A MODELING STUDY OF THE DRYLINE

Results of a modeling study of the 24 May 1989 dryline are presented. A nonhydrostatic, two-dimensional version of the Colorado State Univer sity Regional Atmospheric Modeling System (CSU-RAMS) is used to deduce the impact of east-west variability of soil moisture and vegetation o n convective boundary layer evolution and dryline formation. The effec ts of the initial moisture and wind fields and the impact of the Corio lis force on the model results are also examined. Model output is comp ared with special airborne and sounding observations of the 24 May dry line. Several findings of an earlier observational study of the 24 May dryline are supported in the present study. The modeled drylines are broadly comparable to the observed dryline with respect to the followi ng properties: 1) width, 10 km; 2) strong horizontal moisture and virt ual potential temperature gradients, >4 g kg(-1)/10 km and 2 KI 10 km; 3) strong horizontal convergence, updraft, W-E shear of N-S wind comp onent, 8 X 10(-4) s(-1) 1 m s(-1), 10 m s(-1)/10 km, respectively; 4) bulges of moisture and N-S wind component above the surface dryline lo cation (due to combination of vertical mixing and vertical advection); and 5) elevated moist layer east of the dryline during late afternoon (due to eastward advection of the moisture bulge). A rather diffuse d ryline begins to form by about noon from the combined effects of stron g vertical mixing of moisture and westerly momentum to the west of the dryline. Convective boundary layers (CBLs) of differing depths develo p on either side of the dryline, giving rise to horizontal airflow acc eleration in low levels via a mesoscale hydrostatic pressure gradient east of the dryline. A vorticity dynamics analysis indicates that a ho rizontal virtual potential temperature gradient causes strong solenoid al forcing of horizontal vorticity (order 10(-5) S-2) and a thermally direct vertical circulation that concentrates at the dryline. Low-leve l winds east of the dryline decelerate, generating convergence and res ulting in the development of an easterly, subgeostrophic airflow compo nent during midafternoon. Strong convergent frontogenesis contributes to rapid development of the horizontal thermal gradients at the drylin e during middle to late afternoon. For example, convergent and net fro ntogenesis of Vapor mixing ratio achieves order 10(-7) g kg(-1) m(-1) s(-1). Ageostrophic forcing helps maintain a low-level southerly jet e ast of the dryline during the daytime via the Coriolis term. Virtual t emperature differences across the dryline during late afternoon, imply ing virtual density differences, are consistent with the notion that t he dryline propagates relative to the ambient westerly shear according to the theoretical phase speed of a density current. The formation of the dryline and the evolution of the CBL are sensitive to the east-we st profile of sensible heating, which in turn is sensitive to the soil moisture. A west-to-east change of volumetric soil moisture from 0.35 to 0.5 over 50 km (0.15/50 km), resulting in a sensible heat flux gra dient in the surface layer of about 100 W m(-2)/50 km, is sufficiently large to cause a dryline to form. Drylines do not form in test cases where volumetric soil moisture is horizontally homogeneous with values of 0.5 or 0.35. A mesoscale convergence line, characterized by a stro ng updraft collocated with a weak moisture gradient, forms in the latt er case. Horizontal variations of sensible heat flux in the surface la yer are accentuated by vegetation contrasts, causing additional sensit ivity of CBL and dryline evolution.