Contrasting precipitation gradient characteristics between westerlies and monsoon dominated upstream river basins in the Third Pole

• © 2020, Science Press. All right reserved. Based on precipitation observations from 256 gauges in the westerly and monsoon dominated upstream river basins of the Third Pole (TP), this study determined the relationships between precipitation and elevation. The basins include the upper basins of the Yangtze, Yellow, Lancang, Nujiang, Yarlung Zangbo, Yarkant, Indus, Amu Darya, and Syr Darya. Using the ERA5 data, this work examined the possible reasons for the difference in the characteristics of precipitation gradient, i.e. analyzing the relationships between the total column water vapor (TCWV), convective available potential energy (CAPE), lifting condensation level (LCL) and elevation, respectively. The feasibility of orographic corrections of precipitation data or observation is validated with the improved VIC land surface hydrological model in two mountain basins in the TP. Mean annual precipitation from gauges generally show decreasing trends (17-128 mm/100 m) with increased elevation (2500-5500 m a.s.l.) in the monsoon dominated basins, i.e. upper Yangtze, Yellow, Lancang, Nujiang, and Yarlung Zangbo, while the orographic enhancements are observed at relatively smaller scales, such as, in the very source regions of the upper Lancang and Nujiang, and Rikaze sub-basin with areas of 11000-67740 km2. On the other hand, in the westerly dominated basins, mean annual precipitation tends to increase with elevation (5-64 mm/100 m) in the upper Yarkant, Indus, Amu Darya, and Syr Darya. The precipitation estimates from ERA5 show a good correspondence with the gauge data (R=0.6-0.9, P<0.05), and exhibit a general consistent precipitation gradient pattern with the gauge observations. The ERA5 variables of TCWV, CAPE, and LCL are useful to understand the factors for the spatial pattern of precipitation vertical gradients in the TP basins with different climate control. The TCWV, CAPE, and LCL represent the vertically integrated moisture, instability and condensation necessary for the generation and development of precipitation. The larger TCWV, higher CAPE and lower LCL enhance precipitation. The decrease of precipitation with elevation in monsoon basins is caused by the decrease of TCWV with elevation, while the increase of precipitation with elevation in westerly dominated basins is a result of increasing CAPE and decreasing LCL with elevation. Hydrological modeling results in the upper Yarkant basin and Rikaze basin indicate that the orographic correction of precipitation data significantly improves the model accuracy, reducing the biases to less than 5% relative to flow observations. This work demonstrates that precipitation correction through vertical gradients is an effective way to derive high mountainous precipitation estimates for hydrological modeling from lowland gauges in the TP, especially in the westerly dominated basins, and in monsoon basins at regional or local scales. Knowledge of the spatial and temporal characters and variations of precipitation over the TP is greatly incomplete, which largely hampers the understanding on climate variability and water availability projections in the TP. This study offers a useful reference to derive reliable mountain precipitation through orographic correction, and also provides a scientific basis to establish precipitation observation network in the Second Tibetan Plateau Scientific Expedition and Research.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Nyckelord

Orographic effect

Precipitation gradient

The Third Pole

Tibetan Plateau

Westerly and monsoon circulations

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