Convective Forcing in the Intertropical Convergence Zone of the Eastern Pacific

Abstract

One of the goals of the East Pacific Investigation of Climate, year 2001 process study (EPIC2001), was to understand the mechanisms controlling the forcing of deep atmospheric convection over the tropical eastern Pacific. An intensive study was made of convection in a 4Њ ϫ 4Њ square centered on 10ЊN, 95ЊW in September and October of 2001. This is called the intertropical convergence zone (ITCZ) study region because it encompasses the eastern Pacific intertropical convergence zone. Starting from an analysis of the theoretical possibilities and a plethora of dropsonde, in situ, radar, and satellite data, it is found that newly developing convection occurs where a deep layer of air (of order 1 km deep or deeper) is conditionally unstable with only weak convective inhibition. Shallower conditionally unstable layers are associated with numerous small clouds, but do not seem to produce deep convection. The occurrence of deep convection over the ITCZ study region is presumably related to the propensity of the environment to produce areas of weak convective inhibition over such a deep layer. Three theoretically possible factors control the formation of such convectively unstable areas: 1) the strength of the total surface heat (or moist entropy) fluxes; 2) the advection of moisture into the region; and 3) temperature anomalies caused by dry adiabatic ascent of the inhibition layer, which lies typically between 700 and 850 mb. The areal fraction covered by such instability is small even on highly convective days. In the tropical eastern Pacific, it is found that the total surface entropy flux is the most significant of these factors, with a warm layer in the 700-850-mb range, resulting presumably from subsidence, playing an important suppressive role in certain cases. These two factors account for approximately two-thirds of the variance in satellite infrared brightness temperature averaged over the study region. Moisture (or moist entropy) advection appears to be of less importance. Tropical disturbances such as easterly waves, Kelvin waves, and the Madden-Julian oscillation presumably control convection primarily via these two mechanisms during their passage through this region.

Coupled Impacts of the Diurnal Cycle of Sea Surface Temperature on the Madden–Julian Oscillation

“…The same is true with CF1PS showing more than half of h2v 0 h 0 p i in CF1 despite the lack of dSST. Therefore, the column-integrated MSE and moisture budget analyses suggest that, as in the observations (e.g., Waliser et al 1999;Kemball-Cook and Weare 2001;Raymond et al 2003;Stephens et al 2004), the moisture in the lower atmosphere is accumulated over the warm SST during the calm and sunny period. The resulting increase in the column MSE facilitates deep convection, a process consistent with the recharge-discharge paradigm [e.g., Bladé and Hartmann 1993; see also Xu and Rutledge (2014) for DYNAMO observations].…”

Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

“…The scale height of the saturation humidity following a moist adiabat is about 2 km. This means that column water vapor is largely determined by water vapor in the lower troposphere, between the top of the PBL, g; and z à (Raymond et al 2003). In other words, the observed relationship between column water vapor and precipitation is really a relationship between lower free troposphere humidity and precipitation.…”

Multiple equilibria in a cloud‐resolving model using the weak temperature gradient approximation

“…This dependence on domain size is at least partially related to the distribution of convective inhibition in the modeled domain. Figure 5 shows a histogram of deep convective inhibition (DCIN) [Raymond et al, 2003], where DCIN is defined as…”