Using diurnal surface pressure variations to study the atmospheric circulation in Owens Valley

Diurnal solar heating is an important forcing for the atmosphere. In the upper troposphere and stratosphere, by heating the water vapor and ozone, the sun generates the global atmospheric tide. In the lower troposphere, if the terrain is inhomogeneous, mesoscale circulations can develop, such as plateau-plain circulation, mountain-valley circulation and sea-breeze. A convenient way to monitor and classify such circulations is by a harmonic analysis of pressure and temperature. In our work, harmonic analysis has been applied to nearly 1000 ASOS over the CONUS (Li and Smith 2006). Over the Great Plains and the mid-west, the diurnal surface pressure phase has the characteristic of a “continentally enhanced tide”. By contrast, the large amplitude (150 Pa) and early phase (90 Degree) of some valley stations, such as Bishop and Blythe in California, Rifle in Colorado draw our attention to the unique properties of the valleys in the western United States.

In association with the T-REX project in 2006, a dense network of Automated Weather Stations has been installed and maintained by the Desert Research Institute for a two year period. Their fine spatial and temporal resolutions are very helpful to study the extreme diurnal surface pressure signals in Owens Valley. In our preliminary analysis, the days are categorized according to the directions of the ridge top winds. The amplitude and phase distributions of the diurnal component of surface temperature are similar in different wind classes while the pressure amplitude and phase differ markedly. On days with southeasterly up-valley wind, the pressure phases are similar all across the Owens Valley. Generally, the diurnal component of surface pressure reaches its minimum around 6PM local solar time(LST) around sunset, while surface temperature reaches its maximum around 2:30pm LST when surface insolation reaches its peak.

By contrast, on days with southwesterly wind, the phases show large differences across the valley, with the latest phase for stations on the western slope, and earliest phase for stations at the center of the valley. To understand this, we recall that the surface pressure is the integral of the column air mass above the surface, which, in turn, depends on the temperature distribution along the column. The phase lag between surface temperature and pressure is converted to the phase lag between the surface temperature and the column averaged temperature. The heating mixed-layer depth (H) is determined from the column averaged temperature and density and the ratio of pressure to temperature amplitudes, we call this ratio as flushing index. During quiescent days, the turbulent well-mixed neutral layer develops to almost the ridge top. But during strong synoptic wind days, the ridge top wind incursion flushes out the valley. The mixed boundary layer becomes much thinner than that of the undisturbed days. The estimated H is about 1500m for stations in the valley, and 1200m for stations on the slopes for undisturbed days, and about 800m in the valley and 600 on the slopes for westerly days. Also, the calculated flushing index for each day shows that the day with low H matches the day with strong mountain top westerly well. The phase lag between surface pressure and temperature could also be treated as a time lag factor, which is a function of the vertical turbulent mixing rate.

Linearized Bousinessq equations, solved by FFT methods with confined lateral boundaries, have been used to study the valley surface pressure variation under the diurnal heating. The effect of the synoptic steering wind is considered, including how it modifies the cross-valley phase and amplitude distribution. Idealized WRF simulations are also employed to compare with the observations and the linear solutions.