Using diurnal surface pressure variations to study valley circulation

Harmonic Analysis has been applied to surface hourly observations for nearly 1000 ASOS stations over the United States. The results show that for valley stations, the amplitudes of the diurnal component of surface pressure are relatively large (>150Pa) while phases are relatively early. In contrast, stations from Great Plains and mid-west have a continental enhanced atmospheric tide behavior with phase around 105o. WRF idealized simulation has been utilized to study the character of the diurnal valley circulation.

The detailed study of the valley stations, especially Owens Valley, shows that the depth of the valley is important in predicting its diurnal valley circulation and surface pressure variations. Deep and dry western valleys always have the largest amplitudes and earliest phases of surface pressure. But for shallow valleys, the amplitudes become smaller and phases become later with an obvious tide effect. WRF idealized simulations show that the key factor to determine whether a well-defined valley circulation will exist in late afternoon is the relative height of the daily mixed layer H and the mountain height h. If Hh, the valley will be isolated from the environment. The daily mixed layer depth can be derived from the ratio of the pressure and temperature amplitude.

The seasonal character of pressure phases for deep valleys has also been analyzed. The results show that the phases are earlier in wintertime than in summertime. The time lag between surface temperature maximum and surface pressure minimum is about 2.5h in summer but 40min shorter in winter. WRF simulations and the energy budget analysis explain that difference by different valley heating mechanisms. In the valley, the ground temperature is determined by the incoming solar radiation. The temperature of the valley lower part is determined by the turbulent heating, while the upper part is determined by the adiabatic descent. According to hydrostatic law, the surface pressure approximates the integral of the whole air column and is a function of the vertical temperature profile. In the summer, the solar insolation Q is large; it generates valley circulation with deep mixed layer. Pressure phase is related to the temperature phase of the valley upper part, which reaches its maximum much later than ground temperature. In the winter, Q is small, it fails to generate a strong upslope wind and the mixed layer it generated is shallow. The dominant heating mechanism for valley air temperature is turbulent heating near the ground. Thus the time lag between surface temperature and pressure minimum becomes smaller.