  JRA-55 (2 days behind from JMA suite) |
Updated in near real-time for public use (months behind)ERA-Interim | JRA-55 | MERRA-2
Updated irregularly for public use (years behind): ERA-20C | ERA-20CM | NOAA-CIRES 20CR NOAA-CIRES 20CRv2cNASA MERRA

 

Overview Comparison Table (Reanalyses.org)

Overview Comparison Table (ClimateDataGuide)

Questions and Comments

 

Arctic System Reanalysis (ASR): 2000-2012

The Arctic System Reanalysis (ASR), a high-resolution regional assimilation of model output, observations, and satellite data across the mid- and high latitudes of the Northern Hemisphere for the period 2000 – 2012 has been performed at 30 km (ASRv1) and 15 km (ASRv2) horizontal resolution using the polar version of the Weather Research and Forecasting (WRF) model and the WRF Data Assimilation (WRFDA) System.

Source: Polar Meteorology Group, Byrd Polar & Climate Research Center, The Ohio State University

Time Range: 2000-2012

Assimilation: WRFDA-3DVAR

Dataset Output Times and Time Averaging: 3-hourly for surface and upper air fields, Monthly means of selected variables

ASRv1 – 30 km
Model Resolution: 30 km, 71 sigma levels
Dataset location: http://rda.ucar.edu/datasets/ds631.4/
ASRv2 30 km is expected early 2017

ASRv2 – 15 km
Model Resolution: 15 km, 71 sigma levels
Dataset location: http://rda.ucar.edu/datasets/ds631.4/ - Will become available during second half of 2016

Data Access: Polar Meteorology Group

References | ClimateDataGuide

ECMWF ERA-20C: 1900 - 2010

ERA-20C is ECMWF's first atmospheric reanalysis of the 20th century, from 1900-2010. It is an outcome of the ERA-CLIM project.

ERA-20C was produced with the same surface and atmospheric forcings as the final version of the atmospheric model integration ERA-20CM. A coupled Atmosphere/Land-surface/Ocean-waves model is used to reanalyse the weather, by assimilating surface observations. The ERA-20C products describe the spatio-temporal evolution of the atmosphere (on 91 vertical levels, between the surface and 0.01 hPa), the land-surface (on 4 soil layers), and the ocean waves (on 25 frequencies and 12 directions). The horizontal resolution is approximately 125 km (spectral truncation T159). Note, atmospheric data are not only available on the native 91 model levels, but also on 37 pressure levels (as in ERA-Interim), 16 potential temperature levels, and the 2 PVU potential vorticity level. Monthly means, daily, and invariant data are available. The temporal resolution of the daily products is usually 3-hourly.

The assimilation methodology is 24-hour 4D-Var analysis, with variational bias correction of surface pressure observations. Analysis increments are at T95 horizontal resolution (aprox. 210 km). The analyses provide the initial conditions for subsequent forecasts that serve as backgrounds to the next analyses. A 10-member ensemble was produced initially, to estimate the spatio-temporal evolution of the background errors.

The observations assimilated in ERA-20C include surface pressures and mean sea level pressures from ISPDv3.2.6 and ICOADSv2.5.1, and surface marine winds from ICOADSv2.5.1. The observation feedback from ERA-20C is available. It includes the observations but also departures before and after assimilation and usage flags.

Data Access: ECMWF

References | ClimateDataGuide

ECMWF ERA-20CM Model integration (no data assimilation): 1900 - 2010

The ERA-20CM atmospheric model integrations were produced in the framework of the ERA-CLIM project.

There are two versions, ERA-20CM and ERA-20CMv0, each comprising of a 10-member ensemble. The first version is 'final', the second is 'experimental'. The 'experimental' version was an early production and should not be used to initiate new research.

The model integration is forced by radiative forcing from CMIP5 and also by sea-surface temperature (SST) and sea ice cover from HadISST2.

Access: ECMWF

References | ClimateDataGuide

ECMWF Interim Reanalysis (ERA-Interim): 1979-present

ERA-Interim was originally planned as an 'interim' reanalysis in preparation for the next-generation extended reanalysis to replace ERA-40. It uses a December 2006 version of the ECMWF Integrated Forecast Model (IFS Cy31r2). It originally covered dates from 1 Jan 1989 but an additional decade, from 1 January 1979, was added later. ERA-Interim is being continued in real time. The spectral resolution is T255 (about 80 km) and there are 60 vertical levels, with the model top at 0.1 hPa (about 64 km). The data assimilation is based on a 12-hourly four-dimensional variational analysis (4D-Var) with adaptive estimation of biases in satellite radiance data (VarBC). With some exceptions, ERA-Interim uses input observations prepared for ERA-40 until 2002, and data from ECMWF's operational archive thereafter. See Dee et al. (2011) in the references below for a full description of the ERA-Interim system.

Data Access: ECMWF | NCAR

References | ClimateDataGuide

ECMWF 40 Year Reanalysis (ERA-40): Sep 1957-Aug 2002

Completed in 2003, ERA-40 is a global atmospheric reanalysis of the 45-year period 1 September 1957 - 31 August 2002. It was produced using a June 2001 version of the ECMWF Integrated Forecast Model (IFS Cy28r3). The spectral resolution is T159 (about 125 km) and there are 60 vertical levels, with the model top at 0.1 hPa (about 64 km). Observations were assimilated using a 6-hourly 3D variational analysis (3D-Var). Satellite data used include Vertical Temperature Profile Radiometer radiances starting in 1972, followed by TOVS, SSM/I, ERS and ATOVS data. Cloud Motion Winds are used from 1979 onwards. Various data from past field experiments were used, such as the 1974 Atlantic Tropical Experiment of the Global Atmospheric Research Program, GATE, 1979 FGGE, 1982 Alpine Experiment, ALPEX and 1992-1993 TOGA-COARE.

Data Access: ECMWF  | NCAR

References | ClimateDataGuide

ECMWF 15 Year Reanalysis (ERA-15): Jan 1979-Dec 1993

Completed in 1996, ERA-15 is a global atmospheric reanalysis of the 15-year period 1 January 1979 - 31 December 1993. It was produced using an April 1995 version of the ECMWF Integrated Forecast Model (IFS Cy13r4). The spectral resolution is T106 (about 190 km) and there are 31 vertical levels, with the model top at 10 hPa (about 31 km altitude). Observations were assimilated using a 6-hourly Optimum Interpolation analysis (OI). Satellite data used were limited to cloud-cleared TOVS radiances and Cloud Motion Winds from GOES, GMS, and METEOSAT. Pseudo-observations of surface pressure (PAOBS) were also used, as well various data from past field experiments: 1979 FGGE, 1982 Alpine Experiment (ALPEX), TOGA, SUBDUCTION, and those found in the COADS dataset.

Data Access: ECMWF  | NCAR

References | ClimateDataGuide

Japanese 25-year Reanalysis (JRA-25): 1979-2004, JCDAS: 2005-Jan.2014

The Japanese 25-year Reanalysis (JRA-25) represents the first long-term global atmospheric reanalysis undertaken in Asia. Covering the period 1979-2004, it was completed using the Japan Meteorological Agency (JMA) numerical assimilation and forecast system and specially collected and prepared observational and satellite data from many sources including the European Center for Medium-Range Weather Forecasts (ECMWF), the National Climatic Data Center (NCDC), and the Meteorological Research Institute (MRI) of JMA. A primary goal of JRA-25 is to provide a consistent and high-quality reanalysis dataset for climate research, monitoring, and operational forecasts, especially by improving the coverage and quality of analysis in the Asian region.  JRA-25 was conducted by JMA and CRIEPI (Central Research Institute of Electric Power Industry).  It had been continued as JCDAS (JMA Climate Data Assimilation System) operated by JMA in near real time basis. The data assimilation systems of JRA-25 and JCDAS are the same.  Users can use JRA-25 and JCDAS as one continuous reanalysis dataset. JCDAS data provision was terminated in early Feburay 2014 because it was replaced with JRA-55 in operational. The available data period of JRA-25/JCDAS is for 35 years and 1 month (January 1979 to January 2014).

Data Access: NCAR

Homepage | Atlas | References | ClimateDataGuide

Japanese 55-year Reanalysis (JRA-55): 1958-2012,  and extended to present
     [JRA-55C(1972-2012) and JRA-55AMIP(1958-2012)] <-- not extended to present

JMA has carried out the second reanalysis project named the Japanese 55-year Reanalysis (JRA-55) (nicknamed JRA Go! Go!) using a more sophisticated NWP system, which is based on the operational system as of December 2009, and newly prepared past observations. The analysis period is extended to 55 years starting from 1958, when the regular radiosonde observations became operational on the global basis. Many of deficiencies in JRA-25 have been diminished or reduced in JRA-55 because many improvements achieved after JRA-25 have been introduced. JRA-55 provides a consistent climate dataset over the last half century. JRA-55 has been continued in near real time basis after 2013. If you need real time basis (2 days behind) data, please access to JMA. The real time basis data are also provided to other cooperating organizations (Note: half year behind). Note that the products for extended period after 2013 are also called JRA-55. As "JRA-55 family", there are two subproducts JRA-55C and JRA-55AMIP produced by MRI/JMA. JRA-55C assimilated conventional observations only. JRA-55C covers from 1972 to 2012. Before 1971, use JRA-55 instead because no satellite data was assimilated in JRA-55 before 1971. JRA-55AMIP (AMIP type run of JRA-55, with no observations) covers 1958 to 2012. JRA-55C and JRA-55AMIP data are available from DIAS and NCAR. Note that JRA-55C and JRA-55AMIP are not extended to present. JRA-55 Atlas (climate charts) is now available.

Data Access: JMA | DIAS (JRA-55) (JRA-55C) (JRA-55AMIP) | NCAR (JRA-55: Daily 3-Hourly and 6-Hourly DataMonthly Means and Variances) (JRA-55C: Daily(3-hourly,6-hourly), Monthly) (JRA-55AMIP: Daily(3-hourly,6-hourly), Monthly) | ESGF/NASA/WCRP | ECMWF |

Homepage | References | JRA-55 Atlas |

 

NASA Modern Era Reanalysis for Research and Applications (MERRA): 1979-2016(Feb)

MERRA is a NASA reanalysis for the satellite era using a major new version (circa 2008) of the Goddard Earth Observing System Data Assimilation System Version 5 (GEOS-5) produced by the NASA GSFC Global Modeling and Assimilation Office (GMAO). The Project focused on historical analyses of the hydrological cycle on a broad range of weather and climate time scales and placed the NASA EOS suite of observations in a climate context.

Data Access: GES MDISC | ESGF

Home Page | References | FAQ | Atlas | ClimateDataGuide | AMS Special Collection

NASA Modern Era Reanalysis for Research and Applications Version-2 (MERRA-2): 1980-present

MERRA-2 is a NASA reanalysis for the satellite era using a major new version of the Goddard Earth Observing System Data Assimilation System Version 5 (GEOS-5) produced by the NASA GSFC Global Modeling and Assimilation Office (GMAO). MERRA-2 assimilates observations not available to MERRA during the 2010s, and therefore, will continue processing in real time longer than MERRA.  There are numerous improvements and updates to the data assimilaiton, model and observing system. One notable change is the assimilation of aerosol observations, including black and organic carbon, sulfate and dust. Production began in the spring of 2014 and is presently available for access.

Data Access: GES MDISC | FTP Subsetter

Home Page | File Specification | Documentation

NCEP Climate Forecast System Reanalysis (CFSR): 1979-present

The National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) spans 1979 to present. The CFSR was designed and executed as a global, high resolution, coupled atmosphere-ocean-land surface-sea ice system to provide the best estimate of the state of these coupled domains over this period. The T382 resolution atmospheric data spans 1979 to 2010. The current T574 analysis is an extension of the CFSR as an operational, real time CFSv2 product from 2011 into the future.

Data Access: NCEP | NCDC NOMADS | NCAR (includes real time CFSv2) | ESGF

References | ClimateDataGuide

NCEP/DOE Reanalysis II: 1979-near present

NCEP produced a second version of their first reanalysis starting from the beginning of the major satellite era. More observations were added, assimilation errors were corrected and a better version of the model was used.

Data Access: NCEP NOMADS | NCAR | ESRL | KNMI | IRI

References | ClimateDataGuide

NCEP/NCAR Reanalysis I: 1948-present

This reanalysis was the first of its kind for NOAA. NCEP used the same climate model that were initialized with a wide variety of weather observations: ships, planes, RAOBS, station data, satellite observations and many more. By using the same model, scientists can examine climate/weather statistics and dynamic processes without the complication that model changes can cause. The dataset is kept current using near real-time observations.

Data access: NCEP NOMADS| NCAR | ESRL | IRI | KNMI 

References | FAQ | FGDC | ClimateDataGuide

NCEP North American Regional Reanalysis (NARR): 1979-near present

The NARR reanalysis was done to produce very high resolution output over the North American region. Observational inputs were similar to NCEP I with the addition of assimilated precipitation. The NARR model region was nested in a global, lower resolution model. Outputs are similar to the NCEP I and II models but with more snow, ice and precipitation related variables.

Data Access: NCDC and NCEP NOMADS | NCAR | ESRL

References | FAQ | ClimateDataGuide

NOAA-CIRES 20th Century Reanalysis version 2 (20CRv2): 1871-2012

The 20th Century Reanalysis version 2 (20CRv2) dataset contains global weather conditions and their uncertainty in six hour intervals from the year 1871 to 2012. Surface and sea level pressure observations are combined with a short-term forecast from an ensemble of 56 integrations of an NCEP numerical weather prediction model using the Ensemble Kalman Filter technique to produce an estimate of the complete state of the atmosphere, and the uncertainty in that estimate. The uncertainty is approximately inversely proportional to the density of observations. Additional observations and a newer version of the NCEP model that includes time-varying CO2 concentrations, solar variability, and volcanic aerosols are used in version 2. The long time range of this dataset allows scientists to examine better long time scale climate processes such as the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation as well as looking at the dynamics of historical climate and weather events. Verification tests have shown that using only pressure creates reasonable atmospheric fields up to the tropopause. Additional tests suggest some correspondence with observed variations in the lower stratosphere.

Data Access: ESRL | KNMI | IRI | NCAR | NERSC | BADC | ESGF

Homepage | References | Related Publications | ClimateDataGuide

NOAA-CIRES 20th Century Reanalysis version 2c (20CRv2c): 1851-2012 [2013-2014]

The 20th Century Reanalysis version 2 (20CRv2c) dataset contains global weather conditions and their uncertainty in six hour intervals from the year 1851 to 2012. Surface and sea level pressure observations are combined with a short-term forecast from an ensemble of 56 integrations of an NCEP numerical weather prediction model using the Ensemble Kalman Filter technique to produce an estimate of the complete state of the atmosphere, and the uncertainty in that estimate. The uncertainty is approximately inversely proportional to the density of observations. Additional observations from ISPDv3.2.9 and new boundary conditions from the Simple Ocean Data Assimilation with sparse observational input (SODAsi.2) pentad sea surface temperature and COBE-SST2 monthly sea ice concentration are used.  For the 2013 and 2014 extension, SSTs are from the NOAA daily 1/4 degree Optimal Interpolation version 2. 2014 does not include tropical cyclone pressure observations after April. 

Data Access: ESRL |  NCAR | NERSC (every member)

Homepage | References | Related Publications ClimateDataGuide

 


Have a question or comment on a specific reanalysis dataset or in general?

Use this link to Post Notes, Questions, and Comments on specific reanalyses


 

 

bastin (not verified)

Tue, 07/26/2011 - 00:39

So which is the best analysis for wind related long term calculations.......NCEP/NCAR reanalysis or NOAA-CIRES 20th Century Reanalysis V2 (20CR): 1871-2008

It depends on your needs, and which winds you are referring to. Ocean/SHem will have diffs in N/N when satellite data becomes available. 20CR is an ensemble of surface pressure only assimilation, no wind assimilation. the pressure gradients where obs are available should help the low level winds, but there is no upper level wind assimilation.

bottom line is that the 20CR is so new, you will likely have to determine this for your self, and we hope that you can share that information back here, as others may have similar questions. Be sure to search for new papers coming out and conference papers, 20CR is getting a lot of attention!
MB

Stergios (not verified)

Tue, 03/20/2012 - 03:48

Some questions regarding the 20th century reanalysis Which reconstruction of solar irradiance is used to force the model? Is the model forced with total or spectral solar irradiances? Is there a 11-yr solar cycle? Was the stratospheric one kept constant? Thank you very much.

The reconstruction is described in http://www.cpc.ncep.noaa.gov/products/people/wd51hd/vddoolpubs/solar_reconstruction.doc The NCEP model is forced by a spectral distribution of solar radiation. We do include the abt 11 yr cycle, even with a forecast to 2020. Not sure what is meant by the stratosphere question. Huug van den Dool (no need for anonymity on my part)

Thank you very much for the quick reply.

The NCEP model is forced by spectral irradiances but, if i understood correctly, the solar cycle variation refers to total solar irradiance only. This means that every spectral band of the radiation code increases equally (~0.1%) from the minimum to the maximum phase of the 11-yr solar cycle.
The last question was referring to stratospheric ozone. There is a prognostic equation for ozone, right? So, i guess there is a weak ozone variation in the course of the 11-yr solar cycle.

Stergios Misios

Huug van den Dool (not verified)

Thu, 03/22/2012 - 08:31

Yes, you understand correctly. The solar constant varies, but by the same % irradiance variation for each freq band. Moorthi or Yu-Tai can answer as to whether ozone varies in that set-up. Huug

megha (not verified)

Tue, 03/27/2012 - 00:27

kindly suggest me that can we take two different parameters from two different grid data set of NCEP/NCAR reanalysis 2 to calculate a variable in which both the parameters have been involved? for example-can i take skin temperature from surface data grid and all other from pressure data grid to calculate latent heat flux ?

If I understand correctly, you would like to use data from N/N surface files and pressure level files, together, to make a new calculation of Latent heat flux? Ultimately, I don't think there is a specific reason that would prevent it. You would need to be careful about time step an grid centers (if all not identical). However, the reanalysis should produce it's own latent heat flux. Is the LE not sufficient? Can you offer more details or clarification?

The typical output would provide data centered on a grid box, and at the listed pressure level. However, for certain purposes, data offset to the edges are more accurate, but these should be easily identifiable. When in doubt, check the model or system documentation for a description of the grid and data format.

Linden Ashcroft (not verified)

Thu, 07/05/2012 - 23:52

Has anyone worked with the uncertainties of monthly values derived from the 20th Century Reanalysis product? I know there are monthly mean values of sub-daily ensemble spread available, but these are not the same as the spread of the monthly averages calculated using each ensemble. That sentence is confusing, because I am confused! Would be very grateful for any advice or ideas.

Dear Linden, You are right that monthly means of sub-daily ensemble spread are not the same as the ensemble spread of the monthly averages. The "correct" spread will always only results from doing all analyses for each member separately and then analyse the spread of the results (e.g., averages, trends, any other statistics). The monthly means of sub-daily ensemble spread can only tell you something about the ensemble spread of the monthly averages if you have a way to consider the autocorrrelation. In my view, it is still valuable as an upper-limit estimation. Maybe I am too simplistic, but some part of the spread will be close to random and thus its contribution is expected to decrease with the square root of n, so the true spread of the monthly means will be lower.

gregoire.leroy

Fri, 08/03/2012 - 03:47

Hello all,

I have a question regarding the CFSV2. UCAR website states that it is a continuation of CFSR. But is it a reanalysis like CFSR or is it an analysis? Can we consider that the quality of CFSV2 data are the same than CFSR?

Thanks,

G.L

prakashb2002

Fri, 09/07/2012 - 00:39

Hello all,
I am studying applicability of met parameter (P, T & Rh) derived from reanalysis products to GPS PWV estimation. I selected NCEP R1, NCEP FNL and ERA-Interim for a decade and use the values (P, T & Rh) derived from them for inter-comparison. R1 and ERA-Interim are reanalysis products whereas FNL is operational GDAS analysis. Is it feasible to use them for inter-comparison? Any help will be highly appreciated.

Prakash

Dick Dee (not verified)

Tue, 09/18/2012 - 05:15

Dear anonymous, your question would make a good subject for a paper. It is probably too general for this forum. I can say that we are reasonably confident about lower-tropospheric humidity in ERA-Interim, especially over land areas, based on the quality of the precipitation estimates generated by the model - see Dee et al QJ 2011 (http://onlinelibrary.wiley.com/doi/10.1002/qj.828/abstract). ERA-Interim humidity near the surface compares very well with independent observations - see Simmons et al JGR 2010 (doi:10.1029/2009JD012442). We have also looked at the variability of upper-tropospheric humidity and temperature in the tropics which seems reasonably good - see my presentation at the WCRP Open Science Conference in Denver last year (http://conference2011.wcrp-climate.org/orals/B4/Dee_B4.pdf) Dick

Anonymous (not verified)

Tue, 10/23/2012 - 20:51

Hi all, I am studying the surface energy balance problem and using the ERA-Interim solar radiation data, the fields are accumulated from the start of the forecast, so in order to get averaged values I divided the data by the length of the forecast step as the ERA-Interim website FAQs : ( FLD( STEP2)- FLD( STEP1))/(( STEP2-STEP1)*3600) while I get some negative values which I think is impossible,so can you tell me the right to handle the accumuliate field?Thanks. X.L

Paul (not verified)

Thu, 10/25/2012 - 06:08

In reply to by Anonymous (not verified)

In order to obtain average values from accumulated values you need to divide by the number of seconds in the STEP ie STEP*3600 because the accumulation is from the beginning of the forecast to the STEP (in hours) in question. If you require the average value from STEP1 to STEP2 then the formula you quote is correct provided that STEP2 is larger than STEP1. If you obtained netCDF data from the ECMWF data server at http://data-portal.ecmwf.int/ then you need to be aware that the data are packed, see the data FAQ at http://www.ecmwf.int/products/data/archive/data_faq.html#netcdfintegers, and that the scale_factor and add_offset vary with date, time and STEP etc.

matthias.jerg

Mon, 01/21/2013 - 03:23

Hello,

I would like to compare globally gridded monthly means of cloud products derived from satellite observations (AVHRR,MODIS) with reanalysis products. I am tending towards using ERA-Interim but would be interested in the general opinion on this question. I would also be very interested in any recommendations which products to look at. I am aiming for cloud fractional coverage, cloud liquid and ice water path and possibly cloud top height. Any comments on this also wrt common traps and problems, parameters to investigate etc. are highly appreciated.

Thanks,

Matthias

I can't say which you should compare, as it ultimately depends on your metrics and purpose. However, direct cloud data comparisons have been tricky, in my experience, since there are inherent differences in what is observed, and how the background models compute cloud quantities. Part of this is also the motivation for data simulators (e.g. ISSCP Simulator). I would encourage you to include radiation observations into the comparison, as those should be another variable where the feedback from clouds to the atmosphere manifests.

Thanks for your answer. You are certainly raising an important point. We might include direct radiation comparisons but the current scope of the project will not allow us to do much. Therefore, we may have to stick with direct cloud data comparisons for the time being.

Thanks,

Matthias

Brian Walsh (not verified)

Thu, 01/31/2013 - 13:52

I'm looking for high resolution surface wind (10 metres above ground) for 5 years (2008-2012) off Western Greenland. We'd prefer something less than 20 km in spatial resolution and at least 6 or 12 hourly time steps. Is that available from any reanalysis dataset?

Gijs de Boer (not verified)

Wed, 02/20/2013 - 14:02

I have a couple of quick questions about ERA-Interim. The documentation is a bit fuzzy on this, but as far as I can tell, the radiation scheme used in Interim is RRTM, is that correct? Also, am I reading correctly that SST and sea ice come from the NCEP analysis for years prior to June 2001, then the NOAA OISST from July 2001-December 2001, the NCEP RTG from January of 2002-January 2009 and finally OSTIA from February 2009 onwards? Thanks, Gijs de Boer

Please take a look at http://www.ecmwf.int/research/era, specifically the links to the IFS documentation, which has all the details about the radiation scheme used in the model, and links to 'known quality issues' which include the inconsistent use of SST products.

I did look at the documentation before posting my questions. However, I was not convinced that I had all of the answers to the questions I posed. For longwave radiation, the model documentation states: "Since cycle Cy22r3, two longwave radiation schemes are available in the ECMWF model, the pre-cycle Cy22r3 by Morcrette (1991), and the current longwave radiation transfer scheme, the Rapid Radiation Transfer Model (RRTM)." It is not stated (as far as I can tell), which version is used for the ERA-Interim runs, which is what I am trying to figure out... While those at ECMWF are likely versed in which model versions are used for the ERA-I production runs, unfortunately I'm not. If I am reading the documentation correctly, the shortwave portion of the radiative transfer scheme is described in Fouquart and Bonnel (1980), and RRTM is NOT used for shortwave radiation. As far as SST goes, the model documentation states: "For the operational ten-day forecasts, ECMWF uses the NCEP daily real-time global SST product (RTG_SST) at 0.5 degree resolution." In the Dee et al. paper, it states that ERA-Interim uses NCEP RTG between 01/02 and 01/09. I was just trying to confirm that this is the case, given the documentation's description. Thanks for providing the hint about searching under the "known quality issues" section, where what I describe above appears to be confirmed. Thanks again for any insight that folks may be able to provide on the longwave radiation scheme used for the ERA-I runs.

Sorry about that - there is a lot of information available but unfortunately it is not all in one place, and not always clear, and it could certainly be much better organised. We tried to be thorough in Dee et al 2011, which is the basic reference for ERA-Interim. In fact it is just as you say - in ERA-Interim LW radiation is computed using RRTM while SW is still based on Fouquart and Bonnel with six spectral intervals.

I just took a quick look at the MERRA assimilated observations in Jan2000. I can see data near Summit in surface pressure, but not near Humbolt, GITS, Tunu-N or Petermann. I'm not so familiar with the stations data availability. If they were in the GTS, then they would be input. Note, that MERRA assimilates surface pressure from surface meteorology stations, not temperature, moisture or wind.

To look at this, I used MERRA's Gridded Innovations and Observations data, on openDAP at: http://opendap.nccs.nasa.gov/dods/MerraObs

It can also be downloaded from GES DISC: http://disc.sci.gsfc.nasa.gov/mdisc/data-holdings/merra-innov

These are not yet well documented.

We did not use these data in ERA-Interim - they are also not used by the ECMWF operational forecast system. Most of the Greenland station data that currently arrive at ECMWF are from coastal stations. Those inland stations could be very valuable for forecasting (and reanalysis, of course).

For the 20th Century Reanalysis (20CR, http://reanalyses.org/atmosphere/overview-current-reanalyses#TWENT ), you can obtain all of the observations used from the International Surface Pressure Databank version 2 ( http://reanalyses.org/observations/international-surface-pressure-datab… ) for the entire global domain and time period (1871-2010) or for a subset period or region using the tools
courtesy of the Data Support Section of the Computational and Information Systems Laboratory at the National Center for Atmospheric Research from http://rda.ucar.edu/datasets/ds132.0/.

Maps of the stations available to the 20CR can be viewed at http://www.esrl.noaa.gov/psd/data/ISPD/v2.0/.
A text file with the stations available is at http://reanalyses.org/sites/default/files/groups/users/gilbert.p.compo/… .
See the ISPD home page http://reanalyses.org/observations/international-surface-pressure-datab… for more information.

Please let me know if I can of more help.

best wishes,
gil compo

Anonymous (not verified)

Mon, 06/17/2013 - 06:39

Hi Dears, Where can I find the list of surface station observations used in ERA-Interim? Where can I find the list of radiosonde station observations used in ERA-Interim? Thanks, Celeste

Tade (not verified)

Mon, 07/08/2013 - 14:59

hello there, I'm wondering if anyone out there could provide and /or confirm on the availability of ERA interim reanalysis at 0.25 degree spatial resolution and sub daily or daily temporal resolution for public access. Apparently, I read from ECMWF portal that ERA interim reanalysis data has approximately 80 km resolution. I'm interested to use a global reanalysis data to force hydrological models such as SWAT for a meso watershed with sparse hydrometeorological stations. I would appreciate if you drop me any piece of information on the availability of global reanalysis data with resolution in the order of 20 -30km at daily scale or shorter. So far I saw CFSR with 38 km which has a potential use but I'm hoping if there is more resoluted one. Many thanks, Tade

Tade, To date, there is no global reanalysis dataset at the resolution you are requesting. See the overview table at http://reanalyses.org/atmosphere/comparison-table There are regional reanalyses that come close to your desired resolution. Again, see the table. best wishes, gil compo (University of Colorado/CIRES and NOAA/Earth System Research Laboratory/Physical Sciences Division)

asimnicol (not verified)

Tue, 07/23/2013 - 04:30

Has anyone worked with the uncertainties of monthly values derived from the 20th Century Reanalysis product? I know there are monthly mean values of sub-daily ensemble spread available, but these are not the same as the spread of the monthly averages calculated using each ensemble. That sentence is confusing, because I am confused! Would be very grateful for any advice or ideas.

Dear Asimnicol, Your comment had spam lines, which have been deleted. At portal.nersc.gov, you can find each member of selected variables averaged to a daily mean and to a monthly mean. From this, you can construct your own ensemble spread or other statistic. Please let me know of any additional questions. best wishes, gil compo (University of Colorado/CIRES and NOAA/Earth System Research Laboratory/Physical Sciences Division)

Christopher (not verified)

Sat, 07/27/2013 - 05:29

Hello, I am looking for the PFT maps used in NCEP I (and other reanalysis, but NCEP I is the first one I'd like to use)? So, the underlying classification of land grid cells according to vegetation or similar used in the reanalysis. I have had no luck finding these online so wanted to ask here? Thanks, Christopher

Cathy.Smith@noaa.gov

Mon, 07/29/2013 - 11:26

In reply to by Christopher (not verified)

I think the files are here (monthly) ftp://ftp.emc.ncep.noaa.gov/mmb/gcp/sfcflds/test/fixed/README_albedo_gfrac.txt NCAR has ncep R1 files and lists vegetation (grib 225). I'm not sure which set of products it is in. http://rda.ucar.edu/datasets/ds090.0/#description Cathy

It looks like NCAR has the variable Vegetation Species for the NCEP/NCAR Reanalysis at http://rda.ucar.edu/datasets/ds090.0/#description The University of Maryland table describing the land cover for 1 degree data from which the NCEP/NCAR vegetation species is presumably derived is at http://glcf.umd.edu/data/landcover/ (after consulting with Mike Ek of NOAA). best wishes, gil compo

Dick Dee (not verified)

Tue, 07/30/2013 - 04:05

The main ECMWF reanalysis webpage (www.ecmwf.int/research/era) has some information about observations used in ERA-Interim, including a timeline (http://www.ecmwf.int/research/era/do/get/index/29/29?showfile=true) and a complete inventory of radiosonde observations (http://www.ecmwf.int/research/era/do/get/index/29/28). We are working on a system that will allow users to view and download all observations used in our reanalyses - stay tuned.

David (not verified)

Thu, 08/22/2013 - 15:07

I didn't quite formulate my question earlier. Let me retry. The general question is what the kg/m2 means in NARR in monthly files? My assumption is that for these "transports" that the value is an accumulation over 3hr. So, to get to a daily value of, say, ssrun, one would simply multiply by 8. And to a monthly integral just days per month. Could someone confirm this? Regards, David

ndashora (not verified)

Fri, 09/06/2013 - 03:05

While working for estimation of IWV using GPS, I learned that the diurnal variations in GPS-IWV can be obtained if we use high resolution (better than 6 hours i.e. 3 hourly) surface temperature and pressure values. These values, in turn are required to obtain the hydrostatic delay over the GPS site for estimation of wet delay. I would really appreciate if somebody can provide information about a reanalysis product that has temporal resolution of 3 hours and spatial resolution about 100 km or better. I downloaded MERRA 3-hour product, but observations from our AWS do not match with this data for shorter scales (like diurnal variations and range of variations of parameters).

Dear ndashora, It would be very helpful if you would make a Page on the site and post a figure illustrating the issue. If you do not have an account, go to https://reanalyses.org/user/register to make one. A listing of the spatial and temporal resolution of the atmospheric reanalysis datasets is given at https://reanalyses.org/atmosphere/comparison-table see the Model Output Resolution column and Publicly Available Dataset Resolution column. Links to Data Access can be found under the descriptions at https://reanalyses.org/atmosphere/overview-current-reanalyses Oh, you may want to spell out the acronym IWV. Other users refer to is as Water Vapor Path or other names.

Nirvikar Dashora (not verified)

Thu, 09/12/2013 - 08:16

Thanks a lot to all of you who responded to my query regarding GPS based water vapor estimation and high res. data. I, indeed, am thankful to you all and at the moment feeling overwhelmed to receive four replies and all are supporting and relevant. So now, let see how all comments work-out. I would like to come back with my findings again to this forum. It has been useful to post a comment here.

Paul Trimble (not verified)

Wed, 09/25/2013 - 06:31

Is the NARR going to be run back to 1900 with 20th Century Reanalysis being used for its boundary coniditions? This would be very useful for studying regional climate and hydrologic within the early portion of the 20th Century. Thanks Paul

Dear Paul, There are no plans that I know of do anything like this specifically. You may be interested in the dynamically downscaled data described in DiNapoli, S. M. and V. Misra, 2012: Reconstructing the 20th century high-resolution climate of the southeastern United States, J. Geophys. Res., 117, D19113, doi:10.1029/2012JD018303. best wishes, gil compo

Anonymous (not verified)

Thu, 09/26/2013 - 06:02

Is NCEP reanalysis more accurate (lower bias and RMS) in summer (JJA) than in winter (DJF) in the stratosphere and troposphere of the northern hemisphere? If so, why? How about in the southern hemisphere? Thanks!

What variables are referring to? In general, your question is a research question that depends on your interests. More broadly, the paper Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J. Woollen, Y. Zhu, A. Leetmaa, R. Reynolds, M. Chelliah, W. Ebisuzaki, W. Higgins, J. Janowiak, K. C. Mo, C. Ropelewski, J. Wang, R. Jenne, D. Joseph, 1996: The NCEP/NCAR 40-year reanalysis project, Bull. Amer. Meteor. Soc., 77, 437-470, doi: 10.1175/1520-0477(1996)077&lt;0437:TNYRP&gt;2.0.CO;2. describes the expected level of reliability of the various variables. With more than 10,000 papers having used the dataset, it is possible that someone has already looked into your question, but without more detail, it is difficult to provide any additional direction. Links to additional information (FAQ, ClimateDataGuide) can be found at http://reanalyses.org/atmosphere/overview-current-reanalyses#NCEP1 best wishes, gil compo U. of Colorado CIRES &amp; NOAA Earth System Research Laboratory

Remo Beerli (not verified)

Mon, 12/09/2013 - 02:35

Hi everyone, Is there a mask of the elevation of the topography of every grid point in 20CR? If yes, where can I access it? Thanks in advance for any help or suggestions!

Dear Remo, One source is to see the files listed at http://www.esrl.noaa.gov/psd/data/gridded/data.20thC_ReanV2.monolevel.html You want the file from the table for Time-Invariant Either Geopotential Height Surface 2x2 ftp://ftp.cdc.noaa.gov/Datasets/20thC_ReanV2/time_invariant/hgt.sfc.nc or Geopotential Height Surface (gaussian grid) ftp://ftp.cdc.noaa.gov/Datasets/20thC_ReanV2/gaussian/time_invariant/hgt.sfc.nc best wishes, gil compo

Hi Remo, The 20CR surface geometric height is contained in the file sflxgrbfg_orography.t62.grib, which can be downloaded at the following URL: http://rda.ucar.edu/datasets/ds131.1/index.html#sfol-wl-/data/ds131.1?g=309 You will need to sign in at the NCAR RDA website before downloading the file. Additionally, the ISPDv2, which was assimilated into the 20CR, contains the station elevation data and modified elevation for all stations in the ISPDv2. These data may be accessed from the NCAR RDA website at the following URL: http://rda.ucar.edu/datasets/ds132.0/ - Tom Cram

Alima (not verified)

Thu, 02/27/2014 - 22:25

Hi everyone, I am looking for the land sea mask for 288*145, please let me know where I can have access it. Thanks in advance

Dear Alima, Is there a particular dataset whose land/sea mask you are trying to get? Depending on the dataset, the mask could be different. NCL could possibly be used to make a general conversion from the built-in mask http://www.ncl.ucar.edu/ but I have not test this. best wishes, gil

Mehdi (not verified)

Mon, 03/03/2014 - 06:09

Hi everyone, I'm trying to understand the vertical wind velocities in the ERA Interim data set in model levels ("eta" hybrid coordinates). According to the netCDF metadata, the provided vertical velocity is in Pa.s-1 which corresponds to a vertical velocity in pressure coordinates. Hence the following question : do we have d(p)/dt instead of d(eta)/dt in the data in model levels ? Thanks for any piece of advice.

SA (not verified)

Tue, 04/01/2014 - 03:07

Hi everyone, I am analyzing hourly rainfall records in many regions of the world. However, I would like to know which reanalysis data is suitable to make some comparison. Thanks in advance for any suggestions :D

Dear SA, What you are asking turns out to be a research question. Several papers have looked into this. See a list at http://reanalyses.org/atmosphere/observational-studies and http://reanalyses.org/atmosphere/inter-reanalysis-studies-0#Precipitation Please report back with what you find. best wishes, gil compo

I can't say anything specific, since your comment has very little detail about where and when you are working. One thing that will be universal when considering reanalysis precipitation is that the quantity is derived from the background model forecast of the assimilation process. So, while the environment that produces precipitation is affected by the observational analysis, precipitation relies on the model physics, and has significant uncertainty. If observations of precipitation are available, you'll want to use those, at the least, in a comparison with the reanalysis data. Some reanalyses produce 1 hourly data, and some assimilate rainfall (see the North American Regional Reanalysis). Biases and uncertainty in reanalysis precipitation depends greatly on where and when you are looking. The papers listed in the pages Gil provided are a start, but be sure to follow those that they cite as well.

andre (not verified)

Wed, 04/23/2014 - 11:16

Hello, What are the NCEP1 products that I should use to calculate Net Heat Flux at the air-sea interface? Qnet = SW- LW - LH - SH. where SW denotes net downward shortwave radiation, LW net upward longwave radiation, LH latent heat flux, and SH sensible heat flux I can find these products at http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.surfaceflux.html Regarding the latent and sensible fluxes I don't have a problem (since there are only two in the NCEP list), but regarding the others I have several. For example: "Net longwave radiation", "Net shortwave radiation", "Upward longwave radiation flux" , "Downward solar radiation flux",... Hope you can help on this. Thanks!

Yes, that equation is correct and your abbreviations are correct. According to someone in our group who looked at your question "I think they should use the net shortwave and net longwave (if there are also upward and downward fluxes). Note that the shortwave from reanalysis has some real problems (it depends a lot on the cloud cover, which is not simulated well in the AGCM used in the reanalsyis). We did a paper on this a while back: Scott, J. D. and M. A. Alexander, 1999: Net shortwave fluxes over the ocean. J. Phys. Oceanogr., 29, 3167-3174. "

Anonymous (not verified)

Fri, 06/20/2014 - 06:56

I would like to know what is the vertical height levels for the reanalysis data like CFSR,MERRA, &amp; ERA-I? Would there be different height levels possible? Also I want to know about Sigma levels ?

Dear User, The vertical levels for the post-processed data from the sources listed on http://reanalyses.org/atmosphere/overview-current-reanalyses under Data Access are described on each page. All of the datasets you list have multiple levels, some with more than 60 in the vertical. All of the datasets are available in self-describing formats that contain the level information. Sigma levels describe a vertical coordinate that is the level where the pressure is a specified fraction of the surface pressure. For example, sigma 0.995 is the level where the pressure is 0.995 times the surface pressure. If the surface pressure is 1000 hPa, then the pressure at the sigma level 0.995 is 995 hPa. Please reply back if you have additional questions.

Anonymous (not verified)

Mon, 08/25/2014 - 07:51

I've looked at the surface latent and sensible heat flux averaged for a polar cap (north of 70N). Both show odd seasonal cycles, with maximum values during winter, and minimum during summer. Is this mainly due to the sea ice misspecification in the 20CR dataset? The other fluxes (shortwave and longwave radiation at both surface and top of atmosphere) show more "normal" cycles (though somewhat higher values).

Dear Anonymous, As noted at https://reanalyses.org/atmosphere/inter-reanalysis-studies-0 perhaps what you are looking for is in the paper Lindsay, R., M. Wensnahan, A. Schweiger, J. Zhang, 2014: Evaluation of Seven Different Atmospheric Reanalysis Products in the Arctic*. J. Climate, 27, 2588–2606. doi: http://dx.doi.org/10.1175/JCLI-D-13-00014.1 Many additional seasonal intercomparison maps (many more than could be included in the publication) showing the median from the seven reanalyses and the deviations of each from the median are found at http://psc.apl.uw.edu/arctic_reanalyses. Both seasonal means and seasonal trends for the period 1981-2010 are shown. 34 different variables are included: surface fluxes, temperature, humidity, wind, precipitation, and pressure; layer heights and temperatures; and TOA radiative fluxes. best wishes, gil compo for Reanalyses.org

Keith (not verified)

Wed, 09/10/2014 - 14:29

I'm curious whether sounding data from field campaigns (specifically the TEPPS cruise in 1997) would have been incorporated into the reanalyses. According to Serra and Houze (2000) pt I, the data was "transmitted via the Shipboard Envi- ronmental (data) Acquisition System (SEAS) to NOAA and were incorporated into the National En- vironmental Satellite, Data and Information Service model initialization." However, I have been unable to figure out if any (or all) of the reanalyses would have included this data.

Gary Barnes (not verified)

Tue, 09/16/2014 - 19:13

We are looking at Hurricane Felicia (2009) and have used the NCEP-NCAR reanalysis wind fields to look at the large scale environment. A colleague suggested that we look at CFSR. Does the CFSR reanalysis access different finer scale data? I understand it offers finer resolution but without finer resolution inputs we would necessarily see much difference? thanks, gary

Yes, CFSR uses finer scale observations than NCEP/NCAR Reanalysis. Automated aircraft reports: at cruising altitude 5-6 minutes between reports 737 cruises at 780 km/hr -&gt; reports every 65-78 km see: http://amdar.noaa.gov/FAQ.html#bytes More reports during ascents/decents. So CFSR can get finer details from the aircraft data. Of course, commercial aircraft won't be flying near Hurricane Felicia. Satellite data: NCEP/NCAR Reanalysis uses temperature retrievals and cloud track winds. The temperature retrievals are reduced to a fixed horizontal and vertical resolution This kept the amount of satellite data more or less constant and prevented the satellite data from overwhelming the conventional data as the spatial and frequency resolution of the satellite data improved. The CFSR uses a direct assimilation of the satellite radiances as well at cloud track winds. Much satellite radiance data is thinned or "averaged" because it is of a higher resolution than the assimilation system. So CFSR can access the finer details from the satellite data. (BTW direct assimilation of the radiances is better than temperature retrievals.) However, the CFSR doesn't assimilate radiance data in cloudy "pixels" and hurricanes tend to be cloudy. So the CFSR has observations to support the finer resolution. However, too near the hurricane, the observation systems don't do a good job of sampling.

Thank you very much for your input. We pulled up the NCEP-NCAR reanalysis and compared it to CFSR. Looking at the 250 u and v components one can see that virtually all the larger scale features (&gt; 5 deg latitude) are similar. The CFSR does generate many more fine scale features that I would be a little worried about, lots of closed contours covering length scales of 1-2 degrees. These tend to appear more frequently as one moves toward the equator. Of course subtle changes in the estimate will trigger a contour to appear or disappear. We see some nuances in the sub-tropical jet in CFSR but I think our interpretation of one vs . the other product would be close. Didn't see anything odd around the hurricane, at least at 250 hPa. Given the increased use of the aircraft data I imagine that the CFSR fields are better from Honolulu toward the west coast. regards, gary

Alina (not verified)

Fri, 10/24/2014 - 11:36

Hello Everyone, I would need to know whether either or all of the following 3 reanalyses: - NCEP/NCAR - ERA-Interim - JRA25 assimilates data from the Tropical Moored Buoy System. I am particularly interested in data between 1979 and the present. Thank you in advance for any information on this subject!

michael.fiorino

Mon, 11/03/2014 - 17:24

whither ERA-15 in this section?

I know ERA-15 is kind of 'OBE' (overtaken by events) relative to ERA-40 (I worked on both reanalyses)...However...

ERA-15 would be useful in assessing observing system variability and the benefits of observation data set 'cleansing'/improvement that were brought about from the initial comparisons of older obs to modern models in the pioneering reanalyses (NCEP/NCAR I NCEP/DOE II and ERA-15).

from an old reanalysis guy...

best /R Mike Fiorino

And the DAO reanalysis too! (Sorry, I think that was lost some time ago) ERA-15 is still available at the ECMWF data server (http://apps.ecmwf.int/datasets/data/era15/) and is also described at the Climate Data Guide (https://climatedataguide.ucar.edu/climate-data/era-15). As you say, it is probably best for marking the progression of reanalyses, but there are known issues that require new investigations to update to more recent data.

Paul Poli (not verified)

Tue, 11/04/2014 - 01:53

Hello Mike, Glad to exchange with you via this forum. Generally reanalysis producers prefer to see only their latest and shinest product used, not just because it tends to give them 'better press', but rather for the immediate feedback they get from users regarding the new products, and so as to improve them next time around. And thus we collectively tend to instruct users 'please do not use earlier products for new research' (i.e. feel free to continue using it in ongoing projects, but do not start *new* work with these data-- please use instead the latest product). If we went along with this we would actually shorten the list above! That said, in the user survey conducted last year (responded by 2500+, the survey mostly reached ECMWF users though), there were 170 respondents who said they were using ERA-15. I doubt these have all been using ERA-15 for the same work over and over in the past nearly 20 years so this suggests there are probably new users of ERA-15, as there are new users of ERA-40. I'll add ERA-15 there, but we should also add FGGE if everybody found its most ancient data holdings. The whole series of reanalyses makes for a nice historical summary of atmospheric research improvements in the last 3 decades! But this shouldn't confuse users, so we should also probably mark with a special flag here those products that we think are, how to say it better, 'outdated'? Paul Poli

None of the "operational" analyses (JMA, ECMWF, NMC, NCEP, etc) are considered to be reanalyses, simply because in each of these the record contains discontinuities caused by periodic upgrades and changes in model, analysis methods, resolution, observation streams, and whatever else constituted the improvements from which all the operational systems have benefitted over time. In fact, a major rationale for doing reanalysis at all was the possibility to create long records of weather, using modern analysis/forecast systems, and without the operational discontinuities. Comparisons can certainly be made however, between operational systems and reanalyses, for what that might be worth.

Sneha (not verified)

Tue, 12/30/2014 - 04:06

Hello, As per the documentation on MERRA, vertical resolution is 72 levels. However the v-wind given by the dataset has only 42 available levels eventhough the latitude and longitude resolutions remain the same. May I know what is the reason behind this?

The modeling and assimilation in the GEOS5 system that performs the MERRA reanalysis runs at 72 terrain following (eta) coordinates. These are not typically what most meteorological studies would favor, so the models eta coordinate is interpolated to 42 pressure levels. For certain variables it is beneficial to provide data in both coordinates. Please see the MERRA File Specification Document for details: http://gmao.gsfc.nasa.gov/research/merra/file_specifications.php

Cathy.Smith@noaa.gov

Thu, 02/05/2015 - 14:46

Does anyone know why the only surface humidity field from the ERA-Interim dataset is 2m dew point and not the more commonly used fields of specific humidity and/or relative humidity (which are also available as pressure level fields)?

The reason ERA-Interim only has 2m dew point temperature and not specific or relative humidity is historical. There is a FAQ about this, which describes how to calculate the 2m specific and relative humidity: http://www.ecmwf.int/en/does-era-40-dataset-contain-near-surface-humidity-data

What do you mean by "modern" humidity, Cathy? The synoptic surface atmospheric observations assimilated by ERA-Interm are reported as dewpoints and the synoptic message that contains the dew points also reports temperature and surface pressure, whose values are needed to compute other humidity variables. This was past practice, and is current practice. What is actually measured today depends on the instrument that is deployed, but the conversion to dewpoints for uniform reporting purposes should follow procedures laid down in WMO regulations. Dew point is no less modern than any other humidity variable. If reanalysis output is to be made available for only one humidity variable, it seems reasonable to me that one should use the same variable that is used in observational reports. Providing output for alternative humidity variables, using documented conversion formulae is certainly an option, though small numerical inconsistencies between the variables would arise from data encoding and interpolation.

Having specific humidity available in addition to Td (which is also important, as you say) is a valuable thing and makes certain calculations much easier as you are not combining T and q in one variable. To get the actual moisture in the air with Td, you have to also calculate the saturation q for the measured temperature so you have to read both Tsfc and Td and do the additional calculation. And, as many of the other reanalyses do not output dewpoint, it makes comparisons with them much easier if you have the same variable. In addition, the calculation of saturation q is a nonlinear function of T, so if you want to compare monthly means of other reanalyses, you have to calculate q (sfc) for each time step in a monthly average and then average (or compute Td from each time step of q and T in the other reanalyses). The non-linearity is also an issue for reanalyses which output ensemble means. Theoretically, Td ought to be calculated separately for each member and then averaged.

Agreed it is a bit of a nuisance to have to go through these steps. We went through them to compute monthly-mean specific and relative humidities in our 2010 ERA-Interim paper (doi: 10.1029/2009JD012442) and go through them regularly in preparing our annual contributions to the BAMS state-of-the climate article. But there are several possible variables - the HadISDH datset based on direct analysis of the synoptic data caters for seven different ones.

Patrick Boylan (not verified)

Wed, 02/11/2015 - 16:02

Does anyone know why the surface pressures from the surface level files (Sp) do not equal the surface pressure from the model level files (ln(Sp)). I have looked at several months in 2010 at all 4 times and step equal to 0.

The surface pressure (sp) archived at the surface is calculated from the model level data. However, the latter is archived by storing the spectral coefficients of ln(sp) (natural log of sp) whereas at the surface sp is archived as sp in grid point space. Differences between these two fields will arise because of the different packing errors associated with their GRIB packing and due to any differences in the methodologies used for transforming from the spectral representation to the grid point representation.

Alexander Cher… (not verified)

Tue, 02/24/2015 - 07:01

Does anyone know something about precipitation data in the ERA-20C reanalyses. Here is no such variable: http://apps.ecmwf.int/datasets/data/era20c_daily/ but there are new variables like: Total column rain water or Total column snow water (additionally to the standard Total column liquid/ice water). Is the following assumption correct: precipitation rate = Total column rain water + Total column snow water ? Is there any literature on this issue?

In ERA-20C there are various precipitation fields at the surface. There are no mean rate precipitation fields - the precipitation fields are accumulated from the beginning of the forecast for +step hours. To obtain the mean rate quantity you need need to divide by the number of seconds in step. The parameter names are: convective precipitation, convective snowfall, large-scale precipitation, large-scale snowfall, snowfall and total precipitation. Total column fields are vertically integrated quantities, not fluxes.

Alba Cid (not verified)

Tue, 04/28/2015 - 02:21

Hello! I'm using sea level pressure fields from ERA-interim reanalysis and from the 20th century reanalysis. There are some regions where SLP from 20CR is much lower than SLP from ERA-interim. For instance, around lon = 134ºW, lat = 14ºN, there are several drops in the SLP from 20CR not present in ERA-interim, with differences of about 20 mb. Does anybody know if there is a physical explanation for that or if they are outliers? Thank you very much in advance Alba

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