Characteristics of the Water Vapor Transport in the Canyon Area of the Southeastern Tibetan Plateau

Changes in the surface fluxes cause changes in the annular flow field over a region, and they affect the transport of water vapor. To study the influence of the changes in the surface flux on the water vapor transport in the upper layer in the canyon area of southeastern Tibet, in this study, the water vapor transport characteristics were analyzed using the HYSPLIT_v4 backward trajectory model at Danka and Motuo stations in the canyons in the southeastern Tibetan Plateau from November 2018 to October 2019. Then, using ERA-5 reanalysis data from 1989 to 2019 and the characteristics of the high-altitude water vapor transportation, the impact of the surface flux changes on the water vapor transportation was analyzed using singular value decomposition (SVD). The results show that the main sources of the water vapor in the study area were from the west and southwest during the non-Asian monsoon (non-AMS), while there was mainly southwest air flow and a small amount of southeast air flow in the lower layer during the Asian monsoon (AMS) at the stations in southeastern Tibet. The water vapor transmission channel of the westward airflow is higher than 3000 m, and the water vapor transmission channel of the southwestward and southeastward airflow is about 2000 m. The sensible heat and latent heat are negatively correlated with water vapor flux divergence. The southwest boundary of southeastern Tibet is a key area affecting water vapor flux divergence. When the sensible heat and latent heat exhibit downward trends during the non-Asian monsoon season, the eastward water vapor flux exhibits an upward trend. During the Asian monsoon season, when the sensible heat and latent heat in southeastern Tibet increase as a whole, the eastward water vapor flux in the total-column of southeastern Tibet increases.

Water vapor transport in cases of South Atlantic Convergence Zone (SACZ)

This case study analyzes water vapor flux that is vertically integrated into the atmosphere in episodes of the South Atlantic Convergence Zone (SACZ). The scope of this study is two cases that occurred between January and February 2018. We use the ERA-Interim reanalysis data from the European Center for Medium-Range Weather Forecasts (ECMWF) to build the maps of vertically integrated water vapor flux and its divergence. We use two 5º by 5º sub-areas, centralized over Belo Horizonte and São Paulo, as control for water vapor balance. The results point to the existence of water vapor transport from the Amazon region to Southeastern Brazil in association to the SACZ. Convergence areas of vertically integrated water vapor flux predominate along the Northwest-Southeast line. The two cases over the Belo Horizonte area presented an average of water vapor balance of -1.8 and -12.9 mm/day. The average at the São Paulo area was -3.6 and 2.0 mm/day. The negative values indicate that precipitation exceeded evapotranspiration, that is, the area served as a water vapor sink.

Atmospheric Rivers and Associated Precipitation over France and Western Europe: 1980–2020 Climatology and Case Study

Atmospheric rivers are important atmospheric features implicated in the global water vapor budget, the cloud distribution, and the associated precipitation. The ARiD (Atmospheric River Detector) code has been developed to automatically detect atmospheric rivers from water vapor flux and has been applied to the ECMWF ERA5 archive over the period 1980–2020 above the Atlantic Ocean and Europe. A case study of an atmospheric river formed in the East Atlantic on August 2014 that reached France has been detailed using ECMWF ERA5 reanalysis, ground based observation data, and satellite products such as DARDAR, AIRS, GPCP, and GOES. This atmospheric river event presents a strong interaction with an intense upper tropospheric jet stream, which induced stratosphere–troposphere exchanges by tropopause fold. A 1980–2020 climatology of atmospheric rivers over Europe has been presented. The west of France, Iberian Peninsula, and British Islands are the most impacted regions by atmospheric rivers with an occurrence of up to four days per month during the October–April period. Up to 40% of the precipitation observed on the west European coast can be linked to the presence of ARs. No significant trend in the occurrence of the phenomena was found over 1980–2020.

Estimation of the precipitable water and water vapor flux using MAX-DOAS in two typical cities of China

<p>      Water vapor transport affects regional precipitation and climate change. The measurement of precipitable water and water vapor flux is of great significance to the study of precipitation and water vapor transport. In the study, a new method of computing the precipitable water and estimating the water vapor transport flux using multi-axis differential optical absorption spectroscopy (MAX-DOAS) were presented. The calculated precipitable water and water vapor flux were compared to the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data and the correlation coefficient of the precipitable water, the zonal and meridional water vapor flux and ECMWF are r≥0.92, r=0.77 and r≥0.89, respectively. The seasonal and diurnal climatologies of precipitable water and water vapor flux in the coastal (Qingdao) and inland (Xi’an) cities of China using this method were analyzed from June 1, 2019 to May 31, 2020. The results indicated that the seasonal and diurnal variation characteristics of the precipitable water in the two cities were similar. The zonal fluxes of the two cities were mainly transported from west to east, Qingdao's meridional flux was mainly transported to the south, and Xi'an was mainly transported to the north. The results also indicated that the water vapor flux transmitting belts appear near 2km and 1.4km above the surface in Qingdao and appeared around 2.8km, 1.6km and 1.0km in Xi'an. </p>

Improved eddy covariance flux based transpiration estimates at high relative humidity and comparison to sap flux

<p>Eddy covariance (EC) directly measures evapotranspiration (ET), which consists of transpiration and evaporation (E) from the soil and other surfaces. For process understanding it is pivotal to separate ET into its components. Yet, its computation is highly sensitive to the methodology used to estimate T. Among the multiple methods proposed in recent years, T has been estimated from EC via the Transpiration Estimation Algorithm (TEA, Nelson et al., 2020), and from the sap flux measurement network SAPFLUXNET (Poyatos et al., 2020). These methods are applicable to a large number of measurement sites worldwide, and can help constrain the global estimates of the ratio of T to ET, T/ET. While EC measures water and carbon fluxes across ecosystems globally, water vapor flux measurements can be underestimated at high relative humidity (Ibrom et al., 2007; Mammarella et al., 2009) causing errors in the measured ET and propagating into the predicted T.</p><p>Here we report a method to detect and correct the high relative humidity error caused by attenuation of high frequency in water vapor measurements of a closed-path EC system. Our results of the comparison between present water use efficiency (WUE) with previous TEA-based WUE show that the corrected WUE is lower at high relative humidity than that derived from previous TEA at the sub-daily scale. Besides, we compare the corrected T estimates from EC to concurrent SAPFLUXNET sites to show an improved relationship between sap flux and EC based T, T/ET, and WUE. Finally, we explore the main abiotic factors, such as vapor pressure deficit, air temperature, and precipitation, influencing WUE estimated from different T estimation methodologies. These results provide an improved data-driven approach to the ongoing research on ET partitioning and the factors influencing the WUE across ecosystems globally.</p><p> </p><p>Ibrom, A. et al. (2007) ‘Strong low-pass filtering effects on water vapour flux measurements with closed-path eddy correlation systems’, Agricultural and Forest Meteorology. doi.org/10.1016/j.agrformet.2007.07.007.</p><p>Mammarella, I. et al. (2009) ‘Relative humidity effect on the high-frequency attenuation of water vapor flux measured by a closed-path eddy covariance system’, Journal of Atmospheric and Oceanic Technology. doi.org/10.1175/2009JTECHA1179.1.</p><p>Nelson, J. A. et al. (2020) ‘Ecosystem transpiration and evaporation: Insights from three water flux partitioning methods across FLUXNET sites’, Global Change Biology. doi: 10.1111/gcb.15314.</p><p>Poyatos, R. et al. (2020) ‘Global transpiration data from sap flow measurements: the SAPFLUXNET database’, Earth System Science Data. doi:10.5194/essd-2020-227.</p>

Increasing warm-season precipitation in Asian drylands and response to reducing spring snow cover over the Tibetan Plateau

Precipitation is crucial for life and the ecological environment in Asian drylands. This study investigated precipitation trends in Asian drylands in previous four decades and simulated its possible linkage with snow cover reduction over the Tibetan Plateau. The results show that precipitation has been increasing and contributing to wetter conditions in Asian drylands. The increasing trends can be attributed to the deepened quasi-stationary wave trough around the Lake Balkhash and the meridional water-vapor flux originated from the Arabian Sea and the Bay of Bengal. The mid-latitude waves and eddy disturbances correspond to the northward upper-level Tibetan Plateau (TP) mode of the South Asian High (TP-SAH) and the Afro-Asia jet with cyclonic rotation. Both SAH and Afro-Asia jet anomalies strengthen the ascending motion and northward water-vapor convergence in Asian drylands, and those are favorable for summer precipitation. The anomalous circulations are linked to the following: (1) the reduced snow cover (SC) over the west TP in the late spring results in decreasing soil moisture and increasing diabatic heating in summer and favors northward extension of TP-SAH and the Afro-Asia jet; (2) the reduced TP/SC increases surface temperature over TP and northeast Asia, which decreases the temperature gradient between the TP and the Indian Ocean, between northeast Asia and East Asia. Decreased temperature gradients are beneficial to the southwest-northeast cyclonic rotation of Afro-Asia jet and consequently strengthen the southerly wind and northward water-vapor flux over TP and surrounding regions. This study emphasizes important effects of the reducing TP/SC on intensifying summer precipitation in Asian drylands.

Gap-Filling of Surface Fluxes Using Machine Learning Algorithms in Various Ecosystems

Five machine learning (ML) algorithms were employed for gap-filling surface fluxes of CO2, water vapor, and sensible heat above three different ecosystems: grassland, rice paddy field, and forest. The performance and limitations of these ML models, which are support vector machine, random forest, multi-layer perception, deep neural network, and long short-term memory, were investigated. Firstly, the accuracy of gap-filling to time and hysteresis input factors of ML algorithms for different ecosystems is discussed. Secondly, the optimal ML model selected in the first stage is compared with the classic method—the Penman–Monteith (P–M) equation for water vapor flux gap-filling. Thirdly, with different gap lengths (from one hour to one week), we explored the data length required for an ML model to perform the optimal gap-filling. Our results demonstrate the following: (1) for ecosystems with a strong hysteresis between surface fluxes and net radiation, adding proceeding meteorological data into the model inputs could improve the model performance; (2) the five ML models gave similar gap-filling performance; (3) for gap-filling water vapor flux, the ML model is better than the P–M equation; and (4) for a gap with length of half day, one day, or one week, an ML model with training data length greater than 1300 h would provide a better gap-filling accuracy.

Interactive Radiation Accelerates the Intensification of the Midlevel Vortex for Tropical Cyclogenesis

Interactive radiation helps accelerate tropical cyclogenesis, but the mechanism is still unclear. Using idealized numerical modeling in the radiative–convective equilibrium framework, it is revealed that interactive radiation can bring forward tropical cyclogenesis by accelerating the development of the midlevel vortex. A strong horizontal longwave radiative warming anomaly in the layer between 6 and 11 km altitudes in the vortex region, caused by large concentration of ice-phased particles at high levels, is critical to the development of the midlevel vortex. This longwave radiative warming anomaly induces more upward water vapor flux (mainly in the nonconvective region) and then results in more latent heating at upper levels and more sublimation and melting cooling at lower levels. This leads to an increase of the vertical diabatic heating gradient, and then the intensification of the midlevel vortex. A stronger upward water vapor flux also produces more condensates at upper levels and further enhances the horizontal longwave radiative warming anomaly in the upper troposphere, constituting a positive feedback, and then accelerates tropical cyclogenesis.