Evaluation of the Surface Radiation Budget in the Atmospheric Component of the Hadley Centre Global Environmental Model (Hadgem1)

15 SEPTEMBER 2008 BODAS-SALCEDO ET AL. 4723 Evaluation of the Surface Radiation Budget in the Atmospheric Component of the Hadley Centre Global Environmental Model (HadGEM1) A. BODAS-SALCEDO,M.A.RINGER, AND A. JONES Met Office, Hadley Centre, Exeter, United Kingdom (Manuscript received 13 June 2007, in final form 1 February 2008) ABSTRACT The partitioning of the earth radiation budget (ERB) between its atmosphere and surface components is of crucial interest in climate studies as it has a significant role in the oceanic and atmospheric general circulation. An analysis of the present-day climate simulation of the surface radiation budget in the atmospheric component of the new Hadley Centre Global Environmental Model version 1 (HadGEM1) is presented, and the simulations are assessed by comparing the results with fluxes derived from satellite data from the International Satellite Cloud Climatology Project (ISCCP) and ground measurements from the Baseline Surface Radiation Network (BSRN). Comparisons against radiative fluxes from satellite and ground observations show that the model tends to overestimate the surface incoming solar radiation (Ss,d). The model simulates Ss,d very well over the polar regions. Consistency in the comparisons against BSRN and ISCCP-FD suggests that the ISCCP-FD database is a good test for the performance of the surface downwelling solar radiation in climate model simulations. Overall, the simulation of downward longwave radiation is closer to observations than its shortwave counterpart. The model underestimates the downward longwave radiation with respect to BSRN measurements by 6.0 W mϪ2. Comparisons of land surface albedo from the model and estimates from the Moderate Resolution Im- aging Spectroradiometer (MODIS) show that HadGEM1 overestimates the land surface albedo over deserts and over midlatitude landmasses in the Northern Hemisphere in January. Analysis of the seasonal cycle of the land surface albedo in different regions shows that the amplitude and phase of the seasonal cycle are not well represented in the model, although a more extensive validation needs to be carried out. Two decades of coupled model simulations of the twentieth-century climate are used to look into the model’s simulation of global dimming/brightening. The model results are in line with the conclusions of the studies that suggest that global dimming is far from being a uniform phenomenon across the globe. 1. Introduction tion regarding the top-of-the-atmosphere (TOA) radiation budget, and they have been extensively used as The earth radiation budget (ERB) is the ensemble of validation tools for climate models (Li et al. 1997; radiative fluxes entering and leaving the earth– Chevallier and Morcrette 2000; Bony et al. 2004; Ringer atmosphere system, which drives the earth’s climate. and Allan 2004). However, TOA ERB measurements Therefore, measurements of this balance are needed to do not provide a complete constraint on the atmo- improve our knowledge of the earth’s climate and cli- sphere’s radiative properties. This means that the par- mate change (Ramanathan 1987; Ramanathan et al. titioning of the ERB between the atmosphere (ATM) 1989). Since the 1970s, great efforts have been made to and surface (SFC) components is of crucial interest in measure this budget globally and with sufficient accu- climate studies. Gleckler (2005) showed that this parti- racy by means of broadband sensors aboard satellites. tioning has a significant role in the oceanic and atmo- All these measurements provide invaluable informa- spheric general circulation. In addition, the balance between longwave radiative cooling and latent heating establishes a link between radiative processes and the hydrological cycle. Therefore, any changes in the radia- Corresponding author address: Dr. A. Bodas-Salcedo, Met Of- fice, Hadley Centre, FitzRoy Rd., Exeter EX1 3PB, United King- tive budget of the atmosphere will have an impact on dom. the response of the hydrological cycle (Stephens 2005). E-mail: [email protected] These two reasons highlight the importance of knowing DOI: 10.1175/2008JCLI2097.1 Unauthenticated | Downloaded 10/10/21 04:08 PM UTC JCLI2097 4724 JOURNAL OF CLIMATE VOLUME 21 both the surface and the atmospheric radiation budgets lation of global dimming/brightening in section 7. (SARB), not only for their direct impact on the general Conclusions are presented in section 8. circulation, but also for their role in climate feedback problems. 2. Model description and experimental design As satellites provide TOA measurements, the SRB has to be modeled from those measurements, together We use present-day climate simulations from the at- with information about the state of the atmosphere and mosphere-only version of the new Hadley Centre cli- the surface. A global perspective of the surface radia- mate model, HadGEM1, referred to as HadGAM1. tion budget, both in the shortwave and longwave parts HadGAM1 uses a horizontal resolution of 1.25° lati- of the spectrum, can only be obtained from satellites. tude by 1.875° longitude, and has 38 vertical levels, the Several studies have used the International Satellite top level being at around 39 km. The simulations used Cloud Climatology Project (ISCCP) C1 data to provide are from a five-member ensemble of model runs of a global perspective of the surface radiation budget HadGAM1, forced with observed sea surface tempera- (Pinker and Laszlo 1992; Darnell et al. 1992; Whitlock tures (SSTs) from the second Atmospheric Model In- et al. 1995; Zhang et al. 1995; Gupta et al. 1999). Li and tercomparison Project (AMIP-II; Gates et al. 1999), Leighton (1993) computed a global climatology of the each member using different initial conditions. The solar radiation budget using data from the Earth Ra- runs start on December 1978, and we use a 20-yr cli- diation Budget Experiment (ERBE), not relying on matology from 1981 to 2000. We use monthly mean ISCCP data. Li (1995) intercompared the net surface diagnostics of the different components of the SARB as well as other diagnostics that help the interpretation of shortwave radiation (NSSR) as derived from ERBE the results (e.g., cloud cover and precipitable water against the NSSR derived from ISCCP by Whitlock et content). Here we give some details of the physical al. (1995). More recently, these datasets have been up- processes relevant for the simulation of the surface ra- graded by using ISCCP-D1 data and improved algo- diation budget, but a more detailed description of Had- rithms (Stackhouse et al. 1999; Zhang et al. 2004). In GAM1, and its performance in terms of global clima- addition, these satellite-based databases, along with tology, variability, and regional climate can be explored surface observations, have been used to evaluate the in Martin et al. (2006) and Ringer et al. (2006). performance of the simulations of the surface radiation The radiation code is that of Edwards and Slingo budget by climate models (Garrat 1994; Li et al. 1997; (1996) used in the third climate configuration of the Wild et al. 1998; Wild 2005). However, interest in the Met Office Unified Model (HadCM3; HadAM3 for the SRB is not confined to global scales. Smith et al. (2002) atmospheric component), with some developments. showed that the regional climate and surface radiation The longwave band from 1200 to 1500 cmϪ1 has been are related, and Bolle et al. (2006) used the surface net split at 1330 cmϪ1 in order to better represent the over- radiation as a possible indicator of changes in the Medi- lap between CH4 and N2O; gaseous absorption is based terranean region. on the updated High-Resolution Transmission (HIT- Note that we usually refer to the atmospheric com- RAN) 2000 database (Rothman et al. 2003); the water ponent of the Hadley Centre Global Environmental vapor continuum is version 2.4 of the Clough–Kneizys– Model version 1 (HadGEM1) as the Hadley Centre Davies (CKD) formulation (Clough et al. 1992) and has Global Atmospheric Model (HadGAM1), and we will been included in the shortwave region; ice crystal use this notation throughout the paper. This paper is sizes are determined using the parameterization by organized as follows: The data used in this study and a Kristjánsson et al. (2000); the sea surface albedo is brief description of the model are presented in section based on the functional form of Barker and Li (1995), 2. Section 3 compares the climatologies of SRB pro- modified in the light of aircraft data; and the land sur- vided by HadGAM1 and ISCCP-FD. The perfomance face albedo is described by Essery et al. (2003). of HadGAM1’s simulation of the surface radiation bud- The simple aerosol climatology used previously (Cu- get is evaluated against surface observations in section sack et al. 1998) has largely been superseded in Had- 4. Section 5 looks at the representation of the interan- GAM1 by schemes to interactively simulate sulfate, nual variability of surface incoming radiation over the fossil-fuel black carbon, biomass-burning and sea-salt tropical Pacific, and section 6 compares HadGAM1’s aerosols, as detailed in Martin et al. (2006). Only the land surface albedo with that from the Moderate Reso- stratospheric sulphuric acid aerosol component of the lution Imaging Spectroradiometer (MODIS). Two de- earlier climatology has been retained. The direct radia- cades of coupled model simulations of the twentieth- tive effect (scattering and absorption of radiation) of all century climate are used to look into the model’s simu- aerosols is included; this means that the “semi-direct” Unauthenticated | Downloaded 10/10/21 04:08 PM UTC 15 SEPTEMBER 2008 BODAS-SALCEDO ET AL. 4725 effect (the impact on clouds of the warming caused by terval (i.e., S for shortwave, L for longwave, and T for absorbing aerosols; Hansen et al. 1997) is also included. total), the first subscript denotes the level (i.e., t for Parameterizations of both first and second indirect TOA, a for atmosphere, and s for surface), and the aerosol effects (impact on cloud droplet size and on second subscript denotes the direction (i.e., u for up- precipitation efficiency, respectively) are also included, welling, d for downwelling, and n for net).

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