Melting and evaporation of precipitation particles result in cooling of air and thus subsidence. In the absence of other forces than gravity, the subsiding air concentrates in river valleys, which act as air drainage. Down-valley flow generated this way is driven by gravitational force and may thus be termed drainage flow, similar to the nocturnal drainage flow under clear sky and weak synoptic conditions. The difference between the wet and dry drainage flows is the mechanism causing the air to subside, namely cooling by melting and evaporation during wet conditions and cooling by long-wave radiation under dry conditions, respectively. The cooling rates from melting and evaporation exceed those resulting from nocturnal, clear sky radiational cooling. While dry drainage flow may occur at nighttime during periods of weak synoptic forcing, wet drainage flow is not bound to nighttime and may overcome moderate synoptic forcing.
Observations made during the MAP SOP show that a down-valley drainage flow can develop underneath an opposite-directed flow of moist air that is lifted onto the topographic barrier. In the absence of atmospheric instability, this results in widespread upslope (orographic) precipitation, where particles are formed in the air ascending the slope of the terrain. The melting layer (indicated by the radar bright band), dynamically marked by a layer of convergence, tends to separate the upward motion above from the downward motion below it. Sustained orographic precipitation will be accompanied by subsiding air that gets concentrated in valleys and result in a drainage flow. This precipitation-driven drainage flow may reach a maximum depth limited primarily by the height of the melting layer and secondarily by the valley confines. The drainage flow strength and depth are related to the rainfall amount. Typical examples of wet drainage flow documented in MAP are IOPs 8, 9, 10, and 14.
Supplementary URL: