An assessment of radiative flux biases in the climate forecast system model CFSv2
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An assessment of radiative flux biases in the climate forecast system model CFSv2 Siddharth Kumar1,2 · R. Phani1 · P. Mukhopadhyay1 · C. Balaji2 Received: 13 August 2020 / Accepted: 17 November 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract An extensive analysis of radiative flux biases in the Climate Forecast System Model Version 2 (CFSv2) is done. Annual mean and seasonal variations of biases at the surface and top of the atmosphere (TOA) are reported in the global domain. Large regional biases in shortwave (SW) and longwave (LW) radiation are observed over convectively active zones in the tropics. The relative contribution of various processes responsible for the reported biases is quantified. The poor simulation of clouds and inadequate representation of surface properties seem to be major contributors. Over certain regions, errors due to different processes add up, whereas, over other regions, errors tend to nullify each other. Surface and atmospheric variables taken as input parameters in the radiative transfer modules are compared with satellite-based observations. The maximum biases in SW and LW radiation are observed over the regions of persistent low clouds. The magnitude of the SW and LW biases at the TOA is in phase with the biases in cloud fraction by and large. However, the error in the radiative fluxes due to errors in surface radiative properties is of equal importance. The cold bias in near-surface air temperature reported in other studies may partly be attributed to an underestimation in the net SW radiation at the surface. In the present study, a plausible prescription is also provided to correct the source of the biases. Keywords Radiation · CFS · Cloud · Bias · Physical process
1 Introduction Radiation from the Sun is the only external source of energy on the Earth, responsible for driving atmospheric and oceanic circulation. A fraction of incident energy in the form of shortwave (SW) radiation is reflected into space by the earthatmosphere system, termed as the planetary albedo. The most recent estimate of the Earth’s albedo from the Cloud and Earth’s Radiant Energy System (CERES) data is ~ 0.29 (Stephens et al. 2015; Wielicki et al. 1996). The remaining part of incident energy gets absorbed by the system and reemitted as the longwave (LW) radiation. On longer timescales, a radiative balance needs to be maintained at the top of the atmosphere (TOA) (Trenberth et al. 2009).
* Siddharth Kumar [email protected] 1
Indian Institute of Tropical Meteorology, Ministry of Earth Sciences, Pune, India
Indian Institute of Technology, Madras, Chennai, India
2
Global climate models (GCMs) are expected to replicate the radiative balance inferred from the observations because the model’s mean climate depends on the energy distribution among its various components (Wild et al. 2015). Several authors have documented radiation biases in Coupled Model Intercomparison Project (CMIP) models (e.g., Pincus et al. 2008; Wild et al. 2013, 2015; Lauer and Hamilton,
