An observational view of rainfall characteristics and evaluation of ERA5 diurnal cycle in the Yarlung Tsangbo Grand Canyon, China

Abstract

The Yarlung Tsangbo Grand Canyon (YGC), one of the world's deepest canyons, is located within the East Himalayas, which are remote and poorly instrumented. In this study, a rain gauge network was established around the YGC region. Data collected from the network for more than three years disclose that the rainfall spatial pattern in the south of the YGC region is relatively homogeneous. Diurnal cycles showed some variations among sites, but a clear floor was visible in the afternoon and peak values were exhibited in the early morning.The monthly precipitation in the YGC region shows two peaks, in April and July.There are two regions (500 m and 2,500 m AMSL) with high precipitation in the YGC. For rainfall extremes, vertical convection and vapor transport are important in this region. The diurnal cycle from our observations provides a key test of the physical parameterizations for use in the fifth-generation ECMWF (European Centre for Medium-Range Weather Forecasts) reanalysis product (ERA5).Our observations indicate that ERA5 cannot reproduce the diurnal patterns of precipitation in the YGC region. ERA5 showed a wet bias when estimating light cumulus rainfall and a dry bias when estimating heavier (convective) precipitation. The erroneous diurnal variation of ERA5 precipitation (false afternoon rainfall) was due to the CAPE (Convectively Available Potential Energy)-based convective precipitation scheme. The higher ERA5 precipitation than observed was due to the large-scale rainfall scheme in the Integrated Forecasting System (IFS) of ERA5. These findings indicate that there still is room for improvement of the ERA5 reanalysis data in the eastern Himalaya region.