New insights into mesoscale convective system updrafts and downdrafts
Wang, Die — Brookhaven National Laboratory
Giangrande, Scott — Brookhaven National Laboratory
Area of research:
As among the largest and most intense convective storms, mesoscale convective systems (MCSs) are often recognized for their extreme precipitation potential, as well as their implications for the global-scale water cycle and general circulation. One of the most fundamental properties of these MCSs is the convective vertical air motions, in the form of up- and downdrafts. These motions are among the most difficult aspects of deep convective clouds to measure, and our lack of information on these drafts inhibits proper representation of MCSs in numerical weather and climate prediction models. Our study presents a multi-year, multi-site use of radar wind profilers (RWPs) to characterize mature MCS kinematic structure and contrast MCS properties from two archetypal MCS locations (Amazon basin, BR; Oklahoma, USA).
Observations of convective updrafts within extreme deep convection and MCS conditions are costly, hazardous, and infrequent. This study provides an extended analysis from relatively cost-effective RWP instrumentation to address deficiencies in our understanding of MCS kinematic structures, cold pool properties, and coupled precipitation processes. We find that MCSs in Amazonia and Oklahoma often share similar rainfall rates, cold pool properties, and inferred mixing rates for low-level downdrafts. However, Oklahoma systems show more intense up- and downdrafts, and higher stratiform rain fractions than Amazon events.
This study contrasts characteristics of mature squall-line MCSs observed by extended ground-based RWP deployments from the U.S. DOE ARM facility. This analysis compares the kinematic structure, precipitation, and cold pool properties associated with MCSs over RWP sites in Oklahoma, U.S., (mid-latitude) to those observed during a 2-year deployment to Manaus, Brazil, during the GoAmazon 2014/15 campaign (tropical). The results indicate similar convective rainfall rates and total rainfall accumulations for MCSs in these two regions. For both regions, convective cold pools are associated with sharp decreases (~ 10 K) in the surface equivalent potential temperature (θe) near the time of convective line passages. Surface θe properties for both regions suggest a modest relationship between rainfall rate and the probability of observing measurable surface rainfall. The probability of observing convective updrafts in both tropical and mid-latitude MCSs is found to be similar as a function of low-level radar reflectivity. Mid-latitude MCSs are associated with more intense convective updrafts, with upward air motions peaking at higher altitude. The most pronounced contrast is the propensity for deeper and more intense downdrafts in mid-latitude systems. An analysis based on observed downdraft properties is performed using simple mixing assumptions. For these events, the vertical gradient of θe in the lower troposphere is relatively consistent between the Amazon and Oklahoma, suggesting similar mixing rates for downdrafts originating below 3 km (~ 0.1 km-1). If downdrafts originate nearer to the level of minimum θe in Oklahoma, mixing may be occurring at rates comparable to 0.3 km-1. As deep convective observations are rare, these RWP insights are anticipated to inform convective simulations, as well as air motion retrievals from other ground- and space-based platforms.