EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Response of Orographic Precipitation to Subsaturated Low-Level Layers

Shizuo Fu1, Richard Rotunno2, and Huiwen Xue3
Shizuo Fu et al.
  • 1Department of Atmospheric and Oceanic Sciences, Peking University, Beijing, China (
  • 2National Center for Atmospheric Research, Boulder, USA (
  • 3Department of Atmospheric and Oceanic Sciences, Peking University, Beijing, China (

Orographic precipitation is, on the one hand, an important source of fresh water, and on the other hand, a potential cause of floods and other disasters. Previous studies have focused on the situation where the whole atmosphere is saturated and nearly moist-neutral. However, there are times when subsaturated low-level layers are observed to be below saturated, nearly moist-neutral, upper-level layers.

A series of idealized two-dimensional simulations are performed here to investigate the impact of this subsaturated low-level layer on orographic precipitation. It is found that the impact is mainly controlled by a nondimensional parameter and two competing effects. The nondimensional parameter is N2zt/U, where N2 and zt are, respectively, the dry Brunt–Väisälä frequency and depth of the subsaturated low-level layer, and U the cross-mountain wind speed. When the nondimensional parameter exceeds a critical value, the decelerated region on the upwind side of the mountain moves upwind, resulting in weak surface precipitation near the mountain peak. When it is smaller than the critical value, surface precipitation occurs near the mountain peak.

The two competing effects are: 1) the vapor-transport effect, meaning that increasing zt decreases the amount of vapor transported to the mountain, and hence tends to decrease surface precipitation; and 2) the updraft width effect, meaning that increasing zt enhances flow blocking, producing a wider updraft over the upwind slope, and hence tends to increase surface precipitation. When the vapor-transport effect dominates, surface precipitation decreases with zt. When the updraft-width effect dominates, surface precipitation increases with zt.

How to cite: Fu, S., Rotunno, R., and Xue, H.: Response of Orographic Precipitation to Subsaturated Low-Level Layers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3266,, 2020

This abstract will not be presented.