Near-surface Density Currents Observed in the Stratocumulus-topped Marine Boundary Layer

Matt Wilbanks North Carolina State University
Sandra Yuter North Carolina State University
Simon de Szoeke Oregon State University
Matthew Miller North Carolina State University
Andrew Hall North Carolina State University
Casey Burleyson Pacific Northwest National Laboratory

Category: Mesoscale Convective Organization and Cold Pools

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Doppler lidar RHI through a density current observed on 26 October 2008 at 0824 UTC showing mean wind corrected radial velocity along a plane approximately orthogonal to the frontal boundary. Negative velocities (blue) indicate flow towards the ship located at the origin, and positive velocities (red) indicate flow away from the ship. The frontal boundary of the density current is located at the surface at ≈0.5 km horizontal range and slants backward with height up to the depth of the head (≈400 m) near 1 km range.

Results from large eddy simulations and cloud-system resolving models suggest that density currents (i.e. cold pools or outflows) may be a driver of mesoscale organization in drizzling marine stratocumulus convection. Ship-based scanning and in situ observations from the southeast Pacific permit an examination of density currents and their relation to evolving mesoscale cloud and precipitation structures. An objective method identifies 71 density current fronts over 30 days using an air density criterion and isolates each density current’s core (peak density) and tail (dissipating) zone. Compared to front and core zones, most density current tails exhibited weaker density gradients and wind anomalies elongated about the axis of the mean wind. The mean cloud-level advection relative to the surface layer wind (1.9 m s^-1) nearly matches the mean density current propagation speed (1.8 m s^-1). The similarity in speeds allows drizzle cells to deposit cool tails in their wakes. High-resolution scanning Doppler lidar data captured prefrontal updrafts that had a mean intensity of 0.91 m s^-1, reached an average altitude of 800 m, and were often topped by low-lying shelf clouds. These shelf clouds were typically not connected to the stratocumulus cloud layer above them. Nearly 90% of density currents were identified when C-band radar estimated areal average rain rates exceeded 1 mm d^-1 over a 30-km diameter area. Rather than peaking when rain rates are highest overnight, density current occurrence peaks between 0600 and 0800 local solar time when enhanced local drizzle co-occurs with shallow subcloud dry and stable layers. The dry layers may contribute to density current formation by enhancing subcloud evaporation of drizzle. Density currents preferentially occur in regions of open cells but also occur in regions of closed cells. The ship-based observations are direct evidence for cold-pool–induced updrafts beneath drizzling marine stratocumuli. However, these updrafts did not routinely initiate new vigorous drizzle cells above them. Hence, our observations suggest that cold pools are not the primary process controlling mesoscale organization in marine stratocumulus. Once the ENA site is fully functional, our analysis will be extended to explore the relationship between cold pools and mesoscale organization in the northeast Atlantic.

This poster will be displayed at ASR Science Team Meeting.

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