Does convective aggregation need to be represented in cumulus parameterizations?

Abstract

[1] Tropical deep convection exhibits a variety of levels of aggregation over a wide range of scales. Based on a multisatellite analysis, the present study shows at mesoscale that different levels of aggregation are statistically associated with differing large-scale atmospheric states, despite similar convective intensity and large-scale forcings. The more aggregated the convection, the dryer and less cloudy the atmosphere, the stronger the outgoing longwave radiation, and the lower the planetary albedo. This suggests that mesoscale convective aggregation has the potential to affect couplings between moisture and convection and between convection, radiation, and large-scale ascent. In so doing, aggregation may play a role in phenomena such as ''hot spots'' or the Madden-Julian Oscillation. These findings support the need for the representation of mesoscale organization in cumulus parameterizations; most parameterizations used in current climate models lack any such representation. The ability of a cloud systemresolving model to reproduce observed relationships suggests that such models may be useful to guide attempts at parameterizations of convective aggregation.

An Observational View of Relationships Between Moisture Aggregation, Cloud, and Radiative Heating Profiles

“…Observational studies of convective self-aggregation to date (Tobin et al 2012;Stein et al 2017;Tobin et al 2013) have quantified the degree of aggregation based on the degree to which cold clouds observed with a domain are spatially coherent, and these studies have also noted that clear areas tend to be less humid when convection is more aggregated. We invert this logic using a measure of aggregation defined by the inhomogeneity of the integrated water vapor field.…”

“…As reviewed carefully by Holloway et al (2017;this issue), the organization of convection has been thought of for many years as being intimately linked to mesoscale systems organized by gravity and other convectively coupled waves (e.g., Mapes 1993) or large-scale circulations. The Earth's atmosphere does exhibit some characteristics of convective self-aggregation, including the tendency of more organized atmospheres to have lower humidity in clear areas, reduced domain-mean high cloudiness, and increased low cloudiness in non-convective areas, with corresponding impacts on surface and radiative fluxes (Tobin et al 2012;Stein et al 2017;Tobin et al 2013).…”

“…Here, we expand on the relatively small literature (Tobin et al 2012(Tobin et al , 2013Stein et al 2017) examining how the structure of the atmosphere, and the clouds embedded in it, depends on the degree of organization. We exploit a range of colocated observations from the A-Train satellite constellation (Stephens et al 2002;L'Ecuyer and Jiang 2010) to identify aggregated states in the tropical atmosphere and examine the cloud, precipitation, and radiative structure of these states.…”

Thermodynamic control of anvil cloud amount

“…Numerical (21,23,24) and observational studies (25,26) show that when convection is more clustered, the free atmosphere is drier and the area covered by anvil clouds is smaller. Because this clustering, or aggregation, is expected to occur more easily at high surface temperatures (27), such a process might affect cloud feedbacks and climate sensitivity (28,29).…”

Diagnosing the average spatio-temporal impact of convective systems – Part 2: A model intercomparison using satellite data

“…The detailed representation of deep convection through parameterisation in a GCM remains a challenge, particularly for the temporal evolution and life cycle of organised convection (e.g. Moncrieff et al, 2012;Tobin et al, 2013). As such, there is certainly scope for further in-depth evaluations to inform parameterisation development and improve global climate models.…”