Radar Characteristics of the Cloud Populations of Tropical Disturbances in the Northwest Atlantic

Radar information from several northwestern Atlantic tropical disturbances has been analyzed in order to determine the radar characteristics of the cloud populations typically present in these systems. Frequency distributions of the radar echoes by size, duration, height, and separation have been constructed. These distributions are very skewed, the majority of the echoes failing in the classes having the smallest values. The echoes analyzed can be classified into 3 different scales according to the maximum cross-sectional area attained. Using GATE nomenclature, D, C, and B/C scales can be identified. Radar echoes in each areal–extent class have different overall characteristics as follows: D-scale echoes are numerous, small, short lived, shallow, and usually do not merge; C-scale echoes are larger, longer lived, deeper, and usually form from aggregations of smaller echoes; B/C-scale echoes are very large and long lived, originate from mergers, and often take the form of bands. The relative importance of each scale is discussed. Individual disturbances differ from each other in the total area covered by or-how and the mean size and standard deviation of the echoes present. However, the shape of the frequency distributions of the above mentioned echo characteristics is the same for all cases. The frequency distributions of echo area and height are lognormally distributed. It is hypothesized that the large scale conditions determine the formation of a number of convective elements of a certain size and that these elements develop by a process whereby growth is a random proportion of the previous size of the element.