Cirrus cloud parameterizations for global climate models (GCMs) and North American (Mexican) monsoon modeling study

Cirrus clouds have an important impact on earth's climate by influencing the radiation balance and hydrological cycle. Ice clouds play an important role in the climate system, but they are often poorly represented in climate models. Improved parameterizations of cirrus clouds in GCMs require good understanding of the cloud properties and especially of the role of small ice crystals in cirrus. One of the goals of our research is to help improve climate prediction through better representation of the microphysical cirrus properties in GCMs. We present here a GCM parameterization for bimodal size spectra in mid-latitude cirrus clouds and a trimodal size distribution parameterization for tropical cirrus. Both schemes predict different behavior of the size distributions (SD) for the same ice water content (IWC) and temperatures. As temperature decreases beyond -35°C, the magnitude of the small mode is enhanced with the tropical scheme, but the opposite occurs with the mid-latitude scheme. This is a fundamental finding and indicates that the radiative properties of tropical and mid-latitude cirrus are considerably different for the same IWC. This finding may also point to the different mechanisms by which convective and non-convective cirrus are generated.; Another important climate prediction and climate variability problem is our ability to understand and to forecast the strength of the North American monsoon (NAM). The climate and economy of northwest Mexico, Arizona and New Mexico are strongly linked to the NAM, which provides about 65%, 35% and 45% of the annual rainfall of these regions, respectively. The Great Basin also derives a portion of its annual precipitation from the NAM, depending on location. Monsoon precipitation is dominated by convective events and it is often associated with intense rainfall and damaging flash floods---namely these related to the Gulf of California (GC) and attendant sea surface temperatures (SSTs). We use a regional scale model MM5 that resolves the GC, as well as SST data that accurately represent SST in GC. Our empirical study supports the hypothesis that northern Gulf of California SSTs play a critical role in the timing and amount of monsoon rainfall over the U.S. southwest. Our MM5 modeling experiments explore further the idea and show that the northern GC SSTs are an important factor affecting the boundary layer, circulation and stability parameters in the monsoon region.