Simulated Convective Invigoration Processes at Trade Wind Cumulus Cold Pool Boundaries

Abstract

Observations of precipitating trade wind cumuli show convective invigoration on the downwind side of their cold pools. The authors study convection and cold pools using a nested-Weather Research and Forecasting Model simulation of 19 January 2005-a day from the Rain in Cumulus over the Ocean experiment. The temperature and water vapor mixing ratio drops in simulated cold pools fall within the envelope of observed cases, and the wind enhancement matches observations more closely. Subcloud updrafts downwind and near the cold pool boundary are statistically compared to updrafts further from cold pools. Updrafts near cold pool outflows are moister than the other updrafts and are more likely to originate from overall moister regions. Cold pool-influenced updrafts tend to exceed the other updrafts in vertical velocity and are associated with more cloud liquid water. The strength of circulation within the cold pool boundary is unable to match that because of the low-level environmental wind shear, and the lifted updrafts advect faster than the environmental wind, thereby accessing the ambient environmental moisture converged by cold pool expansion. Cases with higher rain rates correspond to larger cloud cover through the shearing off of the upperlevel cloud, consistent with observations. This study suggests that it is the ability of cold pools to lift thermodynamically favorable air that is critical for secondary convection of trade wind cumuli.

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

“…2 Studies that use larger domain sizes to represent cold pool mesoscale organization are more likely to attribute anomalous moisture sources for secondary convection outside of the cold pool (Li et al 2014;Schlemmer and Hohenegger 2016). This is also consistent with the observed inhomogeneity of the secondary convection around the mesoscale arc (Figs.…”

“…Dynamical forcing of boundary layer air is stronger when the cold pool gust front is aligned with the mean wind at the downwind side of a trade wind region cold pool, and the vorticity interactions help explain the more pronounced convection on the downwind side of a cold pool (e.g., in Figs. 3 and 4, further explored in Li et al 2014). Stronger dynamical forcing is also related to higher downdraft heights in Jeevanjee and Romps (2015).…”

“…The sensitivity of the convective downdraft to condensate loading and mixing above the boundary layer is also important. In addition, this means further evaluating the effects of low-and high-wind shear, as wind shear can interact dynamically with the convective structure (Li et al 2014). Related to all these questions is also the timescale of the boundary layer recovery within a cold pool, and how often cold pools are able to fully recover before being impinged upon by a subsequent cold pool; observations suggest cold pools occur in clumps (e.g., Fig.…”

“…The local cooling can serve to destabilize the subcloud layer and reinforce the updrafts in stratocumulus clouds (e.g., Feingold et al 1996) and can also be visualized in lower-level scud clouds. In the shallow cumulus regime, such an elevated enhanced moisture layer may aid future convection (e.g., Li et al 2014;Schlemmer and Hohenegger 2014). As drops reach larger sizes through collision-coalescence (e.g., Baker et al 2009) and their corresponding size-dependent fall velocities increase, drops can reach the ground with only partial evaporation.…”

Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

“…The boundary between the cold pool and the surrounding environmental air, known as the outflow boundary or gust front, is key to sustaining multi-cellular deep convection (e.g., Weisman and Klemp, 1986). It has also been shown to trigger new convective cells in marine stratocumulus (Wang and Feingold, 2009;Terai and Wood, 2013) and trade-wind cumulus clouds (Zuidema et al, 2012;Li et al, 2014). Downdrafts also have implications for new particle formation in the outflow regions, which contribute to maintaining boundary layer cloud condensation nuclei (CCN) concentrations in unpolluted environments .…”