Dynamical controls on the diurnal cycle of temperature in complex topography

Abstract

We examine the climatological diurnal cycle of surface air temperature in a 6 km resolution atmospheric simulation of Southern California from 1995 to the present. We find its amplitude and phase both have significant geographical structure. This is most likely due to diurnally- varying flows back and forth across the coastline and ele- vation isolines resulting from the large daily warming and cooling over land. Because the region's atmosphere is generally stably stratified, these flow patterns result in air of lower (higher) potential temperature being advected upslope (downslope) during daytime (nighttime). This suppresses the temperature diurnal cycle amplitude at mountaintops where diurnal flows converge (diverge) during the day (night). The nighttime land breeze also advects air of higher potential temperature downslope to- ward 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 the diurnal winds are not uniform, gen- erating spatial structures in the phase and shape of the temperature diurnal cycle as well as its amplitude. We confirm that the simulated characteristics of the tempera- ture diurnal cycle as well as those of the associated diurnal winds are also found in a network of 30 observation sta- tions in the region. This gives confidence in the simula- tion's realism and our study's findings. Diurnal flows are probably mainly responsible for the geographical structures in the temperature diurnal cycle in other regions of