Evaluating CFSR Air-Sea Heat, Freshwater, and Momentum Fluxes in the context of the Global Energy and Freshwater Budgets

Principal Investigator:

Dr. Lisan Yu, Woods Hole Oceanographic Institution, lyu@whoi.edu

Co-Investigator:

Dr. Yan Xue, NOAA, National Centers for Environmental Prediction, Yan.Xue@noaa.gov

Collaborators:

Dr. Arun Kumar, NOAA, National Centers for Environmental Prediction, arun.kumar@noaa.gov

Dr. Xiangze Jin, Woods Hole Oceanographic Institution, xjin@whoi.edu

Dr. Caihong Wen, NOAA, National Centers for Environmental Prediction, Caihong.wen@noaa.gov

Abstract

The project aims at providing a comprehensive assessment of the Coupled Forecast System Reanalysis (CFSR) by NOAA NCEP in representing air-sea heat, freshwater, and momentum fluxes in the context of the global energy and water budgets. 

CFSR is the first and only reanalysis that operates a coupled atmosphere-ocean-land climate system with an interactive sea-ice component, and the one that has the finest spatial resolution (~0.5°) ever produced by any reanalysis. Evidence has clearly pointed to the advantages and strengths of the finer-resolution coupled CFSR system in characterizing air-sea interaction processes at regional and global scales. However, biases/errors in the CFSR flux components at various temporal scales are also evidenced. The biases/errors appear to have significant impact on the estimates of the energy and water budgets over the global oceans. Currently, CFSR produces a global energy imbalance of 15 Wm^-2, which is about 10 Wm^-2 unexplainably higher than the estimates from the earlier NCEP reanalyses. Balancing the global energy/water budgets has long been a challenging issue for the air-sea flux community, with estimates ranging considerably, from 2 to 30 Wm^-2, when computed using available reanalyzed, ship-, and satellite-based flux products (see Figure 1). The global energy/water budgets are central to the understanding of climate variability and change produced by the reanalyses. Knowledge of the potential impact of biases/errors in surface flux estimates on the global budget estimates will be highly beneficial to not only the users of CFSR products but also the development of the next-generation Earth System reanalysis.

Figure 1. The energy (heat) budget over the global ice-free oceans constructed from 12 net heat flux products, including CORE2, three early reanalyses (NCEP1, NCEP2, ERA40), three latest reanalyses (CSFR, ERAinterm, MERRA), the ship-based NOCS2, the net heat flux from the combination of OAFlux-1° with satellite radiation products (ISCCP, SRB, CERES), and from OAFlux-0.25°+CERES. The averages were constructed for the period of 1985-2009 except for ERA40 (1985-2002), CERES+OAFlux(1°) and CERES+OAFlux(0.25°) (2000-2009). 

The primary objectives of the project are

 (i)  to identify the strength and weakness of the CFSR surface flux components by comparison with in situ flux measurement, satellite-based analyses and other reanalyses products and  understand the sources of biases,

(ii) to investigate the effect of spatial resolution in improving the accuracy and spatial structure of CFSR  fluxes on regional and global scales,

(iii) to investigate the use of physical constraints together with ocean state variables to diagnose and understand the uncertainties in CFSR air-sea fluxes.

The study will include the analysis of the earlier and the latest reanalyses as value-added evaluation. The significance of the project research is in the potential to  (i) establish a baseline that can be used to help determine the scope and extent of the CFSR surface fluxes to be applied; (ii) improve our understanding of the state-of-estimation of air-sea fluxes in latest reanalyses; (iii) obtain new insights on the cause of the discrepancies in global energy/freshwater budget estimates based on air-sea fluxes; and (iv) obtain practical recommendations for future improvement of air-sea flux estimation in reanalyses.

Progress