Water Vapor Sink Research Articles

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.

Water vapor and cloud microphysical characteristics of a sea-effect snowstorm in Shandong Peninsula, China

Sea-effect snowfall is a special form of snow in the coastal areas of the Bohai Sea in China. However, a quantitative understanding of its water vapor and cloud microphysical mechanisms is still lacking. In this study, the Weather Research and Forecasting model version 4.0 with three microphysics schemes was employed to investigate a sea-effect snowstorm that occurred in the north of Shandong Peninsula on 9–10 January 2018. The simulated snowfall amount, temperature and humidity profiles, and radar reflectivity were in general agreement with the observations. The moisture budget revealed that the water vapor came completely from evaporation of the surface of Bohai Sea. The largest water vapor sink term was the moisture advection, followed by the deposition and condensation (accounting for only 15% of the sea surface evaporation). This phenomenon is analogous to heating water in a kettle and then the water vapor drifting away with the wind. In the main snowfall area, the total water vapor transport was positive (i.e., inflow) below the height of 1.5 km, and was almost zero above 3.0 km. The period of maximum moisture inflow in the low layer was about 30 min ahead of the maximum surface snowfall. In addition, the hydrometeor distributions in this sea-effect snowstorm were relatively simple compared to those in non-sea-effect snowstorm; the snow content was the most (0.09 g kg−1), followed by the supercooled cloud water (0.01 g kg−1), owing to the weak convection in it. Snow growth was dominated by the vapor deposition, and the process of rimming accounted for only 1/4 of the vapor deposition. The evidently weaker vapor condensation than vapor deposition is quite different from that in summertime heavy rainfall.

Summertime atmospheric water vapor transport between Tibetan Plateau and its surrounding regions during 1990–2019: Boundary discrepancy and interannual variation

The summertime atmospheric water vapor transport (WVT) of the Tibetan Plateau (TP) and surrounding regions is investigated based on the ERA5 reanalysis data during 1990–2019. It is found that the ascending motion on the south of the TP is conducive to the transport of water vapor from the low-latitude monsoon regions to the TP, showing strong WVT at 500 hPa on the southern TP. Strong water vapor transporting into the TP exists on the southern boundary (Bs) which is from Kashmir to the trumpet-shaped topography (TST) region, and from upper to lower layers. The WVT of the other three boundaries exhibit differently: (1) The prevailing westwards WVT exists on the western boundary. (2) The water vapor flows into the TP below 700 hPa on the Sichuan Basin and transports away from the TP above 700 hPa on the eastern boundary. (3) A regular water vapor “flows out of-into-out of” the TP pattern is found on the northern boundary. The most active water vapor transporting towards the TP appears on the Sichuan Basin, TST and eastern Bs in the lower layer, while the most active water vapor transporting away from the TP occurs on the eastern boundary and middle-east Bs in the middle layer. In general, water vapor inflows of the TP are larger than its outflows, resulting in summertime TP becoming a water vapor sink (153.28 × 106 kg s−1). The result is mainly attributed to the water vapor inflows in the middle-lower layer on the Bs. The WVB and PW over the summer TP present the increasing trends during the recent years and the trend of the PW is significant.

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.

Variation Characteristics of Summer Water Vapor Budget and Its Relationship with the Precipitation over the Sichuan Basin

Based on the daily precipitation data from the meteorological stations in Sichuan and the monthly average ERA-Interim reanalysis data from 1979 to 2016, the variation characteristics of summer water vapor budget in the Sichuan Basin and its relationship with precipitation are discussed in this study. The results show that, in summer, the water vapor in the Sichuan Basin and its four sub-basins flows in from the southern and western boundaries and flows out through the eastern and northern boundaries, and the basin is obviously a water vapor sink. From 1979 to 2016, the water vapor inflow from the southern and western boundaries significantly decreased, as well as the water vapor outflow through the eastern boundary. The summer precipitation in the Sichuan Basin is significantly positively correlated with the water vapor inflow at the southern boundary and net water vapor budget of the basin in the same period, and it is negatively correlated with the water vapor outflow at the northern boundary. The southern and northern boundaries are the two most important boundaries for the summer precipitation in the Sichuan Basin. Additionally, this study reveals that, under the multi-scale topography on the east side of the Tibet Plateau, the spatio-temporal distribution of precipitation in the Sichuan Basin results from the interactions between the unique topography of the Sichuan Basin and the different modes of water-vapor transport from low latitudes. The atmospheric circulation over the key area of air–sea interaction in the tropical region and its accompanying systems, as well as the anomalies of regional circulations and water vapor transport over the eastern China and Sichuan Basin, are the main reasons for the variation in summer precipitation in the Sichuan Basin.

Mean state of global and regional ocean water cycle based on ERA-interim reanalysis

Based on ERA-interim reanalysis dataset from 1979 to 2018, this study revisits the global and regional water cycle, as well as the water vapor transport between ocean basins. The surface of the Earth is divided into five continents and four ocean basins, water vapor budget, and its seasonal variability of each continent and ocean basin are calculated. The Indian Ocean and the Atlantic are major net water vapor source regions, ~ − 0.47 Sv and ~ −0.82 Sv, respectively, while the Pacific and the Arctic are weak net water vapor sink regions, ~ 0.04 Sv and ~ 0.06 Sv, respectively. A total of 85% of evaporation and 78% of precipitation occur at the ocean-atmosphere interface while 15% of evaporation and 22% of precipitation occur at the land-atmosphere interface. About 1.19 Sv water vapor is carried out from oceans to lands. The water vapor transport is about 0.07 Sv from the Indian Ocean to the Pacific, and 0.22 Sv from the Atlantic to the Pacific through the segments. At low latitudes, both the Atlantic and the Indian Ocean transport water vapor to the Pacific, with a total of 0.29 Sv. Those results update the estimate of global and regional water vapor budget.

Investigating the Interannual Variability of the Boreal Summer Water Vapor Source and Sink over the Tropical Eastern Indian Ocean-Western Pacific

Using the four-times daily and monthly-mean reanalysis datasets of NCEP/NCAR for the 1958 to 2018 period, we investigate the interannual variability of the June-July-August (JJA)–mean water vapor source and sink over the tropical eastern Indian Ocean-Western Pacific (TEIOWP) and the underlying mechanism. It is found that the two major modes (EOF1 and EOF2) of the water vapor source and sink anomalies over the TEIOWP present a southwest-northeast oriented dipole and a southwest-northeast oriented tripole. Specifically, when the western maritime continent shows an anomalous water vapor source, the northwestern Pacific is characterized by anomalous water vapor sink and source in EOF1 and EOF2 modes, respectively. The EOF1 and EOF2 modes are primarily driven by a single and a double meridional cell anomaly, which corresponds to the in-phase and out-of-phase linkage between evaporation anomalies over the western maritime continent and precipitation anomalies over the northwestern Pacific, respectively. Furthermore, the EOF1 mode is regulated by the quick transition of the El Niño-Southern Oscillation (ENSO) phase, whereas the EOF2 mode probably originates from internal atmospheric variability. Considering that the standard deviation of PC1 is much higher during ENSO years than that during non-ENSO years, it is probable that the water source and sink anomalies over the TEIOWP tend to be dominant by EOF1 mode during ENSO years. In contrast, the EOF2 mode may play an important role in the water source and sink anomalies over the TEIOWP during non-ENSO years. Accordingly, the water vapor source and sink anomalies over the TEIOWP may be well predicted based on the ENSO state in the previous December-January-February. These results are useful for understanding the predictability of water vapor source and sink anomalies over the TEIOWP.

Solid-solid hydration and dehydration of Mars-relevant chlorine salts: Implications for Gale Crater and RSL locations

Water vapor is likely being exchanged between the regolith and the atmosphere on Mars, according to evidence from multiple landing sites (including Gale Crater), satellite measurements and numerical modeling. The mechanism of this exchange is largely unknown but could involve the formation of water frost, the adsorption of thin films of water onto mineral surfaces, or the deliquescence or hydration of salts. Hydration, a solid-solid phase transition during which water molecules are incorporated into a crystal structure in stoichiometric amounts, is possible for many salts found at Gale Crater and elsewhere on Mars. These salts may therefore be acting as a source and sink for water vapor in the Martian regolith. Furthermore, salt hydration state may be used as a marker for the presence of liquid water on present-day Mars. For example, the hydrated, crystalline perchlorate and chloride salts detected at recurring slope lineae (RSL) locations have been proposed to form from liquid brines rather than by experiencing hydration by atmospheric water vapor. Here we use an environmental cell coupled to a Raman microscope to experimentally study the hydration and dehydration of magnesium chloride (MgCl2), calcium perchlorate (Ca(ClO4)2), and calcium chloride (CaCl2) with the goal of determining which of these salts are capable of experiencing hydration on diurnal time scales. Other potential hydration phase transitions of chlorine-containing salts are thought to be less likely. Specifically, we study the transition between magnesium chloride tetrahydrate and hexahydrate, anhydrous calcium perchlorate and hydrated calcium perchlorate, and calcium chloride dihydrate and hexahydrate. We find that under conditions measured by the REMS instrument at Gale Crater, some chlorine-containing salts can readily hydrate and other salts can readily dehydrate, but no salt system studied here is likely to undergo both processes at the surface on diurnal time scales. With respect to RSL formation, these experiments suggest that atmospheric hydration of these salt systems may be too slow or otherwise not feasible. Hence, hydrated salts formed recently at active RSL locations on Mars may indeed be an indicator of the presence of liquid water.

New measurements of D/H on Mars using EXES aboard SOFIA

The global D/H ratio on Mars is an important measurement for understanding the past history of water on Mars; locally, through condensation and sublimation processes, it is a possible tracer of the sources and sinks of water vapor on Mars. Measuring D/H as a function of longitude, latitude and season is necessary for determining the present averaged value of D/H on Mars. Following an earlier measurement in April 2014, we used the Echelon Cross Echelle Spectrograph (EXES) instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) facility to map D/H on Mars on two occasions, on March 24, 2016 (Ls = 127°), and January 24, 2017 (Ls = 304°), by measuring simultaneously the abundances of H2O and HDO in the 1383–1391 cm−1 range (7.2 μm). The D/H disk-integrated values are 4.0 (+0.8, −0.6) × Vienna Standard Mean Ocean Water (VSMOW) and 4.5 (+0.7, −0.6) × VSMOW, respectively, in agreement with our earlier result. The main result of this study is that there is no evidence of strong local variations in the D/H ratio nor for seasonal variations in the global D/H ratio between northern summer and southern summer.

Interdecadal Climate Variations Controlling the Water Level of Lake Qinghai over the Tibetan Plateau

Abstract Using observed and reanalysis data, the pronounced interdecadal variations of Lake Qinghai (LQH) water levels and associated climate factors were diagnosed. From the 1960s to the early 2000s, the water level of LQH in the Tibetan Plateau has experienced a continual decline of 3 m but has since increased considerably. A water budget analysis of the LQH watershed suggested that the water vapor flux divergence is the dominant atmospheric process modulating precipitation and subsequently the lake volume change . The marked interdecadal variability in and was found to be related to the North Pacific (NP) and Pacific decadal oscillation (PDO) modes during the cold season (November–March). Through empirical orthogonal function (EOF) and regression analyses, the water vapor sink over the LQH watershed also responds significantly to the summer Eurasian wave train modulated by the low-frequency variability associated with the cold season NP and PDO modes. Removal of these variability modes (NP, PDO, and the Eurasian wave train) led to a residual uptrend in the hydrological variables of , , and precipitation, corresponding to the net water level increase. Attribution analysis using the Coupled Model Intercomparison Project phase 5 (CMIP5) single-forcing experiments shows that the simulations driven by greenhouse gas forcing produced a significant increase in the LQH precipitation, while anthropogenic aerosols generated a minor wetting trend as well.

A map of D/H on Mars in the thermal infrared using EXES aboard SOFIA

On a planetary scale, the D/H ratio on Mars is a key diagnostic for understanding the past history of water on the planet; locally, it can help to constrain the sources and sinks of water vapor through the monitoring of condensation and sublimation processes. To obtain simultaneous measurements of H2O and HDO lines, we have used the Echelle Cross Echelle Spectrograph (EXES) instrument aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) facility to map the abundances of these two species over the Martian disk. High-resolution spectra (R = 6 × 104) were recorded in the 1383−1390 cm-1 range (7.2 μm) on April 08, 2014. Mars was very close to opposition and near northern summer solstice (Ls = 113°). Maps of the H2O and HDO mixing ratios were retrieved from the line depth ratios of weak H2O and HDO transitions divided by a weak CO2 line. As expected for this season, the H2O and HDO maps show a distinct enhancement toward polar regions, and their mixing ratios are consistent with previous measurements and with predictions by the global climate models, except at the north pole where the EXES values are weaker. We derive a disk-integrated D/H ratio of 6.8 (+1.6, −1.0) × 10-4. It is higher than the value in Earth’s oceans by a factor 4.4 (+1.0, −0.6). The D/H map also shows an enhancement from southern to northern latitudes, with values ranging from about 3.5 times to 6.0 times the VSMOW (Vienna standard mean ocean water) value. The D/H distribution shows a depletion over the Tharsis mountains and is consistent with observed latitudinal variations. The variations in D/H with latitude and altitude agree with the models and with the isotope fractionation expected from condensation and sublimation processes.

Water vapor sources for Yangtze River Valley rainfall: Climatology, variability, and implications for rainfall forecasting

The method of calculating water vapor flux can show the paths of moisture transport but cannot easily identify the sources and sinks of water vapor. In this study, we estimate the evaporative moisture sources for the Yangtze River Valley (YRV) rainfall with a water vapor back‐trajectory method, using meteorological data from the Modern Era Retrospective‐analysis for Research and Applications (MERRA). The major moisture sources and their relative contributions show large seasonal variations. The moisture from the Bay of Bengal and the western Pacific usually compensate each other both during the evolution of YRV wet season (April–September) and interannually for the wet months (peak in August). The major direct moisture sources are over YRV and its major moisture transport pathways over land, rather than over the ocean, but the ocean is important in initiating the moisture transfer. However, over these important land moisture sources, surface evapotranspiration is not controlled by soil wetness and has weak impact on the variability of rainfall. Local moisture recycling over YRV is mostly a passive response to rainfall and circulation changes. The prediction of YRV rainy season rainfall thus depends more on the knowledge of large‐scale circulations and monsoons than land surface conditions.

Closing the Global Water Vapor Budget with AIRS Water Vapor, MERRA Reanalysis, TRMM and GPCP Precipitation, and GSSTF Surface Evaporation

Abstract The authors investigate if atmospheric water vapor from remote sensing retrievals obtained from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and the water vapor budget from the NASA Goddard Space Flight Center (GSFC) Modern Era Retrospective-analysis for Research and Applications (MERRA) are physically consistent with independently synthesized precipitation data from the Tropical Rainfall Measuring Mission (TRMM) or the Global Precipitation Climatology Project (GPCP) and evaporation data from the Goddard Satellite-based Surface Turbulent Fluxes (GSSTF). The atmospheric total water vapor sink (Σ) is estimated from AIRS water vapor retrievals with MERRA winds (AIRS–MERRA Σ) as well as directly from the MERRA water vapor budget (MERRA–MERRA Σ). The global geographical distributions as well as the regional wavelet amplitude spectra of Σ are then compared with those of TRMM or GPCP precipitation minus GSSTF surface evaporation (TRMM–GSSTF and GPCP–GSSTF P − E, respectively). The AIRS–MERRA and MERRA–MERRA Σs reproduce the main large-scale patterns of global P − E, including the locations and variations of the ITCZ, summertime monsoons, and midlatitude storm tracks in both hemispheres. The spectra of regional temporal variations in Σ are generally consistent with those of observed P − E, including the annual and semiannual cycles, and intraseasonal variations. Both AIRS–MERRA and MERRA–MERRA Σs have smaller amplitudes for the intraseasonal variations over the tropical oceans. The MERRA P − E has spectra similar to that of MERRA–MERRA Σ in most of the regions except in tropical Africa. The averaged TRMM–GSSTF and GPCP–GSSTF P − E over the ocean are more negative compared to the AIRS–MERRA, MERRA–MERRA Σs, and MERRA P − E.

The Apparent Water Vapor Sinks and Heat Sources Associated with the Intraseasonal Oscillation of the Indian Summer Monsoon

Abstract The possibility of using remote sensing retrievals to estimate apparent water vapor sinks and heat sources is explored. The apparent water vapor sinks and heat sources are estimated from a combination of remote sensing, specific humidity, and temperature from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and wind fields from the National Aeronautics and Space Administration (NASA)’s Goddard Space Flight Center (GSFC)’s Modern Era Retrospective-Analysis for Research and Applications (MERRA). The intraseasonal oscillation (ISO) of the Indian summer monsoon is used as a test bed to evaluate the apparent water vapor sink and heat source. The ISO-related northward movement of the column-integrated apparent water vapor sink matches that of precipitation observed by the Tropical Rainfall Measuring Mission (TRMM) minus the MERRA surface evaporation, although the amplitude of the variation is underestimated by 50%. The diagnosed water vapor and heat budgets associated with convective events during various phases of the ISO agree with the moisture–convection feedback mechanism. The apparent heat source moves northward coherently with the apparent water vapor sink associated with the deep convective activity, which is consistent with the northward migration of the precipitation anomaly. The horizontal advection of water vapor and dynamical warming are strong north of the convective area, causing the northward movement of the convection by the destabilization of the atmosphere. The spatial distribution of the apparent heat source anomalies associated with different phases of the ISO is consistent with that of the diabatic heating anomalies from the trained heating (TRAIN Q1) dataset. Further diagnostics of the TRAIN Q1 heating anomalies indicate that the ISO in the apparent heat source is dominated by a variation in latent heating associated with the precipitation.

Parameterization of middle atmospheric water vapor photochemistry for high-altitude NWP and data assimilation

Abstract. This paper describes CHEM2D-H2O, a new parameterization of H2O photochemical production and loss based on the CHEM2D photochemical-transport model of the middle atmosphere. This parameterization accounts for the altitude, latitude, and seasonal variations in the photochemical sources and sinks of water vapor over the pressure region from 100–0.001 hPa (~16–90 km altitude). A series of free-running NOGAPS-ALPHA forecast model simulations offers a preliminary assessment of CHEM2D-H2O performance over the June 2007 period. Results indicate that the CHEM2D-H2O parameterization improves global 10-day forecasts of upper mesospheric water vapor compared to forecasts using an existing one-dimensional (altitude only) parameterization. Most of the improvement is seen at high winter latitudes where the one-dimensional parameterization specifies photolytic H2O loss year round despite the lack of sunlight in winter. The new CHEM2D-H2O parameterization should provide a better representation of the downwelling of dry mesospheric air into the stratospheric polar vortex in operational analyses that do not assimilate middle atmospheric H2O measurements.

Energia estática sobre o norte e nordeste do Brasil relacionada com a temperatura da superfície do mar

A partir de fontes aparentes de calor e do sumidouro aparente de umidade, foram investigadas as características da circulação atmosférica de grande escala, nas regiões Norte e Nordeste do Brasil (NEB), relacionadas com a variabilidade da temperatura da superfície do mar (TSM) nos oceanos Atlântico e Pacífico Tropicais. Foram utilizados dados mensais das variáveis temperatura, umidade específica, componente zonal e meridional do vento e altura geopotencial (gpm) gerados pelo National Centers for Environment Prediction/National Center for Atmospheric Research (NCEP/NCAR). Os dados utilizados encontram-se espaçados numa grade de 2,5°x 2,5° latitude e longitude, para os níveis padrão de 925 a 100 hPa. Além disso, também foram utilizados dados de TSM, numa grade de 2°x2° de latitude e longitude. Foram escolhidos quatro episódios de El Niño-Oscilação Sul (ENOS) e quatro de padrão de dipolo. O estudo refere-se ao trimestre de fevereiro-abril do período chuvoso. Durante os episódios quentes (frios), El Niño e dipolo positivo (La Niña e dipolo negativo), a atmosfera das áreas 1 e 2 (área 1 - centro Norte e área 2 - centro do Nordeste do Brasil) apresentaram-se menos (mais) úmidas. As variações observadas acima (abaixo) da média na energia estática seca, foram mais fortes durante os anos de El Niño (La Niña) do que durante os anos de dipolo positivo (dipolos negativos). Foram encontradas, menores (maiores) áreas de convergência de energia estática em baixos níveis e divergências em altos níveis durante os anos de El Niño (La Niña). Durante a fase positiva (negativa) dos dipolos, a faixa de convergência de energia estática sobre o Oceano Atlântico, foi observada mais ao norte (ao sul). A região Amazônica, apesar da atuação dos episódios de El Niños, La Niña, dipolos positivos e negativos, comportou-se como um sumidouro de vapor d'água. Já, região do NEB, como uma fonte aparente de vapor d'água durante os anos de El Niño e dipolo positivo e sumidouro para os anos de La Niña e Fase negativa do dipolo.

Characterizing Tropical Cirrus Life Cycle, Evolution, and Interaction with Upper-Tropospheric Water Vapor Using Lagrangian Trajectory Analysis of Satellite Observations

Abstract Tropical cirrus evolution and its relation to upper-tropospheric water vapor (UTWV) are examined in the paper by analyzing satellite-derived cloud data, UTWV data from infrared and microwave measurements, and the NCEP–NCAR reanalysis wind field. Building upon the existing International Satellite Cloud Climatology Project (ISCCP) data and the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) product, a global (except polar region), 6-hourly cirrus dataset is developed from two infrared radiance measurements at 11 and 12 μm. The UTWV is obtained in both clear and cloudy locations by developing a combined satellite infrared and microwave-based retrieval. The analysis in this study is conducted in a Lagrangian framework. The Lagrangian trajectory analysis shows that the decay of deep convection is immediately followed by the growth of cirrostratus and cirrus, and then the decay of cirrostratus is followed by the continued growth of cirrus. Cirrus properties continuously evolve along the trajectories as they gradually thin out and move to the lower levels. Typical tropical cirrus systems last for 19–30 ± 16 h. This is much longer than cirrus particle lifetimes, suggesting that other processes (e.g., large-scale lifting) replenish the particles to maintain tropical cirrus. Consequently, tropical cirrus can advect over large distances, about 600–1000 km, during their lifetimes. For almost all current GCMs, this distance spans more than one grid box, requiring that the water vapor and cloud water budgets include an advection term. Based on their relationship to convective systems, detrainment cirrus are distinguished from in situ cirrus. It is found that more than half of the tropical cirrus are formed in situ well away from convection. The interaction between cirrus and UTWV is explored by comparing the evolution of the UTWV along composite clear trajectories and trajectories with cirrus. Cirrus are found to be associated with a moister upper troposphere and a slower rate of decrease of UTWV. Moreover, the elevated UTWV has a longer duration than cirrus. The amount of water in cirrus is too small for evaporation of cirrus ice particles to moisten the upper troposphere significantly (but cirrus may be an important water vapor sink). Rather, it is likely that the same transient motions that produce the cirrus also transport water vapor upward to maintain a larger UTWV.

Moisture Flux and Water Balance over the South China Sea During Late Boreal Spring and Summer

The moisture flux and water balance over the South China Sea (SCS) during late boreal spring and summer are studied using the reanalysis data of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP/NCAR). It is shown that the SCS is a water vapor sink during that period of time. Climatologically, the SCS works like a “reservoir of water vapor” for its special geographical location. In early May, water vapor is brought into the SCS area through its eastern, southern, and western boundaries, and is transported out of that area through its northern boundary. From June to August, the western and southern boundaries of the SCS are inflow passes of moisture flux to that region, while the northern and eastern boundaries are outflow passes. It is the intense convergence of moisture to the SCS area that maintains the local heavy precipitation. The northward branch of moisture flux feeds the precipitation over eastern China.

Seasonal variation of global surface pressure and water vapor

Previous studies have shown that the seasonal variation of global-mean surface pressure (ps) results from variation of global-mean water vapor pressure (pw). The current study, employing the global data generated by Version 1 of the Goddard Earth Observing System (GEOS-1) Data Assimilation System, shows that seasonal variations of regional ps and pw tend to be out of phase (particularly in the subtropics of the two hemispheres) and that the magnitude of the former variation is generally much larger than that of the latter. The seasonal variations of these two quantities are maintained by airmass and water vapor transports by the global divergent circulation, which is driven by the latent heat released by cumulus convection over the water vapor sink, as the “water mass forcing” mechanism predicted. Since pwand ps are used often in depicting the climate system, assessments of climate change in terms of the global-mean and regional variations of these two variables should be interpreted with caution.

Annual variation of the global precipitable water and its maintenance: observation and climate-simulation

The annual variation of the global-mean precipitable water 〈W〉and the associated hydrological cycle were analyzed with the upper-air data generated by the Global Data Assimilation System of the National Meteorological Center for 1981–1991 and the European Centre for Medium Range Weather Forecasts from 1983–1991. It was found that the annual variation of 〈W〉 coincides with that of the Northern Hemisphere precipitable water [W]NH. The hemispheric-mean ([ ]) water budget analysis shows that water vapor is transported from the winter to the summer hemisphere across the equator by the Hadley circulation, and that the annual variations in the water vapor sink [P – E] for both hemispheres also follow the same seasonal march. The amplitudes of the annual variations in these two hydrological processes are comparable in both hemispheres. Thus, the annual variations of [W]NH and [W]SH are the result of slight imbalances between the cross-equator water vapor transport and the water vapor sink, particularly in the spring and fall. The climatological hemispheric-mean water budgets reveal that the Southern Hemisphere is a water vapor source and the Northern Hemisphere is a water vapor sink. The cross-equator water vapor transport constitutes a major source acting to maintain [W]NH, and in turn 〈W〉. The hydrological mechanism maintaining the observed 〈W〉 annual variation is consistent with that obtained from the hydrological cycle in a 10-year (1979–1988) climate simulation done at the Goddard Laboratory for Atmospheres as part of their participation in the Atmospheric Model Intercomparison Project (AMIP). DOI: 10.1034/j.1600-0870.1996.00001.x