Seasonal and diurnal variation of convective available potential energy (CAPE) using COSMIC/FORMOSAT‐3 observations over the tropics

Abstract

The global pattern of convective available potential energy (CAPE) at seasonal and diurnal time scales is discussed using 1 year of COSMIC/FORMOSAT‐3 satellite observations. The calculation of CAPE using temperature and humidity measurements of COSMIC is described. The estimated CAPE is grouped into 5 × 5 grid and is further classified into four seasons, namely, winter, spring, summer, and autumn. The CAPE magnitudes in general have high values over land as compared to oceanic region, which confirmed the consistency of CAPE calculations. The systematic migration of CAPE from Northern Hemisphere to Southern Hemisphere is observed during Northern Hemisphere summer to winter, coinciding with the movement of intertropical convergence zone. Once the seasonal pattern is established, the composite diurnal patterns of CAPE with 2 h resolution are obtained by combing all the observations in one season. Diurnal variation of CAPE has shown domination of semidiurnal variations at some latitudes (12 h) and diurnal variation (24 h) at some other latitudes. The mean removed CAPE is then subjected to Fourier analysis to extract the diurnal variation amplitudes. During the observational period, larger CAPE magnitudes are observed over the Indian Ocean during most of the seasons, comparable in magnitude to that of the land regions. As the CAPE and precipitation patterns have correlation, the present study demonstrated the capability of COSMIC/FORMOSAT‐3 to study the diurnal patterns of CAPE, which will have implications in interpreting the tropical diurnal precipitation patterns.

Short-term variations of vapor isotope ratios reveal the influence of atmospheric processes

“…For example, using a tunable diode laser (TDL), Lee et al (2006) investigated the temporal variations of the vapor δ 18 O in surface air in New England and suggested that water vapor mixing ratio (w) was a better predictor for the vapor δ 18 O than the air temperature on time-scale shorter than a few days. In contrast, Wen et al (2010) found that w was a poor predictor of the temporal variability of the δ v during the peak summer monsoon activities in Beijing. Lee et al (2007) reported the continuous measurements of δ 18 O of atmospheric water vapor and ET over a temperate forest, and observed a diurnal variation of 60‰ or more of the δ 18 O flux ratio on some days.…”

“…The temporal variations of δ v near the surface are related to the atmospheric processes on various time-scales such as air mass advection, precipitation, ET and entrainment from the free atmosphere (Lai et al, 2006;Lee et al, 2006;Wen et al, 2010). Most of the seasonal variability in δ v can be understood through the Rayleigh distillation mechanisms (Gat, 1996).…”

“…The temperature and more importantly, the water content of an air mass are indicators of its rainout history (Jacob et al, 1991;Lee et al, 2006). Seasonal variations of δ v are attributed to the entrainment at the top of the planetary boundary layer (Angert et al, 2008), the intensity of monsoon activities (Yu et al, 2005;Wen et al, 2010), and synoptic events, such as precipitation events (Lee et al, 2006), passage of fronts (Wen et al, 2010) and tropical depression (Fudeyasu et al, 2008). The factors that contribute to the diurnal variations of δ v may be different geographically.…”

“…The factors that contribute to the diurnal variations of δ v may be different geographically. It may be local ET, diurnally variable entrainment in the boundary layer, sea breeze, or a combination of these processes that drives the diurnal cycles of δ v (Lai et al, 2006;Lee et al, 2006;Welp et al, 2008;Wen et al, 2010).…”

“…It is suggested that δ v can be estimated based on the monthly or event-based precipitation because δ v can potentially reach equilibrium with the isotopic composition of the falling rain droplets during rainy periods (Stewart, 1975;Jacob et al, 1991). However, the equilibrium method for predicting δ v in surface air may suffer large errors in arid and semiarid climates, and even fail during the non-rainy periods (Wen et al, 2010).…”