A Low-Level Circulation in the Tropics

Abstract

Deep convective tropical systems are strongly convergent in the midtroposphere. Horizontal wind measurements from a variety of rawinsonde arrays in the equatorial Pacific and Caribbean are used to calculate the mean dynamical divergence profiles of large-scale arrays (Ն1000 km in diameter) in actively convecting regions. Somewhat surprisingly, the magnitude of the midtropospheric divergence calculated from these arrays is usually small. In principle, the midlevel convergence of deep convective systems could be balanced on larger scales either by a vertical variation in the radiative mass flux of the background clear sky atmosphere, or by a divergence from shallow cumuli. The vertical variation of the clear sky mass flux in the midtroposphere is small, however, so that the offsetting divergence must be supplied by shallow cumuli. On spatial scales of ϳ1000 km, the midlevel convergent inflow toward deep convection appears to be internally compensated, or "screened," by a divergent outflow from surrounding precipitating shallow convection. Deep convective systems do not induce a large-scale inflow of midlevel air toward actively convecting regions from the rest of the tropics, but instead help generate a secondary low-level circulation, in which the net downward mass flux from mesoscale and convective-scale downdrafts is balanced by a net upward mass flux from precipitating shallow cumuli. The existence of this circulation is consistent with observational evidence showing that deep and shallow convection are spatiotemporally coupled on a wide range of both spatial and temporal scales. One of the mechanisms proposed for coupling shallow convection to deep convection is the tendency for deep convection to cool the lower troposphere. The authors use radiosonde temperature profiles and the Tropical Rainfall Measuring Mission (TRMM) 3B42 gridded rainfall product to argue that the distance over which deep convection cools the lower troposphere is approximately 1000 km.

Chemical isolation in the Asian monsoon anticyclone observed in Atmospheric Chemistry Experiment (ACE-FTS) data

“…The left panel highlights the main convective outflow level near ∼12 km, with the strongest closed circulation of the anticyclone somewhat above this level (∼12-16 km; Randel and Park, 2006). The mechanism(s) of vertical transport above 12 km are a subject of ongoing research, but likely involve large-scale circulation in the anticyclone , overshooting deep convection (Dessler and Sherwood, 2004), or the dynamical divergence mechanism discussed in Folkins et al (2007). Although the detailed mechanism(s) leading to formation and maintenance of the tracer profiles are not understood at present, the ACE-FTS data provide detailed observations to test idealized or more realistic models.…”

Quantifying the deep convective temperature signal within the tropical tropopause layer (TTL)

“…Their constructed deep convective life cycle is marked by cooling in the lower to mid troposphere, warming in the mid to upper troposphere, and cooling at TTL altitudes. More recently, Folkins et al (2008) and Mitovski et al (2010) successfully revealed similar local convective temperature signals associated with high rainfall rates from the Tropical Rainfall Measuring Mission (TRMM) at tropical radiosonde stations. These earlier studies were limited by (1) using proxies for deep convective clouds (brightness temperature, rain rate), (2) the limited spatial coverage given by the small number of radiosonde station locations.…”

Aerosol Effects on Cumulus Congestus Population over the Tropical Pacific: A Cloud-Resolving Modeling Study

“…However, with the exception of van den Heever et al (2011), previous studies did not specifically address how aerosols (serving as CCN) affected the frequencies of cumulus congestus. Nevertheless, congestus are an important component of tropical convection (e.g., Johnson et al 1999;Folkins et al 2008). Recent studies have emphasized their crucial role in moistening mid-troposphere, setting up the optimal condition for deep convective systems, especially in the context of the Madden̶Julian oscillation (MJO) lifecycle (Kemball-Cook and Wearethe cumulus congestus population over the tropical oceans have been studied by Johnson et al (1996), Mapes and Zuidema (1996), Folkins et al (2008), and Folkins et al (2009).…”

“…Nevertheless, congestus are an important component of tropical convection (e.g., Johnson et al 1999;Folkins et al 2008). Recent studies have emphasized their crucial role in moistening mid-troposphere, setting up the optimal condition for deep convective systems, especially in the context of the Madden̶Julian oscillation (MJO) lifecycle (Kemball-Cook and Wearethe cumulus congestus population over the tropical oceans have been studied by Johnson et al (1996), Mapes and Zuidema (1996), Folkins et al (2008), and Folkins et al (2009). These factors include the relatively small convective available potential energy (CAPE) and the weak stable layer around the melting layer, which is sometimes accompanied by a dry air layer.…”