Diurnal Variability of Tropical Deep Convection across Global Tropics Captured By the GNSS and Satellites Data.

Convective variability in the global tropics is dominated by the diurnal cycle. Although tropical deep convection has been analyzed using various techniques, the lack of water vapor information with a sufficient temporal resolution over longer time span, constituted an important limitation in such studies. To better understand the coupling between tropospheric moisture, clouds, and precipitation on the diurnal time scale, we utilized 16 years of observations of tropospheric integrated moisture derived from the 42 global navigation satellite system (GNSS) permanent stations, distributed across continents and islands within the global tropics. These data, together with satellite precipitation estimates (TRMM) and cloud top temperatures (merged IR) were used to analyze a diurnal variability in tropospheric deep convection. Results have shown that GNSS integrated moisture was characterized by a mono-modal distribution and its maximum occurred in local afternoon, similar as TRMM precipitation and IR cloud top temperature. Such situation was found at most of the analyzed locations, which confirms that on the diurnal time scale, afternoon precipitation maximum over land is driven primarily by a solar heating, which leads, via enhanced surface fluxes, to increased atmospheric moisture, which was well captured by the GNSS technique. In addition, the highest diurnal variability of integrated moisture was found on continental and large-islands stations, thus reflecting a landside subregimes of the tropical deep convection. In contrast, stations located at small islands were characterized by rather small diurnal variability of integrated moisture and in several cases by two precipitation maxima, with secondary maximum during night and early morning, thus showing an imprint of an oceanic climate regime. However, this secondary precipitation maximum tends to occur during daily minimum of tropospheric integrated moisture. Even though clouds tops appear lower during the seconday, nighttime precipitation maximum than during the primary on in the afternoon, decrease in cloud top temperature is observed during both. Therefore it most likely represents advection of precipitating cloud systems from surrounding oceans.

Overall, daily changes of the tropical integrated moisture are small compared to the average water vapor content, and barely exceed 5% of its value. Nevertheless, even such small changes (up to 2 mm of integrated moisture) in favorable large-scale conditions lead to the convection and therefore affect precipitation. Additionally, their mono-modal distribution does not change with seasons, although mean values of integrated moisture clearly reflects seasonality and, in several cases, changes related to the large-scale flow such as e.g., Asian monsoon. Conducted analysis have proved that GNSS technique can be successfully used in the tropical deep convection studies, especially considering its high temporal resolution, weather independency and a generally sparse network of water vapor monitoring infrastructure in global tropics.