Fine-scale vertical structure of clear-air echoes during the International H2O Project as detected by an airborne Doppler radar

Characteristics of the cloud-free convective boundary layer (CBL) are examined by means of combined University of Wyoming Kingair (WKA) and Cloud Radar (WCR) measurements collected in the central Great Plains during IHOP (International H2O Project, May-June 2002). Clear air echoes are sufficiently strong for the radar to detect most of the CBL, at a resolution of ∼30 m. Vertical radar transects across the CBL are remarkably dominated by well-defined plumes of higher reflectivity. It is indicated 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 insects.; The close-range Doppler radar velocities are compared to gust probe vertical velocities under synoptically quiescent condition. It is found that the radar vertical velocities have a downward bias of 0.5 +/- 0.2 m s-1 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.; A simple numerical simulation of the airflow field and insect concentration in the CBL is developed to further demonstrate a consistency of the observed insect flight behavior with the presence of well-defined insect plumes in the otherwise well-mixed CBL, and with the tendency of insects to cluster in updrafts. Other studies have proposed that the response of insects to being lofted high into the CBL is controlled by air temperature. Our simulation shows that a temperature-threshold dependence alone does not yield well-defined plumes in regions of rising air currents.; Vertically-pointing radar reflectivity and Doppler velocity data collected above and below the aircraft, flying along fixed tracks, are used to define echo plumes and updraft plumes respectively. Updraft plumes are generally narrower than echo plumes, but both types of plumes have the dynamical properties of buoyant eddies, especially at low levels. This buoyancy is driven both by a temperature excess and a water vapor excess over the ambient air. Plumes that are better defined in terms of reflectivity or updraft strength tend to be more buoyant.; The existence of the CBL echo plumes and radar "fine-lines", sustained by low-level air convergence, has long been interpreted as due to a biotic response to updrafts. Several radar fine-lines, all with a humidity contrast, were examined to dynamically interpret their vertical structure. In all cases the fine-line represents a convergence zone within a deep CBL. This convergence sustains a sharp contrast in mixing ratio, and usually in potential temperature, across the fine-line. The question addressed herein is whether, at the scale examined here (∼10 km), the airmass contrast itself, in particular the difference in virtual potential temperature and resulting solenoidal circulation, is responsible for the sharp definition of the boundary and the sustained convergence, leading to a radar fine-line.