Procedure for the automatic estimation of mixed layer height.

摘要

The daytime mixed layer results from mechanical and thermal turbulence processes driven by differences in air-surface temperature and moisture. As such, the height of the mixed layer (z(sub i)) is a measure of the effectiveness of energy transfer from the sun to the earth's surface and, in turn, to the lower atmosphere (Stun, 1989). Maximum daytime values for z(sub i) in the region of the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) vary from less than 100 m in cloudy, moist, calm, stable conditions to nearly 3 km in clear, dry, unstable conditions. The principal characteristic of the mixed layer is that scalar quantities such as moisture and temperature are mixed throughout. Thus, z(sub i) becomes one of the principal scaling parameters used to describe the structure of the lower planetary boundary layer. Normally, a stable layer (a potential temperature inversion) at the top of the mixed layer interfaces between processes in the lower atmosphere and in the free atmosphere above. The strength of this inversion limits the rate of growth of z(sub i) with time and the vertical transfer of energy and moisture. When and if z(sub i) reaches the condensation level, clouds can form; hence, cloud base height (particularly for fair-weather cumulus clouds) often coincides with z(sub i) later in the day. Although the concept of the mixed layer height is straightforward, its measurement can be relatively difficult, or at least awkward. The most reliable method is an analysis of potential temperature and mixing ratio profiles retrieved from balloon ascents. (The potential temperature changes from constant to increasing with height; the mixing ratio changes from constant to decreasing with height.) Often however, the profiles of temperature and moisture are ambiguous, with multiple inversions or none at all. In addition these profiles supply only a snapshot of the atmospheric structure that may well be unrepresentative of the average, either in time or space. In some instances, the term 'well mixed' should not be applied to the lower atmosphere at all; during and after precipitation, for instance, the changes in surface and lower atmospheric conditions cause large ambiguities. This paper describes an automatic estimation method using radar profiler data and discusses a one-year climatology of z(sub i) over the SGP CART site.