(Hadgem1). Part I: Model Description and Global Climatology

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1274 JOURNAL OF CLIMATE VOLUME 19

The Physical Properties of the Atmosphere in the New Hadley Centre Global Environmental Model (HadGEM1). Part I: Model Description and Global Climatology

G. M. MARTIN,M.A.RINGER,V.D.POPE,A.JONES,C.DEARDEN, AND T. J. HINTON Hadley Centre for Climate Prediction and Research, Met Office, Exeter, United Kingdom

(Manuscript received 28 April 2005, in final form 28 July 2005)

ABSTRACT

The atmospheric component of the new Hadley Centre Global Environmental Model (HadGEM1) is described and an assessment of its mean climatology presented. HadGEM1 includes substantially improved representations of physical processes, increased functionality, and higher resolution than its predecessor, the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). Major developments are the use of semi-Lagrangian instead of Eulerian advection for both dynamical and tracer fields; new boundary layer, gravity wave drag, microphysics, and sea ice schemes; and major changes to the convection, land surface (including tiled surface characteristics), and cloud schemes. There is better coupling between the atmosphere, land, ocean, and sea ice subcomponents and the model includes an interactive aerosol scheme, representing both the first and second indirect effects. Particular focus has been placed on improving the processes (such as clouds and aerosol) that are most uncertain in projections of climate change. These developments lead to a significantly more realistic simulation of the processes represented, the most notable improvements being in the hydrological cycle, cloud radiative properties, the boundary layer, the tropopause structure, and the representation of tracers.

1. Introduction Lagrangian dynamical core, which has recently been implemented in the Met Office’s operational forecast Advances in our understanding of the climate sys- models. In addition, continuing research into climate tem, together with the need to reduce the uncertainties processes has yielded a number of new physical param- associated with predictions of future climate change, eterizations, improving the representation of these pro- inevitably mean that global models will need to repre- cesses and adding new functionality to the model. Fur- sent the physical processes that determine climate with thermore, increases in computing resources have al- increasing levels of complexity. The development of the lowed us to increase the horizontal and vertical Met Office’s Hadley Centre Global Environmental resolutions in both the atmosphere and ocean com- Model (HadGEM) represents a major step in this di- pared to our previous model, the Third Hadley Centre rection, in which greater emphasis is placed on the in- Coupled Ocean–Atmosphere General Circulation teractions between climate, chemistry, and ecosystems. Model (HadCM3). Taken together, these develop- HadGEM1 marks the first stage of this process, which ments mean that HadGEM1 is thus very different from will eventually result in a suite of models incorporating its predecessor. many different subcomponents. Climate model development carries with it the re- A key aspect of our strategy is for a unified model quirement for continuing evaluation of all aspects of (UM), used for both numerical weather prediction and the simulated climate (e.g., the mean climatology, spa- climate modeling. It has therefore been a particular aim tial and temporal variability, and extreme events) to- in building HadGEM1 to incorporate the new semi- gether with a thorough investigation of model systematic errors. In Part I of this paper we evaluate the mean climatology of the atmospheric component of Corresponding author address: Dr. G. M. Martin, Hadley Cen- tre for Climate Prediction and Research, Met Office, FitzRoy HadGEM1 and highlight the most important new fea- Road, Exeter EX1 3PB, United Kingdom. tures of the model. In Part II (Ringer et al. 2006) we E-mail: [email protected] assess several aspects of the global variability and re-

JCLI3636 1APRIL 2006 MARTIN ET AL. 1275 gional climate. These include model energetics, North- monotone treatment of tracer transport; predictor–cor- ern Hemisphere storm tracks and blocking activity, syn- rector implementation of a two-time-level semi-implicit optic variability over Europe, tropical waves and the time integration scheme; and better geostrophic adjust- Madden–Julian oscillation, the Asian summer mon- ment properties, bringing better balance (and reduced soon, and the quasi-biennial oscillation (QBO). time step dependence) to the coupling with physical The purpose of this paper is to focus on key aspects parameterizations, which now calculate increments in of the model atmosphere in order to provide informa- parallel based on balanced states rather than in se- tion that will help the scientific community to interpret quence. The new dynamical core brings the capability studies and predictions made using HadGEM1 within to run at much higher resolution in the future without the context of an understanding of its limitations and the need for further major revisions to the dynamics sensitivities. A detailed analysis of changes in model itself. HadGEM1 is thus one of the few fully coupled fields compared with the previous Hadley Centre models running without flux correction to use semi- model, HadCM3, and of the impacts of increasing hori- Lagrangian dynamics to advect the dynamical fields. zontal resolution and coupling between atmosphere Other examples are described in Douville et al. (2002) and ocean models, is given in Martin et al. (2004). A and Lazar et al. (2005), while models such as that de- paper by Johns et al. (2006, hereafter JOH) focuses on scribed in Kiehl et al. (1998) use such schemes solely to aspects of the coupled model such as sea surface tem- advect moisture. perature and salinity, ocean circulation, heat transport, The model uses the Arakawa-C grid in which the sea ice distribution, the El Niño–Southern Oscillation, zonal and meridional wind components are staggered cloud–climate interactions, and climate sensitivity, as as well as the momentum and thermodynamic vari- well as considering the overall skill of the model against ables. This arrangement has the best geostrophic ad- HadCM3. justment properties overall for atmospheric flows and The rest of this paper is arranged as follows: section tends to work better with semi-implicit time schemes 2 provides a description of the major new features of (Davies et al. 2005). The previous model, HadCM3, the atmospheric component of HadGEM1; section 3 used a resolution typically adopted in other climate describes the experiments performed and the datasets models, namely 2.5° latitude by 3.75° longitude (usually used for the model evaluation; section 4 provides an referred to as “N48”). HadGEM1 uses double this reso- overall statistical assessment of the model; section 5 lution in the atmosphere, that is, 1.25° latitude by 1.875° presents a more detailed assessment of the model’s cli- longitude (“N96”). The higher resolution has been matology compared to its predecessor, HadCM3; and shown in previous model versions to improve some as- section 6 provides a summary of the most important pects of the model’s climatology and its variability by results and conclusions. resolving smaller-scale features and improving the representation of dynamics (Pope and Stratton 2002). However, it can also reveal compensating errors in the 2. Model description model and may lead to degradation in some model fields while others improve. It is therefore important to The major differences between HadGEM1 and evaluate the benefits of higher resolution against the HadCM3 are summarized in Table 1. This section pre- increased cost of the simulations. Although most of the sents an overview of the most important new features results shown in this paper come from HadGEM1 at of the model’s atmospheric component. A full descrip- N96 resolution, we have compared HadGEM1 runs at tion of the changes to the ocean model is provided in both resolutions with those from HadCM3 in order to JOH. separate resolution effects from those of changing the model formulation. The new model runs on a Charney–Phillips vertical a. Dynamical core, configuration, and resolution grid in which the momentum and thermodynamic vari- The new dynamical core is described by Davies et al. ables are staggered. This avoids having undesirable (2005). It is a nonhydrostatic, fully compressible, deep computational modes and has more accurate normal atmosphere formulation using a terrain-following, modes. The latter avoids the possibility of oscillations height-based vertical coordinate. It includes the follow- across atmospheric boundaries such as subsidence in- ing features: semi-Lagrangian advection of all prognos- versions and the tropopause. The vertical resolution is tic variables except density, permitting relatively long twice that of HadCM3 and the model top extends to time steps (the model currently uses a time step of 30 over 39 km in height. The vertical levels are defined in min) to be used at high resolution; a conservative and Table 2. 1276 JOURNAL OF CLIMATE VOLUME 19

TABLE 1. Summary of HadCM3 and HadGEM1 configurations.

HadCM3 HadGEM1 Atmospheric grid Arakawa-B grid Arakawa-C grid Hydrostatic Yes No Horizontal resolution 2.5° latitude ϫ 3.75° longitude 1.25° latitude ϫ 1.875° longitude Vertical resolution 19 levels; hybrid pressure; Lorenz grid 38 levels; hybrid height; terrain-following near bottom (atmosphere) boundary; Charney–Phillips grid Physics–dynamics Sequential Parallel split (slow processes), sequential split (fast coupling processes). (Dubal et al. 2004) Dynamics Eulerian advection, split-explicit time Semi-Lagrangian advection, conservative monotone integration (Cullen 1993) treatment of tracers; semi-implicit time integration (Staniforth et al. 2003; Davies et al. 2005) Radiation Edwards and Slingo (1996); Cusack et Edwards and Slingo (1996); Cusack et al. (1999a) al. (1999a) Boundary layer Local Richardson number mixing Nonlocal mixing scheme for unstable BLs (Lock et al. scheme (Smith 1990, 1993) 2000). Local Richardson number scheme for stable layers (Smith 1990, 1993) Microphysics Senior and Mitchell (1993); evaporation Mixed phase scheme including prognostic ice content; of precipitation as in Gregory (1995) solves physical equations for microphysical processes using particle size information (Wilson and Ballard 1999) Convection Mass flux scheme (Gregory and Revised scheme including diagnosed deep and Rowntree 1990); convective shallow convection; new thermodynamic closures at downdraughts (Gregory and Allen lifting condensation level; new CMT 1991); convective momentum parameterization based on flux–gradient transport (Gregory et al. 1997) relationships; parameterized entrainment /detrainment rates for shallow convection. Based on ideas in Grant and Brown (1999) and Grant (2001). Convective anvil scheme (Gregory 1999) Gravity wave drag Gregory et al. (1998) GWD scheme with low-level flow blocking Webster et al. (2003) Orography Derived from U.S. Navy 10Ј dataset Derived from Global Land-Based 1-km Base Elevation (GLOBE) dataset at 1Ј resolution Hydrology MOSES-I (Cox et al. 1999) MOSES-II (Essery et al. 2001); nine surface tile types plus coastal tiling; seasonally varying vegetation (Lawrence and Slingo 2004) Clouds Smith (1990); prescribed critical relative Smith (1990); parameterized RH-crit (Cusack et al. humidity for cloud formation 1999b); vertical gradient area cloud scheme (Smith (RH-crit) et al. 1999) River routing Simple basin aggregate output Embedded 1° ϫ 1° river transport/hydrology instantaneously to ocean submodel (Oki and Sud 1998) Aerosols Interactive sulphate (anthropogenic Interactive sulphate, sea salt, black carbon, and sources only, direct effect only); all biomass-burning aerosol schemes; direct/indirect other aerosols and effects prescribed radiative forcing (Jones et al. 2001; Roberts and Jones 2004; Woodage et al. 2003) Ocean horizontal 1.25° ϫ 1.25° 1° ϫ 1° except in Tropics where it increases smoothly resolution to 1⁄3° in latitude at equator Ocean vertical 20 levels 40 levels resolution Ocean model Gordon et al. (2000) HadGOM1 (JOH): EVP sea ice dynamics; multiple-category ice thickness distribution; linear free-surface scheme; high-resolution coast/bathymetry; fourth-order advection of active tracers

The impact of the increased vertical resolution alone resolution, to the extent that some schemes (e.g., the is not investigated here. Experience with previous ver- boundary layer, convection, and cloud) behave very dif- sions of the climate model (e.g., Pope et al. 2001; Martin ferently at different resolutions. This is particularly the et al. 2000; Bushell and Martin 1999) has shown that the case for changes in the lower-tropospheric resolution. model physics is particularly sensitive to the vertical Martin et al. (2000) showed that increasing the resolu- 1APRIL 2006 MARTIN ET AL. 1277

TABLE 2. Vertical levels in HadGEM1. The hybrid height rep- troposphere. However, both of those studies suggested resents the true height above a surface at mean sea level. Layer that the changes observed were a result of an underly- ϭ boundary heights depend on surface orographic height up to n ing sensitivity to vertical resolution in model interac- 29 but are at constant real height above this. Note that the height corresponds to a World Meteorological Organization standard tions between boundary layer and convective pro- atmosphere. cesses. Although many of the changes to the boundary layer and convection schemes in HadGEM1 are de- Layer boundary Hybrid “eta” Hybrid signed to address such sensitivities, the nature of the (n) coordinate height (m) new schemes is such that a certain minimum level of reso- 38 (lid) 1.0 39 254.8 lution is required, higher than that of HadCM3. We thus 37 0.838 334 8 32 908.7 consider the increased vertical resolution in HadGEM1 36 0.744 343 5 29 219.1 35 0.677 206 8 26 583.6 as part of the change in the model formulation. 34 0.623 064 3 24 458.3 33 0.576 389 7 22 626.1 b. Physical processes 32 0.534 716 0 20 990.2 31 0.496 830 8 19 503.0 1) RADIATION AND OZONE 30 0.462 073 7 18 138.6 The radiation code is that of Edwards and Slingo 29 0.429 891 3 16 875.3 28 0.399 814 2 15 694.6 (1996) used in HadCM3 with some developments. The 27 0.371 439 6 14 580.8 band structure remains unchanged, except that the Ϫ 26 0.344 416 2 13 520.0 longwave band from 1200 to 1500 cm 1 has been split at 25 0.318 432 1 12 500.0 1330 cmϪ1 in order to better represent the overlap be- 24 0.293 467 0 11 520.0 tween CH and N O; gaseous absorption is based on 23 0.269 520 9 10 580.0 4 2 22 0.246 593 8 9680.0 the updated High-Resolution Transmission (HITRAN) 21 0.224 685 7 8820.0 2000 database (Rothman et al. 2003); the water vapor 20 0.203 796 6 8000.0 continuum has been updated to version 2.4 of the 19 0.183 926 4 7220.0 Clough–Kneizys–Davies (CKD) formulation (Clough 18 0.165 075 2 6480.0 et al. 1992) and has been included in the shortwave 17 0.147 243 0 5780.0 16 0.130 429 8 5120.0 region; the simple linear fits to the single-scattering 15 0.114 635 6 4500.0 properties of water droplets have been replaced by 14 0.099 860 3 3920.0 higher-order Padé approximants; ice crystal sizes are 13 0.086 104 0 3380.0 determined using a new parameterization based on the 12 0.073 366 8 2880.0 relationship between effective dimension and tempera- 11 0.061 648 5 2420.0 10 0.050 949 1 2000.0 ture described in Kristjansson et al. (2000), although 9 0.041 268 8 1620.0 that was expressed in terms of mean maximum dimen- 8 0.032 607 4 1280.0 sion; the sea surface albedo is based on the functional 7 0.024 965 1 980.0 form of Barker and Li (1995) modified in the light of 6 0.018 341 7 720.0 aircraft data (J. Haywood 2005, personal communica- 5 0.012 737 3 500.0 4 0.008 151 9 320.0 tion), and a representation of the frequency depen- 3 0.004 585 4 180.0 dence of the albedo is included. 2 0.002 038 0 80.0 The ozone distribution in HadCM3 was imposed 1 0.000 509 5 20.0 and did not evolve with changes in the tropopause. In 0 (surface) 0.0 0.0 HadGEM1 we introduce a scheme that moves ozone to follow the tropopause while conserving the column tion in the lowest 2.5 km of the atmosphere resulted in mass in the troposphere and stratosphere separately. major changes to the precipitation distribution. Large 2) AEROSOLS decreases in equatorial precipitation, particularly over Indonesia, and increases in precipitation to the north A significant new feature of HadGEM1 is the inclu- and south, especially over the Arabian Sea, South sion of interactive schemes for various aerosol species. China Sea, and western Pacific Ocean, resulted from These include sulphate, sea salt, black carbon, and bio- the same increase in lower-tropospheric resolution as is mass-burning aerosols; only a simple treatment of the made between HadCM3 and HadGEM1. Bushell and first of these was present in HadCM3. Martin (1999) showed that the representation of the The sulphate scheme is a development of that de- amount and vertical distribution of stratocumulus cloud scribed in Jones et al. (2001). It simulates two modes of was improved with increased resolution in the lower free aerosol particle (designated the Aitken and accu- 1278 JOURNAL OF CLIMATE VOLUME 19

TABLE 3. Forcing datasets used in the HadGEM1 and HadCM3 atmosphere-only integrations described in this paper. The data used for the HadGEM1 coupled model runs shown are identical except for ozone, which uses the time series of the Stratospheric Processes and Their Role in Climate (SPARC) data from 1870 to 1990, and trace gases, which uses annual mean time series assembled from a number of sources (J. Lowe 2005, personal communication).

Dataset HadCM3 HadGEM1 Orography U.S. Navy 10Ј orography data 1Ј GLOBE data Soil parameters Wilson and Henderson-Sellers (1985) Wilson and Henderson-Sellers (1985); Soil carbon from Post et al. (1982), processed by Woodward et al. (1995) Vegetation Wilson and Henderson-Sellers (1985) International Geophysical Biophysical Program (IGBP; derived from AVHRR) Sea surface temperature AMIP-II (www-pcmdi.llnl.gov/projects/ AMIP-II (www-pcmdi.llnl.gov/projects/amip) amip) Sea ice AMIP-II (www-pcmdi.llnl.gov/projects/ AMIP-II (www-pcmdi.llnl.gov/projects/amip) amip) Land DMS emissions — Spiro et al. (1992) Seawater DMS concentrations — Kettle et al. (1999) Anthropogenic sulphur From the 1B inventory compiled under Smith et al. (2004) dioxide emissions the Global Emissions Inventory Activity of the International Global Atmospheric Chemistry project and Orn et al. (1996)

Volcanic SO2 emissions — Andres and Kasgnoc (1998) Sulphate oxidants — Derived from STOCHEM Soot emissions — T. Nozawa (2003, personal communication) Biomass aerosols — T. Nozawa (2003, personal communication) Ozone AMIP-II (www-pcmdi.llnl.gov /projects/ From SPARC Web site for 1990. Assembled amip) from Randel and Wu (1999), Randel et al. (1999), and Kiehl et al. (1999) from the Stratospheric Aerosol and Gas Experiment and Total Ozone Mapping Spectrometer ozone trends, together with a mixture of tropospheric monitoring and model results ϫ Ϫ4 Ϫ1 ϫ Ϫ4 Ϫ1 ϫ CO2 CH4 NO2 CFC-11 CFC-12 Fixed values: 5.287 10 kg kg Fixed values: 5.241 10 kg kg 9.139 9.116 ϫ 10Ϫ7 kg kgϪ1 4.861 ϫ 10Ϫ7 kg 10Ϫ7 kg kgϪ1 4.665 ϫ 10Ϫ7 kg kgϪ1 1.053 ϫ kgϪ1 1.053 ϫ 10Ϫ9 kg kgϪ1 1.595 ϫ 10Ϫ9 kg kgϪ1 1.595 ϫ 10Ϫ9 kg kgϪ1 10Ϫ9 kg kgϪ1 mulation modes) and also includes a mode for sulphate al. 2003) aerosol schemes include modes for freshly dissolved in cloud water. These three modes, along with emitted particles that gradually age over time into an- the mass mixing ratios of sulphur dioxide and dimethyl other, more hygroscopic, mode ; there is also a mode sulphide (DMS), are advected by the model’s tracer for aerosol that has become incorporated into cloud advection scheme and undergo the processes of wet and droplets. As well as their different size distributions and dry removal. The scheme has an improved treatment of optical properties, a key difference between these two the diffusional scavenging of Aitken-mode sulphate aerosol types is that aged black carbon is considered to aerosol (Roberts and Jones 2004) and now includes the be only slightly hygroscopic, only becoming incorpo- coagulation of Aitken- to accumulation-mode sulphate rated into cloud droplets by diffusion, whereas bio-

(Woodage et al. 2003). The oxidation of SO2 and DMS mass-burning aerosols are considered to act as cloud is performed using oxidant concentrations derived from condensation nuclei (CCN). Transport and deposition the offline STOCHEMchemical transport model (Col- processes are performed in a similar fashion to sul- lins et al. 1997). The SO2 emissions used are given in phate; the emissions used are given in Table 3. Table 3, and the model now includes a more interactive The sea salt scheme, unlike those described above, is scheme for the emission of DMS from the ocean based a simple diagnostic scheme depending on wind speed on sea surface temperature and low-level wind speed and height above the surface to determine the number (Jones and Roberts 2004). concentration of sea salt particles in two size modes; see Both the black carbon (Roberts and Jones 2004) and Jones et al. (2001) for details. the biomass-burning (Davison et al. 2004; Woodage et The model includes the direct effect (scattering and/ 1APRIL 2006 MARTIN ET AL. 1279 or absorption of radiation) of all simulated aerosols and is adjusted to ensure that the buoyancy consumption of thereby also includes the so-called “semi-direct” effect turbulence energy in cloud-capped boundary layers is (the impact on atmospheric temperature and clouds limited. Mixing across the top of the boundary layer is due to absorption by aerosols). All aerosol species ex- through an explicit entrainment parameterization. This cept black carbon also contribute to both the first and is coupled to the radiative fluxes and the dynamics second indirect effects on clouds (modifying cloud al- through a subgrid inversion diagnosis. There is an ad- bedo and precipitation efficiency, respectively), based ditional nonlocal flux of heat. For stable boundary lay- on the treatment of Jones et al. (2001). ers the stability limits the vertical transport of energy, motivating use of a closure based on local gradients. 3) LAND SURFACE PROCESSES AND HYDROLOGY The local Richardson number scheme described by Smith (1990, 1993) is used, although in HadGEM1 the The second version of the U.K. Met Office Surface SHARPEST functions (a variant on the SHARP Exchange Scheme (MOSES-II) (Cox et al. 1999; Essery scheme of King et al. 2001) for the stability dependence et al. 2001) is used. This allows tiling of land surface are used, which give less mixing at a given stability than heterogeneity using nine different surface types. A in HadCM3. separate surface energy balance is calculated for each tile and area-weighted grid box mean fluxes are com- 5) CONVECTION puted, which are thus much more realistic than when a The convection scheme in HadGEM1 is based on the single surface type is assumed. In addition, vegetation mass flux scheme of Gregory and Rowntree (1990) but leaf area is allowed to vary seasonally, providing a more with major modifications. The scheme is now explicitly realistic representation of seasonal changes in surface coupled to the boundary layer scheme, and cumulus fluxes. convection is similarly diagnosed using the mean hu- Tiling of coastal grid points allows separate treat- midity profile. If cumulus convection is diagnosed, then ment of land and sea fractions. This in combination the boundary layer scheme is capped at convective with the increased ocean model resolution used in cloud base. The convection scheme is then triggered HadGEM1 (see JOH) greatly improves the coastline, from the lifting condensation level in order to param- particularly in island regions such as Indonesia where eterize transports from cloud base upward. Deep and the total number of islands has increased from 6 to 35 shallow convection are diagnosed separately and differ- between HadCM3 and HadGEM1. ent thermodynamic closures are applied. In shallow Realistic river flow is important for the freshwater convection the closure is based on Grant (2001); for contribution to the thermohaline circulation in deep convection a CAPE closure is used, based on HadGEM1. In HadCM3 river routing was represented Fritsch and Chappell (1980a,b). A new convective mo- by a simple basin aggregate output instantaneously to mentum transport (CMT) parameterization is used for the ocean. This neglects seasonal variation in the flow. both deep and shallow convection, based on a flux– The new Total Runoff Integrating Pathways (TRIP) gradient relationship obtained from the stress budget. dynamic river routing scheme (Oki and Sud 1998) ad- A new cloud-base closure for CMT is used, based on vects runoff along prescribed channels using an embed- the assumption that large-scale horizontal pressure gra- ded 1° ϫ 1° river transport submodel. dients should be continuous across cloud base. Entrain- ment and detrainment rates for shallow convection are 4) BOUNDARY LAYER TURBULENT MIXING parameterized as in Grant and Brown (1999). The ra- The boundary layer scheme is that of Lock et al. diative effects of convective anvils are represented by (2000). It is a first-order turbulence closure mixing specifying (for radiation) a vertically varying convective adiabatically conserved variables. For unstable bound- cloud amount (Gregory 1999). ary layers strong transport by eddies on the scale of the 6) CLOUD AND PRECIPITATION MICROPHYSICS layer depth motivates a nonlocal dependence. The ex- istence and depth of unstable layers is diagnosed ini- The large-scale cloud scheme for liquid cloud is that tially by moist adiabatic parcels. Diffusion coefficients of Smith (1990), in which cloud water and cloud amount (“K-profiles”) are specified functions of height within are diagnosed from total moisture and liquid water po- the boundary layer, related to the strength of the tur- tential temperature using a triangular probability dis- bulence forcing (formulation developed from large- tribution function. In HadCM3 the width of this distri- eddy simulation data). Two separate K-profiles are bution was fixed globally for each model level, while in used, one for surface sources of turbulence and one for HadGEM1 the width is diagnosed from the variability cloud-top sources. The vertical extent of the K-profiles of the moisture and temperature of the surrounding 1280 JOURNAL OF CLIMATE VOLUME 19 grid points (Cusack et al. 1999b); a similar scheme was identify the impacts and feedbacks of coupled processes described in Hansen et al. (1983). A representation of and developing sea surface temperature errors on the the difference between cloud area fraction and cloud atmospheric performance. Reference is made to similar volume fraction is made by subdividing a single model comparisons of HadAM3 and HadCM3. layer into three. A major difference from HadCM3 is Evaluation of model biases is made by comparison the use of a prognostic ice content. HadGEM1 uses an with observational and reanalysis climatologies. Much updated version of the Wilson and Ballard (1999) mi- of this is done using the European Centre for Medium- crophysics scheme. Transfers between water categories Range Weather Forecasts (ECMWF) Re-Analyses, (ice, liquid water, vapor, and rain) are calculated based ERA-15 (Gibson et al. 1997) and ERA-40 (Uppala et on physical process equations using particle size infor- al. 2006). (Note that although ERA-15 does not cover mation. the entire period of the model integrations, this has very little impact on the comparison of long-term cli- 7) GRAVITY WAVE DRAG matologies presented here.) Reanalyses have the ad- The gravity wave drag (GWD) scheme is that of vantages of global coverage, using the same analysis Webster et al. (2003) and includes low-level flow block- method throughout, and providing self-consistent ing. The actual gravity wave drag is that due to air fields. However, in regions of the globe where obser- flowing over orography; it is deposited where wave vations are sparse, or for fields not measured directly, breaking is diagnosed (typically in the lower strato- the results may be dominated by model biases. It is thus sphere). The remaining drag (about 80% of the total) is important to compare reanalyses from different sources attributed to flow around the orography and is depos- and to view the model Ϫ reanalysis differences in this ited uniformly between the surface and the subgrid- context. Basic comparisons between ERA-15 and Na- scale orographic height. tional Centers for Environmental Prediction (NCEP) reanalyses, and also between ERA-15 and ERA-40, 3. Experimental details have been made and only those model biases that fall outside of such comparisons are discussed. Other The evaluation of the atmospheric component of datasets used include: the Climate Prediction Center HadGEM1 is made mainly using atmosphere-only in- (CPC) Merged Analysis of Precipitation (CMAP) (Xie tegrations of the model forced by observed sea surface and Arkin 1997), which is derived from five types of temperatures (SSTs) and sea ice concentrations. This satellite estimate combined with rain gauge data; the eliminates feedbacks from any developing SST errors in the coupled model but also prevents any coupled atmo- Southampton Oceanography Center Air–Sea Heat and sphere–ocean processes from operating. It is therefore Momentum Flux climatology derived from the Com- important to examine the performance of the atmo- prehensive Ocean–Atmosphere Data Set, COADS1a sphere in the coupled model in comparison with the 1980–93dataset enhanced with additional metadata atmosphere-only runs. Note that we refer to the atmo- from ships (Josey et al. 1996); and a surface tempera- sphere-only version of HadGEM1 as the Hadley Cen- ture climatology (Legates and Willmott 1990) derived tre Global Atmospheric Model (HadGAM1) and the from terrestrial observations of screen-height air tem- atmosphere-only version of HadCM3 as the Hadley perature and shipboard measurements. Centre Atmospheric Model (HadAM3), the latter be- The scope of the changes introduced in moving from ing described in Pope et al. (2000). HadCM3 to HadGEM1 is so great that in most cases The HadGAM1 runs use prescribed SSTs and sea ice it is not possible to analyze the direct impact of any from the second Atmosphere Model Intercomparison individual change on the model results. Instead, we Project (AMIP-II) (Gates et al. 1999) from 1979 to generally evaluate the model as a whole against its 1996. The other boundary forcing datasets used are predecessor and the observations. In some cases, how- summarized in Table 3. A five-member ensemble of ever, sensitivity tests have allowed us to attribute cer- runs was created and the results averaged to produce an tain improvements or deterioration to a particular overall climatology. Comparisons are made with similar model change or changes. In addition, we have made runs of HadAM3. The climatology of the HadGAM1 use of “spinup” runs to assess the contribution of dif- atmosphere is also compared with that from the 1979– ferent physical parameterization schemes to the differ- 99 period of a HadGEM1 run that includes historical ences between HadGAM1 and both HadAM3 and the time-varying forcing due to greenhouse gases, ozone, reanalyses. This is achieved by averaging the tendencies aerosol emissions, and land surface/vegetation and cur- from each scheme over the first and second days of a rently extends from 1860 to 2000. This allows us to series of 52 five-day integrations, each starting from 1APRIL 2006 MARTIN ET AL. 1281

FIG. 1. Taylor diagrams (see text for explanation) summarizing the comparison of HadAM3 (N48) with HadGAM1 (N96). The arrows indicate the evolution of the fields shown from HadAM3 to HadGAM1. Key: T, temperature; q, water vapor; rh, relative humidity; q, water vapor; z, geopotential height; u, ␷, horizontal wind components; numbers refer to the pressure level of the field; tcloud, total cloud; precip, precipitation; swout, outgoing shortwave radiation; csswout, clear-sky outgoing shortwave radiation; olr, outgoing longwave radiation; csolr, clear-sky outgoing longwave radiation; lwcl, longwave cloud forcing; swcf, shortwave cloud forcing; pmsl, pressure at mean sea level; ke, transient eddy kinetic energy; uv, vt, vq, other transient eddy fields.

operational analyses and scattered evenly through four viation of the model is the same as that of the clima- winters (December to February). This approach has tology, then the radius is unity. The root-mean-square been used previously by Pope et al. (2001) and Pope difference (model minus climatology) less the differ- and Stratton (2002). ence in the global averages is the distance from the reference point to the plotted point. The correlation 4. General statistical assessment of results between the model and the climatology is the cosine of the polar angle: if the correlation between the model Taylor diagrams (Gates et al. 1999; Taylor 2001) and the climatology is unity, then the point will lie on compare the global distribution of multiannual mean the horizontal axis. In the figures that follow the results fields from the models with corresponding monthly for all four seasons are combined so that any errors in means from observations or reanalyses. The distance the seasonal variation are also included. from the origin is the standard deviation of the field Figure 1a shows that the impact of the new model on normalized by the standard deviation of the observa- radiative parameters is broadly neutral with the excep- tionally based climatology; that is, if the standard de- tion of deteriorations to the longwave cloud forcing and

Fig 1 live 4/C 1282 JOURNAL OF CLIMATE VOLUME 19 outgoing longwave radiation (OLR). These are associ- resolution leads to significant improvements in the ated with a worsening of the precipitation climatology. properties of the free atmosphere in HadGEM1 com- Figure 1b shows that the zonal wind errors are similar pared to its predecessor, HadCM3. The atmosphere- in the two models but the meridional winds, particu- only and fully coupled versions of the model are very larly at upper levels, are worse in HadGAM1. The er- similar in most respects in the free atmosphere and, rors in the thermodynamic variables for HadGAM1 with the exception of 200-hPa winds, only results from and HadAM3 are shown in Fig. 1c. In general, the new the atmosphere model (HadGAM1) are shown in this model improves on the previous version at upper levels section. and is similar or slightly worse at mid and low levels. In common with many other climate and weather The improvements at upper levels can be attributed to forecast models, previous Hadley Centre models have the increased upper-tropospheric vertical resolution, suffered from large cold and moist biases in the upper the new dynamical core, and changes in the microphysi- troposphere, which are associated with incorrect posi- cal parameterization (discussed in more detail in sec- tioning of the tropopause. This error is greatly reduced tion 5). Finally, Fig. 1d shows that the transient eddy in HadGAM1 (Figs. 2 and 3) although the basic pattern fields are generally better in HadGAM1. This is mainly is similar. The midlatitude troposphere remains rather due to the increased horizontal resolution in the new cold, but the bias in the tropical troposphere is re- model. versed, making it now slightly too warm and dry. Coupling the atmosphere model to the ocean model Improvements in the tropopause structure and in the has a small detrimental impact on the precipitation and stratospheric circulation are evident in the winds (Fig. OLR, mid- to lower-tropospheric humidities, and me- 4) even though the stratosphere is still not particularly ridional winds but has a broadly neutral impact on the well resolved. A large westerly wind bias at upper levels other fields. Some of these changes (e.g., precipitation, in the Southern Hemisphere is reduced significantly, OLR, meridional winds) are related to the develop- and the stratospheric jet in the Northern Hemisphere ment of equatorial SST errors in the coupled model has moved poleward, in better agreement with ERA (see section 5). (Fig. 4). The easterly bias around the tropical tropo- Comparison of the coupled model HadGEM1 (at pause is also reduced, although a slight westerly bias is N96 resolution) with HadCM3 (N48 resolution), plots introduced in the upper troposphere (between 200 and not shown, confirms the improvements in upper-level 400 hPa). Changes in the troposphere are more mixed. temperatures and humidities, while the impact on the Tropical wind biases at lower levels are reduced in low-level temperatures and humidities is broadly neu- HadGAM1. Detrimental changes are the slight tral. Precipitation and related cloud fields remain worse strengthening of the midlatitude westerly jets through- in the new model, but total cloud amount is improved. out the troposphere, particularly in the Northern Hemi- The transients are also improved in the new (higher sphere (Fig. 4). The tropical meridional circulation (not resolution) model. Overall, the impact on the winds is shown) is still too strong although the biases are signifi- neutral compared with HadCM3. cantly reduced in the zonal mean. Taylor diagrams give an overview of the statistical The substantial improvement in the upper-tropo- comparison of global fields from the model with obser- spheric temperatures and moisture in HadGAM1 vations and are useful for comparing the performance comes from three main sources: increased vertical reso- of different models. However, they do not reveal any lution around the tropopause (Pope et al. 2001), the information about the vertical or horizontal distribu- more accurate semi-Lagrangian advection scheme, and tion of errors or about different mechanisms operating a modification to the microphysics that allows more in different models. These aspects are examined in the efficient conversion of vapor to ice, thus restricting the following sections and reasons for the model differ- amount of vapor that reaches high levels. The warming ences are discussed. and drying in the tropical midtroposphere comes from the modified convection scheme, which produces a 5. Improvements and changes in model more intense intertropical convergence zone (ITCZ). climatology and processes The intense convection enhances surface wind stresses, which are thought to drive tropical sea surface tem- a. General circulation perature errors in the coupled model (see section 5b and JOH). The increased horizontal resolution in 1) ZONAL MEAN HadGAM1 also contributes to the tropical midtropo- The combined effect of the new dynamical core, the spheric warm bias, while at the same time limiting the improved physical parameterizations, and the increased subtropical cold and moist bias in the midtroposphere. 1APRIL 2006 MARTIN ET AL. 1283

FIG. 2. DJF zonal mean temperature (K) in HadGAM1 (N96) compared with HadAM3 (N48) and ERA.

FIG. 3. As in Fig. 2 but for relative humidity (%).

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Increased horizontal resolution leads to changes in the erly bias in the North Pacific. The Icelandic low is also hydrological cycle through the increased transient ver- strengthened and extends too far eastward. Improve- tical velocities, which promote more rapid condensa- ments in mean sea level pressure in HadGAM1 occur tion and less rapid evaporation of precipitation. It is mainly due to the change in dynamical core from Eu- also partly associated with a poleward movement of the lerian to semi-Lagrangian. The improvement is particu- westerly jets, which occurs as horizontal resolution is larly marked near the poles, where the Eulerian advec- increased. Both of these effects were seen in HadAM3 tion scheme breaks down owing to the close concentra- (Pope and Stratton 2002). tion of grid points, and drastic filtering is necessary to The improvements in zonal winds are in the same maintain stability. Increased resolution also contributes direction as those seen between HadAM2b and to the improvements, as eddies and their associated HadAM3 (Pope and Stratton 2002) and were there at- heat and momentum transports are more accurately tributed to the inclusion of convective momentum represented. transports (CMT). The revisions to the convection The reduction in high pressure biases and strength- scheme in HadGAM1 include a new parameterization ening of the Aleutian and Icelandic lows are associated of CMT based on flux–gradient relationships. Compari- with strengthening of the midlatitude westerlies at 850 sons of HadGAM1 with and without the revised con- hPa in HadGAM1 in both December–February (DJF) vection scheme suggest that at least some of the addi- and June–August (JJA). This removes easterly biases, tional improvement in the zonal winds in HadGAM1 but is rather overdone in places, leading to a westerly comes from improving the representation of CMT. This bias from the Atlantic into northern Europe and in the is confirmed by the spinup tendencies in the zonal wind northeast Pacific. It also contributes to a slight westerly (Fig. 5). Changes in the total zonal wind tendency be- bias in the zonal mean (Fig. 4). The easterly biases in tween HadGAM1 and HadAM3 on the second day of HadAM3 were tending to reduce the advection of each ensemble (the first day is omitted as each model warm air from the Atlantic, leading to cold biases near adjusts to slightly differing initial conditions) are shown the surface in Siberia during winter. Stronger westerlies in Fig. 5a with the contributions to this change from the in HadGAM1 increase advection and reduce these cold advection, convection, and boundary layer and gravity biases. Over North America, the removal of a south- wave drag schemes shown in Figs. 5b–d, respectively. easterly wind bias associated with a strengthening of The change in convection zonal wind tendency shows the low-level westerly winds in HadGAM1 has resulted increased westerly tendencies in the tropical upper tro- in reduced surface temperatures in the northeast of the posphere and in the Tropics and midlatitudes between continent and a small cold bias there (see Martin et al. 200 and 400 hPa. It is also apparent that changes in the 2004). representation of gravity wave drag (whereby drag as- Tropical winds in the upper troposphere show sociated with breaking gravity waves in the lower marked differences between the coupled and atmo- stratosphere is reduced greatly), and the associated ad- sphere-only versions of the model (Fig. 7). In the justment of the advection tendencies, contribute to the coupled model a large westerly bias appears over the improvement in the lower stratosphere. In addition, equatorial Pacific, with a corresponding subtropical both the changes in CMT and in gravity wave drag easterly bias on either side, in both DJF and JJA. This contribute to the strengthening of the westerly jet in the is associated with a split ITCZ and cold equatorial SST Northern Hemisphere. anomalies in HadGEM1 (JOH).

2) HORIZONTAL CIRCULATION PATTERNS 3) TRACER TRANSPORT Significant improvements are also found in the hori- The semi-Lagrangian advection scheme has a signifi- zontal circulation patterns. Figure 6 shows a decrease in cant positive impact on the distribution of tracers in the the high pressure bias at northern high latitudes. There model. The dry mass of the atmosphere is formally con- is evidence from 500-hPa heights that high pressure bi- served in the model. Tracers (including moisture) are ases at southern high latitudes are also reduced. (Biases also conserved, although a suitable correction may in mean sea level pressure in the Antarctic should be need to be applied, particularly if there are sources. treated with caution as mean sea level pressure where Calculations using a long-lived tracer indicate that, in there is orography is diagnosed differently in the two practice, the model drift due to nonconservation is very models.) The Aleutian low is deeper and more realistic, small (the rate of increase of the burden is less than if a little too strong. A strong high pressure bias to the 0.008% per year). The most important tracer is water southeast means that there is an associated southwest- vapor; however, in the troposphere it is difficult to dis- 1APRIL 2006 MARTIN ET AL. 1285

Ϫ FIG. 4. As in Fig. 2 but for zonal wind (m s 1).

Ϫ Ϫ FIG. 5. The change in spinup tendencies in zonal wind (m s 1 day 1) in HadGAM1 compared with HadAM3. Results from day 2 averaged over 52- and 68-member ensembles, respectively.

Fig 4 5 live 4/C 1286 JOURNAL OF CLIMATE VOLUME 19 entangle improvements that arise through improved were subsequently included in an intermediate model advection and those that arise through improvements in (HadAM4: Webb et al. 2001), which used both the in- the microphysics parameterization. One indicator of creased vertical resolution and some of the physics the transport properties of an advection scheme is the changes in HadGAM1 but retained the Eulerian dy- so-called “tape recorder” signal (Mote et al. 1996) of namical core. HadAM4 shows a general drift of tracers water vapor transport in the stratosphere (Fig. 8). Sea- into the stratosphere as a result of poor representation sonal injections of moist and dry tropospheric air into of the tropopause. This is corrected in HadGAM1, the tropical lower stratosphere gradually drift upward largely as a result of the move to semi-Lagrangian dy- over the following seasons. Gregory and West (2002) namics. There is a well-defined stratification of black showed that the speed of transport was dependent on carbon as it enters the stratosphere, as it rapidly de- the choice of tracer advection scheme, even when the creases with height. Differences in the tropospheric dis- same winds were used in different schemes. In particu- tribution arise from changes in the emissions inventory lar, they showed that transport by the new semi- used and possibly from differences in precipitation. Lagrangian scheme was more accurate than by the old HadAM4 used fossil fuel emissions from Cooke et al. Eulerian scheme. Our results confirm this: the tape re- (1999), whereas HadGEM1 uses fossil fuel and biofuel corder is too fast in both models but is slower in emission inventories (T. Nozawa 2003, personal com- HadGAM1. Note that the Halogen Occultation Experi- munication) that have different rates and distribution ment (HALOE) observations (Russell et al. 1993) are of emissions: emissions over India are substantially in- multiannual means repeated and therefore do not creased, for example, while those over Europe are con- show the substantial interannual variability noted by siderably reduced. In addition, reductions in monsoon Mote et al. rainfall mean that less black carbon will be scavenged, The magnitude of the moist and dry signals depends allowing concentrations to build up further. Changes in on the accuracy of the representation of the tropopause transport are likely to be a secondary effect in the tro- and of the hydrological cycle in the upper troposphere. posphere. In the standard 19-level HadAM3 model the hydro- pause was very poorly represented, and negative water b. Hydrological cycle vapor was generated above it, which had to be corrected by redistributing water vapor. The 30-level ver- Figure 10 compares the mean precipitation for DJF sion of this model (used in Fig. 8) was much better, but in HadGAM1 with HadAM3 and CMAP observations. the stratosphere was still much too dry (Pope et al. There is an overall increase in the hydrological cycle in 2001). The 38-level HadGEM1 model has similar reso- HadGAM1 compared with HadAM3 and the ITCZ has lution around the tropopause, but the new dynamics both narrowed and intensified. The main region where significantly improves the transport across the tropo- the precipitation errors increase, in both DJF and JJA pause and its position and temperature. These factors, (not shown), is around the Maritime Continent, where combined with the improved hydrological cycle (in par- the convective activity is one of the major driving forces ticular the cloud microphysics in the upper tropo- of the global atmospheric circulation. The lack of rain- sphere), give rise to a significant improvement in the fall in this region is due to deficiencies in the model magnitude of the dry and moist injections into the physics, particularly the convection scheme. A detailed stratosphere. Further improvements are gained by im- analysis of the spinup ensemble is currently underway proving the representation of stratospheric processes. to try to understand the development of these errors. Figure 8d shows results from a 60-level version of the Initial indications are that the tendency for the convec- model (this is the same as the 38-level model in the tion scheme to overstabilize the region around the lift- troposphere but has more levels extending into the me- ing condensation level, combined with developing low- sosphere). The extra levels improve the circulation and level wind errors over the Pacific and other aspects that hence the transport. In addition, methane oxidation in remain poorly represented in HadGAM1 (including the the mesosphere releases water vapor and improves the treatment of cloud anvils detraining from the top of overall water vapor structure. deep convection, the treatment of detrainment and its Further evidence of the improved transport around sensitivity to vertical resolution, the diurnal cycle of and below the tropopause comes from examination of convection over land, and the complex circulation pat- tracers that are less active than water vapor. A good terns generated by land–sea contrasts), inhibit convec- example is black carbon generated from fossil fuel and tion over the Maritime Continent and promote it over biofuel emissions (Fig. 9). Anthropogenic sulphate the western Pacific. Note that in the case of the diurnal aerosol was included in HadCM3 and aerosol tracers cycle of precipitation over land, for example, changes in 1APRIL 2006 MARTIN ET AL. 1287

FIG. 6. As in Fig. 2 but for mean sea level pressure (hPa).

Ϫ FIG. 7. DJF mean 200-hPa winds (m s 1) in HadGEM1 (N96) and differences from HadGAM1 (N96) and ERA.

Fig 6 7 live 4/C 1288 JOURNAL OF CLIMATE VOLUME 19

FIG. 8. Time–pressure sections of tropical (12.5°S–12.5°N) monthly mean specific humidity (ppmv) for HALOE observations (repeated multiannual means), HadAM3 (N48L30), HadGAM1 (N96L38), and HadGAM1 (N48L60). the convection scheme have not been specifically aimed atmosphere-only model originates. We again make use at resolving the issues identified in HadAM3 (Yang and of the 52-member ensemble of 5-day spinup runs (de- Slingo 2001), and the biases are essentially unchanged scribed above). The noise is minimized by averaging in HadGAM1. the daily mean fields over the whole ensemble. To solve this problem we need to understand how the Figure 11 shows differences in daily mean precipita- precipitation bias over the Maritime Continent in the tion over the Maritime Continent region from days 2 to

Fig 8 live 4/C 1APRIL 2006 MARTIN ET AL. 1289

ing of the causes of this error and suggest possible so- lutions. Figure 12 compares the DJF mean precipitation in the coupled and atmosphere-only versions of HadGEM1. The error patterns are rather different in the coupled model, particularly around Indonesia, where the bias is positive rather than negative, and over the western equatorial Pacific, where the bias is negative. This was also the case in HadCM3 and was associated with a warm SST error around the Maritime Continent and a cold equatorial Pacific (Inness and Slingo 2003). A similar pattern of SST errors occurs in HadGEM1, although the errors around the Maritime Continent are smaller and the equatorial cooling is larger. As in HadCM3 (and indeed many other coupled models), there is also evidence of a so-called “split” or “double” ITCZ in the western Pacific. The changes in the pattern of precipitation in both coupled models arise through the lack of convection over Indonesia and the excessive easterly wind stresses over the equatorial Pacific in their atmospheric components. The lack of convection over Indonesia allows warmer SSTs to develop, which then promote additional convection locally. At the same time, the excessive easterly wind stresses promote upwelling of cold water through Ekman divergence, resulting in a cold equatorial SST anomaly that helps to confine convection to the Maritime Continent. A new balance is FIG. 9. Annual and zonal mean total black carbon concentration reached whereby significant convection in the region Ϫ (kg kg 1) in HadGAM1 and an intermediate model run can occur. This then drives a stronger Walker circula- (HadAM4) with 38 levels and including the HadGAM1 aerosol tion that further strengthens the low-level easterly scheme, but using the Eulerian dynamical core. trade winds over the equatorial Pacific. The impact of these errors on model interannual variability is dis- 5 of the spinup ensemble against the first day. This cussed by JOH. illustrates the development of a systematic bias in the There is evidence of a change in the balance between precipitation distribution, with less rainfall over the large-scale and convective cloud in HadGAM1. In the eastern Indian Ocean and the ocean areas around In- tropical upper troposphere HadGAM1 has less layer donesia and more over the western Pacific to the north cloud than HadAM3, but much more convective cloud. of the equator. Comparison with Fig. 10, which shows This results from the inclusion of a parameterization for the precipitation bias in this region in the HadGAM1 convective anvils. At lower levels, convective cloud climatology, reveals that the climatological bias is es- amounts are reduced in the Tropics and midlatitudes tablished within the first few days of the integration. but increased in the subtropics where shallow convec- This suggests that feedbacks on longer time scales, from tion dominates. These changes are largely a result of other model biases, either have little impact on this improved diagnosis and treatment of shallow convec- region or simply exacerbate the preexisting error pat- tion in HadGAM1. tern. These results are encouraging because they imply In the mid and lower troposphere, layer cloud wa- that the problem originates through inadequate repre- ter and ice contents are reduced in HadGAM1 com- sentation of convective activity over this region, rather pared with HadAM3. A new microphysics scheme that than through a complex combination of feedbacks. Ex- includes prognostic ice contents was introduced in amination of convection and boundary layer tendencies HadGAM1, the direct impact of which is to increase in this region, and of the development of biases in the the cloud water paths, primarily through a decrease in tropical circulation during the spinup period, is cur- fall speeds at midlatitudes and the subgrid-scale model rently under way and should improve our understand- changes in convective regions. The decreases in cloud

Fig 9 live 4/C 1290 JOURNAL OF CLIMATE VOLUME 19

Ϫ FIG. 10. DJF mean precipitation (mm day 1) in HadGAM1 (N96) compared with HadAM3 (N48) and CMAP/O.

FIG. 11. Development of precipitation bias in 5-day spinup runs of HadGAM1.

Fig 10 11 live 4/C 1APRIL 2006 MARTIN ET AL. 1291

Ϫ FIG. 12. DJF mean precipitation (mm day 1) in HadGEM1 (N96) and differences from HadGAM1 (N96) and CMAP. water and ice contents in HadGAM1 must therefore be cloud top pressure (CTP) and visible optical depth due to less efficient vertical moisture transport. (TAU). The model results used from HadAM3 and HadGAM1 correspond to this same 5-yr period. These include diagnostics that are directly comparable with c. Radiation budget and clouds the ISCCP data, derived using the ISCCP simulator The representation of clouds and cloud feedbacks (Klein and Jacob 1999; Webb et al. 2001). Here we continues to provide a major source of uncertainty in consider the nine basic ISCCP cloud types (Table 4): climate modeling and climate change studies (Cubasch the full ISCCP dataset (and the ISCCP simulator) pro- et al. 2001). It is therefore desirable to compare a mod- vides information on 42 CTP–TAU categories. The el’s cloud and radiation fields with observations as this analysis is restricted to 60°N–60°S as the satellite data provides information on the simulation of cloud pro- are less reliable at higher latitudes. cesses that is useful for improving the simulation of the present-day climate and may also help increase our 1) TOP-OF-ATMOSPHERE RADTIATION BUDGET confidence in predictions of climate change. Figure 13 compares HadGAM1 and HadAM3 simu- The radiation budget data are taken from the Earth lations of the annual mean TOA radiation budget with Radiation Budget Experiment (ERBE) (Harrison et al. ERBE. It should be remembered that the radiation 1990) and cover the period from January 1985 to De- budget components depend not only on the cloud simu- cember 1989. The data are the continuous record from lations but also on the atmospheric temperature and the ERB satellite (ERBS) and provide information on water vapor distributions, the surface reflectivity, and both the total and clear sky top-of-atmosphere (TOA) other atmospheric constituents such as aerosols. Gen- fluxes, allowing calculation of the cloud radiative forc- erally, it is reasonable to say that the HadGAM1 simu- ing (CRF). We combine these with International Sat- lation compares either as well or better with the ERBE ellite Cloud Climatology data (ISCCP-D2) (Rossow observations than HadAM3. Note that, although the and Schiffer 1999) for the same period. The ISCCP data horizontal resolution of the HadGAM1 (N96) simula- provide information on clouds classified according to tion is double that of HadAM3 (N48), we find that the

Fig 12 live 4/C 1292 JOURNAL OF CLIMATE VOLUME 19

TABLE 4. Definition of the nine basic ISCCP cloud types. CTP is cloud top pressure; TAU is visible optical depth.

CTP Ͻ 440 hPa (high) Cirrus Cirrostratus Deep convective cumulus 440 Ͻ CTP Ͻ 680 hPa (middle) Altocumulus Altostratus Nimbostratus CTP Ͼ 680 hPa (low) Cumulus Stratocumulus Stratus 0.3 Ͻ TAU Ͻ 3.6 (thin) 3.6 Ͻ TAU Ͻ 23 (intermediate) TAU Ͼ 23 (thick)

differences resulting from the increase in resolution are flected flux in HadGAM1 compared to HadAM3. The generally much smaller than the differences between OLR distribution is broadly similar in the two models, the two models. with some improvement being seen over the tropical The most notable improvements in the reflected warm pool and the South Pacific convergence zone. shortwave radiation (RSW) are found over areas of The OLR is overestimated in the subtropical subsi- widespread marine stratocumulus cloud (off the west dence areas, particularly in the Southern Hemisphere. coasts of Peru, California, southern Africa, and Austra- As with HadAM3 (see Pope et al. 2000), this may well lia), over the tropical Indian and Pacific Oceans, over be indicative of an overvigorous Hadley circulation the Southern Hemisphere midlatitude oceans, and over leading to excessive drying in these regions. The distri- the major desert regions, most clearly over the Sahara. bution of the annual mean net TOA flux in both models In all cases there is an increase in the shortwave re- compares well with ERBE.

FIG. 13. Comparison of the annual mean (top) TOA reflected shortwave, (center) outgoing longwave, and (bottom) net radiation from ERBE with HadGAM1 and HadAM3 (W mϪ2).

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2) CLOUD RADIATIVE FORCING AND TOTAL creased cloud amounts. A more detailed examination CLOUD AMOUNT of the cloud properties is required in order to understand this. Further insight on the model cloud simulations can be gained by looking at the cloud radiative forcing—the 3) ISCCP CLOUD TYPES difference between the total (or all-sky) radiation bud- Figure 16 compares the HadAM3 and HadGAM1 get fields shown above and those for cloud-free condi- simulations of high-level clouds (CTP Ͻ 440 hPa) tions. Figure 14 shows comparisons of the annual mean with ISCCP observations. The high cloud is divided shortwave and longwave CRF from HadGAM1 and into three broad optical depth categories: “thin” (TAU HadAM3 with ERBE data. Ͻ 3.6), “intermediate” (3.6 Ͻ TAU Ͻ 23), and The shortwave CRF fields indicate that the improve- “thick” (TAU Ͼ 23). The most striking improvement in ments to the reflected shortwave radiation discussed HadGAM1 is the simulation of cloud of intermediate above result primarily from an improved cloud simula- optical depth. HadAM3 produces only very small tion. Note that the better simulation of the reflected amounts of this type of cloud, and the improvement is shortwave radiation over the desert regions is not cloud primarily due to the inclusion of a parameterization for related but is the result of a more realistic specification convective anvils (Ringer and Allan 2004). This in- of the surface reflectivities. The longwave CRF shows crease in intermediate thickness high cloud is accom- clear improvements over the midlatitudes (in both panied by a significant reduction in the optically thick- hemispheres) and over the tropical oceans. Over tropi- est cloud that is also beneficial, although this cloud type cal land areas, however, there is no discernable im- is generally still overestimated by the model, particu- provement in the HadGAM1 simulation and the long- larly at midlatitudes. The optically thin high cloud (“cir- wave CRF continues to be too low compared with rus”) is underestimated in general, particularly over ERBE. land and over tropical oceans. Note that what is dis- Figure 15 shows comparisons with the net CRF from played here is the model cloud that has TAU Ͼ 0.3, as ERBE and the total cloud amount from ISCCP. The this is considered to be the minimum optical depth de- most marked improvement in the net CRF simulation tectable by ISCCP: the model often produces signifi- in HadGAM1 is the representation of the near cancel- cant amounts of cloud optically thinner than this lation of shortwave and longwave CRF over the Trop- threshold, but this of course cannot be verified using ics, particularly the tropical oceans (Kiehl 1994). ISCCP data. The clearest improvement in the HadGAM1 simula- Figure 17 shows similar plots for low-level (CTP Ͼ tion of the total cloud amount is the increase in cloud 680 hPa) cloud. In this case only the thick and interme- coverage in both Northern and Southern Hemisphere diate optical depth categories are shown as the ISCCP midlatitudes, leading to a much more favorable com- thin cloud tends to be less reliable at these levels. parison with the ISCCP data. There are also significant Again, there are clear improvements in HadGAM1 increases in regions of marine stratocumulus cloud; this compared with HadAM3: generation of previously ab- type of cloud was largely absent in HadAM3 (see dis- sent intermediate optical depth cloud and reduction of cussion below). It is still the case, however, that the the optically thickest cloud, both of which lead to an cloud amount is underestimated in these areas. Indeed, improved comparison with the ISCCP data. As noted it is evident that the total cloud coverage continues to above, HadAM3 achieved a reasonable simulation of be underestimated by HadGAM1 in many areas, over the cloud radiative forcing in many areas by generating both land and ocean. A good example of this is the excessive amounts of optically thick cloud that compen- subtropical oceanic subsidence regions, where the sated for a general lack of cloudiness. The middle-level model generates very little cloud but the satellite ob- cloud changes (not shown) are similar and also lead to servations often indicate cloud coverage of around 40% an improved comparison with the ISCCP data. The im- or greater. provements in the simulation of these low- and middle- It is possible that compensating errors (or even tun- level cloud types are particularly encouraging, as they ing of model cloud properties) can lead to apparently continue to be a source of difficulty for many current reasonable simulations of cloud radiative forcing even climate models (e.g., Lin and Zhang 2004). This aspect though the representation of clouds is in error (Webb et is discussed further in the following section. al. 2001). The areas of marine stratocumulus provide a 4) STRATOCUMULUS REGIONS good example of this: the HadAM3 and HadGAM1 shortwave and net CRF appear to be quite similar de- The persistent stratocumulus clouds that exist on the spite the fact that HadGAM1 produces significantly in- eastern margins of the oceans can strongly influence the 1294 JOURNAL OF CLIMATE VOLUME 19

Ϫ FIG. 14. Comparison of annual mean (top) shortwave and (bottom) longwave cloud radiative forcing (W m 2) from ERBE with HadGAM1 and HadAM3. global climate. Underestimates of stratocumulus cloud SST errors of around 3–4 K in these areas (Gordon et amount lead to excessive solar heating of the ocean al. 2000; JOH). surface and significant local surface temperature depar- The tendency for marine stratocumulus clouds to tures from observed climatologies when atmosphere occur in shallow layers beneath a marked tempera- and ocean GCMs are coupled. HadCM3 showed warm ture inversion makes the cloud very sensitive to the

FIG. 15. As in Fig. 14 but for (top) ERBE net cloud radiative forcing and (bottom) ISCCP total cloud amount.

Fig 14 15 live 4/C 1APRIL 2006 MARTIN ET AL. 1295

FIG. 16. As in Fig. 13 but for ISCCP high-level (top) thick, (center) intermediate, and (bottom) thin optical depth clouds (%). vertical temperature and moisture structure in the Charney–Phillips vertical grid, and the same changes to boundary layer. In a GCM, this structure is itself de- the way the physics is coupled to the dynamics as are pendent on model details such as the vertical resolution used in HadGAM1. These tests showed that deeper and the representation of boundary layer mixing. In cloud-capped boundary layers and more realistic cloud HadGAM1, both of these aspects have been improved amounts could be achieved in this model. Subsequently, (see section 3). Lock (2001) revised the numerical implementation of Lock et al. (2000) and Martin et al. (2000) described the fluxes at the top of the boundary layer in the Lock the new boundary layer scheme used by HadGAM1 et al. scheme, including a realistic coupling between and its performance in HadAM3 (with 31 vertical lev- turbulent, radiative, and subsidence fluxes across the els, which matches the resolution used in HadGAM1 in inversion grid levels. In the context of the HadGAM1 the mid to lower troposphere). They found improve- dynamical core, these revisions showed further signifi- ments in the vertical structure of the stratocumulus- cant improvement in the model’s representation of stra- capped boundary layer, and in the transition from stratocumulus. tocumulus to trade cumulus. However, the cloud Figure 17 showed that low-level cloud amounts are amounts, underestimated in HadAM3, were further un- increased everywhere in HadGAM1, but there are derestimated with the new scheme. Tests of the same particularly large increases in the persistent stratocu- scheme were also made with a version of HadAM3 that mulus regions off the coast of California, Peru, south- incorporated the semi-Lagrangian, semi-implicit dy- ern Africa, and Australia. These are regions where namical core, the Arakawa-C horizontal grid and the cloud amounts were consistently underestimated in

Fig 16 live 4/C 1296 JOURNAL OF CLIMATE VOLUME 19

FIG. 17. As in Fig. 14 but for ISCCP (top) low-level thick and (bottom) intermediate optical depth clouds.

HadAM3, but they are far more realistic in the new to collapse of the cloud layer. It should be emphasized model. Consequently, there is far less warming of SSTs that it has not been possible to test this last hypothesis in these regions in the coupled model, HadGEM1 (see rigorously, so it remains speculative. JOH), although a warming of 1–2 K still occurs close to d. Aerosols and cloud microphysics the coasts, where the boundary layer structure is af- fected by having mixed land/sea grid boxes. As described in section 2, a significant new feature of The reasons behind the substantial improvement in HadGEM1 is the inclusion of interactive schemes for stratocumulus cloud amounts in HadGAM1 are still not aerosols capable of acting as cloud condensation nuclei, clear. One of the largest contributors to the balance of namely sulphate, sea salt, and biomass-burning aerosols forcing on stratocumulus clouds is subsidence. The way and that, thereby, affect clouds via the first and second in which the boundary layer scheme interacts with the indirect effects. The parameterization used to deter- model dynamics is crucial for the correct contribution mine cloud droplet effective radii (re) for stratiform and of subsidence to this balance. There are several changes shallow convective clouds (Martin et al. 1994) calcu- to the model that are thought to contribute to the ob- lates re as a function of cloud liquid water content and served improvements: the numerical treatment of the cloud droplet number concentration (Nd). In HadCM3, inversion within the boundary layer scheme (Lock prescribed values of Nd were used (one value over land 2001), the use of vertical diffusion coefficients calcu- and another over ocean), but in HadGEM1 Nd is a lated from balanced fields (rather than ones that have function of aerosol concentration (Jones et al. 2001), been sequentially updated during the time step, as in which should lead to a more realistic simulation of HadAM3), and the Charney–Phillips vertical stagger- cloud droplet sizes. HadGEM1’s precipitation scheme ing of the momentum and thermodynamic variables in the calculation of autoconversion rates also uses Nd. (rather than the Lorenz staggering used in HadAM3). Figure 18 compares the distribution of cloud-top re,as The Charney–Phillips staggering in the vertical not only a mean of the months of January, April, July, and Oc- avoids having undesirable computational modes (which tober 1987, from the satellite retrievals of Han et al. are present when using Lorenz staggering) but also has (1994) and from HadGEM1; Fig. 19 shows a compari- more accurate normal modes. The removal of compu- son of column-integrated cloud droplet number for the tational modes avoids the possibility of oscillations be- same period against the retrievals of Han et al. (1998). tween the distinct regimes above and below the subsi- Both comparisons show that, in general, HadGEM1’s dence inversion, which could upset the balance be- simulation of cloud microphysical quantities is reason- tween the subsidence and the turbulent fluxes leading able.

Fig 17 live 4/C 1APRIL 2006 MARTIN ET AL. 1297

FIG. 18. Droplet effective radii (␮m) simulated by HadGEM1 compared with retrieved values from Han et al. (1994).

6. Summary and conclusions eral new physical parameterizations, better coupling between the physics and dynamics, and increases to The characteristics and major new features of the both the horizontal and vertical resolutions. New func- atmospheric component of the new Hadley Centre cli- tionality includes interactive aerosol schemes (provid- mate model, HadGEM1, have been described. The ing a stronger basis for climate change predictions), model performs well and improves upon our previous dynamic river routing (improving the surface hydrol- model, HadCM3, in many respects. The use of semi- ogy), and tiling of the surface and coastal regions (al- Lagrangian dynamics provides considerably improved lowing even higher resolution of surface characteristics tracer advection, necessary for the transport of aerosols to be coupled to the atmosphere). The infrastructure and other species. In addition, the model includes sev- for including an interactive carbon cycle and online

Fig 18 live 4/C 1298 JOURNAL OF CLIMATE VOLUME 19

Ϫ FIG. 19. Column-integrated droplet concentration (ϫ 106 cm 2) from HadGEM1 compared with that retrieved by Han et al. (1998). chemistry has also been added. Although these are cur- pressure. These improvements can be attributed to the rently too costly for general use, they provide the basis increased resolution and the new dynamics and physics for modeling greenhouse gas and air quality changes. packages. Some of the most impressive improvements As detailed in the previous sections, evaluation of are in the tropopause structure and the reduced surface HadGEM1 against observations and reanalyses indi- pressure bias in the Arctic. The transport of both water cates that most aspects of the simulation are signifi- vapor and tracers is also dramatically improved. cantly improved compared to HadCM3. The basic There are considerable improvements in the repre- model variables of temperature, winds, and moisture are sentation of cloud in HadGEM1 compared with improved in the free atmosphere, as is mean sea level HadCM3. Perhaps the most significant of these is the

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