Regional climate research

The local atmospheric circulation in a super-high-resolution regional model of Southern California

As part of our ongoing effort to understand the local climate in Southern California (map), we examined the primary modes of atmospheric circulation variability in a regional atmospheric model (MM5). The 6-km grid spacing of the model is high enough to resolve the main mountain complexes of the region (see contour plot of the model's terrain and coastline). At its lateral boundaries, the model is forced by the weather forecasting community's best guess of the actual large-scale weather conditions over the period 1995-2003, a product called the eta model reanalysis. The region is approximately the same size as a typical grid box of the current generation of general circulation models used for global climate prediction, and so can be thought of as a laboratory for the study of climate at spatial scales smaller than those resolved by global climate simulations.

We find that the simulated atmospheric circulation during the October to March wet season, when variability is most significant, can be understood through an objective classification technique in terms of three wind regimes. The most common we term the "Common Northwesterly" regime. It is characterized by alongshore flow from the northwest. The other two regimes are the "Santa Ana" regime, when strong offshore flow brings dry air to the coast from the desert interior, and the "Onshore" regime, when moist air is transported from the ocean over the coastal mountain ranges. The composite surface wind patterns associated with these regimes exhibit significant spatial structure within the model domain, consistent with the complex topography of the region. These regimes also correspond nearly perfectly with the simulation's highly-structured patterns of variability in critical climate variables such as relative humidity, precipitation and temperature, and therefore are the main modes of local climate variability.

The regimes are approximately equally likely to occur regardless of the phase of the classical large-scale modes of atmospheric variability prevailing in the Pacific--North American sector, such as the Pacific North American pattern. The high degree of spatial structure of the local regimes and their tightly-associated climate impacts, as well as their ambiguous relationship with the primary modes of large-scale variability demonstrates that the local perspective offered by the high resolution model is necessary to understand and predict the climate variations of the Southern California region, including its response to global-scale increases in greenhouse gases.

download the publication (Conil and Hall 2005) describing these results in more detail.