On the seasonal variation of various types of precipitation over global tropical ocean region: A perspective from TRMM measurements

Abstract

The radar reflectivity and precipitation rate from Precipitation Radar (PR) onboard Tropical Rainfall Measuring Mission (TRMM) are obtained over the global tropical regions (35°S-35°N) during the period from 2007 to 2012, combined with coincident vertical velocity at 400 hPa (ω400 hPa), relative humidity at 850 hPa (RH850 hPa) and lower tropospheric stability (LTS) from European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. First of all, the seasonal and spatial distribution of meteorological factors, including ω400 hPa, RH850 hPa, and LTS, together with rain rate are investigated. Next, four typical Regions of Interest (ROIs) and their individual seasons (i.e., favorable and non-favorable seasons for rainfall) are identified for further analysis by determining whether there exist most pronounced seasonal differences observed in ω400 hPa. Meanwhile, rainy area, and rainfall of three precipitation types (i.e., shallow, stratus, and convection precipitation regimes) over the ROIs has been calculated. The three dimensional structures of individual precipitating system are analyzed, based on normalized contoured frequency by altitude diagram (NCFAD) and other statistical methods. Finally, with a focus on convection precipitation system, we give a quantitative description of the response of precipitation vertical structure to meteorological factors. Namely, how the rain echo top height (RTH), echo top height with reflectivity of 30 dBZ (ZTH30 dBZ), and reflectivity center of gravity (ZCOG) vary with ω400 hPa, RH850 hPa, and LTS. In particular, (1) high rain rate dominates over intertropical convergence zone, which is characterized by strong updraft, sufficient moisture, and low LTS, showing the average rain rate is negatively associated with ω400 hPa at both temporal and spatial scales. That is to say, lower ω400 hPa comes with more intensive rain rate. (2) The meteorological factors, along with average rain rate, exhibit appreciable seasonal variation. To be specific, negative (positive) ω400 hPa anomalies is mostly found over the northern (southern) hemisphere in summer and autumn, as opposed to the patterns found in winter and spring. (3) In terms of the area with rainfall, the stratiform precipitating system accounts for more than 50% of the total area under investigation, followed by the convective precipitating system (about 30%), and the shallow precipitating system (less than 20%). In contrast, convective precipitating system, among others, takes the lead (about 65%) in contributing to the accumulated rainfall amount in the ROIs studied, followed by stratiform precipitating system (about 25%), and the shallow precipitating system (about 10%). (4) In the season with relatively higher ω400 hPa, both rainy area and accumulated rainfall amount for all three types of precipitation show a increasing trend, irrespective of ROI. By comparison, rain rate and its vertical structures show large discrepancy, i.e., the intensity of convective precipitation system in favorable season tends to systematically increase as compared with that in non-favorable season. (5) The bulk precipitation system parameters used to describe convective precipitating system, including RTH, ZTH30 dBZ, and ZCOG, are observed to be elevated sharply with increasing ω400 hPa and RH850 hPa. The same holds for the increasing LTS but with a smaller magnitude in the elevated height. This implies that ω400 hPa and RH850 hPa most likely play a dominant role in dictating the vertical development of convection.