The Role of Diurnal Hysteresis between Near-surface Temperature and Relative Humidity on Mitigating Near-surface Atmospheric Dryness

Diurnal hysteresis, characterized by a time lag or phase difference between diurnal cycles of two variables, provides valuable insights into land-atmosphere interaction and ecological processes. While it is widely accepted that diurnal surface temperature and relative humidity variations are inversely locked in phase, recent studies have reported diurnal hysteresis between relative humidity and temperature. This phenomenon arises from contributions related to diurnal water vapor variation resulting from local evapotranspiration and water vapor advection from other regions. This study employed a multi-linear regression approach to quantify the phase lag time between temperature and relative humidity by distinguishing linear and non-linear contributions of diurnal temperature variation to relative humidity. Our analysis utilized data from the FLUXNET2015 dataset, which offers ecosystem-scale meteorological measurements worldwide, the HadISD dataset, providing quality-controlled surface weather data globally, and ERA5-Land, offering finer-scale and accurate land reanalysis data on a global scale, to explore the regions exhibiting diurnal hysteresis globally. Our findings reveal the presence of diurnal hysteresis between relative humidity and temperature in coastal, lakeside, and montane regions, notably in New Guinea, southwestern Arabia, the Andes, and the Himalayas. In these regions, the vapor pressure deficit (VPD) is mitigated due to the out-of-phase diurnal relationship between temperature and relative humidity, leading to a smaller underestimation bias in daily VPD when estimated by daily temperature and relative humidity. Even though lower VPD might reduce evapotranspiration in regions showing diurnal hysteresis due to decreased atmospheric demand, the non-local moisture transport provides these regions with additional water vapor. This counters the temperature rise in the morning and aids in sustaining the diurnal hysteretic relationship, maintaining the VPD mitigation in these regions. Furthermore, climate change is likely to change the diurnal hysteretic relationship, posing a threat to VPD mitigation. This study underscores the importance of identifying regions with diurnal hysteresis and highlights the potential implications of changing diurnal hysteretic relationships on local microclimates.