Diurnal Precipitation Features over Complex Terrains along the Yangtze River in China Based on Long-Term TRMM and GPM Radar Products

Abstract

Based on the 20-year high-resolution precipitation data from TRMM and GPM radar products, diurnal features over complex terrains along the Yangtze River (YR) are investigated. Using the Fast Fourier Transform (FFT) method, the first (diurnal) and second (semi-diurnal) harmonic amplitude and phase of precipitation amount (PA), precipitation frequency (PF), and intensity (PI) are analyzed. The diurnal amplitudes of PA and PF have a decreasing trend from the west to the east with the decreasing altitude of large-scale terrain, while the semi-diurnal amplitudes of PA and PI depict the bimodal precipitation cycle over highlands. For the eastward propagation of PA, PF is capable of depicting the propagation from the upper to the middle reaches of YR, while PI shows the eastward propagation from the middle to the lower reaches of YR during nighttime and presents sensitivity to highlands and lowlands. According to the contribution of different-sized precipitation systems to PI over the highlands and lowlands, the small (<200 km2) ones contribute the least while the large ones (>6000 km2) contribute the most, but the medium ones (200–6000 km2) show a slightly larger contribution over the highlands than over the lowlands. The propagation of each scaled precipitation system along the YR is further analyzed. We found that small precipitation systems mainly happen in the afternoon without obvious propagation. Medium ones peak 2–4 h later than the small ones, with two eastward propagation directions at night from the middle reaches of YR to the east. The large ones are mainly located in lowlands at night, with two propagation routes in the morning over the middle and lower reaches of YR. Such a relay of the propagation of the medium and large precipitation systems explains the eastward movement of PI along the YR, which merits future dynamic studies.

Evolutionary Characteristics of Daytime and Nocturnal Precipitation Heterogeneity in Gansu Province, Northwest China

“…It can be seen that daytime precipitation, although usually slightly lower than nocturnal precipitation, has played a greater contribution in the evolutionary process of increasingly abundant precipitation in Gansu province, benefiting from the substantial increase in the frequency of daytime precipitation. In terms of mechanism, this may be due to the greater temperature range during the day, especially in the afternoon, which is very conducive to the formation of local convective rain [39][40][41].…”

“…For example, the special topography is conducive to blocking the increasing abundance of external water vapor in the atmospheric circulation, and favorable hydrological conditions can make full use of the local internal circulation of water vapor. Once sufficient water vapor reaches the plummeting temperature, it is natural for water vapor to condense into precipitation, especially in the early morning and afternoon [39][40][41]. With global warming, precipitation events should form more easily at higher elevations, and the increase in precipitation amounts is also likely to be significantly ahead of that at lower elevations.…”

“…The above results confirm that the spatial distribution of precipitation concentration in China has complex meridional and zonal variability, and the correlation index generally shows a downward trend. It is well known that precipitation has a distinct diurnal variation process [39][40][41] due to the alternation of day and night, but a series of scientific issues related to it, such as whether its response to climate change is consistent in different periods and which period is more critical, have yet to be determined. Given this, we selected Gansu province, which has diverse climate types and is located deep in the interior of Northwest China, as the research object, to bridge the cognitive gap of precipitation concentration and its evolution on the diurnal scale in such regions, based on the data set of daily values of 23 national meteorological stations from 1970 to 2019.…”