Establishing a Holistic Understanding of the Circulations of Mesoscale Convective System Stratiform Regions

Abstract

Mesoscale convective systems (MCSs) significantly influence large-scale circulations as well as radiation and precipitation budgets across the globe (Houze 2004). This study uses DoE Atmospheric Radiation Measurement (ARM)-sourced observations, complemented by large-eddy-scale (LES) numerical simulations, to examine how the characteristics of midlatitude MCS stratiform precipitation regions are generated and modified by low-frequency gravity waves, line-end vorticity-induced wind flows, and large-scale environmental conditions.

Much research has been devoted to these widespread systems and their stratiform regions due to their wide- ranging impacts on the global zonal wind field as well as the radiation balance and hydrologic cycle. The ability to simulate stratiform regions that closely match observations remains elusive and is further complicated because the characteristics of the stratiform region are closely tied to the detrainment of hydrometeors from the convective updraft as well as evaporation and sublimation induced by mid-level rear inflow. These intra- and extra-system circulations are products of three processes: low-frequency gravity waves, line-end vortex-induced flows, and environmental wind. Therefore, it is these processes that ultimately control the characteristics of the stratiform region, but no study has yet attempted to formally quantify their individual contributions to the MCS wind field.

We hypothesize that the poor representation of MCS stratiform regions in model simulations is a result of an incorrect balance between gravity wave, line-end vortex, and environmentally induced flows that results from an incorrectly simulated latent heating profile. The ARM-supported Plains Elevated Convection at Night (PECAN) and Mid-Latitude Continental Convective Clouds Experiment (MC3E) field campaigns offer unprecedentedly detailed low-level observations coincident with multi-Doppler storm wind field characterization so that quantification of individual contributions is now possible. To test this hypothesis, we will focus on three objectives:

1. Identify low-frequency gravity waves generated by midlatitude MCSs observed by the PECAN and MC3E field campaigns.
2. Isolate the impacts of low-frequency gravity waves, line-end vortex, and large-scale environmental flows on the development and characteristics of the observed PECAN and MC3E MCS stratiform regions.
3. Identify potential errors in the stratiform region of LES-simulated PECAN and MC3E MCSs resulting from incorrect circulation causal partitioning compared to (2).

The first two objectives will use PECAN and MC3E datasets, including lower-tropospheric Atmospheric Emitted Radiance Interferometer (AERI) and profiler observations, in situ and remotely sensed microphysical observations, and multiple-Doppler-derived three-dimensional wind fields to identify low-frequency gravity waves, line-end vortices, the environmental wind field, and their associated circulations. Any associated stratiform region microphysical modifications will also be examined. The third objective involves comparing the processes contributing to the MCS circulation in the observations to high-resolution case-study MCS simulations. It will also use a novel technique to separate the direct microphysical from the indirect dynamical impact of microphysical parameterization changes on the stratiform region.

This project directly addresses the “Convective Cloud Processes” FOA research topic. Studying the relative impacts of the three controlling processes on MCS system-scale circulations and stratiform structure will addresses “interactions between dynamics and microphysics”, the “organization of convective systems”, and the “understanding of convective processes controlling the…microphysical and macrophysical properties of convective clouds.” Given the key contributions MCS stratiform regions make to global circulation and precipitation, the project will lead to “improved understanding of processes that are…fundamental to the Earth’s radiative balance and/or hydrologic cycle.”