Abstract : I.4
Using diurnal surface pressure variations to study atmospheric circulation in Owens Valley

Yanping Li, Ronald Smith, Vanda Grubisic
Yale University

The T-REX project was hold in Mar-Apr, 2006, in Owens Valley, CA. 16 AWS has been installed and maintained by DRI. By using harmonic analysis, the amplitude and phase distributions of the diurnal component of surface temperature, pressure are calculated and categorized as quiescent/westerly days according to ridgetop wind. Generally, the diurnal surface pressure component reaches its minimum around sunset, while temperature reaches its maximum around 14:30LST. Since surface pressure is the integral of the column air mass above the ground, the mixing layer depth can be estimated by the column averaged temperature, density, and the ratio of pressure to temperature amplitudes. During quiescent days, the turbulent well-mixed neutral layer can develop to almost ridge top. But during strong westerly days, the ridgetop wind flushes out the valley thus carries away part of the heating transported from the ground. The mixing boundary layer becomes much thinner.

WRF model 2d idealized simulations have been applied to study the diurnal valley circulation and how the sun shadow and ridgetop wind affect it. The vertical energy budget shows that the surface temperature variation is dominated by the sensible heating and the eddy heat transport. But the pressure variation is determined by the vertical heat advection in the valley center. In the late afternoon, although the temperature near the ground start to fall, the descending in the upper part of the valley still warms up the atmosphere there, thus the surface pressure keeps decreasing until after sunset. With the shadow effect, the strong cross-valley wind will form around sunrise/sunset time, and the sensible heating on one slope will cause the surface temperature to be higher on this slope than the other one, but lower surface pressure on the other slope instead of the heated slope. With the existence of the ridgetop westerly, the derived mixing layer depth become much lower compared with that of the quiescent day.