| Clouds and precipitation play key roles in the climate change. Using joint observations of multiple satellite instruments to study clouds and precipitation has become a hot topic in the field of climate change. The Tibetan Plateau (hereafter TP) can affect the weather and climate over regions even the globe through its mechanical and thermodynamic effects. Thus, it is important to study the clouds and precipitation on the TP. Moreover, better understanding of thermodynamic processes of different types of clouds and precipitation provide factual basis and improve the accuracy for the weather and climate prediction.Using the combining measurements of the tropical rainfall measuring mission (TRMM) precipitation radar (PR) and visible and infrared scanner (VIRS) from 1998 to 2012, the climatological characteristics of different precipitations in summer including their horizontal distribution, vertical structures, infrared signal characteristics, and the diurnal variation in TP are investigated, and the relationship between different precipitation types are also revealed. In addition, this thesis studies the horizontal distributions, vertical structures, micro-physical characteristics, cloud radiation and the daytime-nighttime difference for different cloud layers, and the corresponding atmospheric circulation based on the CloudSat dataset. At last, the horizontal distribution, vertical structures, infrared signal characteristics, genernal circulations and the diurnal variation of the precipitation over different terrain are studied, revealing the influence of topograpghy on precipitation. The major findings can be summarized as follows:1. Climatological characteristics of deep convection and shallow precipitationMost of the precipitation over the TP is week deep convection, which occupies 67.8%, followed by shallow precipitation with 26.4%, and strong deep convection with 5.8%. The rain rate of the strong deep convection, week deep convection and shallow precipitation are 9.5,1.4 and 1.3 mm/h, respectively, and the corresponding contributions to the total precipitation are 18.5,51.2 and 30.3%. All of the deep convective rain rate profiles different cloud infrared radiant temperature experience an increasing-reducing process from the storm top altitude to the ground level, with the inflection points at around 7.5 km. On the other hand, the shallow precipitation only has an increasing process. Both the precipitation frequency peaks for strong deep convection and weak deep convection appear at 16:00 LT (local time), while their precipitation intensities reach the maxima at 13:00 LT and 18:00 LT. Besides, the strong deep convection has a secondary peak at 00:00 LT. For the shallow precipitation, the precipitation frequency and intensity peaks appear at 20:00 LT synchronously. The diurnal variation of radiant temperature of deep and shallow precipitation are similar to each other, with the minima around 19:00 LT. It is found that the strong deep convection and weak deep convection have the characteristics of eastward movement, and the week deep convection is more obvious, while there is no significant meridional propagation characteristics observed in the shallow precipitation.2. Climatological characteristics of different cloud phases of precipitation over the TPBased on the brightness temperature (TB10.8) of cloud top (i.e., cloud phases) observed by the thermal infrared channel of VIRS, the precipitation over the TP can be categorized into ice cloud (TB10.8<233 K), ice-water mixed cloud (phase 1 with TB10.8 between 233 and 253, phase 2 with TB10.8 between 253 and 273 K), and water cloud (TB10.8≥273 K). The results indicate that precipitation cloud over the TP is mainly in the form of ice cloud, which occupies 43.01% followed by the form of mixed phase 1 cloud with 38.85% and mixed phase 2 cloud with 17.79%, and water cloud is least with 0.35%. The rain rates of different cloud phases of precipitation are small, concentrating in the range of 0.5-2.0 mm/h. The spatial distributions indicate that the frequency and intensity of precipitation over the TP increase from the west towards the east, while the storm top altitudes decrease from the west towards the east. The vertical structures of precipitation can be divided into 2 layers. In the first layer (echo top to 7.25 km altitude), the reflectivity increases with the altitude decreasing. The second layer is from 7.25 km to near surface ground, where the reflectivity decreases with the altitude decreasing. The diurnal cycle shows that the frequencies of all precipitation in the western TP reach the maxima near 16:00 LT, which indicates that precipitation has no dramatic developing process. However, the maximum frequencies of mixed 2, mixed 1 and ice clouds delay about 2 hours in turn in central and eastern TP. This indicates a significant convection developing process. In addition, the phases for rain frequencies between different parts of the TP delay from 90°E, revealing the convection propagating from the west to the cast.3. Climatological characteristics of vertical structures of cloudThe fraction of clouds over the TP in summer can reach up to 86.79%. Clouds over the TP is mainly in the form of single layer cloud, which occupies 56.86% followed by the form of double layer cloud with 24.47%. The spatial distribution of occurrence frequency shows that the single layer cloud is mainly located in the northern plateau, while fractions of multilayer cloud decrease gradually from the southeast to the northwest. The cloud top height can reach about 17 km. The cloud thickness of single layer cloud is larger than that of multilayer clouds, and that of upper cloud is smaller than that of the lower cloud. The values of microphysical characteristics including particle number concentration, cloud water content and effective radius of single layer are larger than those of multilevel clouds, and the values of upper cloud are lower than those of lower cloud. Moreover, the values of microphysical characteristics and the reflectivity show remarkable logarithmic function relationships for different layer clouds. The shor (long) wave radiation has heating (cooling) effect, and the cooling (heating) effect of single layer is greater (smaller) than that of multilayer clouds. The precipitation is mainly produced by liquid precipitation, and then followed by the solid precipitation, and the drizzle. Furthermore, the drizzle occurs mainly in the multilayer clouds. In addition, higher surface temperature, larger surface specific humidity and higher surface pressure is found to be contributed to the formation of multilayer clouds. The single layer fraction in the daytime (62.99%) is larger than that at night (51.00%), whereas, multilayer clouds are opposite. The larger multilayer cloud fraction at night is related to the larger specific humidity and higher pressure near the surface. The fraction of liquid precipitation is larger during the daytime than that at night. Conversely, the fractions of solid precipitation and drizzle are larger at night.4. Climatological characteristics of precipitation along slope HimalayasThe precipitation characteristics over the steep Himalayas and adjacent regions, including the flat Gangetic Plains (FGP), foothills of Himalayas (FHH), steep slope of south Himalayas (SSSH), and Himalayas-Tibetan Plateau tableland (HTPT), are investigated using merged dataset from the TRMM PR and VIRS for May-August 1998-2012. Results indicate that the rain frequency increases significantly from the FGP via FHH to the lower elevations of the SSSH (2.5 km), and then decreases as the elevation further increases up to HTPT, with the minimum over the HTPT. Along with such spatial variation of the rain frequency, rain rate distributes 4 mm/h over FGP,5.5 mm/h over FHH,2-4 mm/h over SSSH, and less than 2 mm/h over HTPT. The 20-dBZ echo top is highest in FGP with 16 km, and then followd by FHH (15.5 km), SSSH (14 km) and HTPT (14 km). More than 60% of precipitation over the FGP, FHH and HTPT is produced by ice-phase topped clouds, while more than 70% over the SSSH is from mixed-phase topped clouds. Analysis suggests that the highest rain frequency over the SSSH may be caused by a strong upward motion over the SSSH as oceanic air flow interacting with the terrain of the Himalayas, while the largest rain rate over the FHH may result from a low-level convergence over the FHH where moist oceanic air flow is blocked by the SSSH. Instead of the elevation, a topography index we defined in this study has linear relationships with precipitation parameters, indicating that it is a better indicator of precipitation distribution over complex topography. Diurnal variations of precipitation have significant regional difference, which are strongly relevant to topography. Precipitations in HTPT and FGP are dominantly characterized by afternoon maxima and morning minima. At SSSH, rain is characterized by highest RFs with double peaks in the afternoon and midnight. The meteorological parameters also show significant diurnal variations, and the spatial distribution are well related with the precipitation. The upward motion resulting from the low-level convergence and upper-level divergence is contributed to the rain occurring and development. |
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