Retrieval of Vertical Air Motion in Shallow Convective Clouds

Abstract:

In-cloud vertical air motion [V_air,in-cloud] is a key parameter for determining the strength of the vertical transport of heat and moisture in convective clouds: processes that impact cloud fraction and lifetime and ultimately the global radiative budget. Despite its importance, measurements of V_air,in-cloud are sparse, especially in precitating shallow convection. V_air,in-cloud measurements by in-situ sensors on aircraft are of high quality but limited to few field campaigns. Alternatively, retrieving V_air,in-cloud from Doppler radars in convective clouds is challenging, because it requires seperating the contribution of the sendimentation velocity from the observed Doppler velocity.

In this study, we improve the lower-bound method first proposed by Battan [1964] to retrieve V_air,in-cloud in lightly precipitating shallow convective clouds. The lower-bound method is built on the assumption that small drops (air motion tracers) are always present within radar observation volume such that the slower downward falling edge of the Doppler spectrum can be used to retrieve vertical air motion. However, several factors limit the applicability of the lower-bound method, particularly the sensitivity of the radar, the noise level in the Doppler spectrum and the turbulent broadening of the Doppler spectrum. The current study capitalizes on improved estimates of the turbulence broadening Borque et al., [2016] and on well-established techniques for the removal of the spectral broadening due to turbulence, wind shear and radar beamwidth. To address outstanding biases identified by Shupe et al., [2008], we propose an additional correction for spectral broadening due to signal-to-noise ratio.

Using 3-years of independent observations collected at the Eastern North Atlantic (ENA) Atmospheric Radiation Measurement (ARM) program observatory, we confirm that the revised lower-bound method can be used to retrieve V_air,in-cloud in precipitating shallow convective clouds with an uncertainty of 0.2 m s^-1. Our V_air,in-cloud retrievals open up opportunities to investigate the interplay of air velocity with microphysical process. For instance, preliminary resuls suggests that clouds with larger liquid water path (LWP) are characterized by stronger vertical air motions compared with those having lower LWP values.