The Influence of Microphysics Parameterizations on Medium-Range Forecasts of Hemispheric Waviness

The release of latent heat in the cloud and precipitation structures characterizing mid-latitude weather systems can exert a substantial impact on the downstream Rossby wave pattern. Recent work has demonstrated that the nature of these impacts is strongly dependent upon the manner in which cloud and precipitation processes are represented in numerical forecast models. Models approximate the processing of water substance within such disturbances using cloud microphysical parameterizations of varying complexity.

Much prior work has considered the impact of latent heat release in organized cloud systems in terms of its contribution to forecast errors in the phase and/or amplitude of individual synoptic waves. Such consideration overlooks a related but perhaps more global question – namely, what is the impact of such latent heat release on the aggregate waviness of the evolving, larger-scale flow? We conduct experiments aimed at gaining insight into this question.

First, we present results from 151 daily runs of a triad of low resolution, 120-h WRF-ARW simulations spanning the Northern Hemisphere cold season of 2016-17 in which only the model’s cloud microphysical package is different between the runs. Next, we employ sinuosity as a measure of the aggregate 200 hPa waviness over the Northern Hemisphere and consider the waviness differences in the light of the varying microphysical parameterizations in the model. Finally, a comparison of synoptic environments with and without robust, landfalling atmospheric rivers over North America is made to consider the impact of such features on the hemispheric evolution of upper-tropospheric waviness.