Tracking the life cycle of thunderstorms: Perspectives from GoAmazon2014/15
Submitter
Giangrande, Scott — Brookhaven National Laboratory
Area of research
Vertical Velocity
Journal Reference
Science
The Amazon Basin has been at the forefront of impactful deep convective cloud (DCC) studies. Our study investigates daytime DCC observations to document changes in storm characteristics within the context of the larger-scale environmental shifts found between the Amazon wet and dry seasons. The focus is on the use of surveillance weather radar “cell tracking,” coupled with Atmospheric Radiation Measurement (ARM) user facility radar wind profiler-based vertical air velocity estimates. The Amazon Basin setting offers a natural laboratory for convective studies, providing the frequent clouds necessary for documenting storm life cycle under a variety of conditions. The unique observations collected during the GoAmazon2014/15 field campaign also help address gaps in our understanding of the factors that regulate DCC size, frequency, and updraft or precipitation intensity. Moreover, these coupled observations are rare, but they are critical for the development of high-resolution cloud models that lack coupled microphysical/dynamical constraints.
Impact
The unique aspect of our study is its emphasis on radar-tracked isolated DCCs that pass over the ARM radar wind profiling equipment, yielding direct measurements of vertical hydrometeor and, by proxy, air motions. This coupled use of profiling-based vertical air velocity information with radar tracking has been missing from previous analyses of the DCC life cycle, allowing new perspectives on the evolution of updrafts and other storm-intensity measures. Performing such analysis within the context of distinct Amazon meteorological wet and dry regimes enables advanced interpretation of the role of bulk environmental controls on storm properties -- specifically, how larger-scale regime shifts may modify storm initiation, evolution, and coupled microphysical-dynamical properties. From this work, observations and parcel model simulations argue that dry-season DCCs rapidly develop and contain stronger updrafts at low levels because of factors including elevated low-level convective available potential energy (CAPE). In contrast, wet-season DCCs were longer-lived, with accompanying strong updrafts aloft because of larger free-tropospheric relative humidity and reduced entrainment-driven dilution. Stronger DCC downdrafts are also consistently observed at storm mid-levels during the dry season, which is partially attributed to increased graupel loading, as well as enhanced evaporation, dry-air entrainment mixing, and negative buoyancy in regions adjacent to sampled dry-season cores.
Summary
This study tracks thunderstorms observed during the wet and dry seasons of the Amazon Basin using U.S. Department of Energy ARM GoAmazon2014/15 campaign data. We couple radar-based precipitation tracking with opportunistic overpasses of ARM’s radar wind profiler (RWP) and other ground-based observations to add unique insights into the upward and downward air motions within these clouds at various stages in the storm life cycle. The results of a simple updraft model are provided to lend physical explanations for observed seasonal differences in storm behaviors.
Figure 1. Composite mean and standard deviation of radar cell-tracked properties (area and mean radar reflectivity factor Z) for Amazon wet (blue/solid) and dry (red, dashed) season isolated cells.
(Bottom) Cumulative vertical air velocity retrievals (CFADs) from the ARM profiler for the (a) wet season, (b) dry season, and (c) wet-minus-dry differences for storm overpasses with Z > 25 dBZ. From journal.