Assessing Convective Organization in Tropical Radar Observations

Abstract

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.

Identifying Relations Between Deep Convection and the Large‐Scale Atmosphere Using Explainable Artificial Intelligence

“…We quantify the convective state of the atmosphere in two ways. We use the total area that is experiencing convection, or TCA, as a measure of the overall convective activity within the radar domain, and ROME (Retsch et al., 2020) to quantify convective organisation. It has long been known that the TCA relates very strongly to area average convective rainfall (e.g., Byers & Union, 1948; Davies et al., 2013; Doneaud et al., 1984).…”

“…The details of ROME are provided in Retsch et al. (2020), but we will briefly summarize the main idea here.…”

“…We use ROME to assess convective organization. This measure of organization has been specifically defined and validated for use with radar data sets and has shown skill in identifying relationships of convective organization with different states of the Australian monsoon (Retsch et al., 2020). ROME quantifies the degree of organization of a radar scene based on the arrangement of contiguous regions of convective activity.…”

A Relationship Between ITCZ Organization and Subtropical Humidity

“…Quantifying organization is not straightforward (see, e.g., Figure 1 in Brueck et al, 2020), as evidenced by the various organization indices that have been proposed recently (Brune et al, 2018; Kadoya & Masunaga, 2018; Retsch et al, 2020; Tobin et al, 2012; Tompkins & Semie, 2017; White et al, 2018). Different indices emphasize distinct aspects of organization and do not necessarily agree (Pscheidt et al, 2019; Xu et al, 2019).…”

Toward Form-Function Relationships for Mesoscale Structure in Convection

“…Radar data studies use similar measures and metrics (e.g. Retsch et al 2020). Such conglomeration measures are quite nonunique or arbitrary (Xu et al 2019).…”