On Trade Wind Cumulus Cold Pools

Abstract

Shallow precipitating cumuli within the easterly trades were investigated using shipboard measurements, scanning radar data, and visible satellite imagery from 2 weeks in January 2005 of the Rain in Cumulus over the Ocean (RICO) experiment. Shipboard rainfall rates of up to 2 mm h 21 were recorded almost daily, if only for 10-30 min typically, almost always from clouds within mesoscale arcs. The precipitating cumuli, capable of reaching above 4 km, cooled surface air by 1-2 K, in all cases lowered surface specific humidities by up to 1.5 g kg 21 , reduced surface equivalent potential temperatures by up to 6 K, and were often associated with shortlived increases in wind speed. Upper-level downdrafts were inferred to explain double-lobed moisture and temperature sounding profiles, as well as multiple inversions in wind profiler data. In two cases investigated further, the precipitating convection propagated faster westward than the mean surface wind by about 2-3 m s 21 , consistent with a density current of depth ;200 m. In their cold pool recovery zones, the surface air temperatures equilibrated with time to the sea surface temperatures, but the surface air specific humidities stayed relatively constant after initial quick recoveries. This suggested that entrainment of drier air from above fully compensated the moistening from surface latent heat fluxes. Recovery zone surface wind speeds and latent heat fluxes were not higher than environmental values. Nonprecipitating clouds developed after the surface buoyancy had recovered (barring encroachment of other convection). The mesoscale arcs favored atmospheres with higher water vapor paths. These observations differed from those of stratocumulus and deep tropical cumulus cold pools.

Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations

“…Within cold pools convection is suppressed, but at the downwind (colliding) boundaries of cold pools new convection can be triggered. This leads to arcshaped cloud formations with clear skies in between as seen from satellite imagery (Snodgrass et al 2009;Zuidema et al 2012), and which the LES reproduces. The cold pools from shallow cumuli and congestus are mostly dry in their center, similar to deep convection, because rain rates are sufficiently strong to bring down relatively dry air from higher altitudes.…”

“…In these cells cloud droplets have been scavenged by drizzle drops, leading to very low cloud droplet number concentrations. Observations have also demonstrated that fields of shallow cumuli accompanied by significant rain are organized into arc-shaped formations (Snodgrass et al 2009;Zuidema et al 2012). These are representative for the presence of cold pools, which are produced by the evaporation of rain and convective downdrafts, similar to the cold pools that accompany deep convection (Tompkins 2001).…”

Observations of the temporal variability in aerosol properties and their relationships to meteorology in the summer monsoonal South China Sea/East Sea: the scale-dependent role of monsoonal flows, the Madden–Julian Oscillation, tropical cyclones, squall lines and cold pools

“…With the drop in temperature, there was a sharp spike in wind speed, relative humidity (and at times precipitation), as well as a drop in both particle concentration and water vapor mixing ratio. These characteristics are indicative cold pool events related to convective downdrafts (Wakimoto, 1985;Atkins and Wakimoto, 1991;Miller et al, 2008;Zuidema et al, 2012). Over 20 such events are observable in the time series, with significant variability in amplitude.…”

Time-dependent, non-monotonic response of warm convective cloud fields to changes in aerosol loading

“…Some previous studies have demonstrated clouds' alteration of their environment (Zhao and Austin, 2005;Heus and Jonker, 2008;Malkus, 1954;Lee et al, 2014;Zuidema et al, 2012;Roesner et al, 1990). One example of such an effect is the "preconditioning" or "cloud-deepening" effect (Nitta and Esbensen, 1974;Roesner et al, 1990;Stevens, 2007;Stevens and Seifert, 2008), where clouds cool and moisten the upper cloudy and inversion layers and in this way encourage the development of the next generation of clouds that encounter improved environmental conditions.…”

“…Another way clouds affect their environment is by the evaporation of rain below the cloud base, which induces cooling of the sub-cloud layer (Zuidema et al, 2012;Heiblum et al, 2016a). Lee et al (2014) demonstrated the aerosol effects on the field's CAPE (convective available potential energy) (as distributed above cloud base or below it).…”