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Convective Clouds Continue to Confound a GCM

Stratton, R.A. and Stirling, A.J. 2012. Improving the diurnal cycle of convection in GCMs. Quarterly Journal of the Royal Meteorological Society 138: 1121-1134.
The development of clouds and attendant precipitation occurs on time-scale of minutes to hours. Clouds require lift, moisture, and condensation nuclei in order to form. It is no surprise that these processes generally occur on time and space scales smaller than model resolution and must therefore be parameterized. Parameterizations are "educated guesses," and as such, are subject to the knowledge of the model designers. In spite of this, the formation of clouds can be can be controlled or influenced by processes on almost any time-scale.

Convective type clouds and precipitation, especially over land areas, in the tropics, or during the warm season, have a very strong diurnal cycle. Most people are familiar with the diurnal cycle via the sea-breeze phenomenon. Models have difficulty with the diurnal component of convection. Stratton and Stirling (2012) devised a better way to represent this phenomenon in an atmospheric general circulation model (GCM). In particular, they improved the rate at which cloud air and the environmental air mix (entrainment), along with the mass flux over land in order to improve the timing and strength of convection influenced by diurnal heating.

In order to test the impact of the changes made to the diurnal convection scheme on the precipitation climatology, the authors used a GCM with a horizontal resolution of less than two degrees in latitude and longitude. They ran two ten-year simulations. The first was a control run with the model as it was provided to them. Then a ten-year run was performed with the new convective parameterizations added. They compared this to observed precipitation as derived from output provided by the Tropical Rainfall Measurement Mission (TRMM) satellite.

The results showed that control run (Fig. 1) "tends to lack precipitation over India and have too much precipitation over tropical land in Africa and in South America. The new run has generally reduced the precipitation over tropical land, tending to improve agreement with CMAP." CMAP is an acronym for the observations. In the mid-latitudes though, there were regions (e.g., western Russia) where the new parameterization improved the model results and places where the new parameterizations were worse (e.g., Europe). The results also do not alter the general circulation to great degree.

Figure 1. Adapted from Fig 3 from Stratton and Sterling (2012). The mean summer season precipitation for (a) new convective climatology, (b) new run minus the control, (c) control minus CMAP (observations), (d) new run minus CMAP.

The authors improved the attendant physics associated with convection and in general improved the climatological representation of precipitation. But there were still differences from the observed precipitation, and, in some regions, the new convective scheme performance was not as good. In modeling, improvement does not mean perfection, even though the model is overall closer to reality.

This new parameterization may do a good job in representing precipitation over short time-scale, but over longer time-scales it still does not represent the climatology of precipitation very well. It is a small improvement, but improvement nonetheless. But it is best to understand that the situation here is a microcosm of model performance overall. The models may do a reasonable job on the short time-scale, but can provide very unrealistic scenarios over the time-scale of decades and centuries.

Archived 14 August 2012