Sub-cloud Evaporation Research Articles

Meteorological factors associated with dry thunderstorms and simultaneous lightning-ignited wildfires: The 15 June 2022 outbreak in Catalonia

Summer heatwaves and extended dry spells create optimal meteorological conditions for occasional dry thunderstorms to produce simultaneous lightning-ignited wildfires (LIW). Concurrent ignitions put a significant burden on the firefighting's initial attack, potentially allowing incipient LIWs to escape and grow into large fires. While we can reasonably forecast lightning activity, predicting dry thunderstorm conditions and potential LIW outbreaks remains challenging. In the present study, we analyze the meteorological factors associated with a LIW outbreak that took place in Catalonia on 15 June 2022, with 22 LIW reported in three consecutive days. The fire hazard was high, but not different from past LIW episodes. ERA5-derived indices related to low-level moisture showed extreme values compared to previous studies. Atmospheric conditions with an elevated lifting condensation level coupled with the synoptic framework were set for the formation of dry thunderstorms. Radar reflectivity profiles revealed sub-cloud evaporation, and rain-gauge records corroborated the occurrence of dry lightning. In the context of global warming, we expect an increase in the frequency of LIW outbreaks in the European Mediterranean region due to an increase in lightning-ignition efficiency, which refers to the average chance of fire per lightning stroke.

Temporal variability of the isotope compositions of plum rain in Nanjing from event to interannual time scales

Precipitation in the plum rain period accounts for 40%-50% of annual precipitation in the monsoon region. To clarify the temporal variability of the isotopic composition of precipitation during the plum rain period from event to interannual time scale and identify the influencing factors, we analyzed the isotopic composition of precipitation and its influencing factors in Nanjing from 2015 to 2022. By using the Hybrid Single-particle Lagran-gian Integrated Trajectory (HYSPLIT) model with specific humidity analysis, we investigated the water vapor source and influencing factors. The results showed that 1) the isotopic abundance of atmospheric precipitation was depleted in the summer and enriched in winter. dx was lower in summer and higher in winter. The isotopic abundance of precipitation from the plum rain was depleted compared to mean value of the whole-year. 2) There was no significant correlation between δ2H and δ18O of the plum rain (precipitation) with local meteorological factors. However, dx was lower in light rain, reflecting the effect of sub-cloud evaporation. The average dx was higher during plum rain period in years with more total plum rain precipitation. 3) The low-latitude South China Sea and the western Pacific Ocean source area provided water vapor for the plum rain. The shift of moisture source region led to abrupt changes in precipitation isotopes. Our results could provide data support for studies on precipitation isotopes in the monsoon region, as well as a reference point for further understanding the precipitation mechanism of the plum rain and stu-dying the seasonal variability of atmospheric circulation in the East Asian monsoon region.

Effect of oasis and irrigation on mountain precipitation in the northern slope of Tianshan Mountains based on stable isotopes

Water is one of the scarcest resources in arid regions, and irrigation is an important means to improve crop yield and ensure food security. The Xinjiang features an integrated irrigation agriculture and oasis economy paradigm. However, the understanding of the impact mechanism and quantitative analysis of irrigation on precipitation is inadequate now. With the aim of clarifying the response relationship between mountain precipitation and irrigation in oasis, we clarify and quantify the moisture sources of precipitation using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and multisource data (including field-collected isotopes data, C-Isoscape, ERA5, and GDAS). The results obtained show: (1) Isotope analysis provides valuable insights into the climate background and precipitation recycling. The Local Meteoric Water Line's lower slope and intercept than Global Meteoric Water Line suggest a severe arid climate and strong sub-cloud evaporation. The intercept (13.44) of Local Meteoric Water Line in Mountain Areas exceeds the global average, indicating significant moisture recycling in mountain areas (MA). The d-excess in MA is prominent year-round, highlighting the influence of recycled moisture. (2) The oasis and irrigation influence the local precipitation through offset, remote, combination, and delayed effects. The offset effect hinders precipitation formation in irrigation areas (IA) due to the counteracting irrigation-induced humidification and cooling effect. Through the interaction of westerly circulation and local mountain-oasis hydrological cycle, irrigation-induced moisture is transported to MA and uplifted, which is remote effect. The combination effect synergistically enhances precipitation intensity, especially in intricate MA, through the combined influence of terrain's forced uplift and convective cells. Furthermore, the mountainous precipitation influenced by the mountain-oasis hydrological cycle often exhibits a delayed effect. (3) Based on HYSPLIT model tracking results, the contribution ratio of oasis and irrigation induced evapotranspiration to summer precipitation in MA is approximately 19.60%, ranging from 13.68% to 28.49%. The findings provide insights into the mechanisms of irrigation and water resource management in the northern slope of Tianshan Mountains.

Seasonal Variations of Modern Precipitation Stable Isotopes over the North Tibetan Plateau and Their Influencing Factors

Based on 360 event-based precipitation samples collected at six stations on the North Tibetan Plateau (NTP) in 2019–2020, we analyzed the influence of meteorological parameters, sub-cloud evaporation, moisture sources, and moisture transmission pathways on precipitation and its seasonal variations. The results show that precipitation δ18O, δ2H, and d-excess values show obvious seasonal variations, being depleted in winter and enriched in summer. Although temperature is an important variable that affects the change in δ18O values of precipitation, the results of the sub-cloud evaporation effect and moisture tracing show that differences in moisture sources caused by seasonal changes in large-scale water moisture transport are an important cause of seasonal changes in δ18O and d-excess of precipitation at NTP. Depleted δ18O and enriched d-excess in winter represent the source of moisture transported by the westerlies from the Mediterranean area and Central Asia. Enriched δ18O and d-excess values in summer precipitation are related to the temperature effect. In addition, the meridional motion of the atmospheric flow has an effect on the precipitation isotope values in the NTP. When the meridional circulation is enhanced, the water vapour from low latitudes is easily transported northwards, enriching the summer precipitation isotope values in the central and eastern parts of the plateau. This provides a new insight into the explanation of stable oxygen isotopes in climate proxies across the westerlies-dominated Tibetan Plateau.

Isotopic and Remote Sensing-Based Characterisation of a Rainfall Event over Western Sierra de Gádor (Spain): Implications for Carbonate Aquifer Recharge

Stable water isotopes are widely recognised as essential tools to trace processes within the hydrological cycle and to disentangle complex phenomena, such as regional–local atmospheric patterns, infer water balances or characterise the aquifer recharge. In this work, we characterised two post-summer precipitation events over the western sector of Sierra de Gádor through the analysis of the rainwater δ18O and δ2H ratios, ionic composition and, complementarily, remote sensing products to define the atmospheric circulation during the rainfall episodes. This information enabled us to discern the moisture source, formation conditions and orographic influence on the rain episodes, and to better define the origin of the groundwater recharge and its dynamics in the study area. Although the samples share a common moisture source (W Mediterranean), they display substantial differences in their isotopic compositions. Such differences are attributable to sub-cloud evaporation processes that mask their Mediterranean origin, and to the strong influence of the orography on the hydrochemical and isotopic characteristics of the rainfall at the local level. The groundwater presents a very stable isotopic content and is strongly depleted in heavy isotopes, evidencing that most of the aquifer recharge takes place at very high altitudes and primarily during winter, when the influence of evaporation is attenuated.

What causes site-specific intra-event variations of stable isotopes in precipitation in Beijing?

Stable oxygen isotope composition (δ18Op) and deuterium excess (dp) in precipitation carry important climate and hydrological information on the history of water from evaporation, transport to precipitation. This study aims to quantify the effect of potential factors on site-specific intra-event δ18Op and dp dynamics during three precipitation events in a mid-latitude region Beijing, China. The tested factors included moisture source (Ms) and upstream rainout fraction (Up) obtained by HYSPLIT backward trajectory model, radar reflectivity factor (Ze) that corresponds to raindrop size by weather radar, sub-cloud evaporation fractionation (fe) by an isotope-evaporation model, local precipitation amount (Lp) and precipitation intensity (I) by rain gauge, and relative location of the sampling sites (downwind or upwind). Intra-event δ18Op had an amplitude of 10.3‰, and 84% of the variance could be explained, in the descending order of importance, by Ms, site, Ze, fe and I. Intra-event dp had an amplitude of 16.5‰, and 57% of the variance could be explained by fe, site, Ms and I, in the descending order of importance. Absolute values of intra-event δ18Op and dp differed between two sites, 30 km apart, by up to 4.7‰ and 9.7‰, respectively. This site-specificity in δ18Op and dp was mainly caused by differences in rainout effects and I. This work provides new insights into separating the effects of various climatic and hydrological variables on high-frequency dynamics of δ18Op and dp.

Moisture sources and climatic effects controlling precipitation stable isotope composition in a western Mediterranean island (Pianosa, Italy)

The Mediterranean basin is indicated as a hot spot of climate change, which is an area whose climate is especially responsive to variations. The insular environment is one of the most threatened by the current climate change, especially in terms of drought events, with serious consequences for water scarcity and water stress. This issue is even enhanced in small islands, whose ecosystems are among more sensitive to climatic changes and water availability. The stable isotope composition of hydrogen (δ2H) and oxygen (δ18O) in precipitation is globally recognized as a powerful natural tracer in the water cycle and represents the starting point to investigate hydrological processes. The understanding of the prevailing factors that drive the isotopic variability of precipitation in the Mediterranean is therefore essential to unravel the hydrological processes and to ensure proper and sustainable management of potentially vulnerable resources to climate change. Here, we discuss the results of multi-year isotopic monitoring in the period 2014–2021 of monthly precipitation collected on Pianosa Island (Italy), a small island located in the northern Tyrrhenian (western Mediterranean). The lower slope and intercept of the Local Meteoric Water Line of the island compared to the Global Meteoric Water Line indicated warmer and drier climatic conditions, suggesting the existence of sub-cloud evaporation processes of raindrops during precipitation, especially in summer. The mean δ18O of precipitation was lower with respect to other sites placed at higher elevation in this Mediterranean region, due to the lack of summer precipitation which were generally enriched in heavy isotopes. Temperature and amount effects may explain part of the δ18O variability observed at the monthly and seasonal scale. An HYSPLIT-based moisture uptake analysis indicated the area between the western Mediterranean basin, Italy, and the Adriatic Sea as the region that supplied most of the humidity associated with monthly precipitation samples on Pianosa Island. Less moisture was picked from the northwestern areas of Europe, the North Atlantic Ocean, the proximal Atlantic Ocean, the Iberian Peninsula and North Africa. Consistently with the rainout effect, the higher the moisture fraction picked from the more proximal regions, the more positive the δ18O of precipitation occurring on Pianosa Island; conversely, the higher the percentage of moisture sourced from more distal regions, the more negative the δ18O. A multiple linear model was proposed to predict the δ18O of monthly precipitation from temperature, precipitation amount and moisture origin data, which explained 45% of the δ18O variability. The deuterium excess variability on the island was partly controlled by the local climatic variables, whose effect potentially modifies the original d-excess signature imprinted at the moisture source. No relationship was found between the precipitation deuterium excess and moisture sources, suggesting that more attention should be paid when using the deuterium excess as a tracer of moisture origin, especially in the Mediterranean.

Earth-system-model evaluation of cloud and precipitation occurrence for supercooled and warm clouds over the Southern Ocean's Macquarie Island

Abstract. Over the remote Southern Ocean (SO), cloud feedbacks contribute substantially to Earth system model (ESM) radiative biases. The evolution of low Southern Ocean clouds (cloud-top heights < ∼ 3 km) is strongly modulated by precipitation and/or evaporation, which act as the primary sink of cloud condensate. Constraining precipitation processes in ESMs requires robust observations suitable for process-level evaluations. A year-long subset (April 2016–March 2017) of ground-based profiling instrumentation deployed during the Macquarie Island Cloud and Radiation Experiment (MICRE) field campaign (54.5∘ S, 158.9∘ E) combines a 95 GHz (W-band) Doppler cloud radar, two lidar ceilometers, and balloon-borne soundings to quantify the occurrence frequency of precipitation from the liquid-phase cloud base. Liquid-based clouds at Macquarie Island precipitate ∼ 70 % of the time, with deeper and colder clouds precipitating more frequently and at a higher intensity compared to thinner and warmer clouds. Supercooled cloud layers precipitate more readily than layers with cloud-top temperatures > 0 ∘C, regardless of the geometric thickness of the layer, and also evaporate more frequently. We further demonstrate an approach to employ these observational constraints for evaluation of a 9-year GISS-ModelE3 ESM simulation. Model output is processed through the Earth Model Column Collaboratory (EMC2) radar and lidar instrument simulator with the same instrument specifications as those deployed during MICRE, therefore accounting for instrument sensitivities and ensuring a coherent comparison. Relative to MICRE observations, the ESM produces a smaller cloud occurrence frequency, smaller precipitation occurrence frequency, and greater sub-cloud evaporation. The lower precipitation occurrence frequency by the ESM relative to MICRE contrasts with numerous studies that suggest a ubiquitous bias by ESMs to precipitate too frequently over the SO when compared with satellite-based observations, likely owing to sensitivity limitations of spaceborne instrumentation and different sampling methodologies for ground- versus space-based observations. Despite these deficiencies, the ESM reproduces the observed tendency for deeper and colder clouds to precipitate more frequently and at a higher intensity. The ESM also reproduces specific cloud regimes, including near-surface clouds that account for ∼ 25 % of liquid-based clouds during MICRE and optically thin, non-precipitating clouds that account for ∼ 27 % of clouds with bases higher than 250 m. We suggest that the demonstrated framework, which merges observations with appropriately constrained model output, is a valuable approach to evaluate processes responsible for cloud radiative feedbacks in ESMs.

Atmospheric processes control the stable isotopic variability of precipitation in the middle–lower reaches of the Yangtze River Basin, East Asian monsoon region

Paleoclimatic records in the East Asian monsoon region (EAM) depend on precipitation isotopes as proxies for past hydroclimatic variability. However, the potential mechanisms controlling precipitation isotopic variability in this region remain poorly understood. This study collected daily precipitation isotopes in the middle-lower reaches of the Yangtze River Basin from 2017 to 2020 to explore how convective behaviors and moisture sources determining the variability of precipitation stable isotopes. Daily precipitation δ18O presented a weak inverse relationship with local precipitation amount and air temperature. Results of correlation analysis showed that the maximum correlation zones of precipitation δ18O between different cloud levels and outgoing long wave radiation were identified in the upstream regions such as the Indian Peninsula, BoB, and Indo-China Peninsula during the rainy season. Back-trajectory analysis showed that substantial water vapor from maritine sources in the Indian Ocean and the Bay of Bengal (BoB) largely contributed to the lower δ18O values during the rainy season (−7.5 ‰) because of intense rainout processes in the upstream sourced regions, demonstrated by significant regional correlation between precipitation δ18O and regional precipitation amount. The regional correlations were stronger in the upstream regions than the local influences, indicating that precipitation isotopes in this region are more influenced by regional convective processes in the upstream region during the rainy season. Using the modified Stewart model, the mean subcloud evaporation fraction estimated was greater in rainy season (24.1 % vs 36.1 % for HK and HZ, respectively) than that in dry season (15.1 % vs 19.9 % for HK and HZ, respectively). These findings highlight the importance of upstream processes in controlling precipitation isotopes in the EAM region and provide reliable evidence on the interpretation of monsoonal climate variability.

Stable isotopic composition of fog and rainfall in a Macaronesian cloud forest

Stable isotopes of water are commonly used to investigate the role of fog in cloud forests. This first requires the characterization of the isotopic signatures of both fog and local rainfall. The isotopic composition of rain and fog water and their variability were analysed for a Macaronesian cloud forest in Tenerife, Canary Islands, from samples collected over 5 years within the Global Network of Isotopes in Precipitation (GNIP) project. The results showed little differences in the isotopic signature of fog and rain with the former being slightly higher with respect to the latter (differences in absolute value of 1‰ in δ18O, 7‰ in δ2H and 0.7‰ in d-excess, on average) and with a similar statistical variability. Their meteoric lines, LMWL (local meteoric water line for precipitation): δ2H = 6.56 δ18O + 11.56 and FWL (fog water line): δ2H = 5.57 δ18O + 9.98, which are very close to each other and above the GMWL (Global Meteoric Water Line), reveals that rain and fog may originate from water evaporated from the same water sources and under moisture conditions typical of semi-arid regions. Different slopes and interception in LMWL and FWL point to seasonal processes affect fog and rain differently, i.e. summer evaporation of small fog droplets and autumn-winter rainfalls isotopically depleted from frontal or low-pressure systems that are decoupled from fog formation processes. Both isotopic signatures showed low correlation with temperature and total rainfall (R2 <0.3 in all cases). However, the high anticorrelation observed (R2 values >0.6) between the isotopic composition of rain and fog with rainfall totals with values between 100 and 150 mm for both fog and rain, associated with a well-defined synoptic rainfall pattern affecting the Canary Islands, supports the hypothesis that the “amount effect” is related to the moisture source region and transport patterns. Annual variability of the isotopic signatures showed a marked pattern, typical of these latitudes, with enriched values in summer and more depleted in winter for both rainfall and fog. The high amplitude observed in the seasonal pattern of d-excess for both rain and fog, point to the Mediterranean Sea as a potential source of moisture in the winter months or to a mixing process when the moist air mass trajectories pass over North Africa. Despite the similarities observed in these seasonal patterns, some anomalies were found in the summer d-excess, indicating the participation of some local processes to which d-excess is sensitive, such as sub-cloud evaporation or moisture recycling.

Isotopic insights on quantitative assessments of interaction of eco-hydrological processes in multi-scale karst watersheds

The dynamics of hydrological processes and the storage mechanisms of karst water resources are the most important issues in karst hydrology. The impact of environmental changes on water quantity, and the evaluation and quantification of eco-hydrological processes remain poorly addressed. In this study, high-frequency continuous monitoring in multi-scale karst watersheds in Southwest China combined the approaches of water isotopes and the hybrid single-particle lagrangian integrated trajectory (HYSPLIT) model to identify the recharge mechanisms between atmospheric vapor, rainfall, surface water, and groundwater, and to reveal the interaction of eco-hydrological processes. The dominant moisture sources in Puding (PD) County were the Indian Ocean (43–69%) and local moisture (24–33%). The δ18O and deuterium excess (d-excess) values showed a positive correlation indicating that secondary or sub-cloud evaporation was prominent in the wet seasons. Karst water line-conditioned excess (lc-excess) indicated that karst water interacted with recent precipitation, groundwater, and evaporation across seasons. Owing to its specific hydrogeological structure, surface water and rainwater have a higher contribution rate to groundwater replenishment. The Chenqi stream replenished the Houzhai River mainly in the form of groundwater, with percentages ranging from 38.1 to 93.5% in the wet season, and 47.8–80.1% in the dry season. In the Houzhai outlet, surface water and groundwater interconverted frequently with a percentage of 45.6–49.1%. We believe this is the first systematic study to quantify the supply relationship between water vapor transport, rainfall, surface water and groundwater in the Chinese karst zone, making a significant move forward in the field of karst hydrological processes and improving the efficiency of water resource evaluation and management.

Environmental drivers of precipitation stable isotopes and moisture sources in the Mongolian Plateau

Τhe absence of precipitation-isotope stations in the wide flatland of the Mongolian Plateau (MP) hinders the analysis of precipitation processes. For this reason, a precipitation-isotope station was established in Erenhot in 2015. Based on precipitation samples collected during all precipitation events between January 2015 and December 2019, we analyzed the temporal variations of stable isotopes (δ2H, δ18O) in precipitation and their influencing factors, including meteorological factors, sub-cloud evaporation, and moisture sources. Oxygen and hydrogen isotopes in precipitation in Erenhot are richer than those in Ulaanbaatar, which is situated close to the mountains and depleted compared to, e.g., Baotou. In Erenhot, oxygen and hydrogen isotopes in precipitation are controlled by the temperature effect, similarly to Ulaanbaatar, but also by the amount effect in summer. The low slope of the local meteoric water line indicates that raindrops have undergone sub-cloud evaporation. The annual mean sub-cloud evaporation fraction was 5.2% from 2015 to 2019, while sub-cloud evaporation fraction was highest in summer (7.5%). Sub-cloud evaporation is mainly correlated by relative humidity, precipitation amount, regional (pan) evaporation, and air temperature. Relative humidity has the greatest effect and masks the effect of air temperature. The study area is mainly controlled by the west wind and winter monsoon, which brought 75.6% of the total precipitation with −7.75‰ δ18O. During summer, the Pacific monsoon resulted in 13.5% precipitation with −10.34‰ δ18O in the study area, while the Indian monsoon had negligible impact. Except for moisture advection, the moisture recycling fraction accounted for 11.1% of local precipitation, with −10.32‰ δ18O. This study compensates for the lack of global network of isotopes in precipitation (GNIP) stations in the MP, reveals the MP precipitation-isotope signatures and their influencing factors, and provides basic precipitation-isotope information for researching the distribution of precipitation-isotope signatures in Asia and the water cycle in the MP.

Variability of precipitation-stable isotopes and moisture sources of two typical landforms in the eastern Loess Plateau, China

Study regionTwo typical landforms in the eastern Loess Plateau, China Study focusHydrogen and oxygen stable isotopes in precipitation are highly efficient tracers for quantitatively identifying the regional water cycle. Based on 237 precipitation event data points from seven precipitation stable isotope monitoring stations in the eastern Loess Plateau (ECLP), this study first presented the spatial-temporal variation of precipitation isotopes. It explored the mainly controlling factor and source of water vapor for regional precipitation. New hydrological insights for the regionThis research found a consistent temporal trend was observed in the ECLP except for the Jiexiu station, characterized by enrichment from December to May and depletion from June to October. Signals of the summer monsoon reaching the ECLP were recorded using precipitation δ18O (δ18Op), especially in July. Relatively smaller slope of the local meteoric water line (LMWL) of summer precipitation reflects that sub-cloud evaporation strongly influences regional precipitation process. δ18Op values did not exhibited significant relationships with air temperature and an apparent "precipitation amount effect" was observed in precipitation samples with precipitation amount less than 10 mm. An apparent "anti-altitude" effect appeared in precipitation samples with the elevation between 400 and 750 m of the ECLP. The sub-cloud evaporation had stronger influence on the precipitation process of the the Fenhe river basin valley (FRV) especially in the Jiexiu station. The water vapor of precipitation in the ECLP mainly originated from the Bohai Sea and the East China Sea in the near-surface and southeast path directions, especially in summer. These studies are essential in regional water resources allocation, especially under the influence of climate change.

Types of Severe Convective Wind Events in Eastern Australia

Abstract Severe winds associated with thunderstorms and convection are a hazard affecting key aspects of society, including emergency management and infrastructure design. Several studies around the world have shown that severe convective winds (SCWs) can occur due to several different processes, in a range of atmospheric environments, with significant regional and temporal variations. However, in eastern Australia, the types of SCWs and their variability have not been assessed outside of individual case studies. Here, a combination of reanalysis, lightning, radar, and station data are used to characterize a set of 36 SCW events in four locations in eastern Australia. These events are objectively chosen based on the strongest measured wind gusts from station data (greater than 25 m s−1) over a 14-yr period, with 6-hourly lightning data and a 30-dBZ radar reflectivity threshold used to infer moist convective processes. Radar data analysis suggests that these SCW events are produced by several different types of parent thunderstorms, with station observations suggesting a range of temporal characteristics for these different event types. A clustering algorithm applied to environmental data is used to suggest three dominant types of events, based on low-level moisture, low-level temperature lapse rate, and deep-layer mean wind speed and vertical shear. Based on the distribution of synoptic conditions and thunderstorm properties for each environmental cluster, it is suggested that these three event types correspond to the following: 1) shallow vertical transport of strong synoptic-scale winds to the surface, 2) downbursts driven by subcloud evaporation, and 3) intense thunderstorms including supercells. Significance Statement The purpose of this study is to better understand the different types of severe wind events in eastern Australia that are produced by convective storms. We looked at 36 historical cases in four locations and find that severe winds can be produced by very different classes of convective storms. We also suggest that there are three key types of atmospheric environment that are associated with events in this region. These environments vary in terms of the vertical structure of temperature, moisture, and wind speed above the surface. Understanding the different types of environments that lead to severe convective winds can help to reduce uncertainties in future climate projections for this region based on environmental changes.

Comparisons of Precipitation Isotopic Effects on Daily, Monthly and Annual Time Scales—A Case Study in the Subtropical Monsoon Region of Eastern China

The study on precipitation isotope variation can potentially improve the understanding of weather processes, regional water cycle and paleoclimate reconstruction in the subtropical monsoon region. Based on the measured stable isotope composition in precipitation (δ18Op) and daily precipitation from January 2010 to December 2021 in Changsha of the subtropical monsoon region of eastern China, the δ18Op variations, amount effect and local meteoric water line (LMWL) were analyzed and compared on daily, monthly and annual time scales, as well as under different precipitation intensities. The results showed that, on the daily time scale, δ18Op was significantly and negatively correlated with precipitation in the study area. Influenced by subcloud evaporation, small precipitation events (≤5 mm/d) could change the rainout level of precipitation isotopes. There were significant differences in the slope and intercept of the LMWL on different time scales, in different seasons and under different precipitation intensities. On the daily and monthly time scales, the slope and intercept of the LMWL in the cold half of the year were significantly smaller and larger than those in the warm half of the year, respectively, and the slope and intercept of the LMWL increased significantly with precipitation intensity, and then remained largely stable. On the annual time scale, the slope and intercept of the LMWL in the cold half of the year were smaller than those in the warm half of the year. The possible reasons for the differences in the LMWL on different time scales are the combined effects of seasonal differences in precipitation intensity and water vapor sources.

Seasonal Lifting Condensation Level Trends: Implications of Warming and Reforestation in Appalachia’s Deciduous Forest

Lifting condensation level (LCL) has long been used to estimate cloud base heights. However, spatial and temporal patterns of cloud bases embedded within atmospheric currents flowing over mountainous terrain still need to be more adequately described. To advance understanding, hourly observations of barometric pressure and ambient and dew point temperatures from 1948 to 2017 were acquired for seven airports located at 40.21° N (average) and crossing the Allegheny Mountains of the northeastern United States. Daily LCL trends were quantified, and large positive (2.3 m yr−1) and negative (−1.3 m yr−1) LCL trends were found to be greatest near seasonal transition dates (17 April and 9 November 2022). Cool season LCLs (795 m) increased significantly (p &lt; 0.007) at five sites resulting in an average LCL increase of 81 m and implying a deeper and drier sub-cloud layer. Average warm season LCLs (773 m) decreased by 23 m, suggesting a deeper convective cloud layer and less sub-cloud evaporation that may facilitate higher hydrometeor growth and precipitation rates. Collective results indicate divergent seasonally averaged LCLs characterized by more rapid seasonal transitions, warmer and less cloudy cool seasons, and cloudier and more humid warm seasons that may be partly attributable to aggressive reforestation and contribute to more significant rainfall events and higher flood risks.

Is the deuterium excess in precipitation a reliable tracer of moisture sources and water resources fate in the western Mediterranean? New insights from Apuan Alps (Italy)

The deuterium excess is a function of the isotopic composition of oxygen and hydrogen in water. Traditionally, the d-excess is used worldwide to constrain the moisture sources of precipitation, as it mainly depends on the relative humidity and surface sea temperature at the evaporative sources, that control diffusion of the water molecules across the density gradient. A case in point is represented by precipitation deriving from vapor originating over the eastern Mediterranean Sea, characterized by the higher d-excess values (∼22‰) compared to the global average d-excess in precipitation of 10‰. Accordingly, d-excess values of ∼14‰ found in precipitation across the western Mediterranean are generally interpreted as the result of mixing of Atlantic and Mediterranean air masses. Nevertheless, several studies proved the d-excess values of precipitation evolves during transport of vapor and changes depending on local processes (e.g. sub-cloud evaporation of raindrops, continental moisture recycling, seeder-feeder mechanism), thus compromising the reliability of d-excess as a proxy of vapor source regions. With the aim to investigate the pattern of d-excess in precipitation at higher spatial and temporal resolution, and to evaluate its reliability for tracing the moisture source in the western Mediterranean, in this work we present the results of a study performed in the Apuan Alps (Tuscany, Italy), one of the rainiest areas in Italy. 19 single rain events were collected from 2020 to 2021 at two sites placed at different altitudes. Furthermore, monthly precipitations were collected at five sites at different elevations over different time intervals from 2019 to 2021. Event-based and monthly Local Meteoric Water Lines were computed at each site, indicating droplets evaporation beneath the clouds, especially at lower altitude sites. d-excess seasonally and spatially varied, with higher values registered at higher altitudes in winter and autumn, and lower values at lower altitudes in spring and summer. Large differences in d-excess were found among sites placed at different altitudes, and these differences ranged both at monthly and event scales. Mean and weighted mean d-excess vertical gradients of +0.73‰/100 m (r2 = 0.92) and +0.51‰/100 m (r2 = 0.95), respectively, were computed using 9 months in which monthly samples were collected at all five sites. This positive relationship between altitude and d-excess in precipitation is consistent with a process like sub-cloud evaporation that appears to be the most important process controlling the d-excess spatial variability in the mountainous areas. These findings evidence the importance of local processes acting on the d-excess and downsize the reliability of this parameter in tracing moisture sources in this area. Conversely, the d-excess vertical gradient, combined with the δ18O vertical gradient (i.e. altitude effect), could be used to identify the recharge areas of groundwater.

An hourly-scale assessment of sub-cloud evaporation effect on precipitation isotopes in a rainshadow oasis of northwest China

In arid northwest China where the precipitation intensity is relatively low, the stable hydrogen and oxygen isotopes (δ2H and δ18O) in precipitation are usually impacted by the sub-cloud evaporation. To understand the sub-cloud evaporation effect under an arid climate, we used the hourly meteorological data at 14 stations in the Kaxgar-Yarkant River Oasis, a rainshadow oasis of northwest China, and estimated the monthly and hourly isotopic change in falling drops from the cloud base to the ground. The results showed that the hourly meteorological observations are an effective way to assess the spatiotemporal pattern of sub-cloud evaporation effect. Across the 14 stations, the annual mean changes in deuterium excess below cloud base ranged from −19.1‰ to −6.8‰. The impacts of sub-cloud evaporation during spring and autumn are larger than those during winter and weaker than those during summer. The exponential regression (R2 = 0.96), instead of linear regression (R2 = 0.85), is better at describing the relationship between the raindrop remaining fraction and the isotopic changes from the cloud base to the ground. The sensitivity analysis of isotopic changes to different relative humidity scenarios shows that the drying scenario may lead to slightly larger sensitivity than the wetting scenario. The impact of sub-cloud evaporation on stable isotope compositions in precipitation may be underestimated when low precipitation events (especially less than 1 mm/h) are not sampled.

Quantifying the contribution of evaporation from Lake Taihu to precipitation with an isotope-based method

ABSTRACT Moisture recycling plays a crucial role in regional hydrological budgets. The isotopic composition of precipitation has long been considered as a good tracer to investigate moisture recycling. This study quantifies the moisture recycling fractions (f r) in the Lake Taihu region using spatial variations of deuterium excess in precipitation (d P) and surface water vapour flux (d E). Results show that d P at a site downwind of the lake was higher than that at an upwind site, indicating the influence of lake moisture recycling. Spatial variations in d P after sub-cloud evaporation corrections were 2.3, 1.4 and 3.2 ‰, and d E values were 27.4, 32.3 and 31.4 ‰ for the first winter monsoon, the summer monsoon and the second winter monsoon, respectively. Moisture recycling fractions were 0.48 ± 0.13, 0.07 ± 0.03 and 0.38 ± 0.05 for the three monsoon periods, respectively. Both using the lake parameterization kinetic fractionation factors or neglecting sub-cloud evaporation would decrease f r, and the former has a larger influence on the f r calculation. The larger f r in the winter monsoon periods was mainly caused by lower specific humidity of airmasses but comparable moisture uptake along their trajectories compared to the summer monsoon period.

Monsoon Fringe Area Moisture Transportation Revealed by Water Stable Isotopes: A Lanzhou River Valley Case Study

Because of global warming, the water cycle has intensified and precipitation has changed, with monsoons evolving to become more complex, particularly in monsoon fringe areas. Therefore, it is important to use stable isotopes to trace moisture sources in monsoon fringe areas, such as semiarid areas. In this study, in situ measurements of water vapor isotopes were obtained in the monsoon fringe areas of Lanzhou (36.1° N, 103.7° E), between May and November 2019 with the aim of analyzing the stable isotope composition and atmospheric water vapor variation to improve our understanding of moisture transport. The following results were obtained: stable isotopic variation of atmospheric water vapor shows an alternation between west and southwest moisture with wind directions and sources of water vapor, with δ18O and δ2H varying from high to low. In the Lanzhou valley, water vapor isotopes show a distinct daily cycle of δ17O, δ18O, and δ2H and are consistent with temperature changes. Water vapor isotopes, particularly d-excess, are correlated with temperature and absolute humidity. With regard to the local atmospheric water vapor line (LAVL), we show that the slope and intercept of the LAVL are affected by sub-cloud secondary evaporation and topography.