Toward understanding surface sensible heat fluxes during transitional stability over contrasting surfaces

Under transitional stability, where the flow is inherently unsteady, turbulence parameterizations such as Monin-Obukhov Similarity Theory (MOST), perform very poorly. This is a weakness of current numerical weather models because transitional conditions serve as an initial condition for developed flows (e.g. daytime and nocturnal). Furthermore, it is apparent that the physics during transition are not well understood. An example of this is the occurrence of ‘counter-gradient' heat fluxes where the heat flux vector points in the direction of increasing temperature. Here, we use data from the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program to examine this phenomena.

MATERHORN is a multi-institution, multi-disciplinary project designed to improve weather predictability in mountainous terrain. Two field campaigns were conducted at Dugway Proving Ground in Utah's West Desert, USA, from 25 September – 21 October 2012 and from 1 May – 30 May 2013. We present results from three flux towers situated over broadly different surface types. Two of the sites were located on flat terrain and one was situated on 5-7° slope. One of the flat sites and the slope site can be characterized as desert steppe with sparse vegetation (~ 1m tall), the second flat site as playa (dry lake bed with smooth surface and no vegetation). At all three sites, momentum and sensible heat fluxes were measured at multiple heights (5 or 6 levels up to 28 m agl) using sonic anemometers and fine wire thermocouples, and arrays of 20-25 thermocouples measured temperatures between 1 cm and 3.2 m agl.

Although all three sites are geographically close (within ~ 30 km of one another), they display very different transitional evolutions of temperature and flux profiles. The playa surface is characterized by a much higher soil moisture compared to the other sites which leads to a larger thermal capacity of the ground. This may introduce an inertia during transient conditions. As a starting point, we examine evening transitions with weak or no synoptic forcing. We believe that it is necessary to include surface characteristics, such as soil moisture, to successfully parameterize transitional flows.