Abstract : 2D.6
On the causes of unsustainable evening transition temperature falls in valleys

C. David Whiteman, Sebastian W. Hoch, Gregory Poulos, Maura Hahnenberger
University of Utah, Meteorology Department

Observations of near-surface air temperature time series in mountain areas sometimes show a rapid late afternoon temperature fall that then partially recovers over a period of less than two hours. Observations of this unsustainable or excessive temperature fall are most common on valley floors or on the lower sidewalls of valleys. In this paper we investigate the phenomenon using a network of 58 temperature data loggers, three tall flux towers, and a suite of remote sensing sounding devices deployed in the Owens Valley of California during the March/April 2006 T-REX field experiment. The network of data loggers is used to determine the statistical distribution of temperature falls and periods of recovery, as well as the spatial and temporal extent of the phenomenon. Two cross-valley lines (5-10 km in length) and one along-valley line (40 km in length) of these data loggers allow us to investigate the occurrence of the phenomenon over an extensive area. The occurrence of the phenomenon depends on synoptic weather types, and the phenomenon appears to be most common during quiescent conditions. Data from three heavily instrumented flux towers are then used to test hypotheses on the causes of these unsustainable temperature falls, investigating their relationship to temperature falls at higher levels on the towers, to the characteristics of the growing stable boundary layer, to the initiation or strength of downslope and downvalley flows, to intermittent turbulence or turbulence bursts, to components of the surface radiation and energy budgets, and to other possible drivers.