The Tropopause Heat Flux prior to Two Northern Hemisphere SSWs in the High-Altitude Version of the Navy Global Environmental Model

The new high-altitude version of the Navy Global Environmental Model (NAVGEM-HA) has a vertical domain up to ~116 km and assimilates both standard meteorological observations in the troposphere and satellite data in the stratosphere and mesosphere, creating a realistic numerical weather prediction (NWP) system for middle atmosphere research on synoptic timescales. Recent modeling studies have indicated that improved representation of extreme stratospheric regimes, such as sudden stratospheric warming (SSW) events, can lead to improved prediction of the tropospheric response to SSWs, which include land surface anomalies and shifts to the storm track on subseasonal to seasonal time scales. Studies have confirmed that NAVGEM-HA has accurate representations of the winds in the upper mesosphere/lower thermosphere surrounding SSWs. The goal of this analysis is to study how the tropospheric forcing prior to two illustrative SSWs is represented in NAVGEM-HA meteorological analyses.

The two SSWs that will be analyzed occurred on: 9 February 2010 and 7 January 2013. Calculated with NASA’s MERRA-2 dataset, both of these events had similar magnitudes of the 40-day average tropopause 45°-75°N zonal-mean meridional eddy heat flux anomaly, hereinafter heat flux anomaly, that fall in the top half of all 22 SSWs identified from 1980 to 2015. The 9 February 2010 SSW 40-day average heat flux anomaly was 4.34 K m s^-1 and the 7 January 2013 40-day average heat flux anomaly was 5.13 K m s^-1. Although this time-mean tropopause forcing was similar between the SSWs, the variability in the high-frequency tropospheric flow within these 40-days prior to the SSWs was different. These differences were manifest in the number of SSW-precursor tropospheric blocks: prior to the 9 February 2010 SSW there were 3 mid-tropospheric blocks while prior to the 7 January 2013 SSW there was only 1 block about a month prior.

This presentation will show the tropospheric, tropopause, and stratospheric evolution and coupling within the 40-days prior to the onset of the SSWs in NAVGEM-HA and MERRA-2 dataset. The goal is to determine how well NAVGEM-HA analyses represent the tropospheric forcing that play a role in the onset of SSWs compared to the MERRA-2 dataset, which assimilates less stratospheric satellite data and has a lower model top (i.e., ~80 km) than NAVGEM-HA. This presentation will address the questions: Does a higher model top and more middle atmosphere assimilation make a significant difference in the representation of: (1) the synoptic evolution of SSWS, (2) the tropopause forcing prior to SSWs, (3) the tropospheric evolution prior to SSWs, and/or (4) the downward propagation of anomalies from the stratosphere to the troposphere? The answers to these questions will provide a basis for future work exploring NAVGEM-HA forecasts of extreme stratospheric events.