Cycle Of Water Vapor Research Articles

Diurnal Cycle of Water Vapor as Documented by a Dense GPS Network in a Coastal Area during ESCOMPTE IOP2

Abstract Global positioning system (GPS) data from a dense network have been used for the analysis of the diurnal cycle of water vapor over Marseille, France, during the second intensive observation period (IOP2; 21–26 June 2001) of the Expérience sur Sites pour Contraindre les Modèles de Pollution Atmosphérique et de Transport d’Emission (ESCOMPTE) field experiment. Both tomographic analyses and integrated water vapor (IWV) contents from GPS have been used, in addition to wind profiler data and surface observations. Tomographic analysis of data from the dense GPS network and radiosondes provided the continuous temporal evolution of the vertical distribution of water vapor over the city of Marseille. The city is located on the shore of the Mediterranean Sea in southeastern France and is often under the influence of sea-breeze effects. Two different layers of breeze circulation are identified: a shallow sea breeze, blowing perpendicular to the local coastline, and a deep sea breeze, induced by the regional temperature gradient between sea and land. The origin of water vapor is shown to be mainly due to the advection of marine moist air by these sea-breeze circulations. However, the diurnal cycle of water vapor over Marseille is strongly influenced by the synoptic situation, which changes during the IOP2 (between a northerly mistral in the early stage of the IOP and an easterly wind at the end). It is shown that vertical profiles from tomographic analyses (combined with wind profiler data) allow for a proper interpretation of the diurnal cycle observed in IWV. Two-dimensional maps of IWV are also shown to complement the description of the horizontal advection of moisture by the different circulation regimes.

Assessment of temperature, trace species, and ozone in chemistry‐climate model simulations of the recent past

Simulations of the stratosphere from thirteen coupled chemistry‐climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long‐term trends are compared for an extended period (1960–2004). Comparisons of polar high‐latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long‐term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.

Global Changes of the Water Cycle Intensity

Abstract In this study, numerical simulations of the twentieth-century climate are evaluated, focusing on the changes in the intensity of the global water cycle. A new model diagnostic of atmospheric water vapor cycling rate is developed and employed that relies on constituent tracers predicted at the model time step. This diagnostic is compared to a simplified traditional calculation of cycling rate, based on monthly averages of precipitation and total water content. The mean sensitivity of both diagnostics to variations in climate forcing is comparable. However, the new diagnostic produces systematically larger values with more variability. Climate simulations were performed using SSTs of the early (1902–21) and late (1979–98) twentieth century along with the appropriate CO2 forcing. In general, the increase of global precipitation with the increases in SST that occurred between the early and late twentieth century is small. However, an increase of atmospheric temperature leads to a systematic increase in total precipitable water. As a result, the residence time of water in the atmosphere increased, indicating a reduction of the global cycling rate. This result was explored further using a number of 50-yr climate simulations from different models forced with observed SST. The anomalies and trends in the cycling rate and hydrologic variables of different GCMs are remarkably similar. The global annual anomalies of precipitation show a significant upward trend related to the upward trend of surface temperature, during the latter half of the twentieth century. While this implies an increase in the simulated hydrologic cycle intensity, a concomitant increase of total precipitable water again leads to a decrease in the calculated global cycling rate. An analysis of the land/sea differences shows that the simulated precipitation over land has a decreasing trend, while the oceanic precipitation has an upward trend consistent with previous studies and the available observations. The decreasing continental trend in precipitation is located primarily over tropical land regions, with some other regions, such as North America, experiencing an increasing trend. Precipitation trends are diagnosed further using the water tracers to delineate the precipitation that occurs because of continental evaporation, as opposed to oceanic evaporation. These model diagnostics show that over global land areas, the recycling of continental moisture is decreasing in time. However, the recycling changes are not spatially uniform so that some regions, most notably over the United States, experience continental recycling of water that increases in time.

Seasonal and Diurnal Ozone Variations: Observations and Modeling

Ozone mixing ratios observed by the Bordeaux microwave radiometer between 1995 and 2002 in an altitude range 25–75 km show diurnal variations in the mesosphere and seasonal variations in terms of annual and semi-annual oscillations (SAO) in the stratosphere and in the mesosphere. The observations with 10–15 km altitude resolution are presented and compared to photochemical and transport model results. Diurnal ozone variations are analyzed by averaging the years 1995−1997 for four representative months and six altitude levels. The photochemical models show a good agreement with the observations for altitudes higher than 50 km. Seasonal ozone variations mainly appear as an annual cycle in the middle and upper stratosphere and a semi-annual cycle in the mesosphere with amplitude and phase depending on altitude. Higher resolution (2 km) HALOE (halogen occultation experiment) ozone observations show a phase reversal of the SAO between 44 and 64 km. In HALOE data, a tendancy for an opposite water vapour cycle can be identified in the altitude range 40–60 km. Generally, the relative variations at all altitudes are well explained by the transport model (up to 54 km) and the photochemical models. Only a newly developed photochemical model (1-D) with improved time-dependent treatment of water vapour profiles and solar flux manages to reproduce fairly well the absolute values.

GPS water vapour tomography: preliminary results from the ESCOMPTE field experiment

Water vapour plays a major role in atmospheric processes but remains difficult to quantify due to its high variability in time and space and the sparse set of available measurements. The GPS has proved its capacity to measure the integrated water vapour at zenith with the same accuracy as other methods. Recent studies show that it is possible to quantify the integrated water vapour in the line of sight of the GPS satellite. These observations can be used to study the 3D heterogeneity of the troposphere using tomographic techniques. We develop three-dimensional tomographic software to model the three-dimensional distribution of the tropospheric water vapour from GPS data. First, the tomographic software is validated by simulations based on the realistic ESCOMPTE GPS network configuration. Without a priori information, the absolute value of water vapour is less resolved as opposed to relative horizontal variations. During the ESCOMPTE field experiment, a dense network of 17 dual frequency GPS receivers was operated for 2 weeks within a 20×20-km area around Marseille (southern France). The network extends from sea level to the top of the Etoile chain (∼700 m high). Optimal results have been obtained with time windows of 30-min intervals and input data evaluation every 15 min. The optimal grid for the ESCOMTE geometrical configuration has a horizontal step size of 0.05°×0.05° and 500 m vertical step size. Second, we have compared the results of real data inversions with independent observations. Three inversions have been compared to three successive radiosonde launches and shown to be consistent. A good resolution compared to the a priori information is obtained up to heights of 3000 m. A humidity spike at 4000-m altitude remains unresolved. The reason is probably that the signal is spread homogeneously over the whole network and that such a feature is not resolvable by tomographic techniques. The results of our pure GPS inversion show a correlation with meteorological phenomena. Our measurements could be related to the land–sea breeze. Undoubtedly, tomography has some interesting potential for the water vapour cycle studies at small temporal and spatial scales.

Seasonal cycle of water vapor in the atmosphere of mars as revealed from the MAWD/Viking 1 and 2 experiment

The most detailed information on the seasonal evolution of water vapor distribution in the Martian atmosphere was obtained by the Mars Atmospheric Water Detector (MAWD), which was a five-channel spectrometer operating aboard the Viking 1 and 2 spacecraft in 1977–1980, and from the very recent observations by the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft. The TES results show a considerably larger amount of water vapor near the perihelion of Mars (summer in the southern hemisphere) than the estimates based on the MAWD data for this season, which is characterized by the development of global dust storms. The TES and MAWD instruments operated in different spectral regions (20–50 μm and 1.38 μm, respectively), and this could result in the aforementioned difference because of the effect of aerosol scattering on the intensity of the H2O bands, which becomes significant at short wavelengths. We considered the effect of aerosol scattering on the water vapor content measured in the 1.38-μm band, taking into account the different geometries of observations, and restored the H2O content from the MAWD data with allowance for multiple scattering. We obtained a seasonal and spatial distribution of water vapor that showed better agreement with the TES data and, thus, indicated the stability of the hydrological cycle on Mars. Periodic structures, which could possibly be associated with the influence of the stationary planetary waves, are reliably revealed in the zonal distribution of the atmospheric water vapor. The seasonal variability of the wave structures correlates with variations of the latitude gradient of water vapor. This could indicate that wave processes contribute considerably to the meridional water transport in the Martian atmosphere.

The Boreal-Summer Intraseasonal Oscillations Simulated in a Hybrid Coupled Atmosphere–Ocean Model*

Abstract The boreal-summer intraseasonal oscillation (BSISO) simulated by an atmosphere–ocean coupled model is validated with the long-term observations [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall, ECMWF analysis, and Reynolds' SST]. This validation focuses on the three-dimensional water vapor cycle associated with the BSISO and its interaction with underlying sea surface. The advantages of a coupled approach over stand-alone atmospheric approaches on the simulation of the BSISO are revealed through an intercomparison between a coupled run and two atmosphere-only runs. This coupled model produces a BSISO that mimics the one presented in the observations over the Asia– western Pacific region. The similarities with the observations include 1) the coherent spatiotemporal evolutions of rainfall, surface winds, and SST associated with the BSISO; 2) the intensity and period (or speed) of the northward-propagating BSISO; and 3) the tropospheric moistening (or drying) and overturning circulations of the BSISO. However, the simulated tropospheric moisture fluctuations in the extreme phases (both wet and dry) are larger than those in the ECMWF analysis. The simulated sea surface cooling during the wet phase is weaker than the observed cooling. Better representations of the interaction between convection and boundary layer in the GCM and including salinity effects in the ocean model are expected to further improve the simulation of the BSISO. The intercomparison between a coupled run and two atmospheric runs suggests that the air–sea coupled system is the ultimate tool needed to realistically simulate the BSISO. Though the major characteristics of the BSISO are very likely determined by the internal atmospheric dynamics, the correct interaction between the internal dynamics and underlying sea surface can only be sustained by a coupled system. The atmosphere-only approach, when forced with high-frequency (e.g., daily) SST, introduces an erroneous boundary interference on the internal dynamics associated with the BSISO. The implications for the predictability of the BSISO are discussed.

Diurnal cycle of convection, clouds, and water vapor in the tropical upper troposphere: Satellites versus a general circulation model

Global high‐resolution (3‐hourly, 0.1° × 0.1° longitude‐latitude) water vapor (6.7 μm) and window (11 μm) radiances from multiple geostationary satellites are used to document the diurnal cycle of upper tropospheric relative humidity (UTH) and its relationship to deep convection and high clouds in the whole tropics and to evaluate the ability of the new Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere and land model (AM2/LM2) to simulate these diurnal variations. Similar to the diurnal cycle of deep convection and high clouds, coherent diurnal variations in UTH are also observed over the deep convective regions, where the daily mean UTH is high. In addition, the diurnal cycle in UTH also features a land‐sea contrast: stronger over land but weaker over ocean. UTH tends to peak around midnight over ocean in contrast to 0300 LST over land. Furthermore, UTH is observed to lag high cloud cover by ∼6 hours, and the latter further lags deep convection, implying that deep convection serves to moisten the upper troposphere through the evaporation of the cirrus anvil clouds generated by deep convection. Compared to the satellite observations, AM2/LM2 can roughly capture the diurnal phases of deep convection, high cloud cover, and UTH over land; however, the magnitudes are noticeably weaker in the model. Over the oceans the AM2/LM2 has difficulty in simulating both the diurnal phase and amplitude of these quantities. These results reveal some important deficiencies in the model's convection and cloud parameterization schemes and suggest the lack of a diurnal cycle in SST may be a shortcoming in the boundary forcing for atmospheric models.

Diurnal Variation of Precipitable Water over a Mountainous Area of Sumatra Island

Diurnal variations in atmospheric water vapor at Koto Tabang, a mountainous area of Sumatra Island, Indonesia, are studied by analyzing the GPS-derived precipitable water, radiosonde data, and surface meteorological observation data. A permanent GPS receiving station was established at Koto Tabang in March of 2001. Radiosonde soundings were carried out at 3-h intervals to study the atmospheric water and energy cycles in the Maritime Continent of Indonesia. A distinct diurnal variation in water vapor is observed. The precipitable water increases during daytime, reaching its maximum in the late afternoon. The results of the radiosonde observations indicate that the increase in water vapor occurs up to an altitude of approximately 3 km. Water vapor is transported by turbulent mixing with the development of the boundary layer in the afternoon. The diurnal cycle of water vapor is affected by the intensity of incident solar radiation. It is suggested that the diurnal variation in water vapor is caused by the horizontal transport of water vapor by local circulation. The diurnal range of the precipitable water on days with heavy rain was larger than that on days without rain. Rainfall often occurs as intensive showers during a short period in the late afternoon and early evening in the Koto Tabang area; daily shortwave radiation is still sufficiently strong to generate the local circulations and to induce large diurnal variation in precipitable water, even on the heavy-rain days.

Mesospheric ozone response to changes in water vapor

Observations made by the Upper Atmosphere Research Satellite (UARS) now extend in time for almost a decade and allow for an improved understanding of seasonal and interannual variations of middle atmospheric minor constituents. In particular, measurements of mesospheric ozone and water vapor made by the UARS Halogen Occultation Experiment (HALOE) reveal significant seasonal and decadal variability. At midlatitudes, the observed variation in water vapor is primarily an annual cycle that peaks in summertime driven by changes in the residual circulation. In contrast to earlier satellite observations, HALOE ozone sunset observations also have an annual cycle that is anticorrelated with the water vapor cycle. The anticorrelation is a consequence of water vapor photolysis producing hydrogen species that destroy ozone. However, HALOE sunrise observations of ozone show no seasonal cycle. Analysis of monthly anomalies between 1991 and 2001 show a long‐term water vapor increase of approximately 1%/year. The observed response of ozone depends on altitude and time of day and is strongly negative (up to −4%/year) for sunset observations around 80 km. Again, the response of ozone at sunrise is minimal. Sensitivity tests using a three‐dimensional global chemical transport model show good agreement with the observations and confirm that the observed decrease in sunset ozone is a response to increasing water vapor.

Atomic level characterization as applied to developing cathodes of high brightness

The development is briefly reviewed of the cathodes that have brought about epoch-making progresses in surface science and electron beam technology: the carbon filament by Edison(1884), the tungsten filament by Langmuir(1913), the LaB6-cathode by Lafferty(1951) and the Zr-O/W(100) cathode by Swanson(1979). These developments stimulated the surface scientists in those days to devise atomic level characterization tools for elucidating such characteristics of the cathodes as high brightness, high stability and long service life. These approaches have led to many discoveries of practical importance; the water-vapor cycle on the tungsten surface (Langmuir), the double layer formation on the Cs/W(100)-surface (MacRae, et al.), the critical temperature for the stoichiometric evaporation in LaB6-cathode (Aono, et al.) and the self-recovery function on the Zr-O/W(100) surface (Shimizu, et al.). [DOI: 10.1380/ejssnt.2003.106]

The annual cycle of water vapor on Mars as observed by the Thermal Emission Spectrometer

Spectra taken by the Mars Global Surveyor Thermal Emission Spectrometer (TES) have been used to monitor the latitude, longitude, and seasonal dependence of water vapor for over one full Martian year (March 1999 to March 2001). A maximum in water vapor abundance is observed at high latitudes during midsummer in both hemispheres, reaching a maximum value of ∼100 pr‐μm in the north and ∼50 pr‐μm in the south. Low water vapor abundance (<5 pr‐μm) is observed at middle and high latitudes in the fall and winter of both hemispheres. There are large differences in the hemispheric (north versus south) and seasonal (perihelion versus aphelion) behavior of water vapor. The latitudinal and seasonal dependence of the decay of the northern summer water vapor maximum implies cross‐equatorial transport of water to the southern hemisphere, while there is little or no corresponding transport during the decay of the southern hemisphere summer maximum. The latitude‐longitude dependence of annually averaged water vapor (corrected for topography) has a significant positive correlation with albedo and significant negative correlations with thermal inertia and surface pressure. Comparison of TES results with those retrieved from the Viking Orbiter Mars Atmospheric Water Detectors (MAWD) experiments [Jakosky and Farmer, 1982] shows some similar features but also many significant differences. The southern hemisphere maximum observed by TES was not observed by MAWD, and the large latitudinal gradient in annually averaged water vapor observed by MAWD does not appear in the TES results.

Diurnal variation in water vapor over North America and its implications for sampling errors in radiosonde humidity

Diurnal variations in atmospheric water vapor are studied by analyzing 30‐min‐averaged data of atmospheric precipitable water (PW) for 1996–2000 derived from Global Position System (GPS) observations from 54 North America stations. Vertical structures in the diurnal cycle of atmospheric water vapor are examined using 3‐hourly radiosonde data from Lamont, Oklahoma, during the 1994–2000 period. Significant diurnal variations of PW are found over most of the stations. The diurnal (24 hour) cycle, S1, which explains over 50% of the subdaily variance, has an amplitude of 1.0–1.8 mm over most of the central and eastern United States during summer and is weaker in other seasons. The S1 peaks around noon in winter and from midafternoon to midnight in summer. The semidiurnal (12 hour) cycle is generally weak, with an amplitude of a few tenths of 1 mm. At Lamont, specific humidity in the free troposphere is significantly higher in the early morning (0000–0008 local solar time (LST)) than during the day (0800–1800 LST). This diurnal variation changes little from ∼4 to 16 km above the ground. Near the surface, specific humidity tends to be lower in the morning than in the afternoon and evening in all seasons except summer. This near‐surface diurnal cycle propagates upward through the lower troposphere (up to ∼4 km). Errors in seasonal mean humidity due to undersampling the diurnal cycle with twice‐daily synoptic soundings (at 0000 and 1200 UTC) are generally small (within ±3% or ±0.5 mm for PW), but it can easily reach 5–10% if there is only one random sounding per day. Several physical processes are proposed that could contribute to the diurnal variations in atmospheric water vapor.

Constraints on the seasonal cycle of stratospheric water vapor using in situ measurements from the ER‐2 and a CO photochemical clock

We use in situ measurements of CO obtained in the tropics from 1995 to 1997 on the NASA ER‐2 aircraft and a simple photochemical model to calculate the elapsed time between the entry of air into the stratosphere and the observation, which we define as the photochemical “age” of the air. Assuming this age represents the transit time of the air mass from a boundary at 390 K to its measured altitude, we calculate boundary condition values of CO2 derived from in situ measurements of this species from 400 to 480 K. We validate the approach by comparing these CO2 boundary values with an independent representation of the boundary condition from observations of CO2 in air that had recently entered the stratosphere (as indicated by simultaneous measurements of N2O, CO, and potential temperature). For five of the six flights, differences between CO2 boundary condition values determined using the photochemical age of the air and those derived from independent measurements can be accounted for with isentropic mixing of midlatitude stratospheric air into the tropics. Having validated the photochemical ages of the sampled stratospheric air, we use the same analysis of in situ water data to provide water vapor boundary condition values that constrain the seasonal cycle of water vapor entering the stratosphere. On the basis of these constraints we evaluate the seasonal cycle of entry‐level water vapor derived from tropical tropopause temperatures from the radiosonde network between 10°S and 10°N. We conclude that while average saturation mixing ratios provide a suitable boundary condition for water vapor entering the stratosphere, the uncertainties in saturation mixing ratios derived from radiosonde temperatures and the lack of coverage prevents distinguishing between ascent preferentially occurring over the western equatorial Pacific or throughout the tropics. With the assumption that vertical diffusion and midlatitude mixing have a negligible effect on the calculated age, ascent velocities can be inferred from the photochemical ages. These ascent velocities show a seasonal cycle that is inconsistent with our current understanding of the dynamics driving the stratospheric circulation and with independent estimates of tropical vertical ascent rates.

Full year cycle of desert dust spectral optical thickness and precipitable water vapor over Alexandria, Egypt

We study the annual cycle of dust loading in Alexandria, Egypt. Observations were performed from December 1997 to November 1998, including during the Kamaseen storms of March 1998. A ground‐based, manual Sun photometer was used to measure aerosol optical thickness (τa) at four spectral bands in the 340–870 nm wavelength range (namely 340, 440, 675, 870). Total precipitable water vapor (W) was also measured simultaneously, based on the 936 nm channel measurements and extrapolations of the aerosol optical thickness from the neighboring 675 and 870 nm channels. Aerosol optical thickness τa at all the four spectral bands were very high (averaging 2.0–4.0) during the Kamaseen storms of late March 1998. Correspondingly, the Ångström wavelength exponent α of the optical thickness fell close to zero during that time, implying a substantial increase in dust. Overall, the monthly mean and median aerosol optical thickness were highest during January‐May and lowest in June‐October. During the January‐May period total precipitable water vapor and Ångström exponent were lower than during the June‐October period. There is a high correlation between the Ångström exponent and the optical thickness (r = 0.63), with a ranging from 0.0 to 0.5 for τa > 1.0, indicating high dust concentration. Trajectory analysis shows that the presence of dust was associated with air masses arriving predominantly from the Sahara or North Africa. No significant correlation was found between the optical thickness and the precipitable water vapor. These basic systematic observations are vital for assessing dust climatology in this important part of the world and also for validating satellite observations and dust transport models.

Seasonal variation of water vapor in the lower stratosphere observed in Halogen Occultation Experiment data

The seasonal cycle of water vapor in the lower stratosphere is studied based on Halogen Occultation Experiment (HALOE) satellite observations spanning 1991–2000. The seasonal cycle highlights fast, quasi‐horizontal transport between tropics and midlatitudes in the lowermost stratosphere (near isentropic levels ∼380–420 K), in addition to vertical propagation above the equator (the tropical “tape recorder”). The rapid isentropic transport out of the tropics produces a layer of relatively dry air over most of the globe throughout the year, and the seasonal cycle in midlatitudes of both hemispheres (and over the Arctic pole) follows that in the tropics. Additionally, the Northern Hemisphere summer monsoon has a dominant influence on hemispheric‐scale constituent transport. Longitudinal structures in tropical water vapor and ozone identify regions of strong coupling to the troposphere; an intriguing result is that the solstice minima in water vapor and ozone are spatial separated from maximum convection and coldest tropical temperatures. Detailed comparisons with tropical aircraft measurements and the long record of balloon data from Boulder, Colorado, demonstrate the overall high quality of HALOE water vapor data.

Diurnal Variation of Water Vapor over the Central Tibetan Plateau during Summer.

The diurnal variation of water vapor over the central Tibetan Plateau during summer was investigated using various observational data and a numerical model. A systematic diurnal cycle of the precipitable water was found in the rawinsonde data. That is, the precipitable water decreased during daytime at the valley area, while it increased over the mountain range. This diurnal cycle is most significant in the pre-monsoon season but it is not clear in the middle of the monsoon season. The horizontal distribution of this diurnal cycle was examined by global objective analysis datasets and GMS-5 satellite data, and the results were found to be consistent with the topographic dependence of the diurnal cycle described above. The systematic diurnal cycle of the water vapor amount is interpreted to be caused by water vapor transport due to the thermally-induced circulation over complex terrain. This diurnal cycle of water vapor can be qualitatively simulated by a two-dimensional numerical model. The dominant horizontal scale of topography of the central Tibetan Plateau is 100 to 300 km, which is close to the most effective scale for water vapor transport as predicted by the simple two-dimensional model.

The seasonal cycle of water vapor and saturation vapor mixing ratio in the extratropical lowermost stratosphere

Both in situ measurements and satellite remote sensing data show that water vapor in the extratropical lowermost stratosphere has a pronounced seasonal cycle, with a maximum in summer. In this paper we show that the seasonal cycles derived from satellite measurements by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the Microwave Limb Sounder (MLS) and ER‐2 aircraft measurements are in reasonable agreement when compared using potential vorticity (PV) and potential temperature (θ) binned monthly averages. The annual means and amplitudes of the water vapor seasonal cycle are derived from the SAGE II data for 320–360 K and 2–7 PV units (covering latitudes near and poleward of the tropopause). The high moisture content and the summer maxima indicate that the effect of transport from the troposphere across the extratropical tropopause can be seen in nearly the entire range examined and is more significant in the lower PV‐θ bins. We further investigate mechanisms that control the amount and seasonal cycle of water vapor in the lowermost stratosphere. One important issue is whether local temperature near the extratropical tropopause limits isentropic transport of water vapor into the lowermost stratosphere, similar to the freeze‐out of water vapor crossing the tropical tropopause. A quantitative comparison of saturation vapor mixing ratios (derived from National Centers for Environmental Prediction temperature analyses) with the water vapor measurements shows that the amount of water vapor near the extratropical tropopause is substantially lower than that given by saturation over ice. This demonstrates that local temperature effects do not set the upper limit of water vapor in this region. Analyses using ER‐2 measurements confirm that although saturation and supersaturation do occur, on average, air near the extratropical tropopause is under saturated. The seasonal cycle of the fraction of the overworld air in the lowermost stratosphere is inferred using the water vapor and saturation vapor mixing ratio climatology.

Features and trends in Atmospheric Trace Molecule Spectroscopy (ATMOS) version 3 stratospheric water vapor and methane measurements

Mixing ratios of water vapor entering the stratosphere are inferred from Atmospheric Trace Molecule Spectroscopy (ATMOS) measurements to be 3.28 ppm in 1985, 3.53 ppm in 1992, 3.67 ppm in 1993, and 3.65 ppm in 1994, yielding an average increase of 0.041±0.007 (1σ) ppm/yr or 1.2±0.2%/yr between 1985 and 1994. Strong evidence for a seasonal cycle in water vapor is apparent below 45km in the tropics, and ascent rates derived from these tropical profiles are comparable to previous estimates. An enhancement in the sum [H2O]+2[CH4] apparent between 45 and 60km in the tropics appears to be consistent with remnants of a seasonal cycle in [H2O].

The diurnal cycle of convection, clouds, and water vapor in the tropical upper troposphere

Hourly observations of the 6.7 µm water vapor radiances from geostationary satellites are used to document the diurnal cycle in upper tropospheric water vapor and its relationship to cloud cover and convection. A coherent diurnal cycle in tropical water vapor is observed which lags the variations in cloud cover by approximately 2 hours. The variations in upper tropospheric cloud and water vapor occur (roughly) in phase with changes in deep convection over land, but nearly 12 hours out of phase with those over ocean. This feature is shown to be associated with differences in the vertical structure of land and ocean convection and offers a useful test of convective parameterizations in atmospheric models.