The Changing Extreme Values of Summer Relative Humidity in the Tarim Basin in Northwestern China

Abstract

Relative Humidity (RH) in the arid region of the Tarim Basin is crucial for many reasons. The Tarim Basin has experienced a tendency to become wetter in recent decades, and the RH here also shows an increase over the past decade. However, there has been little examination of these RH changes and especially the changes to the extremes. This study investigates the changes in extreme values and the probability distribution function (PDF) of summer RH using quantile regression during 2006-2018 to understand the possible reasons for the increase in the summer RH anomaly. We find that extremely high values of RH show a consistent significant increase, while extremely low values have no regionally consistent tendency. The overall average value of RH in the Tarim Basin becomes higher, driven by the upper half of the PDF. To explore the physical mechanism for these changes, we examine the corresponding regional meteorological anomaly patterns. The patterns indicate that the anomalous southwesterly airflow at 500hPa brings ample moisture into the basin and the ground in the middle of the basin significantly cools down when an extreme wet event occurs, promoting the occurrence of the extreme high RH. In this process, the contributions of water vapor transport and temperature are of equal significance though with different relative timing. These corresponding regional meteorological patterns occur more often in the most recent decade, which coincides with the recent increase in RH extremes in this region.

Brief Communication: Effects of different saturation vapor pressure calculations on simulated surface-subsurface hydrothermal regimes at a permafrost field site

“…The structure of Tetens's formula is similar to the Magnus form. Interestingly, this short equation (over the water surface or without over-ice correction) is considered a general form to calculate the SVP for all temperature ranges and is also commonly used in the literature (e.g., Xu et al, 2012;Valiantzas, 2013;Nian et al, 2022), as below:…”

“…The value of the SVP over ice is smaller than that over water because the molecular forces bind much more tightly in an ice crystal than in the liquid water phase (Gill, 1982;Alduchov & Eskridge, 1996). However, the SVP is often calculated only over the water surface in many cold region studies (e.g., Xu et al, 2012;Valiantzas, 2013;Leppäranta, 2015;Li et al, 2017;Brauner, 2019;Nian et al, 2022;Xie et al, 2022), and the influences of differences in SVP over water and ice on the calculation of the humidity variable are not generally evaluated in these studies.…”