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 devel- opments 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 cou- pling between the atmosphere, land, ocean, and sea ice subcomponents and the model includes an inter- active 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 system- atic 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 rep- resentation 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;