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WRF Nested Large-Eddy Simulations of Deep Convection during SEAC4RS

Title: WRF Nested Large-Eddy Simulations of Deep Convection during SEAC4RS.
Name(s): Heath, Nicholas Kyle, author
Fuelberg, Henry E., professor directing dissertation
Wiedenhöver, Ingo, 1966-, university representative
Hart, Robert E. (Robert Edward), 1972-, committee member
Bourassa, Mark Allan, 1962-, committee member
Misra, Vasubandhu, 1970-, committee member
Turk, Francis J., committee member
Florida State University, degree granting institution
College of Arts and Sciences, degree granting college
Department of Earth, Ocean, and Atmospheric Science, degree granting department
Type of Resource: text
Genre: Text
Issuance: monographic
Date Issued: 2015
Publisher: Florida State University
Place of Publication: Tallahassee, Florida
Physical Form: computer
online resource
Extent: 1 online resource (109 pages)
Language(s): English
Abstract/Description: Deep convection is an important component of atmospheric circulations that affects many aspects of weather and climate. Therefore, improved understanding and realistic simulations of deep convection are critical to both operational and climate forecasts. Large-eddy simulations (LESs) often are used with observations to enhance understanding of convective processes. This study develops and evaluates a nested-LES method using the Weather Research and Forecasting (WRF) model. Our goal is to evaluate the extent to which the WRF nested-LES approach is useful for studying deep convection during a real-world case. The method was applied on 2 September 2013, a day of continental convection having a robust set of ground and airborne data available for evaluation. A three domain mesoscale WRF simulation is run first. Then, the finest mesoscale output (1.35 km grid length) is used to separately drive nested-LES domains with grid lengths of 450 and 150 m. Results reveal that the nested-LES approach reasonably simulates a broad spectrum of observations, from reflectivity distributions to vertical velocity profiles, during the study period. However, reducing the grid spacing does not necessarily improve results for our case, with the 450 m simulation outperforming the 150 m version. We find that simulated updrafts in the 150 m simulation are too narrow to overcome the negative effects of entrainment, thereby generating convection that is weaker than observed. Increasing the sub-grid mixing length in the 150 m simulation leads to deeper, more realistic convection, but comes at the expense of delaying the onset of the convection. Overall, results show that both the 450 m and 150 m simulations are influenced considerably by the choice of sub-grid mixing length used in the LES turbulence closure. Finally, the simulations and observations are used to study the processes forcing strong midlevel cloud-edge downdrafts that were observed on 2 September. Results suggest that these downdrafts are forced by evaporative cooling due to mixing near cloud edge and by vertical perturbation pressure gradient forces acting to restore mass continuity around neighboring updrafts. We conclude that the WRF nested-LES approach provides an effective method for studying deep convection for our real-world case. The method can be used to provide insight into physical processes that are important to understanding observations. The WRF nested-LES approach could be adapted for other case studies in which high-resolution observations are available for validation.
Identifier: FSU_2015fall_Heath_fsu_0071E_12917 (IID)
Submitted Note: A Dissertation submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Degree Awarded: Fall Semester 2015.
Date of Defense: November 10, 2015.
Keywords: convective dynamics, deep convection, large-eddy simulation, WRF
Bibliography Note: Includes bibliographical references.
Advisory Committee: Henry E. Fuelberg, Professor Directing Dissertation; Ingo Wiedenhoever, University Representative; Robert E. Hart, Committee Member; Mark A. Bourassa, Committee Member; Vasu Misra, Committee Member; Francis J. Turk, Committee Member.
Subject(s): Meteorology
Atmospheric sciences
Persistent Link to This Record:
Host Institution: FSU

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Heath, N. K. (2015). WRF Nested Large-Eddy Simulations of Deep Convection during SEAC4RS. Retrieved from