Tuesday, 31 July 2001: 4:15 PM
Microphysical timescales and orographic precipitation
This is a model-based theoretical study. Moist airflow over
a three-dimensional Gaussian type mountain is examined with emphasis
on orographic precipitation induced by
smooth ascent over the windward slope.
A set of timescales are defined based on a zero dimensional box model.
Single pathway snow formation
models are analyzed with both linear and nonlinear aggregation formulations.
The linear model suggests that the precipitation efficiency is determined by
three timescales, the advection timescale
(t a),
snowfall timescale (t f), and a constant timescale for snow generation (t s). Snow generation is
controlled is the ratio of
(t s/
t a)
and the fraction of the snow that falls
to the ground is controlled by the ratio of tf/ta.
Nonlinear aggregation introduces a threshold point, i.e., a critical condensation rate which separates two states; a no-snow state and a snow-rich state. If the condensation rate is below the threshold value, no snow is generated. As it surpasses the threshold value, the snow generation rate increases rapidly. The threshold point is a function of advection and snowfall timescales and an aggregation coefficient, which is further dependent on the geometries, terminal velocity, and density of snow particles. Numerical simulations with a nonhydrostatic mesoscale model with explicit cloud physics show qualitative agreement with the nonlinear model.
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