Abstract
Abstract Using the satellite-infrared-based Simple Convective Aggregation Index (SCAI) to determine the degree of aggregation, 5 years of CloudSat–CALIPSO cloud profiles are composited at a spatial scale of 10 degrees to study the relationship between cloud vertical structure and aggregation. For a given large-scale vertical motion and domain-averaged precipitation rate, there is a large decrease in anvil cloud (and in cloudiness as a whole) and an increase in clear sky and low cloud as aggregation increases. The changes in thick anvil cloud are proportional to the changes in total areal cover of brightness temperatures below 240 K [cold cloud area (CCA)], which is negatively correlated with SCAI. Optically thin anvil cover decreases significantly when aggregation increases, even for a fixed CCA, supporting previous findings of a higher precipitation efficiency for aggregated convection. Cirrus, congestus, and midlevel clouds do not display a consistent relationship with the degree of aggregation. Lidar-observed low-level cloud cover (where the lidar is not attenuated) is presented herein as the best estimate of the true low-level cloud cover, and it is shown that it increases as aggregation increases. Qualitatively, the relationships between cloud distribution and SCAI do not change with sea surface temperature, while cirrus clouds are more abundant and low-level clouds less at higher sea surface temperatures. For the observed regimes, the vertical cloud profile varies more evidently with SCAI than with mean precipitation rate. These results confirm that convective scenes with similar vertical motion and rainfall can be associated with vastly different cloudiness (both high and low cloud) and humidity depending on the degree of convective aggregation.
Highlights
Mean rainfall and convective activity are intrinsically linked (e.g. Arkin and Meisner 1987)
Using 5 years of CloudSat-Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data, we have shown that, over tropical ocean, the vertical structure of clouds is related to the degree of convective aggregation
Changes in convective aggregation are primarily associated with changes in two cloud types: anvil clouds and low level clouds
Summary
Mean rainfall and convective activity are intrinsically linked (e.g. Arkin and Meisner 1987). Several studies have shown a threshold behavior of selfaggregation with respect to sea-surface temperature (SST), with self-aggregation not occurring below an SST threshold; Khairoutdinov and Emanuel (2010) found such a threshold near 297 K, Wing and Emanuel (2014) found a threshold near 300 K (and another near 307 K, above which they did not see self-aggregation unless they increased their domain size), and Emanuel et al (2014) found a critical SST threshold between 303 and 308 K These values are near the current most common observed SST in tropical convective regions, as well as the maximum observed SST. Khairoutdinov and Emanuel (2010) hypothesized that tropical SST may exhibit self-organized criticality, with feedbacks between aggregation state and net surface fluxes that tend to cool SSTs in aggregated conditions (above the SST threshold) and warm SSTs in disaggregated conditions, maintaining SSTs near the threshold in convective regions This hypothesis has been questioned by observational studies (see below) and recent modeling studies (Wing and Cronin 2015; Holloway and Woolnough 2016). The total number of orbit stretches for each SCAI-R combination that meets these three requirements is listed in table 1
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