Water Vapor In Upper Troposphere Research Articles

Does convectively‐detrained cloud ice enhance water vapor feedback?

We demonstrate that coupled Global Climate Models (GCMs) can reproduce observed correlations among ice water path (IWP), upper tropospheric water vapor (UTWV), and sea surface temperature (SST), and that the presence/strength of this correlation has no direct bearing on the strength of water vapor feedback in the model. The models can accurately reproduce a strong positive correlation between IWP and UTWV, a rapid increase of IWP with increasing SST and a 2–3 times increase in the slope of UTWV versus SST for SSTs warmer than ∼300 K. We argue that the relative concentrations of IWP to UTWV in both observations and models is too small to significantly influence the observed moistening of the upper troposphere (UT).

Inferred long term trends in lightning activity over Africa

Abstract Global warming is becoming a reality, with growing evidence that anthropogenic activity on our planet is starting to influence our climate (IPCC, 2001). Due to the increase in significant weather-related disasters in recent years, it is important to investigate the role of global warming on such changes. In this paper we attempt to estimate the long term trends in lightning activity over tropical Africa during the past 50 years, using upper tropospheric water vapor as a proxy for regional lightning activity. We use the NCAR/NCEP reanalysis product available for the period 1948 to the present to estimate the long term trends in lightning activity. Similarity between the long term African lightning variability and observed rainfall and river discharge variability are demonstrated. Since 1950 the inferred lightning activity over Africa shows significant variability, reaching a maximum during the 1960s, followed by a decrease in activity during the following 30 years.

Absolute accuracy of water vapor measurements from six operational radiosonde types launched during AWEX‐G and implications for AIRS validation

A detailed assessment of radiosonde water vapor measurement accuracy throughout the tropospheric column is needed for assessing the impact of observational error on applications that use the radiosonde data as input, such as forecast modeling, radiative transfer calculations, remote sensor retrieval validation, climate trend studies, and development of climatologies and cloud and radiation parameterizations. Six operational radiosonde types were flown together in various combinations with a reference‐quality hygrometer during the Atmospheric Infrared Sounder (AIRS) Water Vapor Experiment‐Ground (AWEX‐G), while simultaneous measurements were acquired from Raman lidar and microwave radiometers. This study determines the mean accuracy and variability of the radiosonde water vapor measurements relative to simultaneous measurements from the University of Colorado (CU) Cryogenic Frostpoint Hygrometer (CFH), a reference‐quality standard of known absolute accuracy. The accuracy and performance characteristics of the following radiosonde types are evaluated: Vaisala RS80‐H, RS90, and RS92; Sippican Mark IIa; Modem GL98; and the Meteolabor Snow White hygrometer. A validated correction for sensor time lag error is found to improve the accuracy and reduce the variability of upper tropospheric water vapor measurements from the Vaisala radiosondes. The AWEX data set is also used to derive and validate a new empirical correction that improves the mean calibration accuracy of Vaisala measurements by an amount that depends on the temperature, relative humidity, and sensor type. Fully corrected Vaisala radiosonde measurements are found to be suitably accurate for AIRS validation throughout the troposphere, whereas the other radiosonde types are suitably accurate under only a subset of tropospheric conditions. Although this study focuses on the accuracy of nighttime radiosonde measurements, comparison of Vaisala RS90 measurements to water vapor retrievals from a microwave radiometer reveals a 6–8% dry bias in daytime RS90 measurements that is caused by solar heating of the sensor. An AWEX‐like data set of daytime measurements is highly desirable to complete the accuracy assessment, ideally from a tropical location where the full range of tropospheric temperatures can be sampled.

Can Lightning Observations be Used as an Indicator of Upper-Tropospheric Water Vapor Variability?

Abstract Lightning activity in thunderstorms is closely related to the intensity of vertical updrafts indeep convective clouds that also transport large amounts of moisture into the upper troposphere. Small changes in the amount of upper-tropospheric water vapor (UTWV) can have major implications for the Earth's climate. New evidence is presented showing a strong connection between the daily variability of tropical lightning activity and daily uppertropospheric water vapor concentrations from the NCEP-NCAR reanalysis. Our results over the African continent show that the NCEP upper-tropospheric water vapor peaks one day after intense lightning activity in the Tropics. Given the many caveats related to the NCEP UTWV product over Africa, these results need to be interpreted with caution. However, since global lightning activity can be monitored from a few ground stations around the world via the Schumann resonances, we suggest the possible use of continuous lightning observations for studying the daily varia...

Enhanced positive water vapor feedback associated with tropical deep convection: New evidence from Aura MLS

Recent simultaneous observations of upper tropospheric (UT) water vapor and cloud ice from the Microwave Limb Sounder (MLS) on the Aura satellite provide new evidence for tropical convective influence on UT water vapor and its associated greenhouse effect. The observations show that UT water vapor increases as cloud ice water content increases. They also show that, when sea surface temperature (SST) exceeds ∼300 K, UT cloud ice associated with tropical deep convection increases sharply with increasing SST. The moistening of the upper troposphere by deep convection leads to an enhanced positive water vapor feedback, about 3 times that implied solely by thermodynamics. Over tropical oceans when SST greater than ∼300 K, the ‘convective UT water vapor feedback’ inferred from the MLS observations contributes approximately 65% of the sensitivity of the clear‐sky greenhouse parameter to SST.

Raman Lidar Measurements during the International H2O Project. Part I: Instrumentation and Analysis Techniques

Abstract The NASA Goddard Space Flight Center (GSFC) Scanning Raman Lidar (SRL) participated in the International H2O Project (IHOP), which occurred in May and June 2002 in the midwestern part of the United States. The SRL received extensive optical modifications prior to and during the IHOP campaign that added new measurement capabilities and enabled unprecedented daytime water vapor measurements by a Raman lidar system. Improvements were also realized in nighttime upper-tropospheric water vapor measurements. The other new measurements that were added to the SRL for the IHOP deployment included rotational Raman temperature, depolarization, cloud liquid water, and cirrus cloud ice water content. In this first of two parts, the details of the operational configuration of the SRL during IHOP are provided along with a description of the analysis and calibration procedures for water vapor mixing ratio, aerosol depolarization, and cirrus cloud extinction-to-backscatter ratio. For the first time, a Raman water vapor lidar calibration is performed, taking full account of the temperature sensitivity of water vapor and nitrogen Raman scattering. Part II presents case studies that permit the daytime and nighttime error statistics to be quantified.

Deriving the tropospheric integrated water vapor from tipping curve–derived opacity near 22 GHz

In this study we present a simple relation between the tropospheric opacity τ near 22.235 GHz and the integrated water vapor (IWV) content of the troposphere. The opacity is measured at Bern, Switzerland, by the radiometer Middle Atmospheric Water Vapour Radiometer (MIAWARA), designed for middle atmospheric water vapor profile measurements. In contrast to typical radiometers for tropospheric monitoring, this middle atmospheric water vapor radiometer only measures in the vicinity of the 22.235 GHz water vapor line with a bandwidth of 1 GHz. With this study we show that it is even possible to derive the integrated tropospheric water vapor (IWV) content of the atmosphere using this limited frequency range if the liquid water content of the atmosphere is negligible. IWV measurements of the tropospheric monitoring instruments Tropospheric Water Vapour Radiometer (TROWARA, two‐channel radiometer), All‐Sky Multi Wavelength Radiometer (ASMUWARA, multichannel radiometer), and GPS, which are operated next to MIAWARA, are used to derive a linear relation between the opacity and the water vapor content of the troposphere. In a second step, the mean tropospheric temperature is taken into account and a slight improvement of the linear relation is achieved. All instruments involved in this study are contributing to the Studies in Atmospheric Radiative Transfer and Water Vapour Effects (STARTWAVE) project of the Climate program of the National Competence Center in Research. The MIAWARA measurements in the subarctic winter in northern Finland during the Lapbiat Upper Tropospheric Lower Stratospheric Water Vapor Validation Project (LAUTLOS/WAVVAP) campaign in 2004 are compared to radiosonde measurements by the Finnish Meteorological Institute using the same algorithm that was derived for Bern. The agreement of MIAWARA IWV and radiosonde IWV is of the same order as for Bern. Finally, Payerne radiosonde measurements and model simulation using the Atmospheric Radiative Transfer Simulator (ARTS) software and the continuum absorption models of Rosenkranz (1998) and Liebe (MPM87/MPM93) confirm the derived opacity‐IWV relation. This study shows that the integrated water vapor content of the troposphere can be measured by a radiometer operating near the 22.235 GHz water vapor line using a bandwidth of 1 GHz, if the liquid water content of the atmosphere is negligible.

Improvement of the vertical humidity distribution in the chemistry-transport model MATCH through increased evaporation of convective precipitation

[1] We compare the vertical monthly mean distributions of model water vapor from the Model of Atmospheric Transport and Chemistry (MATCH) to tropospheric in situ sonde profiles and to upper tropospheric water vapor climatologies from the Microwave Limb Sounder (MLS). We perform two model runs with and without increased evaporation of precipitation in midlevel downdrafts and subsidence regions of convective systems. We demonstrate that higher evaporation rates significantly improve the modelled vertical distribution of specific and relative humidity with respect to what has been observed, by moistening of mid and lower tropospheric levels in regions of strong convection. We further demonstrate that the more efficient evaporation increases the modelled water vapor residence time, reducing the gap to observed mean water-vapor residence times from synergistic use of the Global Ozone Monitoring Experiment (GOME) water-vapor fields and data from the Global Precipitation Climatology Project (GPCP).

Retrieval of upper tropospheric water vapor and upper tropospheric humidity from AMSU radiances

Abstract. A regression method was developed to retrieve upper tropospheric water vapor (UTWV in kg/m2) and upper tropospheric humidity (UTH in % RH) from radiances measured by the Advanced Microwave Sounding Unit (AMSU). In contrast to other UTH retrieval methods, UTH is defined as the average relative humidity between 500 and 200hPa, not as a Jacobian weighted average, which has the advantage that the UTH altitude does not depend on the atmospheric conditions. The method uses AMSU channels 6-10, 18, and 19, and should achieve an accuracy of 0.48 kg/m2 for UTWV and 6.3% RH for UTH, according to a test against an independent synthetic data set. This performance was confirmed for northern mid-latitudes by a comparison against radiosonde data from station Lindenberg in Germany, which yielded errors of 0.23 kg/m2 for UTWV and 6.1% RH for UTH.

Middle Atmospheric Water Vapour Radiometer (MIAWARA): Validation and first results of the LAPBIAT Upper Tropospheric Lower Stratospheric Water Vapour Validation Project (LAUTLOS‐WAVVAP) campaign

We present a validation study for the ground‐based Middle Atmospheric Water Vapour Radiometer (MIAWARA) operating at 22 GHz. MIAWARA measures the water vapor profile in the range of 20–80 km. The validation was conducted in two phases at different geographical locations. During the first operational period the radiometer was operated at middle latitudes in Bern, Switzerland, and the measured water vapor profiles were compared with the HALOE satellite instrument. The agreement between HALOE and MIAWARA was for most altitudes better than 10%. In the second comparison phase, MIAWARA took part in the Lapland Atmosphere‐Biosphere Facility (LAPBIAT) Upper Tropospheric Lower Stratospheric Water Vapour Validation Project (LAUTLOS‐WAVVAP) campaign in early 2004 in the subarctic region of northern Finland. During this campaign, different balloon sondes probed the water vapor content in the upper troposphere and lower stratosphere. The stratospheric water vapor profiles of the fluorescent hygrometer FLASH‐B and the NOAA frost point hygrometer mirror in the range of 20–26 km were compared with the lowermost retrieval points of MIAWARA. The agreement between the balloon instruments and MIAWARA was better than 2% for a total number of 10 comparable flights. This showed the potential of MIAWARA in water vapor retrieval down to 20 km. In addition, the northern Finland MIAWARA profiles were compared with POAM III water vapor profiles. This comparison confirmed the good agreement with the other instruments, and the difference between MIAWARA and POAM was generally less than 8%. Finally, the tipping curve calibration was validated with tipping curve measurements of the All‐Sky Multi Wavelength Radiometer (ASMUWARA) which was operated 10 months side by side with MIAWARA. The agreement of the tropospheric opacity derived from these tipping curves agree within 1%.

Spatial and spectral variability of the outgoing thermal IR spectra from AIRS: A case study of July 2003

Here we present a survey of the spatial variability in different climate zones seen from AIRS data using the spectral EOF analysis. Over the tropical and subtropical oceans, the first principal component (PC1) is mostly due to the thermal contrast between surface and thick cold cloud tops. The second principal component (PC2) is mainly due to the spatial variation of the lower tropospheric humidity (LTH) and the low clouds. The signature of dust aerosol over the Arabian Sea and the Atlantic off the coast of North Africa in the summertime can be clearly seen in the PC2. Both the PC1 and the PC2 capture the upper tropospheric water vapor variability due to the forced orthogonality of EOFs. The third principal component (PC3) is mainly due to the spatial variation of the lower stratospheric temperature. Over the midlatitude oceans, the PC1 is still due to the thermal contrast of emission temperature. During wintertime, the PC2 is mainly due to stratospheric temperature variations. In the summer, the PC2 over the southern hemisphere is still due to stratospheric temperature variations, but in the northern hemisphere it is mainly due to the variations of the LTH and the low clouds. An exploratory study using synthetic spectra based on a NCAR CAM2 simulation shows that the model could account for the essential features in the data as well as provide an explanation of the three leading PCs. Major disagreements exist in the location of the ITCZ, the dust aerosol, and the lower stratospheric temperature.

Longitudinal variability of water vapor and cirrus in the tropical tropopause layer

The Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) are both instruments on the Upper Atmosphere Research Satellite and are sensitive to water vapor and subvisible cirrus in the tropical tropopause layer. We analyze the longitudinal and vertical variations of water vapor in the tropical tropopause layer using a new version of MLS water vapor that has improved vertical resolution. The water vapor fields show similarity with subvisible cirrus and sea surface temperature (SST) on seasonal and intraseasonal timescales. The formation of cirrus is more prevalent over continental landmasses and over warm oceanic areas where the upper troposphere is also moist. The intraseasonal variability in cirrus is correlated with upper tropospheric water vapor and linked to a variability in SST of about 1°C. The incidence of cirrus increases with SST until temperatures exceed about 29°C. A further rise in sea surface temperature does not see a corresponding increase in cirrus. Similarly, the incidence of cirrus increases as 100 hPa temperatures fall, with clouds being more likely when temperatures are about 190K. At temperatures colder than this, when in situ formation of cirrus should be more likely, there is a decrease in the likelihood of finding cirrus. Thus evidence points strongly to a dominant role for the convective formation of these cirrus.

Note on the effect of horizontal gradients for nadir-viewing microwave and infrared sounders

Passive microwave and infrared nadir sounders such as the Advanced Microwave Sounding Unit A (AMSU-A) and the Atmospheric InfraRed Sounder (AIRS), both flying on NASA s EOS Aqua satellite, provide information about vertical temperature and humidity structure that is used in data assimilation systems for numerical weather prediction and climate applications. These instruments scan cross track so that at the satellite swath edges, the satellite zenith angles can reach approx. 60 deg. The emission path through the atmosphere as observed by the satellite is therefore slanted with respect to the satellite footprint s zenith. Although radiative transfer codes currently in use at operational centers use the appropriate satellite zenith angle to compute brightness temperature, the input atmospheric fields are those from the vertical profile above the center of the satellite footprint. If horizontal gradients are present in the atmospheric fields, the use of a vertical atmospheric profile may produce an error. This note attempts to quantify the effects of horizontal gradients on AIRS and AMSU-A channels by computing brightness temperatures with accurate slanted atmospheric profiles. We use slanted temperature, water vapor, and ozone fields from data assimilation systems. We compare the calculated slanted and vertical brightness temperatures with AIRS and AMSU-A observations. We show that the effects of horizontal gradients on these sounders are generally small and below instrument noise. However, there are cases where the effects are greater than the instrument noise and may produce erroneous increments in an assimilation system. The majority of the affected channels have weighting functions that peak in the upper troposphere (water vapor sensitive channels) and above (temperature sensitive channels) and are unlikely t o significantly impact tropospheric numerical weather prediction. However, the errors could be significant for other applications such as stratospheric analysis. Gradients in ozone and tropospheric temperature appear to be well captured by the analyses. In contrast, gradients in upper stratospheric and mesospheric temperature as well as upper tropospheric humidity are less well captured. This is likely due in part to a lack of data to specify these fields accurately in the analyses. Advanced new sounders, like AIRS, may help to better specify these fields in the future.

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.

Characterization of Upper-Troposphere Water Vapor Measurements during AFWEX Using LASE

AbstractWater vapor mass mixing ratio profiles from NASA's Lidar Atmospheric Sensing Experiment (LASE) system acquired during the Atmospheric Radiation Measurement (ARM)–First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Water Vapor Experiment (AFWEX) are used as a reference to characterize upper-troposphere water vapor (UTWV) measured by ground-based Raman lidars, radiosondes, and in situ aircraft sensors over the Department of Energy (DOE) ARM Southern Great Plains (SGP) site in northern Oklahoma. LASE was deployed from the NASA DC-8 aircraft and measured water vapor over the ARM SGP Central Facility (CF) site during seven flights between 27 November and 10 December 2000. Initially, the DOE ARM SGP Cloud and Radiation Testbed (CART) Raman lidar (CARL) UTWV profiles were about 5%–7% wetter than LASE in the upper troposphere, and the Vaisala RS80-H radiosonde profiles were about 10% drier than LASE between 8 and 12 km. Scaling the Vaisala water vapor profiles to match the precipitable water vapor (PWV) measured by the ARM SGP microwave radiometer (MWR) did not change these results significantly. By accounting for an overlap correction of the CARL water vapor profiles and by employing schemes designed to correct the Vaisala RS80-H calibration method and account for the time response of the Vaisala RS80-H water vapor sensor, the average differences between the CARL and Vaisala radiosonde upper-troposphere water vapor profiles are reduced to about 5%, which is within the ARM goal of mean differences of less than 10%. The LASE and DC-8 in situ diode laser hygrometer (DLH) UTWV measurements generally agreed to within about 3%–4%. The DC-8 in situ frost point cryogenic hygrometer and Snow White chilled-mirror measurements were drier than the LASE, Raman lidars, and corrected Vaisala RS80H measurements by about 10%–25% and 10%–15%, respectively. Sippican (formerly VIZ Manufacturing) carbon hygristor radiosondes exhibited large variabilities and poor agreement with the other measurements. PWV derived from the LASE profiles agreed to within about 3% on average with PWV derived from the ARM SGP microwave radiometer. The agreement between the LASE and MWR PWV and the LASE and CARL UTWV measurements supports the hypotheses that MWR measurements of the 22-GHz water vapor line can accurately constrain the total water vapor amount and that the CART Raman lidar, when calibrated using the MWR PWV, can provide an accurate, stable reference for characterizing upper-troposphere water vapor.

The impact of tropical convection and cirrus on upper tropospheric humidity: A Lagrangian analysis of satellite measurements

Geostationary satellite observations are used in conjunction with an objective pattern‐tracking algorithm to describe the Lagrangian evolution of convection, clouds and water vapor in the tropical upper troposphere. This analysis reveals that larger convective events within a Lagrangian air mass are associated with larger and longer‐lived cirrus anvil shields. Convective systems which generate larger cirrus shields are, in turn, associated with higher downstream humidity levels following the anvil's dissipation. In the absence of cirrus, the clear‐sky upper troposphere is shown to dry at a rate consistent with radiatively‐driven subsidence. The presence of cirrus anvils following a convective event is shown to reduce the rate of drying and for large anvils can even change its sign. Analysis of the Lagrangian tendencies suggests that this moistening effect is not attributable to the evaporation of cirrus condensate, but instead results from the same dynamical mechanisms responsible for the formation and maintenance of the cirrus anvil.

Intraseasonal variations of water vapor and cirrus clouds in the tropical upper troposphere

Space‐time variations of tropical upper tropospheric water vapor and cirrus clouds associated with the intraseasonal oscillation (ISO) are investigated using data from the Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS). Composite moisture and meteorological fields based on five ISO events selected in two boreal winters (1991–1993) are analyzed using 20–80 day band‐pass‐filtered data. At 215 and 146 hPa, wet anomalies with frequent appearance of cirrus clouds exist over the convective system and move eastward from the Indian Ocean to the central Pacific, suggesting a direct effect of convective activity up to this level. At 100 hPa, however, the moisture field seems to be indirectly affected by convective activity through the dynamical response to the convective heating. Dry anomalies are observed over the Indian Ocean around the developing stage and over the eastern Pacific around the mature‐to‐decaying stage of the ISO. Cirrus clouds are frequently found over the cold region located to the east of the convective system. These structures around the tropopause level are closely related to the eastward moving Kelvin and Rossby wave responses to the convective heating with the equatorial cold anomaly and with the subtropical anticyclonic gyres. Between the two gyres the easterly wind blowing through the equatorial cold region may cause dehydration through cirrus formation when the convective system develops over the Indian Ocean and the western Pacific. As the northern gyre intensifies, tropical dry air is transported to the subtropical Pacific and eventually to the equatorial eastern Pacific. It is suggested that the temperature and flow variations due to the coupled Kelvin‐Rossby wave structure play an important role in dehydrating air in the tropical and subtropical tropopause region.

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.

Satellite observations of the water vapor greenhouse effect and column longwave cooling rates: Relative roles of the continuum and vibration‐rotation to pure rotation bands

The Clouds and the Earth's Radiant Energy System (CERES) instrument on board the Tropical Rainfall Measuring Mission (TRMM) satellite provides, for the first time, a large‐scale (40 S to 40 N) data set for the atmospheric greenhouse effect and the column‐averaged longwave (LW) radiative cooling rates in the broadband (5–100 microns) and the window (8–12 microns) regions. We demonstrate here that the separation into the window and the nonwindow (5–8 microns and 12–100 microns) fluxes provides the first global‐scale data set to exhibit the sensitivity of the atmospheric greenhouse effect to vertical water vapor distribution. The nonwindow greenhouse effect varies linearly with the logarithm of column water vapor amount weighted with the atmospheric pressure, while the window component varies quadratically with the water vapor partial pressure. The column cooling rates range from about −170 to −210 W m−2 for the nonwindow region. The window cooling rates are only about 10% to 20% of the above range and approach rapidly to near‐zero values for surface temperatures less than 288 K. The nonwindow component of the greenhouse effect and cooling rates are shown to be more sensitive to upper troposphere water vapor, while the window greenhouse effect and cooling rates are shown to be more sensitive to the lower troposphere water vapor amount. In addition, the data reveal that in tropical regions, with warm sea surface temperatures (greater than 297 K) and elevated upper tropospheric water vapor amounts, the continuum emission in the window region leads to enhanced cooling of the column, while the rotational bands in the nonwindow region lead to a net decrease in the longwave cooling of the atmospheric column.

Water Vapor Feedback in the Tropical Upper Troposphere: Model Results and Observations

The sensitivity of water vapor in the tropical upper troposphere to changes in surface temperature is examined using a single-column, radiative–convective model that includes couplings between the moistening effects of convective detrainment, the drying effects from clear-air subsidence, and radiative heating and cooling from water vapor. Equilibrium states of this model show that as the surface warms, changes in the vertical distribution and temperature of detraining air from tropical convection lead to higher water vapor mixing ratios in the upper troposphere. However, the increase in mixing ratio is not as large as the increase in saturation mixing ratio due to warmer environmental temperatures, so that relative humidity decreases. These changes in upper-tropospheric humidity with respect to surface temperature are consistent with observed interannual variations in relative humidity and water vapor mixing ratio near 215 mb as measured by the Microwave Limb Sounder and the Halogen Occultation Experiment. The analysis suggests that models that maintain a fixed relative humidity above 250 mb are likely overestimating the contribution made by these levels to the water vapor feedback.