The Use of Millimeter Doppler Radar Echoes to Estimate Vertical Air Velocities in the Fair-Weather Convective Boundary Layer

Abstract

Vertical velocity characteristics of the optically clear convective boundary layer (CBL) are examined by means of profiling airborne radar data collected in the central Great Plains during the International H 2 O Project, May-June 2002 (IHOP 2002. Clear-air echoes are sufficiently strong for the radar, a 95-GHz cloud radar, to detect most of the CBL at a resolution of ϳ30 m. Vertical radar transects across the CBL are remarkably dominated by well-defined plumes of higher reflectivity. These echo plumes occupy most of the depth of the CBL in the developing and mature stages of the CBL. Gust probe data indicate that the plumes tend to correspond with ascending motion. Evidence exists in the literature, and arises from this study, that the clear-air scatterers are mostly small insects.The close-range Doppler radar velocities, some 100 m above and below the aircraft, are compared to gust probe vertical velocities after both are corrected for aircraft motion.It is found that the radar vertical velocities have a downward bias of 0.5 Ϯ 0.2 m s Ϫ1 on average. This bias is of the same sign as that reported in wind profiler data in the CBL, but it is larger. The difference between aircraft and radar vertical velocities becomes larger in stronger updrafts. This does not happen in cases where the scatterers are hydrometeors: hydrometeors fall out at their terminal velocity, which does not directly depend on updraft speed.The existence of the CBL echo plumes and radar "fine lines," sustained by low-level air convergence, has long been attributed to a biotic response to updrafts. This response has been assumed to be controlled by air temperature; that is, insects subside when they encounter cold air in the upper CBL. The authors propose that the biotic response is not temperature controlled but, rather, is dependent on the vertical displacement.

Multi-platform Observations Characterizing the Afternoon-to-Evening Transition of the Planetary Boundary Layer in Northern Alabama, USA

“…Wilson et al 1994;Martin and Shapiro 2007). Angevine (1997) reports a downward bias in mean UHF profiler vertical velocities up to 0.3 m s −1 in the daytime PBL due to the subsidence of particulate scatterers, and Geerts and Miao 2005 show that insects can actively oppose PBL upward vertical motions. Concentration of insects in the southern Plains peaks in the mid-afternoon and overnight, with minima near sunrise and dusk (Martin and Shapiro 2007).…”

Dual-Doppler Analysis in a Single Plane from an Airborne Platform

“…Russell and Wilson (1997) argued that clear-air echoes at X band were dominated by scattering by insects, a conclusion that is essentially inevitable for the 95-GHz (W band) WCR, where contributions from Bragg scattering are smaller than at X band. The scatterers appear to descend, with a mean speed of ϳ0.55 m s Ϫ1 , with respect to the air in which they are embedded (Geerts and Miao 2005). Geerts and Miao (2005) found that the difference between the vertical air motion and the Doppler velocity increased with increasing vertical velocity.…”

“…The scatterers appear to descend, with a mean speed of ϳ0.55 m s Ϫ1 , with respect to the air in which they are embedded (Geerts and Miao 2005). Geerts and Miao (2005) found that the difference between the vertical air motion and the Doppler velocity increased with increasing vertical velocity. Thus, the vertical Doppler velocities shown in this section most likely underestimate the vertical air velocity.…”

“…The UWKA made 10 transects across the cold front between 1907 and 2114 UTC (Geerts and Leon 2003). Radar reflectivity, vertical, and horizontal velocity components for a southeast-northwest (SE-NW) transect between 2006:30 and 2011:30 UTC are shown in Fig.…”

Combined vertical-velocity observations with Doppler lidar, cloud radar and wind profiler

“…Live insects can even actively counteract the vertical motions induced by the atmosphere. (Geerts and Miao, 2005;Chandra et al, 2010) These measured velocity values are therefore more ambiguous than those of the Doppler lidar and the wind profiler.…”