Regional Climate Dynamics

Dynamical controls on the diurnal cycle of temperature in complex topography

The diurnal cycles of surface air temperature (SAT) and wind are important climatological variables often poorly captured in global climate simulations. Here we characterize these two quantities on scales unresolved in GCMs with a 6km reconstruction of Southern California's climate generated using MM5 to dynamically downscale the ETA model archive from 1995 to 2003. At 6km, all of the major mountain complexes in the region are resolved, and the model resolution coastline is realistic, allowing the model to correctly simulate the diurnally varying winds in the region. We find that the diurnal cycles of surface air temperature and wind have significant spatial structure due to dynamical links between them.

The winds directly forced by the large daily warming and cooling over land primarily flow across the coastline and elevation isolines, as expected. However, because the region's atmosphere is generally stably stratified, this flow pattern also results in air of cooler (warmer) potential temperature being advected upslope (downslope) during daytime (nighttime). This suppresses temperature diurnal cycle amplitude at mountaintops where diurnal flows converge (diverge) during the day (night). The nighttime land breeze also advects air of warmer potential temperature downslope toward the coast. This raises minimum temperatures in land areas adjacent to the coast in a manner analogous to the daytime suppression of maximum temperature by the cool sea breeze in these same areas.

Because stratification is greater in the coastal zone than in the desert interior, these thermal effects of diurnal winds are not uniform, introducing significant spatial structure into the shape of the temperature diurnal cycle as well as its amplitude. The geographical variations in the shape of the temperature diurnal cycle also induce secondary diurnal wind anomalies, which manifest themselves either as rotation in diurnal wind direction or deviations in the timing of maximum flow. These characteristics of temperature and wind diurnal cycles are also found in a network of 30 observation stations in the region, giving confidence in the simulation's realism and our study's findings. These dynamical links between the two diurnal cycles may also be found in other regions of significant topography, their importance depending partly on the degree of atmospheric stratification.

Download the publication (Hughes et. al. 2007) describing these results in more detail.

Mimi Hughes, Alex Hall, and Rob Fovell make up the team that performed this research.