A shallow-water model of the diurnal dryline

This study explores the diurnal variation in the movement and structure of the dryline using a one-dimensional shallow-water model. The model is adapt ed to test some common theories of dryline motion including the diurnal var iation in surface friction, static stability, inversion erosion, and moment um mixing aloft. These mechanisms of diurnal variation are first studied individually and th en in unison. A diurnal variation in the surface friction produces a model dryline that moves westward during the day (in disagreement with observatio ns) and has a southerly wind maximum near midnight. A diurnal variation in the static stability produces a model dryline that steepens in slope and mo ves eastward during the day and then surges westward at night with a southe rly wind maximum 6 to 9 h after the minimum stability. Inversion erosion du ring the day produces a nearly vertical model dry front that moves eastward during the day with surface southwesterlies. At sunset the model dryline s urges westward with a southerly wind maximum before midnight. A diurnal var iation of the momentum mixing aloft has no significant effect on the model dryline. Results show that the combined case with a diurnal variation of su rface friction, inversion erosion, and static stability with terrain most a ccurately describes the observed dryline system. The westward surge depicte d in the model is compared to the flow evolution of the corresponding dam-b reak problem for a rotating fluid.