Convective Meteorology (Mesoscale Dynamics)

Understanding and Predicting Nocturnal Convection Initiation using an Ensemble-based Multi-scale Data Assimilation System

Samuel K. Degelia

School of Meteorology

01 April 2016, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

There exists a nocturnal maximum in precipitation and thunderstorms over the Great Plains during the summer. However, initiation of these storms is notoriously difficult to forecast in numerical models due to the storms often involving interactions across many scales. Additionally, most storms that develop at night are categorized as “elevated” in which the conditionally unstable source air is located above the boundary layer. While convection initiation (CI) in well-mixed, daytime boundary layers often is triggered along boundaries of surface convergence and is moderately well understood, nocturnal CI is relatively unexplored. In this study, a multi-scale, GSI-based EnKF forecast system is applied to a case from 25 June, 2013 to address the following: (1) the impact of radar and in-situ DA for nocturnal CI forecasts; and (2) the understanding of mechanisms that lead to nocturnal CI.

Assimilation of synoptic and other in-situ mesoscale observations can help to improve analyses of any features that may aid in nocturnal CI such as low-level jets, density currents, or gravity waves. Additionally, many previous studies have shown the benefits of radar DA through improved forecasts in the structure and evolution of mesoscale convective systems and other various daytime phenomena. However, its impact in nocturnal CI forecasts has not yet been studied. After 24 hours of in-situ assimilation on an outer grid, results from three experiments will be presented that compare the impact of in-situ DA, radar DA, and both. Additionally, sensitivity studies for different planetary boundary layer and microphysics parameterization schemes will be presented. These results will be used to diagnose the CI forcing mechanisms for this case.

Convective Meteorology (Mesoscale Dynamics) Seminar Series website