Assessing Convective Organization in Tropical Radar Observations

The spatial organization of deep moist convection is known to be an important determinant of the impacts of severe weather, while future changes to convective organization have been linked to various radiative feedbacks under climate warming. Yet there is no unanimously agreed upon definition of convective organization and so there is also no obvious way to objectively define it. In this work, we set out to define a metric for convective organization based on the size and proximity of convectively active regions. The metric is developed based upon tropical radar observations and takes two-dimensional convective objects, which are predefined in a horizontal plane, as input. We call the metric Radar Organization Metric (ROME). In addition to the proximity of different convective objects, which is used in other organization metrics, ROME is also sensitive to object size. As a result, ROME is also defined for the case of only one convective object. Thus, ROME provides a smoothly evolving measure of the degree of convective organization, which compares well to a visual assessment of the convective objects. ROME is found to be sensitive to different regimes of the North Australian monsoon, and its average diurnal cycle is coherent with the daily evolution of tropical rainfall. Through its dependence on area, ROME adds new capabilities that other metrics lack in measuring the degree of convective organization. In particular, ROME permits quantification of the individual contributions of the object size distribution and the spatial clustering of objects to the overall degree of convective organization.