Abstract
The dependence of the annual mean tropical precipitation on horizontal resolution is investigated in the atmospheric version of the Hadley Centre General Environment Model. Reducing the grid spacing from about 350 km to about 110 km improves the precipitation distribution in most of the tropics. In particular, characteristic dry biases over South and Southeast Asia including the Maritime Continent as well as wet biases over the western tropical oceans are reduced. The annual-mean precipitation bias is reduced by about one third over the Maritime Continent and the neighbouring ocean basins associated with it via the Walker circulation. Sensitivity experiments show that much of the improvement with resolution in the Maritime Continent region is due to the specification of better resolved surface boundary conditions (land fraction, soil and vegetation parameters) at the higher resolution. It is shown that in particular the formulation of the coastal tiling scheme may cause resolution sensitivity of the mean simulated climate. The improvement in the tropical mean precipitation in this region is not primarily associated with the better representation of orography at the higher resolution, nor with changes in the eddy transport of moisture. Sizeable sensitivity to changes in the surface fields may be one of the reasons for the large variation of the mean tropical precipitation distribution seen across climate models.
Highlights
It is widely held that scale interactions are an important feature of the climate system, in particular in the tropics (e.g., Slingo et al 2003, 2009)
Over the Maritime Continent, where the modified surface fields have been applied, coarse boundary conditions lead to a reduction of precipitation, both in the N96DA and N144 control and the degraded land surface (N144DA) experiments
We have investigated how tropical precipitation biases in an atmosphere-only general circulation models (GCMs) (AGCM) depend on the model’s horizontal resolution
Summary
It is widely held that scale interactions (between short/ small and long/large scales in time and space) are an important feature of the climate system, in particular in the tropics (e.g., Slingo et al 2003, 2009). Roberts et al (2004) show that increasing the ocean resolution in a coupled global circulation model to 1/3° allows the ocean model to represent eddies and is associated with an improved mean ocean state. At sufficient resolution in the atmosphere and the ocean, Roberts et al (2009) and Shaffrey et al (2009) find for the HiGEM model that small-scale interactions between the atmophere and instability waves in the tropical Pacific can be captured realistically improving the simulation of the mean state of the tropical Pacfic, the Walker circulation, and of all aspects of ENSO. Strachan et al (2012) report improvements in the number and geographical distribution of tropical cyclones in a series of atmosphere-only simulations with the HadGEM1 model; and Demory et al (2013) find, using the same model simulations, that the global water and energy cycles are represented more realistically at higher resolution
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