Water Vapor In Upper Troposphere Research Articles

Contribution of water vapor to observational estimates of longwave cloud radiative forcing

It has been long recognized that cloud‐free regions are typically associated with drier air, whereas cloudy regions typically are associated with more moisture. Using Advanced Microwave Scanning Radiometer (AMSR‐E) passive microwave observations, we estimated the bias between water vapor path (WVP) of clear‐sky and all‐sky conditions. Furthermore, we assessed the impact of these biases on longwave cloud radiative forcing (CRF). Results indicate that, in a zonal mean sense, the difference between all‐sky and clear‐sky WVP estimates is around 2 mm, with highest values in the midlatitudes summer, where differences up to 7 mm occur. The corresponding CRF change forced by these WVP changes is about 2 W m−2 in a zonal mean sense. Highest values occur in the midlatitudes of the northern hemisphere in which a magnitude up to 6 W m−2 is shown. Over the convectively active regions, the WVP contributed CRF appears to be smaller than 5 W m−2, substantially smaller than the CRF contributed by upper tropospheric water vapor only, suggesting that a significant portion of satellite‐estimated CRF over the tropics is contributed by the redistribution of water vapor associated with convection development.

On the diurnal cycle of deep convection, high‐level cloud, and upper troposphere water vapor in the Multiscale Modeling Framework

The Multiscale Modeling Framework (MMF), also called “superparameterization”, embeds a cloud‐resolving model (CRM) at each grid column of a general circulation model to replace traditional parameterizations of moist convection and large‐scale condensation. This study evaluates the diurnal cycle of deep convection, high‐level clouds, and upper troposphere water vapor by applying an infrared (IR) brightness temperature (Tb) and a precipitation radar (PR) simulator to the CRM column data. Simulator results are then compared with IR radiances from geostationary satellites and PR reflectivities from the Tropical Rainfall Measuring Mission (TRMM). While the actual surface precipitation rate in the MMF has a reasonable diurnal phase and amplitude when compared with TRMM observations, the IR simulator results indicate an inconsistency in the diurnal anomalies of high‐level clouds between the model and the geostationary satellite data. Primarily because of its excessive high‐level clouds, the MMF overestimates the simulated precipitation index (PI) and fails to reproduce the observed diurnal cycle phase relationships among PI, high‐level clouds, and upper troposphere relative humidity. The PR simulator results show that over the tropical oceans, the occurrence fraction of reflectivity in excess of 20 dBZ is almost 1 order of magnitude larger than the TRMM data especially at altitudes above 6 km. Both results suggest that the MMF oceanic convection is overactive and possible reasons for this bias are discussed. However, the joint distribution of simulated IR Tb and PR reflectivity indicates that the most intense deep convection is found more often over tropical land than ocean, in agreement with previous observational studies.

Observed and Simulated Upper-Tropospheric Water Vapor Feedback

Abstract Satellite measurements from the Atmospheric Infrared Sounder (AIRS) in the upper troposphere over 4.5 yr are used to assess the covariation of upper-tropospheric humidity and temperature with surface temperatures, which can be used to constrain the upper-tropospheric moistening due to the water vapor feedback. Results are compared to simulations from a general circulation model, the NCAR Community Atmosphere Model (CAM), to see if the model can reproduce the variations. Results indicate that the upper troposphere maintains nearly constant relative humidity for observed perturbations to ocean surface temperatures over the observed period, with increases in temperature ∼1.5 times the changes at the surface, and corresponding increases in water vapor (specific humidity) of 10%–25% °C−1. Increases in water vapor are largest at pressures below 400 hPa, but they have a double peak structure. Simulations reproduce these changes quantitatively and qualitatively. Agreement is best when the model is sorted for satellite sampling thresholds. This indicates that the model reproduces the moistening associated with the observed upper-tropospheric water vapor feedback. The results are not qualitatively sensitive to model resolution or model physics.

Timescale analysis of aerosol sensitivity during homogeneous freezing and implications for upper tropospheric water vapor budgets

Using timescales for the generation and depletion of water vapor, we predict aerosol sensitivity in clouds formed by homogeneous freezing. Our timescale analysis explains why aerosol sensitivity increases dramatically with ice deposition coefficients (αi) ≪ 0.1, and also why aerosol sensitivity increases as vertical velocity increases, temperature decreases, aerosol number decreases, and aerosol size decreases. We combine existing in‐situ observations with adiabatic parcel modeling to constrain αi ≥ 0.1 for small ice crystals forming at high ice supersaturations. Two important implications for understanding and modeling upper tropospheric water vapor budgets emerge from our results: 1) aerosol sensitivity can be appreciable at low temperatures and moderate updrafts (∼5 cm/s) in the upper tropical troposphere, 2) reconciling our results with recent laboratory measurements supports theory that αi increases with ice supersaturation and/or decreases with ice crystal size.

Ten Years of Measurements of Tropical Upper-Tropospheric Water Vapor by MOZAIC. Part II: Assessing the ECMWF Humidity Analysis

Abstract In a recent publication (Part I), the authors introduced a data source—Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC)—for monitoring and studying upper-tropospheric water vapor (UTWV) and analyzed 10 yr (1994–2004) of MOZAIC measurements of tropical UTWV in its climatology, variability, transport, and relation to deep convection. In this study (Part II), MOZAIC is used to assess the ECMWF humidity analysis over the tropics, taking advantage of the unique nature of the MOZAIC data, namely, the long data record, near-global coverage, and high accuracy. In parallel to Part I, the ECMWF UTWV analysis is assessed against MOZAIC in the following five aspects: 1) annual cycle, 2) vertical structure, 3) probability density functions (PDFs), 4) moisture flux divergence, and 5) interannual variability. The annual cycle of the ECMWF UTWV shows a similar pattern as MOZAIC but has an overall dry bias of about 10%–30% relative humidity with respect to ice (RHi). The dry biases are larger in the deep tropics than the subtropics and larger over the Asian monsoon region than the tropical Atlantic region. The increase in RH with height (from about 300 to 200 hPa) as observed by MOZAIC is largely missing in the ECMWF analysis, which has a roughly constant RH profile. The bimodal distribution of tropical UTWV is well established in MOZAIC, but for ECMWF, the moist mode is abruptly cut off at 100% RHi due to the lack of ice supersaturation (ISS) in the forecast model. Lack of ISS capability is, however, not the only cause for the dry bias in the ECMWF; it also has more occurrences of lower humidity compared to MOZAIC. There is also evidence that ECMWF underestimates the range of upper-tropospheric humidity (UTH) variation. A comparison of moisture flux divergence is conducted to assess the ability of ECMWF to capture the divergent transport of water vapor. It is shown that the ECMWF can represent the distribution of this quantity fairly well, although the dry bias leads to some underestimate of the magnitude. Finally, the authors show a comparison of the ECMWF and MOZAIC depictions of the interannual variation of UTWV during the 1997/98 ENSO event as an illustration that UTWV variations are more difficult to capture than those of the UT temperature.

CRISTA-NF measurements of water vapor during the SCOUT-O3 Tropical Aircraft Campaign

The new remote sensing experiment CRISTA-NF (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere – New Frontiers) successfully participated in the SCOUT-O3 Tropical Aircraft Campaign in November and December 2005. CRISTA-NF operated aboard the high-altitude research aircraft M-55 Geophysica. Mid-infrared spectra (4–15 μm) were measured in the limb sounding geometry with high spatial resolution (250 m vertical sampling, 5–15 km along track sampling). Measurements were carried out during transfer flights between Oberpfaffenhofen, Germany, and Darwin, Australia, as well as during several local flights near Darwin. Water vapor volume mixing ratios in the upper troposphere and lower stratosphere were derived from the CRISTA-NF radiance measurements by utilizing a rapid radiative transfer forward model and the optimal estimation retrieval approach. CRISTA-NF water vapor measurements below the hygropause have a total retrieval error of 15–40% (i.e. root mean square of accuracy and precision). The systematic terms are dominating in the retrieval error budget. The contributions of a priori information to the retrieval results are less than 5–10%. The vertical resolution of the observations is about 250–500 m when permitted by instrument sampling. In this paper we present first results for three transfer flights of the campaign. Being generally in good agreement with corresponding ECMWF operational analyzes, the CRISTA-NF measurements show significantly higher variability and local structures in the upper tropospheric water vapor distributions.

What causes the excessive response of clear‐sky greenhouse effect to El Niño warming in Community Atmosphere Models?

To diagnose the causes of an excessive response of the clear‐sky greenhouse effect to El Niño warming in the Community Atmosphere Models (CAMs), the response of both water vapor and temperature to El Niño warming in the models is examined as a function of height. The percentage response of water vapor to El Niño warming in the models is considerably stronger than the response in the NCEP reanalysis in the middle and upper troposphere (700–300 mbar). The maximum discrepancy with NCEP data at 500 mbar reaches 18%/K in CAM3. The discrepancy in the temperature response between the models and NCEP data at all tropospheric levels is within 0.3 K/K, with the maximum discrepancy occurring in the immediate neighborhood of 600 mbar. The comparison between the models and ERA‐40 reanalysis leads to the similar results. Employing a radiative model, we have calculated the contributions of the excessive water vapor response in the middle and upper troposphere and the contributions from the differences in the lapse rate response to the discrepancies seen in the clear‐sky greenhouse effect. The results confirm that the main cause of the excessive response of the clear‐sky greenhouse effect is an excessive response of water vapor in the middle and upper troposphere. The excessive response of upper tropospheric water vapor is found to be accompanied with an excessive response in the upper cloud cover and vertical motion. Biases in both phases of ENSO contribute to these excessive responses to ENSO.

Validation of Aura Microwave Limb Sounder water vapor by balloon‐borne Cryogenic Frost point Hygrometer measurements

Here we present extensive observations of stratospheric and upper tropospheric water vapor using the balloon‐borne Cryogenic Frost point Hygrometer (CFH) in support of the Aura Microwave Limb Sounder (MLS) satellite instrument. Coincident measurements were used for the validation of MLS version 1.5 and for a limited validation of MLS version 2.2 water vapor. The sensitivity of MLS is on average 30% lower than that of CFH, which is fully compensated by a constant offset at stratospheric levels but only partially compensated at tropospheric levels, leading to an upper tropospheric dry bias. The sensitivity of MLS observations may be adjusted using the correlation parameters provided here. For version 1.5 stratospheric observations at pressures of 68 hPa and smaller MLS retrievals and CFH in situ observations agree on average to within 2.3% ± 11.8%. At 100 hPa the agreement is to within 6.4% ± 22% and at upper tropospheric pressures to within 23% ± 37%. In the tropical stratosphere during the boreal winter the agreement is not as good. The “tape recorder” amplitude in MLS observations depends on the vertical profile of water vapor mixing ratio and shows a significant interannual variation. The agreement between stratospheric observations by MLS version 2.2 and CFH is comparable to the agreement using MLS version 1.5. The variability in the difference between observations by MLS version 2.2 and CFH at tropospheric levels is significantly reduced, but a tropospheric dry bias and a reduced sensitivity remain in this version. In the validation data set a dry bias at 177.8 hPa of −24.1% ± 16.0% is statistically significant.

Transport above the Asian summer monsoon anticyclone inferred from Aura Microwave Limb Sounder tracers

Tracer variability above the Asian summer monsoon anticyclone is investigated using Aura Microwave Limb Sounder (MLS) measurements of carbon monoxide, ozone, water vapor, and temperature during Northern Hemisphere summer (June to August) of 2005. Observations show persistent maxima in carbon monoxide and minima in ozone within the anticyclone in the upper troposphere–lower stratosphere (UTLS) throughout summer, and variations in these tracers are closely related to the intensity of underlying deep convection. Temperatures in the UTLS are also closely coupled to deep convection (cold anomalies are linked with enhanced convection), and the three‐dimensional temperature patterns are consistent with a dynamical response to near‐ equatorial convection. Upper tropospheric water vapor in the monsoon region is strongly coherent with deep convection, both spatially and temporally. However, at the altitude of the tropopause, maximum water vapor is centered within the anticyclone, distant from the deepest convection, and is also less temporally correlated with convective intensity. Because the main outflow of deep convection occurs near ∼12 km, well below the tropopause level (∼16 km), we investigate the large‐scale vertical transport within the anticyclone. The mean vertical circulation obtained from the ERA40 reanalysis data set and a free‐running general circulation model is upward across the tropopause on the eastern end of the anticyclone, as part of the balanced three‐dimensional monsoon circulation. In addition to deep transport from the most intense convection, this large‐scale circulation may help explain the transport of constituents to tropopause level.

Monthly mean upper tropospheric humidities in cloud-free areas from meteosat observations

The purpose of this paper is to study the annual variation of the upper tropospheric relative humidity fields extracted from radiance measurements in the water vapour channel (in the thermal infra-red at wavelengths around 6.3 μm) aboard the geostationary satellite METEOSAT. Monthly mean fields for April, July and October 1988 and January 1989 have been created from the operational Upper Tropospheric Humidity (hereafter UTH) product derived routinely at the European Space Operations Centre (ESOC). The present product retrieval is confined to cloud-free areas in the upper troposphere, thus the monthly means represent an average over cloud-free cases. Principally the patterns of the monthly mean UTH fields reflect the large-scale circulation of the atmosphere. In the low latitudes, belts with relatively high and low UTH values are located over the ascending and descending branches of the ‘Hadley cell’, respectively. The distinct patterns of monthly mean UTH fields can be used to characterize the geographical structure of the Hadley cell. The UTH fields complement existing data on atmospheric humidity for use in numerical weather prediction and climatological research. The monthly mean fields presented in this paper may be useful for validating upper tropospheric water vapour fields in climate models.

Nitric acid particles in cold thick ice clouds observed at global scale: Link with lightning, temperature, and upper tropospheric water vapor

Signatures of nitric acid particles (NAP) in cold thick ice clouds have been derived from satellite observations. Most NAP are detected in the tropics (9 to 20% of clouds with T < 202.5 K). Higher occurrences were found in the rare midlatitudes very cold clouds. NAP occurrence increases as cloud temperature decreases, and NAP are more numerous in January than July. Comparisons of NAP and lightning distributions show that lightning seems to be the main source of the NOx, which forms NAP in cold clouds over continents. Qualitative comparisons of NAP with upper tropospheric humidity distributions suggest that NAP may play a role in the dehydration of the upper troposphere when the tropopause is colder than 195 K.

Ten Years of Measurements of Tropical Upper-Tropospheric Water Vapor by MOZAIC. Part I: Climatology, Variability, Transport, and Relation to Deep Convection

Abstract Ten years (1994–2004) of measurements of tropical upper-tropospheric water vapor (UTWV) by the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) are investigated over three regions—the tropical Atlantic, tropical Africa, and the Asian monsoon region—to determine the UTWV climatology and variability on multiple scales and to understand them in relation to moisture transport and deep convection. The seasonal migration of upper-tropospheric humidity (UTH) keeps pace with that of the ITCZ, indicating the convective influence on UTH distribution. Some significant regional differences are identified with the tropical Africa and the Asian monsoon regions being moister than the tropical Atlantic. UTH generally increases with height by 10%–20% relative humidity with respect to ice (RHi) from about 300 to 200 hPa, and the differences are larger in the deep Tropics than in the subtropics. The probability density functions of tropical UTH are often bimodal. The two modes stay rather constant; differences in the mean value are largely due to the variations in the proportion of the two modes as opposed to changes in the modes themselves. In the deep Tropics, the moisture level frequently reaches ice supersaturation, the most notable case being the near-equatorial Asian monsoon region during the wet season when ice supersaturation is observed 46% of the time. Interannual variations are observed in association with the 1997–98 ENSO event. A warming of about 1–2 K is observed for all three regions equatorward of roughly 15°. Specific humidity also increases somewhat for the tropical Atlantic and tropical Africa, but the increase in temperature outweighs the increase in specific humidity such that RH decreases by 5%–15% RHi. In addition to the ENSO-related variation, MOZAIC also sees increases in both RH and specific humidity over tropical Africa from 2000 onward. Moisture fluxes are computed from MOZAIC data and decomposed into contributions from the mean circulation and from eddies. The flux divergence, which represents the moisture source/sink from horizontal transport, is also estimated. Finally, the MOZAIC climatology and variability are revisited in relation to deep convection obtained from the International Satellite Cloud Climatology Project (ISCCP).

Evaluation of the Influence of Saturation Adjustment with Respect to Ice on Meso-scale Model Simulations for the Case of 22 June, 2002

A meso-scale model simulation is sometimes conducted with a saturation adjustment scheme with respect to ice in the upper troposphere. In such a simulation, the super-saturation with respect to ice is not permitted in upper tropospheric clouds, which is contradicting with natural situation.In this study, we evaluate the influence of the application of ice-saturation adjustment to upper tropospheric clouds in a meso-scale model. When the ice-saturation adjustment is applied, all of the excess water vapor in upper troposphere is consumed to produce large amount of cloud ice and subsequently snow. Small amount of excess water vapor remain for the depositional growth of snow. Since the fall velocity of snow becomes smaller, the residence time of snow in the atmosphere becomes longer. Consequently, the surface rainfall intensity, on domain-average, becomes slightly smaller through the integration time, compared to the simulation without ice-saturation adjustment.

Modeling Water Vapor in the Upper Troposphere and Lower Stratosphere

Upper troposphere and lower stratosphere (UTLS) water vapor is investigated using a general circulation model, the Community Atmosphere Model 3.0 (CAM3.0). Seasonal variability in UTLS water vapor, temperature and zonal wind, based on model simulation results for the period 1991-2000, are analyzed. Results are validated against satellite data from the Halogen Occultation Experiment (HALOE) and ERA-40 reanalyzes from ECMWF. The model captures the seasonal cycle in temperature as well as water vapor. The zonal wind deviates from the reanalysis data in the tropics as the model is not able to reproduce the Quasi-Biennial Oscillation (QBO). Outside the tropics, the zonal wind corresponds very well with the ERA-40 zonal wind. The model is able to reproduce the seasonal signal in the tropical stratospheric water vapor; i.e., the Tape Recorder Signal. This indicates a realistic Brewer-Dobson circulation in the model. However, the Tape Recorder signal attenuates too fast compared to the HALOE data, suggesting a too strong horizontal mixing between the tropical stratosphere and mid latitudes. CAM3.0 shows considerable improvements in UTLS temperatures as well as water vapor compared to earlier generations of the NCAR general circulation models.

Relationships of upper tropospheric water vapor, clouds and SST: MLS observations, ECMWF analyses and GCM simulations

The relationships of upper tropospheric water vapor (UTWV), cloud ice and sea surface temperature (SST) are examined in the annual cycles of ECMWF analyses and simulations from 15 atmosphere‐ocean coupled models which were contributed to the IPCC AR4. The results are compared with the observed relationships based on UTWV and cloud ice measurements from MLS on Aura. It is shown that the ECMWF analyses produce positive correlations between UTWV, cloud ice and SST, similar to the MLS data. The rate of the increase of cloud ice and UTWV with SST is about 30% larger than that for MLS. For the IPCC simulations, the relationships between UTWV, cloud ice and SST are qualitatively captured. However, the magnitudes of the simulated cloud ice show a considerable disagreement between models, by nearly a factor of 10. The amplitudes of the approximate linear relations between UTWV, cloud ice and SST vary by a factor up to 4.

Dry Bias in Satellite-Derived Clear-Sky Water Vapor and Its Contribution to Longwave Cloud Radiative Forcing

Abstract In this paper, the amount of satellite-derived longwave cloud radiative forcing (CRF) that is due to an increase in upper-tropospheric water vapor associated with the evolution from clear-sky to the observed all-sky conditions is assessed. This is important because the satellite-derived clear-sky outgoing radiative fluxes needed for the CRF determination are from cloud-free areas away from the cloudy regions in order to avoid cloud contamination of the clear-sky fluxes. However, avoidance of cloud contamination implies a sampling problem as the clear-sky fluxes represent an area drier than the hypothetical clear-sky humidity in cloudy regions. While this issue has been recognized in earlier works this study makes an attempt to quantitatively estimate the bias in the clear-sky longwave CRF. Water vapor amounts in the 200–500-mb layer corresponding to all-sky condition are derived from microwave measurements with the Special Sensor Microwave Temperature-2 Profiler and are used in combination with cloud data for determining the clear-sky water vapor distribution of that layer. The obtained water vapor information is then used to constrain the humidity profiles for calculating clear-sky longwave fluxes at the top of the atmosphere. It is shown that the clear-sky moisture bias in the upper troposphere can be up to 40%–50% drier over convectively active regions. Results indicate that up to 12 W m−2 corresponding to about 15% of the satellite-derived longwave CRF in tropical regions can be attributed to the water vapor changes associated with cloud development.

Multimodel Analysis of the Water Vapor Feedback in the Tropical Upper Troposphere

Abstract Relationships between the mean humidity in the tropical upper troposphere and tropical sea surface temperatures in 17 coupled ocean–atmosphere global climate models were investigated. This analysis builds on a prior study of humidity and surface temperature measurements that suggested an overall positive climate feedback by water vapor in the tropical upper troposphere whereby the mean specific humidity increases with warmer sea surface temperature (SST). The model results for present-day simulations show a large range in mean humidity, mean air temperature, and mean SST, but they consistently show increases in upper-tropospheric specific humidity with warmer SST. The model average increase in water vapor at 250 mb with convective mean SST is 44 ppmv K−1, with a standard deviation of 14 ppmv K−1. Furthermore, the implied feedback in the models is not as strong as would be the case if relative humidity remained constant in the upper troposphere. The model mean decrease in relative humidity is −2.3% ± 1.0% K−1 at 250 mb, whereas observations indicate decreases of −4.8% ± 1.7% K−1 near 215 mb. These two values agree within the respective ranges of uncertainty, indicating that current global climate models are simulating the observed behavior of water vapor in the tropical upper troposphere with reasonable accuracy.

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.