Cold Pools and Their Influence on the Tropical Marine Boundary Layer

Abstract

Cold pools dominate the surface temperature variability observed over the central Indian Ocean (0°, 80°E) for 2 months of research cruise observations in the Dynamics of the Madden–Julian Oscillation (DYNAMO) experiment in October–December 2011. Cold pool fronts are identified by a rapid drop of temperature. Air in cold pools is slightly drier than the boundary layer (BL). Consistent with previous studies, cold pools attain wet-bulb potential temperatures representative of saturated downdrafts originating from the lower midtroposphere. Wind and surface fluxes increase, and rain is most likely within the ~20-min cold pool front. Greatest integrated water vapor and liquid follow the front. Temperature and velocity fluctuations shorter than 6 min achieve 90% of the surface latent and sensible heat flux in cold pools. The temperature of the cold pools recovers in about 20 min, chiefly by mixing at the top of the shallow cold wake layer, rather than by surface flux. Analysis of conserved variables shows mean BL air is composed of 51% air entrained from the BL top (800 m), 22% saturated downdrafts, and 27% air at equilibrium with the ocean surface. The number of cold pools, and their contribution to the BL heat and moisture, nearly doubles in the convectively active phase compared to the suppressed phase of the Madden–Julian oscillation.

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

“…The boundary layer is cooled and moistened over a large area by the evaporation of stratiform precipitation, reducing the buoyancy of individual surface parcels. Nevertheless, cold pools and the surface-originating convection they reflect clearly do occur (de Szoeke et al 2017), perhaps more than originally thought at the time of Houze and Betts (1981). This is also evident in space-based cloud radar observations of cumulus congestus/cumulonimbus occurring underneath upper-level stratiform cloud (e.g., Riley et al 2011).…”

“…The composite-mean includes a near-surface drying of $ 1 g kg À1 and decrease in the equivalent potential temperature of 2 K. The mean wind speed increases by $ 2 m s À1 , but only for 15-30 min. The pressure increase corresponds to a mean cold pool depth of approximately 300 m, is that stronger temperature drops are accompanied by stronger decreases in the water vapor mixing ratio (and h e ), indicating either higher origin heights for the downdrafts or more coherent structures less modified by environmental mixing (de Szoeke et al 2017). The mean near-surface relative humidity of the cold pools indicate subsaturation (not shown), suggesting the downdrafts rarely if ever maintain saturation, despite being initially saturated.…”

“…8). Most studies point to an origin altitude for the downdraft air of 2 km or less, however (Betts 1976;Betts and Dias 1979;Torri and Kuang 2016a;de Szoeke et al 2017;Schiro and Neelin 2017). That higher clouds are associated with stronger cold pools may reflect a correlation between downdraft width and clouds that are wider as a result of organization, discouraging environmental dilution of the downdraft air (Schlemmer and Hohenegger 2014; Schiro and .…”

“…One important compensation is the condensate loading within the downdrafts, as more condensate will decrease the surface parcel buoyancy (James and Markowski 2010; Torri and Kuang 2016a). Tropical cold pools are more frequent during times of lower outgoing longwave radiation, reflecting more organized deep convection with larger cloud covers, cooler sea surface temperatures (SSTs), and cooler near-surface layers that are closer to saturation (de Szoeke et al 2017), and more fully saturated atmospheres. The number of observed cold pools only varies weakly with time of day (de Szoeke et al 2017), though satellite measurements suggest mesoscale downdrafts able to reach the surface occur 8-12 h after the peak rainfall (Kilpatrick and Xie 2015).…”

A short review of numerical cloud-resolving models

“…More generally, observations (Barnes and Garstang, 1982;Zuidema et al, 2012;Dione et al, 2014;de Szoeke et al, 2017) and such simulations both point to the importance of convectively-generated cold pools which are illustrated in Fig. 14.…”