Validation And Analysis Of AIRS Measured Water Vapor Over The Tibetan Plateau

To improve our knowledge of the dynamic and thermodynamic effects of Tibetan Plateau,it is significant to describe the water vapor over the Tibetan Plateau in a more accurate way.The Atmospheric Infrared Sounder (AIRS) providing with long-term available water vaporretrievals with high spatial resolution has become another important mean to study weather andclimate over the plateau by supplementing limited, ground-based observations. This study is (i)to evaluate the applicability of AIRS water vapor retrievals over the plateau; and (ii) toresearch the distribution features of water vapor and its transports over the Tibetan Plateau byusing AIRS data; and (iii) to discuss the relationship of moisture flux over the plateau with theprecipitation over the plateau and its downstream areas; and (iiii) to analyze the effects ofwater vapor transports from the plateau from two continuous heavy rainfall events.The research firstly explores the applicability of the gridded (level3) monthlytropospheric water vapor (version5) retrievals from AIRS instrument on-board the NASAAqua satellite over the Tibetan Plateau by comparing with carefully-processed radiosonde data.Local correlation analyses indicate that below200hPa the AIRS/AMSU monthly water vaporretrievals are highly consistent with radiosondes over the whole plateau region, especially inthe southeastern part and between300-600hPa. Relative deviation analyses further show thatthe differences between monthly mean AIRS/AMSU water vapor retrieval data andradiosondes are in general small below250hPa, in particular between300-600hPa and in highaltitude areas. Combined with a further direct comparison between AIRS/AMSU water vaporvertical retrievals and radiosonde observations averaged over the entire domain, these resultssuggest that the gridded monthly AIRS/AMSU water vapor retrievals can provide a very goodaccount of spatial patterns and temporal variations in tropospheric water vapor content in theTibetan Plateau region in particular below200hPa. However, differences betweenAIRS/AMSU retrievals and radiosondes are seen at various levels in particular above the levelof250hPa. Therefore, for detailed quantitative analyses of water budget in the atmosphere andthe entire water cycle, AIRS/AMSU retrieval data may need to be corrected or trained usingradiosondes. Two fitting functions are derived for warm and cold seasons, respectively, thoughseasonal difference is generally small. Spatial distributions and seasonal variations of tropospheric water vapor over the TibetanPlateau and the surrounding areas are then explored by means of the water vapor products fromthe high resolution Atmospheric Infrared Sounder(AIRS) on-board the Aqua satellite and theNASA Water Vapor Project(NVAP). Because NVAP has serious temporal in-homogeneity issueas found in previous studies, AIRS retrieval product is primarily applied here, though similarseasonal variations can be derived in both datasets. Intense horizontal gradients appear alongthe edges of the Plateau in the lower tropospheric (500-700hPa) water vapor and columnarprecipitable water, in particular over the regions along the southeastern boundary. Richhorizontal structures are also seen within the Plateau, but with weaker gradient. In themiddle-upper troposphere (300-500hPa), horizontal gradients are relatively weaker. It isshown that there is always a deep layer of high water vapor content over the Plateau with apeak around500hPa, which can extend from the surface to roughly300hPa and even to100hPa at some locations. This layer of high water vapor content has consistent influence onprecipitating processes in the downstream regions such as the valleys of Yellow and YangtzeRiver. Estimated vertically-integrated water vapor flux and moisture divergence in the twolayers (500-700hPa and300-500hPa) further confirm the effect of the Tibetan Plateau on thedownstream regions. In particular, the middle-upper layer water vapor (300-500hPa) tends toplay an essential role during both warm and cold seasons, confirmed by the spatial distributionof seasonal mean precipitation.The relationship with water vapor flux from the Tibetan Plateau and precipitation over theplateau and its downstream areas is discussed in the next part. The correlation analyses onmoisture and its transports over the plateau and ground-observed precipitation shows watervapor transport from the plateau may be related to the precipitation of most parts of China,except regions south of the Yangtze River, and Hetao southern areas and its downstream areasare most affected. By using AIRS and TRMM datasets to do Singular Value Decomposition(SVD) analysis on water vapor flux and precipitation over the plateau, we found thatprecipitation over the plateau is highly related to the moisture transports from itself and placesto the south in warm seasons, while the water flux over the northwestern-northern plateau andareas to the north in cold seasons. The SVD analysis on moisture flux and precipitation overthe plateau and its surroundings then indicates most water vapor from the plateau have effectson the precipitation of Northeast China, Hetao and North China，river basins of the YellowRiver and the Yangtze River and Yunnan-Guizhou-Sichuan regions. Hetao and the YellowRiver downstream areas are highly influenced in warm seasons while Northeast China,Southern Hetao and Sichuan Basin are the most related regions.Finally, the continuous heavy rainfall process over Huaihe River basin happened duringJune to July in2007and another intensive snowfall-rainfall event in South China occurred in January and February,2008are selected as representative of long-term rainfall event in warmand cold seasons to explore the specific influence of water vapor transports from the TibetanPlateau. The results point out that the Tibetan Plateau do have great effects on these two eventsexcept from the contribution of moisture flux from Bay of Bengal, South China Sea, westernpacific, and its abnormal signal of water vapor and its transport was not only important but alsooccurred earlier. Accumulative departure analysis on water vapor and its flux over the TibetanPlateau and precipitation of the main rainfall areas further shows that the signal of water vaporfrom the plateau appeared about four days ahead of the first event and six to ten days earlier inthe second event. Therefore the abnormal signal of moisture and its transport from the TibetanPlateau could be a considerable predictor for continuous strong rainfall event.