Influence of convectively-induced secondary circulations on surface wind variability and heat fluxes in the U.S. southern Great Plains
Date
2023-08
Authors
Enzensperger, Travis Xander
Major Professor
Advisor
Williams, Ian
Gallus, William
Hornbuckle, Brian
Committee Member
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Abstract
Small-scale meteorological features are critical to weather and climate prediction because they affect surface wind speed, heat fluxes, temperature, precipitation, and convective storms. However, because of their finer spatial and temporal scales, mesoscale phenomena are more difficult to predict than synoptic scale features, and some atmospheric models cannot adequately resolve mesoscale secondary circulations and their influence on the surface. As a result, models may have biases in surface wind speed and other meteorological variables. This study examines the surface wind speeds and heat fluxes associated with six types of boundary layer phenomena for 74 cases during the warm season (May-August) in the southern Great Plains region of the United States. The observation data was obtained from the Atmospheric Radiation Measurement (ARM) network of sites. The six case types are clear air, fair weather cumulus, horizontal convective rolls, cellular convection, roll-cell transition, and convective rain (with a focus on gust fronts). In this study, “clear air” is defined as disorganized boundary layer convection. In addition, a subset of 17 cases were simulated by the Weather Research and Forecasting (WRF) model with 3 km grid spacing to evaluate the effectiveness of a convection-allowing model in predicting surface wind speeds for each case type. The methodology of the study mostly concerns considerations of time and space to achieve the most reasonable wind speed comparison between the observations and the model. The performance of the model was evaluated on the basis of mean wind speed and the distribution of wind speed variability.
The ARM observations for the clear air, fair weather cumulus, roll, cell, and roll-cell transition case types exhibited similar wind speed variability that is normally distributed, while the convective rain case type had much greater wind speed variability and a strong right skewed distribution. Most convective rain cases with gust fronts had an extended step-up in the wind speed of about 2 m/s that persisted for approximately two hours after the passage of a gust front. For the subset of model simulated cases, the WRF hindcasts performed well in simulating the observed surface wind speeds for each case type. This result is notable due to the spatial and temporal unpredictability of small-scale meteorological features, particularly warm season convection. In addition, the WRF hindcast surface wind speeds responded to secondary circulations, as was seen in patterns of surface wind speed that were remarkably similar to radar reflectivity patterns. These findings of the WRF-ARM surface wind speed comparison indicate that convection-allowing atmospheric models are effective in simulating the effects of mesoscale secondary circulations on surface wind speed. The ARM observation sensible heat fluxes were greater for the clear air, roll, cell, and fair weather cumulus case types compared to the roll-cell transition and convective rain case types. The latent heat fluxes for the clear air, roll, and roll-cell transition case types were greater than the cell, fair weather cumulus, and convective rain case types. The smaller evaporative fraction for the cell and convective rain case types suggests reduced soil moisture or less active vegetation due to dry conditions. Convective rain cases with gust fronts were the only boundary layer feature to exhibit statistically significant influence on surface temperature variability. It is hoped that the findings of this study will be helpful to the advancement of numerical weather prediction, and in particular, the modeling of mesoscale secondary circulations and their influence on the surface.
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