Water Vapor Budget Research Articles

Characteristics of Mesoscale Convective Systems and Their Impact on Heavy Rainfall in Indonesia’s New Capital City, Nusantara, in March 2022

Nusantara, the new capital city of Indonesia, and its surrounding areas experienced intense heavy rainfall on 15–16 March 2022, leading to devastating and widespread flooding. However, the factors triggering such intense heavy rainfall and the underlying physical mechanisms are still not fully understood. Using high-resolution GSMaP (Global Satellite Mapping of Precipitation) data, we show that a mesoscale convective system (MCS) was the primary cause of the heavy rainfall event. The rainfall peak occurred during the MCS’s mature stage at 1800 UTC 15 March 2022, and diminished as it entered the dissipation stage. To understand the large-scale environmental factors affecting the MCS event, we analyzed contributions from the MJO, equatorial waves, and low-frequency variability to column water vapor and moisture flux convergence. Results indicate a substantial influence of the MJO and equatorial waves on lower-level (boundary layer) meridional moisture flux convergence during the pre-MCS stage and initiation, with their contributions accounting for up to 80% during the growth phase. Moreover, while La Niña and the Asian monsoon had negligible impacts on MCS moisture supply, we find a large contribution from the residual term of the water vapour budget during the maturation and decay phases of the MCS. This suggests that local forcing (such as small-scale convection, local evaporation, land-surface feedback, and topography) also contributed to modulation of the intensity and duration of the MCS. The results of this study can help in our understanding of the potential causes of extreme rainfall in Nusantara and could be leveraged to improve rainstorm forecasting and risk management across the region in the future.

Dominant spring precipitation anomaly modes and circulation characteristics in the Tarim Basin, Central Asia

Recently, extreme precipitation has occurred frequently in the Tarim Basin, which has a fragile ecological environment, arousing widespread concern. Using daily precipitation observations from 42 stations in the Tarim Basin during the spring of 1980–2021 and monthly circulation reanalysis data from the European Center for Medium-Range Weather Forecasts Reanalysis v5, as well as statistical analyses and physical diagnostic methods, this study investigated the abnormal modes and the evolution characteristics and differences in atmospheric circulation. The results show that the spring precipitation anomalies in the Tarim Basin can be divided into two independent precipitation modes: the first (EOF1) is a precipitation pattern that is uniform throughout the region and the second (EOF2) is an east–west inverse pattern. Thus, there are distinct differences in the atmospheric circulation characteristics responsible for abnormal spring precipitation modes in the basin. When the precipitation across the entire basin is consistently excessive, the 500 hPa geopotential height is affected by the negative phase of the North Atlantic Oscillation related circulation, and anomalous negative geopotential height at 500 hPa and anomalous cyclone at 700 hPa control the entire Tarim Basin, favoring anomalous upward motion, and western and southwestern water vapor transport. This leaves the Tarim Basin with a net water vapor budget and abnormally high atmospheric precipitable water. The opposite situation occurs when the precipitation across the entire basin is consistently lower than normal. When there is a west-to-east precipitation gradient, the western part of the Tarim Basin is affected by the anomalous cyclone while the eastern part is affected by the amomalous anticyclone, leading to the east-west discrepancy. The western region of the Tarim Basin is dominated by upward airflow, whereas the eastern region is dominated by downward airflow, providing dynamic conditions for the west-to-east precipitation gradient. Under the influence of anomalous water vapor transport from the southwest and water vapor convergence, water vapor conditions favorable for precipitation can occur. The net water vapor in the basin also exhibited an abnormal west-to-east transport pattern. Moreover, the atmospheric precipitable water demonstrated an inverse phase distribution under the EOF2 atmospheric precipitation mode in the Tarim Basin.

Comparison of the Water Vapor Budget Evolution of Developing and Non-Developing Disturbances over the Western North Pacific

Tropical cyclone (TC) genesis prediction remains a major operational challenge. Using multiple satellite datasets and a state-of-the-art reanalysis dataset, this study identifies developing and non-developing tropical disturbances over the western North Pacific from June to November of 2000–2019 and conducts composite analyses of their water vapor budget components and relevant dynamic–thermodynamic parameters in the Lagrangian framework following three-day disturbance tracks. Both groups of disturbances have a similar initial 850 hPa synoptic-scale relative vorticity, while the water vapor budget of developing disturbances exhibits distinct stage-wise evolution characteristics from non-developing cases. Three days prior to TC genesis, developing cases are already associated with significantly higher total precipitable water (TPW), vertically integrated moisture flux convergence (VIMFC), and precipitation, of which TPW is the most important parameter to differentiate two groups of disturbances. One day later, all the water vapor budget components (i.e., TPW, VIMFC, precipitation, and evaporation) strengthened, linked with the enhancement of the mid-to lower-tropospheric vortices. A negative radial gradient of evaporation occurs, suggesting the beginning of the wind−evaporation feedback. On the day prior to TC genesis, the water vapor budget components, as well as the mid-to lower-tropospheric vortices, continue to intensify, eventually leading to TC genesis. By contrast, non-developing disturbances are associated with a drier environment and weaker VIMFC, precipitation, and evaporation during the three-day evolution. All these factors are not favorable for the intensification of the mid-to lower-tropospheric vortices; thus, the disturbances fail to upgrade to TCs. The results may shed light on TC genesis prediction.

Responses of precipitation and water vapor budget on the Chinese Loess Plateau to global land cover change forcing

There have been notable changes in precipitation patterns on the Loess Plateau (LP) of China in recent decades, and numerous attribution studies have focused on sea surface temperature anomalies and atmospheric circulation changes induced by aerosols and greenhouse gases emission. However, the influences of global land use and land cover change (LULCC) as an important forcing factor in the climate system on regional precipitation remains poorly understood. In this study, we quantified the impacts of LULCC on precipitation and the water vapor budget in the LP region, utilizing data from LULCC forcing experiments conducted by the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Although global LULCC forcing exerted a negative effect on long-term mean precipitation on the LP region from 1850 to 2014, the different response characteristics were detected during different time periods. The global LULCC caused a decrease of 14 mm in annual precipitation during the period of 1850–1960. Conversely, from 1961 to 2014, it led to an increase of 6.4 mm, which is largely attributed to the enhanced water vapor transport along the southern boundary and westerly belt of the LP region. Moreover, from the perspective of the net water vapor balance of the entire LP, although LULCC caused net water vapor export during both periods 1850–1960 and 1961–2014, the export during the latter period (0.20 × 104 kg s−1) was smaller than that during the former period (0.28 × 104 kg s−1), indicating that the global expansion of grassland and cropland, along with the continuous rise in the leaf area index from 1961 to 2014, contributed to retaining more water vapor within the LP, which in turn was more favorable for precipitation. These findings provide valuable insights into the reasons behind precipitation variations in the LP region, emphasizing that global vegetation restoration and greening play a significant role in improving precipitation in ecologically fragile areas.

Anomalous Characteristics of Water Vapor Budget on the Tibetan Plateau under the Influence of Tropical Cyclones over the Bay of Bengal during Early Summer

Abstract Early summer is a peak time for tropical cyclone (TC) activities over the Bay of Bengal (BoB) and a period of South Asian monsoon onset, and the TCs during this time have a significant impact on the water vapor transport associated with monsoons. This study investigates the anomalous characteristics of the dynamic–thermal atmospheric circulation structure and water vapor budget over the Tibetan Plateau (TP) under the influence of BoB TCs generated in May from 1979 to 2020 with JTWC best track data and ERA5 data. Results reveal that a significant southerly water vapor channel forms from the BoB to the southeastern TP with a water vapor convergence near the Yarlung Zangbo Grand Canyon. A part of the water vapor is transported directly to the TP by deep southerly jet, while the other part is lifted by TCs and then climbs upward to the TP by two uplift processes occurring on the southern slope of the TP and over the TP respectively, which makes the whole troposphere over the southeastern TP warmer and wetter. It is found that anomalous southeasterly airflow in the northeast of TC circulation turns to anomalous southwesterly airflow forming an abnormal anticyclonic circulation over the southern TP in the middle and upper troposphere due to the diabatic heating effect. In this process, the TP acts as an anomalous water vapor sink with remarkable water vapor inflow through its southern boundary, with the main water vapor outflow through the eastern boundary, but a weak easterly water vapor backflow to the eastern TP in the lower troposphere. Significance Statement This study attempts to investigate the anomalous features of the water vapor budget over the Tibetan Plateau (TP) under the influence of the Bay of Bengal (BoB) tropical cyclones (TCs) during early summer. Results show that a significant southerly water vapor channel forms from the BoB to the southeastern TP with a water vapor convergence near the Yarlung Zangbo Grand Canyon. The TP acts as an anomalous water vapor sink with more and higher water vapor inflows through the southern boundary of the TP. A positive temperature and humidity anomaly can be found over the southeastern TP extending upward into the middle and upper troposphere. The results are helpful to understand how the BoB TCs affect the weather process over the TP.

Aerosol forcing regulating recent decadal change of summer water vapor budget over the Tibetan Plateau

The Tibetan Plateau (TP), known as the Asian water tower, has been getting wetter since the 1970s. However, the primary drivers behind this phenomenon are still highly controversial. Here, we isolate the impacts of greenhouse gases (GHG), aerosols, natural forcings and internal climate variability on the decadal change of summer water vapor budget (WVB) over the TP using multi-model ensemble simulations. We show that an anomalous Rossby wave train in the upper troposphere travelling eastward from central Europe and equatorward temperature gradient in eastern China due to the inhomogeneous aerosol forcing in Eurasia jointly contribute to anomalous easterly winds over the eastern TP. Such anomalous easterly winds result in a significant decrease in water vapor export from the eastern boundary of the TP and dominate the enhanced summer WVB over the TP during 1979-2014. Our results highlight that spatial variation of aerosol forcing can be used as an important indicator to project future WVB over the TP.

Water vapour exchange between the atmospheric boundary layer and free troposphere over eastern China: seasonal characteristics and the El Niño–Southern Oscillation anomaly

Abstract. This study develops a quantitative climatology of water vapour exchange between the atmospheric boundary layer (ABL) and free troposphere (FT) over eastern China. The exchange flux is estimated for January, April, July and October over 7 years based on a water vapour budget equation using simulated meteorological data. The spatiotemporal characteristics and occurrence mechanism of ABL–FT water vapour exchange and its relationship with the El Niño–Southern Oscillation (ENSO) are revealed: (1) the vertical exchange flux varies regionally and seasonally, with downward transport to maintain ABL moisture during winter and autumn in the northern region and persistent output to humidify the FT in the southern region, particularly in summer. Additionally, the vertical exchange flux is also topographic dependent. (2) The vertical motion at the ABL top, which is produced by the dynamic forcing of the terrain on synoptic winds, is the dominant mechanism for the water vapour vertical exchange over the long-term average. The evolution of the vertical exchange flux within 1 d scale is driven by the ABL diurnal cycle. (3) The interannual variation of water vapour vertical exchange is correlated with ENSO. A triple antiphase distribution with negative–positive–negative anomalies from north to south exists in La Niña years (and vice versa in El Niño years), which corresponds to the spatial pattern of anomalous precipitation. This phenomenon is mainly due to the alteration of vertical velocity and water vapour content at the ABL top varying with ENSO phases. These results provide new insight into understanding the atmospheric water cycle.

The dehydration carousel of stratospheric water vapor in the Asian summer monsoon anticyclone

Abstract. During the StratoClim Geophysica campaign, air with total water mixing ratios up to 200 ppmv and ozone up to 250 ppbv was observed within the Asian summer monsoon anticyclone up to 1.7 km above the local cold-point tropopause (CPT). To investigate the temporal evolution of enhanced water vapor being transported into the stratosphere, we conduct forward trajectory simulations using both a microphysical and an idealized freeze-drying model. The models are initialized at the measurement locations and the evolution of water vapor and ice is compared with satellite observations of MLS and CALIPSO. Our results show that these extremely high water vapor values observed above the CPT are very likely to undergo significant further freeze-drying due to experiencing extremely cold temperatures while circulating in the anticyclonic “dehydration carousel”. We also use the Lagrangian dry point (LDP) of the merged back-and-forward trajectories to reconstruct the water vapor fields. The results show that the extremely high water vapor mixed with the stratospheric air has a negligible impact on the overall water vapor budget. The LDP mixing ratios are a better proxy for the large-scale water vapor distributions in the stratosphere during this period.

Characteristics of Water Vapor Transport for Extreme Summer Precipitation in the Eastern Southwest China and Its Impact Mechanism

To improve understanding of the characteristics of extreme summer rainfall and its water vapor transport in the eastern part of southwestern China (ESWC), this study analyzed data on daily precipitation from 118 meteorological stations in the ESWC from 1979 to 2020, as well as daily reanalysis data from ERA5 and daily reanalysis data from NCEP/NCAR. The study employed polynomial fitting, correlation, regression, clustering, and mixed single-particle Lagrangian trajectory (HYSPLITv5.0) modeling methods to simulate extreme summer precipitation and its water vapor transport characteristics in the ESWC and its possible formation mechanism. The results show that: (1) The contribution rate of extreme precipitation in the ESWC from 1979 to 2020 varied significantly on the interannual time scale. When the number of extreme precipitation days is high (low), the contribution rate of extreme precipitation is also high (low), while the contribution rate of general precipitation (the percentage of the sum of general precipitation to the total summer precipitation of that year) is often low (high). (2) When extreme precipitation occurs in the ESWC, compared with general precipitation, the high-level potential vortices are stronger, and the cold air from higher latitude is more likely to move southward. Meanwhile, the amount of water vapor input to the region is significantly larger than that of general precipitation. (3) There are four channels of water vapor sources in the ESWC during the period of extreme precipitation: the Bay of Bengal, the Arabian Sea, the western Pacific, and the northwest. The contribution of water vapor from the Bay of Bengal is the highest. The number of extreme summer precipitation days in the ESWC is significantly negatively correlated with the water vapor budget of the eastern boundary and positively correlated with Indian Ocean Basin-Wide (IOBW) index in the previous winter. (4) When the winter SST is high in the IOBW mode, it can cause the western Pacific subtropical high and the South Asian high to be stronger and shifted southward in summer, resulting in an increase in the number of extreme precipitation days in the ESWC.

Factors affecting interannual variation in late summer rainfall in the Red River Delta of Vietnam

Based on station monthly observational statistics, the major rainfall in the Red River Delta (RRD) of Vietnam occurs in late summer (July–September) with conspicuous year-to-year variation. Using the ± 0.8 of the seasonal total rainfall standard deviation as criteria, seven wet and six dry years are identified over the period 1983–2015. In addition to the 70.5% of the seasonal total rainfall contributed by the heavy rainfall days, the distinct heavy rainfall accumulation difference between wet and dry years seems to fundamentally establish these two separated extreme wet and dry groups. As revealed from further analyses, the large variability in rainfall is attributed to the influence of tropical cyclones (TCs) and 7–24- and 30–60-day intraseasonal oscillations (ISOs); in particular, the number of TCs affecting the RRD and rainfall produced by TCs are more (less) during the wet (dry) years, and the amplitudes of ISOs are also enhanced (reduced). In many cases, heavy rainfall days are induced by the combined effect of both ISOs and TCs, while some heavy rainfall events are mainly triggered by ISOs. It is found from the water vapor budget analyses that an anomalous cyclone (anticyclone) dominates over the Indochina Peninsula in wet (dry) years, resulting in more (less) water vapor being transported to the RRD, whereas the anomalous convergence (divergence) of water vapor flux leads to the maintenance of excessive (insufficient) rainfall over the RRD. However, the El Niño–Southern Oscillation (ENSO) forcing shows minor effects on the interannual variation in rainfall in the RRD.

Increased southerly and easterly water vapor transport contributed to the dry-to-wet transition of summer precipitation over the Three-River Headwaters in the Tibetan Plateau

The Three-River Headwaters (TRH) region in the Tibetan Plateau is vulnerable to climate change; changes in summer (June–August) precipitation have a significant impact on water security and sustainability in both local and downstream areas. However, the changes in summer precipitation of different intensities over the TRH region, along with their influencing factors, remain unclear. In this study, we used observational and ERA5 reanalysis data and employed a precipitation categorization and water vapor budget analysis to quantify the categorized precipitation variations and investigate their possible linkages with the water vapor budget. Our results showed an increasing trend in summer precipitation at a rate of 0.9 mm per year (p < 0.1) during 1979–2020, with a significant dry-to-wet transition in 2002. The category ‘very heavy precipitation’ (≥10 mm d−1) contributed 65.1% of the increased summer precipitation, which occurred frequently in the northern TRH region. The dry-to-wet transition was caused by the effects of varied atmospheric circulations in each subregion. Southwesterly water vapor transport through the southern boundary was responsible for the increased net water vapor flux in the western TRH region (158.2%), while southeasterly water vapor transport through the eastern boundary was responsible for the increased net water vapor flux in the central TRH (155.2%) and eastern TRH (229.2%) regions. Therefore, we inferred that the dry-to-wet transition of summer precipitation and the increased ‘very heavy precipitation’ over the TRH was caused by increased easterly and southerly water vapor transport.

Interannual variation of midsummer precipitation in the middle reaches of Yarlung Zangbo River and its possible mechanisms

AbstractHydroclimatic process in Yarlung Zangbo River (YZR) plays a critical role in the water cycle of the whole Tibetan Plateau (TP), but there is a lack of understanding about it due to restrictive conditions. In this study, we further investigate the interannual variation characteristics and possible factors of midsummer (July–August) precipitation in the middle reaches of YZR (MYZR) based on the monthly precipitation data from four stations (Lhasa, Xigaze, Tsedang and Gyangze) in the Inner TP and ERA5 reanalysis dataset for the years 1961–2017. The results suggest that precipitation shows no obvious linear trend during the research period but is dominated by interannual oscillation with evident cycles of 3 years. Meanwhile, the interannual variation of precipitation is related to local changes of water vapour budget. The anomalous anticyclones over the Indian Peninsula‐Southeast Asia continuously transport water vapour from the western Pacific, South China Sea and Bay of Bengal (BOB) to the TP. Simultaneously, both the monsoon low and South Asia high in southern TP provide dynamic conditions for the convergence and uplift of water vapour.

Convective Impact on the Global Lower Stratospheric Water Vapor Budget

AbstractWater vapor in the stratosphere is primarily controlled by temperatures in the tropical upper troposphere and lower stratosphere. However, the direct impact of deep convection on the global lower stratospheric water vapor budget is still an actively debated issue. Two complementary modeling approaches are used to investigate the convective impact in boreal winter and summer. Convective influence is diagnosed by tracing trajectories through convective cloud top altitude fields derived from global rainfall and brightness temperature data. Backward trajectory model simulations coupled with a detailed treatment of cloud microphysical processes indicate that convection moistens the global lower stratosphere by approximately 0.3 ppmv in boreal winter and summer 2010. The diurnal peak in convection is responsible for about half of the total convective moistening during winter and nearly all of the convective moistening during summer. Deep convective clouds overshooting the tropopause have relatively minor effect on global lower stratospheric water vapor. A forward trajectory model coupled with a simplified cloud module is used to estimate the relative magnitude of the interannual variability of the convective impact. Combining the results from the two models, we find that the convective impact on the global lower stratospheric water vapor during 2006–2016 is approximately 0.3 ppmv with year‐to‐year variations of up to 0.1 ppmv. An important mechanism of convective hydration of the lower stratosphere is via the detrainment of saturated air and ice into the tropical uppermost troposphere and the subsequent upward transport of some of these moist air parcels across relatively warm and subsaturated tropopause.

Enhanced ENSO-Unrelated Summer Variability in the Indo–Western Pacific under Global Warming

Abstract El Niño–Southern Oscillation (ENSO) is an important but not the only source of interannual variability over the Indo–western Pacific. Non-ENSO forced variability in the region has received recent attention because of the implications for rainy-season prediction. Using a 35-member CESM1 Large Ensemble (CESM-LE) and 30 CMIP6 models, this study shows that the ensemble means project intensified interannual variability for precipitation, low-level winds, and sea level pressure under global warming, associated with the enhanced large-scale anomalous anticyclone (AAC) over the tropical northwestern (NW) Pacific after the ENSO signal is removed. A decomposition based on the column water vapor budget reveals that enhanced precipitation variability is due to the increased background specific humidity. The resultant anomalous diabatic heating intensifies the AAC, which further strengthens the precipitation anomalies. Over the tropical NW Pacific, the wind-induced evaporative cooling on the southeastern flank of the AAC is countered by the increased shortwave radiation due to the strengthened precipitation reduction. Tropospheric temperature anomalies in the ensemble means show no significant change, suggesting no apparent change of the interbasin positive feedback between the AAC and northern Indian Ocean SST. Intermodel analysis based on CMIP6 reveals that models with a larger increase in ENSO-unrelated precipitation variability over the NW Pacific are associated with stronger background warming in the eastern equatorial Pacific, due to the modulated Walker and Hadley circulations.

Water Vapor Budget Features and Its Precipitation Effects over the Key Area of Sichuan-Tibet Railway in Summer

In this study, we investigate the variation of characteristics of summer precipitation with different magnitudes and the water vapor budget in different key areas of the Sichuan-Tibet Railway by using the daily precipitation data from meteorological stations and the monthly mean ERA-Interim reanalysis data from 1979 to 2018. The results show that the summer heavy precipitation anomaly in the Brahmaputra valley and mountain (BVM) area is generated by the multi-scale interactions between the unique topography and different water vapor transports at low latitudes. When an intense anticyclonic circulation occurs over the western Pacific and the South China Sea, there is also an anomalous anticyclonic circulation extending from the northern Indian Peninsula to the Bay of Bengal. This circulation pattern is conductive to the increase of the water vapor inflow from the southern boundary and the net water vapor budget in the BVM area. The strong southwesterly wind and water vapor convergence over this area thus leading to heavy precipitation in the summer. The summer heavy precipitation amount, frequency and intensity in the western parts of the steep terrain (ST) area on the east slope of the plateau are positively correlated with the water vapor inflow from its western boundary. However, the correlations of the water vapor budget to the summer heavy precipitation amount and frequency are opposite in the eastern parts of the ST area. The unique terrain and circulation patterns lead to the localization and diversity of heavy precipitation in the ST area.

Diurnal Characteristics in Summer Water Vapor Budget and Transport over the Tibetan Plateau

Using the ERA5 reanalysis dataset during the period 1979–2019, the diurnal variation in summer water vapor budget (Bt) over the Tibetan Plateau (TP) is investigated in this study. It is found that the TP Bt shows a distinct diurnal cycle. It tends to increase in the morning, reaches a peak in the afternoon, and falls to a minimum in the early morning. The diurnal variations in four boundary water vapor budgets of the TP contribute to the growth in the TP Bt from the early morning to the afternoon, of which the western and eastern boundaries are more important. To understand the reasons for the diurnal variations in boundary water vapor budgets, the temporal evolutions of water vapor transports and relevant circulations at the four boundaries are examined. The results show that the temporal evolutions of water vapor transports and budgets at the four boundaries are essentially regulated by the changes in the orographic thermodynamic effect. Specifically, rapid and strong warming (cooling) on the TP slopes generates anomalous water vapor inputs (outputs) by anomalous upslope (downslope) flows during the daytime (nighttime). At the southern and western boundaries, apart from the terrain effects, the diurnal variation in the Indian southerly monsoon also has an effect on the changes in water vapor budgets by modulating the water vapor input towards the TP below 700 hPa. At the northern and eastern boundaries, under the orographic thermodynamic effects, low-level water vapor transports towards the TP accompanying by plateau-scale vertical circulations, exist significant diurnal variations and thereby adjust the boundary water vapor budgets. In this study, it is also found that the deviated water vapor flux vectors over the TP present a daily clockwise rotation, which mainly results from the diurnal variation in wind below 450 hPa. In addition, the largest amount of precipitation over the TP occurs 2–3 h after the Bt peak.

Changes of Water Vapor Budget over East Asia in Response to 4xCO2 Concentration Forcing

Water resources are essential for the economic development and social security in East Asia, especially under global warming. Based on newly released CMIP6 149-year simulation data from a pre-industrial control experiment (piControl) and a forced experiment on the abrupt quadrupling of CO2 concentration (abrupt-4xCO2), changes of water vapor budget over East Asia due to 4xCO2 concentration forcing and their possible mechanisms are investigated. Change of precipitation (P) demonstrates a spatial pattern of “Southern Flood and Northern Drought” (SFND) in eastern China, which can also be seen in the change of evaporation (E), though at a much smaller amplitude. The change of water vapor budget represented by E–P is dominated by P, which is primarily induced by changes of water vapor divergence associated with both moisture-related thermodynamic contribution and atmospheric circulation-related dynamic contribution. Specifically, under global warming, tropical El Nino-like SST warming causes weakened Walker circulation through decreased zonal temperature gradient, while amplified Arctic warming induces a negative Arctic Oscillation pattern via reduced meridional temperature gradient. The combined signals from tropical and mid-high latitudes result in significant long-term changes of water vapor convergence as well as much more precipitation in the Yangtze River Valley, forming the SFND. Furthermore, the intensity of the SFND change pattern could also have notable interdecadal variation, which is mainly attributed to the modulation of interdecadal signals of the Indian Ocean basin mode (IOBM) and Pacific Decadal Oscillation (PDO). Results of this study could provide an important scientific basis for the future planning and management of water resources over East Asia, specifically in eastern China.

Revisiting the variations of precipitation and water vapour budget over the Tibetan Plateau

Owing to the scarcity of observation data in the western Tibetan Plateau (TP), the knowledge of precipitation changes over the entire plateau based only on the limited data in eastern TP is not reliable. Therefore, the alternative high-resolution precipitation data of the China Meteorological Forcing Dataset (CMFD) are used for the comprehensive analysis of precipitation changes over the whole TP (including western and northern TP) to fill in the lack of understanding of precipitation in the western TP. Compared with observations, CMFD can broadly capture the spatial distributions and identify the temporal variabilities of precipitation over the TP. Results with CMFD data suggested that the annual precipitation over the whole TP did not show a uniform humidification trend in 1979–2018 and featured wetting and drying trends in the northern (NTP) and southern TP (STP), respectively. Additionally, the four seasonal regimes of precipitation over the northern TP (NTP, including most areas of western TP) all experienced a noticeable interdecadal shift around the late 1990s, followed by above-normal precipitation. Except for spring, the seasonal precipitation over the southern TP (STP) showed interannual variations. Spring precipitation over the STP has undergone moistening since the late 1990s, which was consistent with that over the NTP. Four different reanalysis datasets, namely JRA55, MERRA2, ERA5 and CRA40, were used to compare the water vapour budget of each boundary over the TP. The increase in spring precipitation over the NTP and STP was found to be related to the decrease in water vapour outflow from the north boundary. The interdecadal increase in summer precipitation over the NTP was mainly due to the reduction of outflow from the east boundary. Finally, the increase in autumn precipitation was related to the increase in inflow from the west boundary.

Tropical cyclone over the western Pacific triggers the record-breaking ‘21/7’ extreme rainfall in Henan, central-eastern China

During 19–21 July 2021, Henan located in central-eastern China experienced torrential rainfall that caused devastating floods and claimed more than 300 casualties. It remains unclear whether and to what extent this extreme precipitation event is contributed by Typhoon In-Fa (TIF). Here we quantify the contribution of TIF to this record-breaking ‘21/7’ rainfall using an air–sea coupled model with ensemble simulations. The modeling results show that the northwestward moisture transport along the confluence front of TIF and the western Pacific subtropical high (WPSH) contribute mostly to the precipitation extremes across Henan. A sensitivity experiment that removes the TIF effect confirms TIF’s role in shaping extreme rainfall. Specifically, without TIF, WPSH shifts the moisture transport northeastward and causes a heavy rainband over the Korean Peninsula, with much less precipitation over Henan. The water vapor budget over Henan suggests that the TIF-induced moisture advection is nine times greater than the local moisture supply and is effectively converted into clouds therein to reinforce precipitation extremes. The contribution of TIF to the ‘21/7’ Henan torrential rainfall on average could be as large as 42% by comparing the differences between simulated results with and without TIF’s effects.

Intercomparison of upper tropospheric and lower stratospheric water vapor measurements over the Asian Summer Monsoon during the StratoClim campaign

Abstract. In situ measurements in the climatically important upper troposphere–lower stratosphere (UTLS) are critical for understanding controls on cloud formation, the entry of water into the stratosphere, and hydration–dehydration of the tropical tropopause layer. Accurate in situ measurement of water vapor in the UTLS however is difficult because of low water vapor concentrations (&lt;5 ppmv) and a challenging low temperature–pressure environment. The StratoClim campaign out of Kathmandu, Nepal, in July and August 2017, which made the first high-altitude aircraft measurements in the Asian Summer Monsoon (ASM), also provided an opportunity to intercompare three in situ hygrometers mounted on the M-55 Geophysica: ChiWIS (Chicago Water Isotope Spectrometer), FISH (Fast In situ Stratospheric Hygrometer), and FLASH (Fluorescent Lyman-α Stratospheric Hygrometer). Instrument agreement was very good, suggesting no intrinsic technique-dependent biases: ChiWIS measures by mid-infrared laser absorption spectroscopy and FISH and FLASH by Lyman-α induced fluorescence. In clear-sky UTLS conditions (H2O&lt;10 ppmv), mean and standard deviations of differences in paired observations between ChiWIS and FLASH were only (-1.4±5.9) % and those between FISH and FLASH only (-1.5±8.0) %. Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at (+0.7±7.6) %. Estimated realized instrumental precision in UTLS conditions was 0.05, 0.2, and 0.1 ppmv for ChiWIS, FLASH, and FISH, respectively. This level of accuracy and precision allows the confident detection of fine-scale spatial structures in UTLS water vapor required for understanding the role of convection and the ASM in the stratospheric water vapor budget.