Using Surface Pressure Variations to Categorize Diurnal Valley Circulations: Experiments in Owens Valley

Harmonic analysis has been applied to data from nearly 1000 Automatic Surface Observation System (ASOS) stations over the United States to extract diurnal pressure signals. The largest diurnal pressure amplitudes (∼200 Pa) and the earliest phases (∼0600 LST for surface pressure maximum) were found for stations located within deep mountain valleys in the western United States. The origin of these unique characteristics of valley pressure signals is examined with a detailed study of Owens Valley, California. Analysis of observational data from the Terrain-Induced Rotor Experiment (T-REX) project shows that the ratio of the valley surface pressure to temperature amplitude can be used to estimate the daily maximum mixed-layer depth H. On days with strong westerly winds above the valley, the mixed layer is found to be shallower than on quiescent days because of a flushing effect in the upper parts of the valley. Idealized two-dimensional Weather Research and Forecasting Model simulations were used to explain the pressure signal. In agreement with observations, the simulations show a 3-h difference between the occurrence of a surface pressure minimum (1800 LST) and a surface temperature maximum (1500 LST). The resolved energy budget analysis reveals that this time lag is caused by the persistence of subsidence warming in the upper part of the valley after the surface begins to cool. Sensitivity tests for different valley depths and seasons show that the relative height of the mixed-layer depth with respect to the valley depth, along with the valley width-to-depth ratio, determine whether the diurnal valley circulation is a “confined” system or an “open” system. The open system has a smaller pressure amplitude and an earlier pressure phase.