Quantifying the Effects of Horizontal Grid Length and Parameterized Convection on the Degree of Convective Organization Using a Metric of the Potential for Convective Interaction

Abstract

The organization of deep convection and its misrepresentation in many global models is the focus of much current interest. A new method is presented for quantifying convective organization based on the identification of convective objects and subsequent derivation of object numbers, areas, and separation distances to describe the degree of convective organization. These parameters are combined into a “convection organization potential” based on the physical principle of an interaction potential between pairs of convective objects. This technique is applied to simulated and observed fields of outgoing longwave radiation (OLR) over the West African monsoon region using data from Met Office Unified Model simulations and satellite observations made by the Geostationary Earth Radiation Budget (GERB) instrument. The method is evaluated by using it to quantify differences between models with different horizontal grid lengths and representations of convection. Distributions of OLR, precipitation and organization parameters, the diurnal cycle of convection, and relationships between the meteorology in different states of organization are compared. Switching from a configuration with parameterized convection to one that allows the model to resolve convective processes at the model grid scale is the leading-order factor improving some aspects of model performance, while increased model resolution is the dominant factor for others. However, no single model configuration performs best compared to observations, indicating underlying deficiencies in both model scaling and process understanding.

Convective Aggregation and Indices Examined from CERES Cloud Object Data

“…Another limitation is the neglect of the predominance of clear sky in an aggregated state (Kadoya & Masunaga, 2018), whose importance is yet to be established, compared to that of the size insensitivity (for identical N and D 1 ). White et al (2018) defined an organization metric that is constructed on the basis of an interaction potential between a set of objects, where it is assumed that objects that are larger and closer together are more likely to interact with or physically influence each Note. Object groups are divided into small, medium, and large sizes according to the largest member of cloud objects within a grid cell.…”

Assessing Convective Organization in Tropical Radar Observations

“…Focusing on the clustering aspect of organized convection, different metrics have been developed to quantify the strength of organization, examples of which are the Simple Convection Aggregation Index SCAI (Tobin et al, ), Index for Organization I org (Tompkins & Semie, ), and the Convective Organization Potential COP (White et al, ). These indices take as input a two‐dimensional binary field representing a set of “convective objects” within the domain of interest, and they output a scalar value that measures the extent to which the convection may be considered to be “organized.” SCAI and COP were developed based on satellite‐derived cloud distributions and I org was developed based on distributions of updrafts in simulations, but in principle, they can be applied to any two‐dimensional binary field.…”

A wavelet‐based analysis of convective organization in ICON large‐eddy simulations

“…(), and White et al. (). Although convective organization is reflected in the indices, SCAI is strongly dominated by the number of clusters, whereas I org is very noisy and hard to interpret if the number of clusters is small.…”

“…), vertical wind velocity at 730 hPa (Tompkins and Semie, ) or outgoing long‐wave radiation (White et al. ).…”

“…To reduce grid dependency, White et al. () introduced the convective organization potential (COP). COP is comparable to SCAI, but it also includes the area of each convective cluster.…”