Global simulations of precipitation

Simulations of precipitation (in mm/day) for the low resolution (lower left) and high resolution (upper left) versions of the EC-Earth global climate model. Also shown is the observed precipitation from the Global Precipitation Climatology Project or GPCP (resolution is 2.5 degrees, in the lower right panel) and the Tropical Rainfall Measuring Mission or TRMM (resolution is 0.25 degrees, in the upper right panel). The model’s grid is around 100 km globally in ocean and 80 km in atmosphere in its low resolution version, and around 25 km in ocean and 40 km in atmosphere in its high resolution version. The video shows the monthly mean precipitation for the 12 months of 1998.

The video illustrates what is commonly known as “the double intertropical convergence zone (ITCZ) problem”. It is characterized by two zonal bands of precipitation in the equatorial central Pacific whereas observations contain only a single band. Increasing the model resolution reduces this bias in the western and central Pacific, and leads to an improved representation of the ITCZ. This improvements is especially clear during the boreal winter months (for example, try to stop the video in February).

Continental dry zones appear too dry and too extensive in climate models; this is particularly noticeable over the Sahara and Arabian deserts. Using a higher spatial resolution improves the simulated rainfall in the poleward margin of subtropical dry zones (see how rainfall amount over the Magreb, Middle-East, Iran and Pakistan is much closer to observation than in the low-resolution simulation).

Finally, rainfall over mountain ranges, for instance in the Himalaya, is simulated much better when increasing spatial resolution.