Near-Surface Density Currents Observed in the Southeast Pacific Stratocumulus-Topped Marine Boundary Layer*

Abstract

Density currents (i.e., cold pools or outflows) beneath marine stratocumulus clouds are characterized using 30 days of ship-based observations obtained during the 2008 Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) in the southeast Pacific. An air density increase criterion applied to the Improved Meteorological (IMET) sensor data identified 71 density current front, core (peak density), and tail (dissipating) zones. The similarity in speeds of the mean density current propagation speed (1.8 m s−1) and the mean cloud-level advection relative to the surface layer wind (1.9 m s−1) allowed drizzle cells to deposit elongated density currents in their wakes. Scanning Doppler lidar captured prefrontal updrafts with a mean intensity of 0.91 m s−1 and an average vertical extent of 800 m. Updrafts were often surmounted by low-lying shelf clouds not connected to the overlying stratocumulus cloud. The observed density currents were 5–10 times thinner and weaker than typical continental thunderstorm cold pools. Nearly 90% of density currents were identified when C-band radar estimated areal average rain rates exceeded 1 mm day−1 over a 30-km diameter. Rather than peaking when rain rates were highest overnight, density current occurrence peaks between 0600 and 0800 local solar time when enhanced local drizzle co-occurred with shallow subcloud dry and stable layers. The dry layers may have contributed to density current formation by enhancing subcloud evaporation of drizzle. Density currents preferentially occurred in a large region of predominantly open cells but also occurred in regions of closed cells.

Drizzle and Turbulence Below Closed Cellular Marine Stratocumulus Clouds

“…Although yielding insights on the depth of the layer affected by drizzle evaporation, it is possible for this layer to be different than the thermodynamic layer impacted by drizzle evaporation. Observations from a collocated Raman lidar, which were not available for the cases analyzed here, might shed some light on the impact of drizzle evaporation on boundary layer thermodynamics and perhaps cold pools (Wilbanks et al, 2015) associated with these systems. These could be used to assess whether drizzle evaporation can lead to thermodynamic decoupling.Stratocumulus topped boundary layers (STBLs) with stronger radiative cooling at the cloud top are often deeper, have higher turbulence, and hence have higher collision-coalescence rates leading to drizzle formation than those with weaker radiative cooling.…”

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

“…Radar observations indicate that the speed of advection of cold pools is similar to that of the cloud layer (Wilbanks et al 2015). This suggests that cold pools are better thought of as tracers or artifacts of stratocumulus precipitation as opposed to drivers.…”

Characterizing convective cold pools

“…Several variables have been used to define convective cold pool boundaries and attributes in previous work, both in observations and in models. These include buoyancy [ Tompkins , ; Grant and van den Heever , ; Seigel , ; Feng et al ., ], along with related metrics such as (virtual or density) (potential) temperature [ Addis et al ., ; Young et al ., ; Terai and Wood , ; Yokoi et al ., ; Torri et al ., ] and density [ Wilbanks et al ., ]; equivalent potential temperature [ Dawson et al ., ; Katona et al ., ; Schlemmer and Hohenegger , ]; humidity [ Redl et al ., ]; and winds [ Uyeda and Zrnić , ; Engerer et al ., ; Li et al ., ; Kilpatrick and Xie , ; Langhans and Romps , ]. In this section we examine several of these variables, along with a few additional ones, and assess each variable's utility for characterizing convective cold pools according to the following criteria: There should exist some physical basis for using this variable to define convective cold pools.…”