Diurnal circulations in Southern California

Mimi Hughes and Alex Hall

Outline

• Motivation

• Experiment design

• Description of climatological diurnal winds

• Explanation for and impact of diurnal winds

• Model Validation

• Conclusions and future work

Motivation

• Why hindcasts of regional climate?– Need to bring climate descriptions down to local scales

for people to appreciate them.– Better understanding of current climate can only lead to

improved descriptions of how the climate could change in the future.

• Why diurnal mountain winds?– A dominant feature of the climate in Southern

California during the summer– Persistent year round

Experiment design

• MM5 forced by ETA Reanalysis run at high resolution (6km) from 1995-2004

• MRF Boundary layer, Simple ice microphysics, no convective scheme in innermost domain

Diurnal winds

Existing theory to explain diurnal mountain circulations

• Two types of circulations: plain/valley and slope winds

• Designed to explain circulations in river valleys

• Would work perfectly for very simple topography -- terrain in Southern California is too complicated for direct application

Taken from Whiteman (1990)

Why the winds blow…

• Slope winds: as the air near the surface heats up, a pressure gradient force develops from the cool surrounding air toward the slope

• Valley winds: there is less air contained within the valley than on the plain, and so the air within the valley heats up more quickly, causing flow from the plain to the valley

QuickTime™ and aGIF decompressor

are needed to see this picture.

54% 39%

54% 39%

Diurnal surface air temperature

96%4%

96%4%

Diurnal composites of SAT

• EOF1 primarily determines the amplitude, with smaller amplitude at high elevations and near water

• EOF2 determines the shape and phase of the diurnal cycle, with high elevations and coastal areas tending to have reduced cooling at night and an earlier daytime peak

What’s causing these two different types of diurnal cycle?

• The air on the coastal side of the mountains remains stratified through the day, meaning at night the downslope flow brings warm air to these regions

• At high elevations, there’s enhanced subsidence because of the downslope flows, causing warming

• The balance between these two advective warming mechanisms and radiative cooling determines the overall shape of the diurnal cycle of SAT

Huh?

An illustrative cartoon…

Model Validation

EOFs of winds

71%

62%

24%

29%

Diurnal surface air temperature

Conclusions• The diurnal winds are a major component of the

summertime climate of Southern California -- they also persist year round

• There are two major modes associated with the diurnal winds– The first mode of the winds is what we’d expect given the

dominant phase of the diurnal cycle of SAT, and impacts the amplitude of the diurnal cycle of SAT

– The second mode of the winds can be explained by the different shapes of the diurnal cycle, represented by the second EOF of diurnal temperature

• The alterations in the phase and shape of diurnal surface air temperature are caused by the balance between radiative cooling and advective warming

Future/Related Work

• Have studied the impact resolution has on the diurnal winds by running 2km simulation for August 2002 and then turning off fine resolution domains -- modes are robust up to 18km resolution, but fine structures are removed

• Would like to see the impact diurnal winds have on ocean circulation by using just the diurnal component of the winds to force an ocean model

• Working on creating a longer time series to study different features of Southern California’s climate