The Vertical Structure of TOGA COARE Convection. Part I: Radar Echo Distributions

Abstract

Radar data collected by the 5-cm MIT radar, which was deployed aboard the R/V Vickers during the intensive observing period of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment, are partitioned into convective and stratiform Cartesian grid columns. The vertical structure of convective echo is examined through the use of two variables: echo top height and the height of the 30-dBZ reflectivity contour. The first of these variables has traditionally been used to describe the vertical structure characteristics of convection, and the second has recently been linked to internal microphysical properties and lightning. Histograms of the relative frequency of convective-only echo top heights and 30-dBZ contour heights were constructed for the three cruises of the Vickers, with each cruise experiencing different phases of the intraseasonal oscillation (ISO). Cruise 1, which was dominated by the convectively ‘‘inactive’’ phase of the ISO was characterized by the highest frequency of shallow convection (based on echo top heights), whereas cruise 2, which was dominated by a particularly well-defined passage of the convective phase of the ISO, exhibited the tallest echo top heights. Cruise 3 convection was influenced by moderate westerly surface winds characteristic of postwesterly wind burst conditions, and convection was of intermediate height. When viewed as a function of ‘‘internal’’ vertical structure (i.e., 30-dB Z contour height), the frequency distributions vary less from cruise to cruise, with cruises 1 and 2 having nearly identical distributions of convective 30-dBZ contour heights. Furthermore, when the contribution to convective rainfall is examined as a function of 30-dBZ contour height, it is seen that relatively more rain fell from vertically ‘‘intense’’ convection (i.e., convection with tall 30-dBZ contours) during cruises 1 and 3 than during cruise 2. Instantaneous correlations between rainfall rate and radar echo height were highly scattered about a mean value of about 0.55, whereas rainfall rate and 30-dBZ contour height correlations peaked at about 0.8 and exhibited much less scatter.