Deep Convection and Column Water Vapor over Tropical Land versus Tropical Ocean: A Comparison between the Amazon and the Tropical Western Pacific

Abstract

The relationships between the onset of tropical deep convection, column water vapor (CWV), and other measures of conditional instability are analyzed with 2 yr of data from the DOE Atmospheric Radiation Measurement (ARM) Mobile Facility in Manacapuru, Brazil, as part of the Green Ocean Amazon (GOAmazon) campaign, and with 3.5 yr of CWV derived from global positioning system meteorology at a nearby site in Manaus, Brazil. Important features seen previously in observations over tropical oceans—precipitation conditionally averaged by CWV exhibiting a sharp pickup at high CWV, and the overall shape of the CWV distribution for both precipitating and nonprecipitating points—are also found for this tropical continental region. The relationship between rainfall and CWV reflects the impact of lower-free-tropospheric moisture variability on convection. Specifically, CWV over land, as over ocean, is a proxy for the effect of free-tropospheric moisture on conditional instability as indicated by entraining plume calculations from GOAmazon data. Given sufficient mixing in the lower troposphere, higher CWV generally results in greater plume buoyancies through a deep convective layer. Although sensitivity of buoyancy to other controls in the Amazon is suggested, such as boundary layer and microphysical processes, the CWV dependence is consistent with the observed precipitation onset. Overall, leading aspects of the relationship between CWV and the transition to deep convection in the Amazon have close parallels over tropical oceans. The relationship is robust to averaging on time and space scales appropriate for convective physics but is strongly smoothed for averages greater than 3 h or 2.5°.

On the role of aerosols, humidity, and vertical wind shear in the transition of shallow-to-deep convection at the Green Ocean Amazon 2014/5 site

“…CCN concentrations are averaged over ±30 min centered on the time of radiosonde launch. These averaging time frames and radiosonde measurements are statistically robust, as shown in Schiro et al (2016), where they show that temporal averaging up to and including 3 h yields robust statistics defining the transition to deep convection. In this study, we show the impacts of CCN based on 30 min average before and after the radiosonde measurement.…”

“…Many studies have investigated the role of total precipitable water and moisture content of the BL on the strength and evolution of deep convections both over tropical land and ocean sites (Schiro et al, 2016;Holloway and Neelin, 2009). In addition, there are ample studies that show that free tropospheric moistening is important for deep convective evolution (Waite and Khouider, 2010;Zhang and Klein, 2010;Kumar et al, 2013;Sherwood et al, 2004).…”

“…A few recent studies have investigated deep convective evolution and buoyancy using ground-based measurements over the Amazonia (Zhuang et al, 2017;Schiro et al, 2016). Schiro et al (2016) found that given sufficient mixing in the lower troposphere, column water vapor can be used as a proxy to understand the impact of free tropospheric humidity on plume buoyancy related to deep convective evolution.…”

“…Schiro et al (2016) found that given sufficient mixing in the lower troposphere, column water vapor can be used as a proxy to understand the impact of free tropospheric humidity on plume buoyancy related to deep convective evolution. Sensitivity of buoyancy to other factors in the Amazon was also suggested, such as BL and microphysical processes, but the role of aerosols or VWS on deep convective evolution from shallow clouds was not analyzed.…”

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

“…Bretherton et al (2004) show that precipitation picks up exponentially with the increase in column relative humidity, defined as W=W Ã where W is the water vapor path, and W Ã is its saturated value given the temperature profile of the column. Initially found just over the tropical oceans this relationship has, with some caveats, been extended to other regions (Holloway and Neelin 2009;Schiro et al 2016;Bergemann and Jakob 2016) and may also correlate with variations in shallow precipitation (Nuijens et al 2009). The scale height of the saturation humidity following a moist adiabat is about 2 km.…”