Mean Vertical Motions Seen by Radar Wind Profilers

Abstract

Radar wind profilers have been used to measure directly the vertical motion above the radar site. Mean values of vertical motions in the troposphere and lower stratosphere reported at sites in and near mountains are often several centimeters per second and have often been attributed to the effects of quasi-stationary lee waves. However, observations now available at sites in the plains, far from any mountains, also show mean values of several centimeters per second. For example, monthly mean values seen by the Flatland VHF radar near Champaign-Urbana, Illinois, range from about ?3 to ?7 cm s?1, with largest magnitudes during the winter. The authors examine several of the hypotheses that have previously been advanced to explain these observations and find that each is inconsistent with the observations in some respect, except that quasi-horizontal flow along gently sloping isentropic surfaces leads to mean downward motion as large as 1?2 cm s?1. In this paper the authors suggest that the effects of vertically propagating gravity waves can account for most of the mean downward motions measured with radars, and the measured mean vertical motions can aptly be termed ?apparent? mean vertical motions. In gravity waves with downward phase propagation (upward energy propagation), the perturbations to the static stability and to the vertical velocity are negatively correlated. Since the radar reflectivity is proportional to the static stability, regions of the radar sampling volume with downward (or less strongly upward) vertical air motion due to gravity waves are weighted more heavily. A model incorporating this suggestion is first developed for a monochromatic gravity wave and is then expanded to a spectrum of gravity waves. This model predicts a correlation between the magnitude of the downward motion seen by the radar and the gravity wave energy density; the predicted relationship is verified by the observations from the Flatland radar. Statistical analysis of data from Flatland suggests that in the midtroposphere about 60% of the gravity wave energy is contained in waves with downward propagation of phase. The present model for w applies to the reflectivity from any refractive-index irregularities that can be treated as passive scalars, whether they are in the neutral atmospheric density, aerosol density, or plasma density and whether they arise from isotropic turbulence, an-isotropic turbulence, Fresnel scattering etc.