Formation and diurnal variation of the dryline

The formation and diurnal evolution of the dryline during fair weather have been investigated through the use of a two-dimensional mesoscale model that includes condensation/evaporation, budget equations of surface energy and moisture field, as well as turbulence and radiation parameterizations. A moderately strong, vertical wind shear was introduced on a sloping terrain, where the soil is very dry on the west side but moist on the east. Initially, a weak easterly geostrophic wind exists to the east but a weak westerly geostrophic wind to the west of the dryline. During daytime, deepening of the mixed layer due to vertical mixing, especially on the west side, forces the dryline to advance eastward. The westerly wind on the west side is maintained by downward transport of westerly momentum due to strong vertical mixing; the easterly wind on the east side is strengthened due to the inland sea-breeze circulation. Therefore, the resulting low-level convergence sustains a strong moisture gradient along the dryline. During the night, convection near the ground ceases, and a rapid surface cooling to the west of the dryline changes the pressure gradient force near the surface, which is able to maintain a weak westerly wind there. In contrast, a strong easterly wind persists on the east side and forces the dryline to retrograde until sunrise. The strong moisture gradient was also maintained by the low-level convergence. Sensitivity tests show that the most important factors in sustaining a strong moisture gradient along the dryline are the low-level wind shear, the sloping terrain, and the horizontal soil moisture gradient, in that order. Under favorable conditions, a very weak moisture gradient zone can develop into an intensive dryline within 36 hours.