The Role of Gravity Waves from Severe US Convection in Driving the Brewer-Dobson Circulation Using a Model with Realistic Thermodynamic Sources

Abstract:

An inter-model comparison of the annual-mean upward mass flux at 70 hPa in comprehensive chemistry–climate models shows reasonably good agreement on the total strength of the Brewer-Dobson circulation (BDC) and its trend in the 21^st century, but there is large variability in terms of the relative contributions by parameterized gravity waves (GWs) versus resolved Rossby waves. This model dependency presents a major weakness in our current understanding of the BDC and can partly be attributed to deficiencies in non-orographic GW drag parameterizations.

We quantify the contribution of small-scale GWs from summer US storm systems to the BDC using a high-resolution, GW-resolving modeling approach. A nonlinear idealized dry version of the WRF model is forced with high-resolution latent heating/cooling derived from precipitation observations. For several case studies, it is shown that this model produces an excellent quantitative comparison to waves observed by satellite and captures the intermittency associated with convective sources of strong GW events. We model the entire month of June 2014 over the continental US.

The contribution of GWs from summer US storm systems to the BDC is quantified and compared to MERRA reanalysis data and the CAM5 model. Non-orographic GW drag parameterizations used in large-scale models are poorly constrained by observational validation and commonly have tuning parameters with large uncertainties. In particular, capturing the high intermittency of the GW field presents an ongoing important challenge, as a given averaged momentum flux carried by a large number of small-amplitude GWs will produce a drag at much higher altitudes than that produced by the same averaged flux carried by a small number of high-amplitude GWs. In studying the relationship between precipitation strength, GW momentum flux amplitude and magnitude of the stratospheric GW drag, we highlight the potential for improving gravity wave drag parameterizations in global models.