New insights into updrafts and downdrafts using unique ARM observations and high-resolution modeling
Submitter:
Wang, Die — Brookhaven National Laboratory
Giangrande, Scott — Brookhaven National Laboratory
Area of research:
Vertical Velocity
Journal Reference:
Science
Mesoscale convective systems (MCSs) are the largest and most intense convective storms that regulate the global-scale water cycle and general circulation. A fundamental property of these MCSs is convective vertical air motions, which are among the most challenging aspects of these thunderstorms to measure, while observations of updrafts and downdrafts within extreme MCS conditions are costly, hazardous, and infrequent. This inability to observe MCSs' kinematic properties directly inhibits global climate model and cloud-resolving model process improvement.
Impact
To address deficiencies in our understanding of MCS kinematic structures, we employ a unique multi-year, multi-site data set to summarize MCS convective draft properties observed by relatively cost-effective ARM radar wind profiler (RWP) measurements. We perform WRF idealized simulations at a range of resolutions and compare to observational statistics in a multidimensional parameter space to understand and quantify model performance/biases in representing convective draft core behaviors and processes.
Summary
This study explores the updraft and downdraft properties of mature stage MCSs in terms of draft core width, shape, intensity, and mass flux characteristics. The observations use extended RWP and surveillance radar data sets from the DOE ARM facility for mid-latitude (Oklahoma, USA) and tropical (Amazon, Brazil) sites. The Oklahoma MCSs indicate larger and more intense convective updraft and downdraft cores, and greater mass flux than Amazon MCS counterparts. However, similar size-intensity relationships and draft vertical profile behaviors are observed for both regions. Additional similarities include weak positive correlations between core intensity and core width (r ∼ 0.5) and increases in draft intensity with altitude. A model-observational comparison for draft properties (core width, intensity, and mass flux) is also performed to illustrate the potential usefulness of statistical observed draft characterizations. Idealized simulations with the Weather Research and Forecasting model aligned with mid-latitude MCS conditions are performed at model grid spacings (△x) that range from 4 km to 250 m. It is shown that the simulations performed at △x = 250 m at similar mature MCS life cycle stages are those that exhibit draft intensity, width, mass flux, and shape parameter performances best matching with observed properties.
Figure 1: Mean vertical velocity (a), maximum vertical velocity (b), mean vertical velocity normalized by the maximum vertical velocity (c), and mass flux (d) for each updraft core width bin for both observations (MAO and SGP) and WRF simulations with different horizontal grid spacings (△x = 1 km and 250 m). Bins are an increment of 1 km and error bars represent standard deviation. Solid lines are the linear least-squares regression fits.
Figure 2: Histograms and cumulative distribution functions (CDFs, dotted lines) of updraft cores mean vertical velocity (a), maximum vertical velocity (b), core width (c), and mass flux (d) for both observations (MAO and SGP) and WRF simulations with different horizontal grid spacings (△x = 1 km and 250 m).