Lee‐wave clouds and denitrification of the polar stratosphere

Abstract

We present a hypothesis that the known formation of nitric acid hydrates in lee‐wave ice clouds produces a “standing crop” of nitric acid trihydrate (NAT) particles, and that growth and sedimentation of these particles efficiently denitrifies the polar stratosphere. Simulations using a simple two‐dimensional model illustrate that the large NAT number concentrations produced in lee waves (>0.1 cm−3) lead to low gas‐phase HNO3 concentration in the cloud layer, limiting subsequent particle growth. Provided the NAT existence temperature is not exceeded, these particles slowly sediment out of the cloud layer, at which point further growth is initiated. As a result of the long residence time and vertical wind shear, NAT particles produced in geographically selective regions over a short duration can cause extensive denitrification throughout the lower stratosphere. Our model illustrates that a single hypothetical lee‐wave nucleation event [10 hours, 1000 km width (along a longitude) × 100 m altitude] is sufficient to produce significant denitrification (∼25%) vortex‐wide.