Cold-Season Embedded Orographic Convection and Its Impact on Precipitation: An Observational and Modeling Case Study

Anecdotal evidence from reflectivity and vertical velocity transects collected by an airborne profiling radar indicates that benign convection is quite common over mountains in the cold season, often embedded in stratiform clouds. Proximity radiosonde data indicate that the environment in which this convection forms only has the smallest amounts of potential instability. In some cases (typically post-frontal), the convection is clearly coupled to the underlying surface and thus sensitive to the surface energy balance, while in other cases it is decoupled, at least from the lower-elevation terrain. While benign, this convection may significantly alter the orographic cloud depth, the abundance of cloud ice, and precipitation growth. Therefore, this convection may significantly alter the amount and distribution of orographic precipitation. It is not clear how well models capture this process, even convection permitting models, given the small size of this convection.

Here, we examine one case of embedded convection over the Payette Range just upstream of the Idaho Central Mountains, observed on 7 Feb 2017 as part of the SNOWIE campaign (Seeded and Natural Orographic precipitation in Winter: the Idaho Experiment). We document the case with data from radiosondes, a Doppler on Wheels (DOW) radar located on a mountain crest, and the profiling Wyoming Cloud Radar (WCR) aboard the Wyoming King Air cloud physics research aircraft. We document the development of convection in the dynamically consistent framework of WRF simulations at increasingly fine resolutions. By the time of the conference, we hope to have completed Large Eddy Simulations on a 100 m grid over a sufficiently large domain to capture the full evolution of the convection. The objective of such refined simulations is to capture the development of potential instability over elevated terrain and the morphological details of the release of this instability, i.e., the depth and spatial organization of convective cells as they evolve while being advected over the terrain. We will compare simulations of different resolution, to quantify the impact of the embedded orographic convection on hydrometeor growth processes, surface precipitation distribution, and thermodynamic profiles downstream of the range.