Cumulonimbus Vertical Velocity Events in GATE. Part I: Diameter, Intensity and Mass Flux

Abstract

This is the first part of a two-part paper defining the nature of the vertical air motion in and around GATE cumulonimbus clouds. The statistics are from a total of 104 km of flight legs, flown on six days in GATE, at altitudes from near the surface to 8100 m. The basic data sets analyzed are time series of vertical velocity at a frequency of 1 Hz. For the purpose of study, convective events are divided into two categories: drafts, requiring only that vertical velocity be continuously positive (negative) for 500 m and exceed an absolute value of 0.5 m s?1 for 1 s; and cores, the stronger portions of the stronger drafts, requiring that upward (downward) vertical velocity be continuously greater than an absolute value of 1 m s?1 for 500 m. The distributions of average vertical velocity, maximum vertical velocity, diameter and mass flux are given for drafts and cores at five altitude intervals between 150 m and 8 km. In all cases, the distributions are approximately log-normal. Above cloud base, updrafts tend to be smaller but more intense than downdrafts. Updrafts and down-drafts near cloud base are comparable in size and intensity. Downdraft cores are smaller than updraft cores at all attitudes. They also are weaker, except near cloud base, where updraft and downdraft cores have comparable intensity. In the middle troposphere, only 10% of the updraft cores have mean vertical velocities greater than 5 m s?1, and only 10% have diameters in excess of 2 km.