On the Origin of Rotation Observed in Super Rapid Scan Geostationary Satellite Data over Severe Deep Convection

Visible imagery of discrete, severe deep convection (DC) from next-generation super rapid scan (SRS; ≤ 1–min) Geostationary Operational Environmental Satellites (GOES) sometimes exhibits rotation at the cloud-tops and, when not obscured, on the sides of the storm. Recent studies have shown that this apparent rotation geometry and magnitude can be objectively derived with pattern matching and optical flow based mesoscale Atmospheric Motion Vector (mAMV) algorithms. Despite dozens of observed case studies of rotating, severe DC, the cause and origin of cloud-top mAMV derived rotation has not yet been determined. Artifacts in common approaches for satellite flow-field derivation may also modify rotation inferred over some DC, which will be detailed in this presentation. It is therefore not yet clear what the relevance of derived rotation geometry and magnitude is to ground reported severe weather (e.g. large hail, strong winds and tornadoes) and mid-level storm kinematics.

To find the origin of this rotation, an idealized simulation of a supercell thunderstorm is performed using the Weather Research and Forecasting (WRF) Advanced Research WRF (WRF-ARW) model and parcels in rotating regions at the cloud-top are tracked to their source with backward trajectories. The impact of artifacts in flow field derivation are also explored with multiple new case studies of severe DC with rotation in the cloud tops GOES–16 and –17 imagery. Early findings suggest that rotation derived directly over the overshooting tops of supercell thunderstorms is likely generated through tilting and subsequent stretching of horizontal vorticity present due to vertical wind shear below 10 km. This rotation is rapidly reduced through the unfavorable stretching environment present in the diverging storm top. Rotation observed downstream of overshooting tops is created though tilting of baroclinically generated rotation near the cloud-top, and is an order of magnitude larger than what occurs below 10 km. Artifacts in flow field derivation also impact rotation more downstream of overshooting tops when Above Anvil Cirrus Plumes are present. The results to be presented here suggest that isolating the cause and origin of rotation in derived flow fields can improve our understanding on how to interpret flow in SRS satellite imagery and infer internal dynamics of DC relevant to severe weather nowcasting.