Devin Bissell

Date of Award

December 2022

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Atmospheric Sciences

First Advisor

Gretchen Mullendore

Abstract

Overshooting convection can significantly impact the chemical and radiative properties of the upper troposphere and lower stratosphere (UTLS) through the transport of various chemical species. Most notably, these impacts include enhancement of stratospheric water vapor and introduction of ozone-depleting halogen compounds, which both have important impacts on climate change. Convective updrafts provide a mechanism for transporting lower-tropospheric air, containing much higher concentrations of these substances, into the UTLS on time scales and in extratropical locations that are not possible via large-scale circulations. Additionally, overshooting tops are often correlated with severe weather at the surface and their identification can aid in forecasting impactful conditions. For these reasons, accurately predicting the presence and depth of overshooting convection is of high importance. To better understand how well convective transport and potential stratospheric impacts are represented in current forecast models, approximately 75,000 updrafts in the Central and Eastern United States were analyzed from High-Resolution Rapid Refresh version 4 (HRRRv4) simulations for May and July 2021. The sample included all updrafts exceeding vertical velocity thresholds of 2 m/s at 4 km and 5 m/s at 8 km during the first 24 forecast hours of each 06Z and 12Z HRRR run. Climatologies of echo top heights and two definitions of the level of maximum detrainment (LMD) were then produced. These distributions and analyses of a few selected cases exposed key errors in how updrafts are represented at upper levels. This led to the highest echo top heights being under-predicted by the model, but LMD heights using one of the definitions potentially being over-predicted. The height distribution for the second LMD definition matches observations nearly exactly. These results persisted at shorter forecast lead times and show that work needs to be done to improve model simulations of overshooting convection.

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