May 1, 2020 - 2:00 pm
May 1, 2020 - 3:00 pm
School of Meteorology Colloquium
Tropospheric Polar Vortices and Impacts on Atmospheric Flow from the Arctic to the Mid-Latitude using a New Global Modeling System
Friday, May 1st
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Meeting ID: 987 0008 3638
The observed rapid changes in the Arctic are important to quantify not only for understanding the Arctic region, but also for understanding how processes between the Arctic and lower latitudes can interact to culminate in high impact weather events. The tropopause polar vortex (TPV) is an Arctic feature that can interact with mid-latitude atmospheric flow, in which the maintenance and intensification of TPVs depends on diabatic processes. Improved knowledge and a better representation of TPV-mid-latitude interactions in numerical prediction models could extend forecast skill beyond the present-day barrier of 7-10 days.
This study investigates TPVs in the Arctic and their interactions with mid-latitude atmospheric flow using a newly developed global modeling system. This modeling system couples an ensemble Kalman filter (EnKF) data assimilation software (DART) with the Model for Prediction Across Scales (MPAS) global model called MPAD-DART. This system utilizes a newly developed non-hydrostatic global model that allows for smooth transitions from coarse to fine mesh resolutions. The EnKF data assimilation technique allows for flow-dependent background error covariances within MPAS-DART, which is especially important in data sparse regions like the Arctic.
Evaluation of MPAS-DART over the Arctic shows reasonable consistency between the model and observations, however, there are some notable points for improvement. There is a cold bias in the upper-troposphere and lower-stratosphere levels where TPVs are often found, which is a result of too much cooling from the model’s longwave radiation scheme. This overactive longwave cooling is associated with a moisture bias found in the same layer. Assimilating special dropsonde observations from a field campaign flight mission through a TPV mitigates the moisture bias, especially in analyses. Implementing an improved moisture initialization procedure is able to alleviate the moisture bias, even in the absence of special observations. The moisture bias and associated longwave cooling in MPAS-DART results in less intense TPVs later in their lifetimes compared to ERA-5.
After quantifying the bias patterns in MPAS-DART, an interaction of TPVs with mid-latitude flow is investigated through the hypothesis that TPVs can initiate Rossby wave packets. Referred to as Rossby wave initiation (RWI), flow patterns relevant to RWI development are more sensitive to TPV position relative to the jet stream than to TPV intensity. The moisture field, a well-documented source of RWI, is not found to be sensitive to TPV characteristics. A surface cyclone that develops downstream of the RWI is sensitive to the position and magnitude of potential vorticity and windspeed in the upper levels. Lastly, it is found that surface cyclone strength is sensitive to moisture with stronger cyclones associated with increased moisture.
This study is one of the first to demonstrate the utility of a state-of-the-art global modeling system in the Arctic for process studies. While room remains for improvement, the tool enabled valuable scientific exploration of a recently documented Arctic feature, TPVs. Using tools such as this one allow for improved understanding of complex atmospheric processes, their evolution, and the the potential feedbacks between processes, which is particularly powerful in a remote and data-sparse region like the Arctic.