Numerical study of the diurnal variation of the dryline
Abstract
The Purdue mesoscale model including atmospheric radiation and the surface energy budget is used to study the diurnal variation of the dryline. In this model, the turbulent kinetic energy is predicted by a prognostic equation. The length scale ($\ell$) of Sun and Ogura (1980) and Deardorff (1980) is modified; the eddy coefficient is proportional to $\surd$E$\ell$, as suggested by Deardorff (1980); and the similarity equations proposed by Businger et al. (1971) are used in the surface layer. In addition, the force restoring method (Bhumralkar, 1975; Blackadar, 1976) is adopted to predict the surface temperature and an analogous method is used to calculate the surface soil moisture (Deardorff, 1978). Two parts are included in this study. First, a simple one-dimensional ensemble average model is used to study the evolution of the planetary boundary layer in both a cloud-free and cloudy atmosphere. The data of Wangara Day 33 are used to test the cloud-free case. The simulated results are in good agreement with both those observed and those produced by the other more complicated models. Fields of temperature and moisture for the cloud-free case, after running 3 hours, are modified to initiate the cloud layer and used as the initial conditions for the cloudy case. Again, the simulated mean fields, as well as eddy fluxes, are comparable with those produced by more complicated models and observations. In the second part of this study, the Purdue mesoscale model is used to simulate diurnal variation of the dryline during fair weather. The diurnal oscillation of the dryline is well simulated. The dryline moves eastward during the daytime, and westward during the night. The model also reproduces the observed strong nocturnal low-level jet along the sloping terrain. Sensitivity tests show that the horizontal soil moisture gradient, sloping terrain, and low-level vertical wind shear across the dryline play important roles in formation and maintenance of the strong moisture gradient along the dryline.
Degree
Ph.D.
Advisors
Sun, Purdue University.
Subject Area
Atmosphere
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.