A Diagnostic Study of Water Vapor during An Extreme Precipitation Event in the Yili River Valley

An extreme precipitation event occurred during 31 July to 1August, 2016, at the Yili River Valley that broke multi-station single-event precipitation records. Based on calculations of (1) the water vapor transport stream function and non-divergent (rotational) components, (2) potential function and divergent (irrotational) components, (3) the water vapor transport budget using 1°×1° NCEP/NCAR reanalysis, and (4) water vapor transport trajectories using the HYSPLIT model based on the Lagrangian method, the large-scale water vapor transport and convergence characteristics were analyzed during the heavy rainfall period. The results showed that: (1) The Atlantic Ocean and the Red Sea contributed to the water supply during the event, with the Indian summer monsoon circulation at low latitudes, and the Atlantic Ocean’s east airflow at mid-latitudes, constituting the water vapor transmission channel; convergence and orographic uplift of the terrain to the west provided a favorable dynamic convergence mechanism for the occurrence of locally heavy rain. (2) The 3000 m water vapor transmission trajectory included westward and eastward paths at the bottom layer,and the northward path at mid-troposphere, as streams of water vapor for the lower troposphere through vertical motion; the southward lower-tropospheric path provided water vapor from the Arabian Sea, while the mid-latitude water vapor transport from the west was the most powerful; The 5000 m water vapor transmission trajectory was dominated by the westward path and the low-pressure trough itself. (3) During the precipitation period, water vapor was concentrated in the lower troposphere, which was transported to the upper levels through vertical motion. The water vapor inflow from the southernmost boundary was rapidly increasing at the lower layers, while the middle and upper layer water vapor inflow derived from the western boundary. (4) Yili River Valley GPS-precipitable water vapor (PWV) values jumped due to the southwest airflow around the upper trough before the rainfall occurred, and remained high due to the influence of the Indian Southwest Monsoon during the heavy rainfall period.