ET Under Strong Advection of Heat: Will Land-Atmosphere Models Work?

Estimating the latent heat flux (LE) is critical for simulating water and energy balance of surfaces, required for various studies and applications of water balance. But surface latent heat flux is a critical process in regional climate models. It not only connects directly with the other components of the energy balance, but also is a key aspect of feedback processes between the regional atmosphere and surface.

A rigorous test of any model of a process is how well it simulates under conditions far from the norm. For latent heat flux, an example would be surfaces that experience strong advection of heat and saturation deficit from dry upwind surfaces.

A study was conducted which examined the surface energy balance of a location in Texas, where irrigated regions experience high winds and strong advection of heat. This was part of a large study of the soil-plant-atmosphere termed BEAREX08. The study site was an irrigated cotton field. There were a number of eddy covariance systems sited in the field, which also had two large lysimeters. In addition, properties were measured into the mixed layer with a tethered balloon system.

The LE values will be examined, and the relative contribution of available energy and advection will be quantified via the coupling factor for a range of atmospheric conditions. Then, the ability of several models to simulate the LE values under the range of conditions will be examined The models will include some common Penman-Monteith type equations, and several currently used in regional climate models.

The hypothesis that they will not validate well under conditions of strong advection will be tested. Several questions will be addressed. How well are they validated against the measurements? How do the validation results change with atmospheric conditions? Using ancillary data collected at the site, the ability of the models to simulate key processes related to the LE calculation will be examined. This will allow us to evaluate sources of problems in these surface sub models.