Title

Integration of surface and upper air measurements for determining evapotranspiration at a high mountain site

Date of Completion

January 1997

Keywords

Agriculture, Agronomy|Biology, Ecology|Hydrology|Physics, Atmospheric Science|Environmental Sciences

Degree

Ph.D.

Abstract

An integrated approach using surface and upper air measurements was used to estimate evapotranspiration in a high mountain area. The first part of this work focused on identifying potential errors introduced into the results due to the characteristics of the sonde instrument and the dynamic environment it was used in. The second part of this work focused on estimating the evapotranspiration.^ Sources of error were reviewed for radiosonde measurements in the Atmospheric Boundary Layer and focusing on two radiosonde models manufactured by Atmospheric Instrumentation Research, Inc. Temperature and humidity lag errors and wind errors were analyzed. Errors in measurement of azimuth and elevation angles and pressure over short time intervals and at higher altitudes introduced wind vector errors greater than 5 m $\rm s\sp{-1}.$ Mean temperature and humidity lag errors were small, but individual large lag errors occurred with dramatic changes in the environment, such as near the surface or at the top of the boundary layer. Dual sonde flights showed mean instrument error comparable to lag error.^ Vertical profiles of potential temperature and specific humidity measured by radiosondes within the surface boundary layer were used to estimate Bowen Ratio at a high elevation alpine site for 2 cases representing moist/windy and dry/calm conditions. The Bowen Ratio was combined with surface global radiation measurements to estimate evaporation (ET) through an energy balance approach. Weather modified vegetation characteristics and their spatial distribution reflect the large local differences in the latent (ET) and sensible heat fluxes measured a 2 surface stations close by. The sonde estimates correlated well $\rm (R\sp2 = 0.90)$ with a weighted average of the surface measured ET in both an undisturbed high pressure ridge system characterized by a dry deep convective boundary layer and during the passage of a weak frontal system characterized by a shallower mechanically mixed moist boundary layer. ^