Tropospheric Water Vapor Profiles Research Articles

Ground-based FTIR water vapour profile analyses

Abstract. Due to a large vertical gradient and strong variability of water vapour, algorithms that are effectively applied for ground-based remote sensing of many different atmospheric trace gases can be insufficient for the retrieval of tropospheric water vapour profiles. We review the most important features of the retrieval and of the radiative transfer modelling required for accurate monitoring of tropospheric water vapour profiles by ground-based FTIR (Fourier Transform Infrared) experiments. These are: a fit of a variety of different water vapour lines with different strength, a logarithmic scale inversion, a speed dependent Voigt line shape model, and a joint temperature profile retrieval. Furthermore, the introduction of an interspecies constraint allows for a monitoring of HDO/H2O ratio profiles.

Comparisons of Raman Lidar Measurements of Tropospheric Water Vapor Profiles with Radiosondes, Hygrometers on the Meteorological Observation Tower, and GPS at Tsukuba, Japan

Abstract The vertical distribution profiles of the water vapor mixing ratio (w) were measured by Raman lidar at the Meteorological Research Institute, Japan, during the period from 2000 to 2004. The measured values were compared with those obtained with radiosondes, hygrometers on a meteorological observation tower, and global positioning system (GPS) antennas near the lidar site. The values of w obtained with the lidar were lower than those obtained with the corrected Meisei RS2-91 radiosonde by 1.2% on average and higher than those obtained with the corrected Vaisala RS80-A radiosonde by 17% for w ≥ 0.5 g kg−1. The lidar data were higher than those radiosondes’ data by 19% or 33% for w < 0.5 g kg−1. The vertical variations of w obtained with the lidar differed from those obtained with the Meisei RS-01G radiosonde and Meteolabor Snow White radiosonde by 5% on average for w ≥ 0.5 g kg−1. The lidar data were lower than those radiosondes’ data by 37% or 39% for w < 0.5 g kg−1. The temporal variations of w obtained with the lidar and the hygrometers on the meteorological tower agreed to within 0.4% at a height of 213 m, although the absolute values differed systematically by 9%–14% due to the incomplete overlap of the laser beam and the receiver’s field of view at heights between 50 and 150 m. The precipitable water vapor obtained with the lidar indicated a mean positive bias of 2 mm (9%–11%) relative to those obtained with GPS. The lidar water vapor calibration coefficient that was calculated using RS2-91 radiosonde data varied by 11% during an 18-month period. Therefore, it is necessary to develop an accurate, yet convenient, method for determining the calibration coefficient for the use of the lidar.

The STARTWAVE atmospheric water database

Abstract. The STARTWAVE (STudies in Atmospheric Radiative Transfer and Water Vapour Effects) project aims to investigate the role which water vapour plays in the climate system, and in particular its interaction with radiation. Within this framework, an ongoing water vapour database project was set up which comprises integrated water vapour (IWV) measurements made over the last ten years by ground-based microwave radiometers, Global Positioning System (GPS) receivers and sun photometers located throughout Switzerland at altitudes between 330 and 3584 m. At Bern (46.95° N, 7.44° E) tropospheric and stratospheric water vapour profiles are obtained on a regular basis and integrated liquid water, which is important for cloud characterisation, is also measured. Additional stratospheric water vapour profiles are obtained by an airborne microwave radiometer which observes large parts of the northern hemisphere during yearly flight campaigns. The database allows us to validate the various water vapour measurement techniques. Comparisons between IWV measured by the Payerne radiosonde with that measured at Bern by two microwave radiometers, GPS and sun photometer showed instrument biases within ±0.5 mm. The bias in GPS relative to sun photometer over the 2001 to 2004 period was –0.8 mm at Payerne (46.81° N, 6.94° E, 490 m), which lies in the Swiss plains north of the Alps, and +0.6 mm at Davos (46.81° N, 9.84° E, 1598 m), which is located within the Alps in the eastern part of Switzerland. At Locarno (46.18° N, 8.78° E, 366 m), which is located on the south side of the Alps, the bias is +1.9 mm. The sun photometer at Locarno was found to have a bias of –2.2 mm (13% of the mean annual IWV) relative to the data from the closest radiosonde station at Milano. This result led to a yearly rotation of the sun photometer instruments between low and high altitude stations to improve the calibrations. In order to demonstrate the capabilites of the database for studying water vapour variations, we investigated a front which crossed Switzerland between 18 November 2004 and 19 November 2004. During the frontal passage, the GPS and microwave radiometers at Bern and Payerne showed an increase in IWV of between 7 and 9 mm. The GPS IWV measurements were corrected to a standard height of 500 m, using an empirically derived exponential relationship between IWV and altitude. A qualitative comparison was made between plots of the IWV distribution measured by the GPS and the 6.2 µm water vapour channel on the Meteosat Second Generation (MSG) satellite. Both showed that the moist air moved in from a northerly direction, although the MSG showed an increase in water vapour several hours before increases in IWV were detected by GPS or microwave radiometer. This is probably due to the fact that the satellite instrument is sensitive to an atmospheric layer at around 320 hPa, which makes a contribution of one percent or less to the IWV.

Water vapor distributions measured with the Michelson Interferometer for Passive Atmospheric Sounding on board Envisat (MIPAS/Envisat)

We present water vapor profiles obtained from infrared limb emission measurements recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the European Environmental Satellite (Envisat). These retrievals are based on constrained nonlinear least squares fitting. The retrievals are very sensitive to the radiative signals of thin transparent clouds and measurements showing any signature of cloud contamination have been rigorously excluded. The vertical resolution of the retrieved water vapor profiles is 4.5 to 6.5 km up to an altitude of approximately 42 km. The resulting total error of the retrieved water vapor profiles, including measurement noise, systematic and random parameter uncertainties like interfering species or preretrieved temperature or spectroscopic data, is in the range of 6 to 9% in the stratosphere. Towards the tropopause, the error increases up to 30% due to the exponential gradient of the tropospheric water vapor profile, where small line of sight uncertainties lead to strong absolute variations in the water vapor profile below the hygropause. Averaged water vapor distributions obtained from measurements taken during 11 days in June, July, and August 2003 show the expected distributions with low water vapor volume mixing ratios (VMRs) above the tropopause, comparatively dry air inside the tropical stratospheric updraft region and indications for strong dehydration above the Antarctic continent inside the polar vortex. Additionally, in the transition from tropics to subtropics, a latitude band was observed where, in higher altitudes, large water vapor VMRs were measured compared to adjacent tropical and midlatitudinal regions. Especially over the Arabic peninsula, a moist region at 18 km altitude was observed, which was probably related to the South Asian monsoon circulation.

Global monitoring of tropospheric water vapor with GPS radio occultation aboard CHAMP

The global positioning system radio occultation (GPS RO) technique provides a powerful tool for atmospheric sounding which requires no calibration, is not affected by clouds, aerosols or precipitation, and provides an almost uniform global coverage. The paper deals with application of GPS RO measurements from CHAllenging Minisatellite Payload (CHAMP) for the retrieval of tropospheric water vapor profiles. CHAMP RO data are available since 2001 with up to 200 high resolution atmospheric profiles per day. We introduce a new direct method for water vapor retrieval from GPS RO data. Additionally, a 1Dvar algorithm is used for this purpose. The so derived CHAMP water vapor profiles are validated with radiosonde data on a global scale. Here, both methods come to statistically comparable results revealing a negative bias of less than 0.1 g/kg and a standard deviation of less than 1 g/kg specific humidity in the mid troposphere. Potentials of CHAMP RO retrievals for monitoring the mean tropospheric water vapor distribution on a global scale are presented.

Lidar measurements of tropospheric water vapor and aerosol profiles over southeastern Italy

A combined Rayleigh‐Raman lidar based on a XeF excimer laser (351 nm) has been used for the regular monitoring of tropospheric aerosols and water vapor profiles over southern Italy (40°20′N, 18°6′E) in the framework of the European Aerosol Research Lidar Network. Coincident measurements of vertical profiles of the aerosol backscattering ratio R(z), the aerosol extinction coefficient αλoaer(z), and the water vapor mixing ratio w(z) have been performed during several months to investigate the correlation of R(z) and αλoaer(z) with water vapor and to characterize the aerosol optical properties. A strong correlation has been found between the spatial and temporal evolution of R(z) and w(z) both in summer and in autumn regime. The experimental results have revealed that R(z) increases with w(z) and that the aerosol and water vapor burdens are lower in autumn than in the summer. The analysis of the data by using the 4‐day analytical backward trajectories has revealed that the dependence of αλoaer(z) and R(z) on w(z) is quite affected by the typical advection patterns over the lidar site. Aerosols providing a larger contribution to backscattering ratio and extinction coefficient as w(z) increases have been monitored when air masses from northern and eastern European countries were advected over Lecce. The findings in terms of the lidar ratio S have revealed that aerosols with average S values ranging from 50 to 63 sr have been monitored when air masses from northern and eastern Europe were advected over Lecce. On the contrary, average S values ranging from 48 to 74 have been retrieved for air masses advected from North Africa.

Tropospheric ozone layers observed during PEM‐Tropics B

In this paper we analyze distinct layers seen in the tropospheric ozone and water vapor profiles taken during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission in the Tropical Pacific (PEM‐Tropics) Phase B campaign. In summary, fewer layers were observed in this campaign than during the PEM‐Tropics A campaign. However, of those layers found, there were relatively more ozone‐rich‐water‐vapor‐poor layers (68% versus 52%). The percentage of the sampled troposphere occupied by layers during PEM‐Tropics B was less than half of that found during PEM‐Tropics A (8% versus 20%). The differences between these two campaigns suggest a seasonal variation in the occurrence of layers. This is confirmed using measurements made by the Southern Hemisphere Additional Ozonesondes (SHADOZ) network.

Sensitivity of radiometric measurements of the atmospheric CO_2 column from space

Anthropogenic emissions of CO2 over the past century has altered significantly the global carbon cycle. Our understanding of the long-term climatic effects of these emissions would be improved greatly by satellite-based remote sounding of CO2. We provide an initial analysis of a simple satellite-based filter radiometer tuned to the spectral region around 6300 cm(-1) (approximately 1.6 microm) to measure the atmospheric column of CO2 from space. We find that such an instrument has potential and that the sensitivity would be limited by our knowledge of the tropospheric temperature and water-vapor profiles.

Physical‐statistical retrieval of water vapor profiles using SSM/T‐2 Sounder data

The feasibility of retrieving water vapor profiles from downlooking passive microwave sounder data is demonstrated by usage of a retrieval algorithm which extends Bayesian optimal estimation. Special Sensor Microwave T‐2 (SSM/T‐2) downlooking sounder data, consisting of brightness temperature measurements sensitive to water vapor, are used together with total water vapor content data for computing tropospheric water vapor profiles. The significant nonlinearity in the cost function, an implication of the corresponding (nonlinear) radiative transfer equation, necessitates several extensions of the well‐known optimal estimation inversion scheme. We supplemented the scheme by simulated annealing and iterative a priori lightweighting and obtained a powerful physical‐statistical hybrid algorithm. Retrievals based on SSM/T‐2 data were compared to atmospheric analyses of the European Centre for Medium‐Range Weather Forecasts (ECMWF). A statistical validation of the retrieved profiles is presented. The comparisons indicate an approximate accuracy of about 15 to 20 percent for relative humidity.

Narrow-band, narrow-field-of-view Raman lidar with combined day and night capability for tropospheric water-vapor profile measurements

We describe a high-performance Raman lidar system with combined day and night capability for tropospheric water-vapor profile measurements. The system incorporates high-performance UV interference filters and a narrow-band, dual-field-of-view receiver for rejection of background sunlight. Daytime performance has been demonstrated up to 5 km with 150-m vertical and 5-min temporal averaging. The nighttime performance is significantly better with measurements routinely extending from 10 to 12 km with 75-m range resolution and a 5-min temporal average. We describe design issues for daytime operation and a novel daytime calibration technique.

Local upper tropospheric/lower stratospheric clear‐sky water vapor and tropospheric deep convection

Local upper tropospheric and stratospheric water vapor profile retrievals by Stratospheric Aerosol and Gas Experiment II for July 1986–1990 are compared with concurrent International Satellite Cloud Climatology Project deep convective clouds to investigate the possible impact of deep convection on upper level moisture distributions. Results show that at low latitudes, clear‐sky water vapor values in the upper troposphere (above 300 mbar level) decrease following the onset of convection, perhaps because of compensatory subsidence. The drying lasts for a few hours and then is interrupted by a moistening trend centered at 6 hours after the convection breaks out, perhaps because of the detrainment of cloud liquid water from the convective tower and spreading anvil clouds. From about 9 hours later the tropical lower stratosphere shows moistening occurring. Largely because of these effects, the tropical lower stratosphere has 20–30% more moisture at sunrise compared to sunset. The later stratospheric moistening has the largest correlation significance with convection as well as the largest sampling number, though the absolute variation is much less than the variation in the troposphere. At northern hemisphere summer middle latitudes, upper tropospheric drying follows moist convection events, without an obvious moistening trend in the upper troposphere and impact in the stratosphere. The results indicate that the relationship between local upper tropospheric moisture and convective events is timescale and latitude dependent. They illustrate the utility of high vertical and temporal resolution data for proper averaging and interpretation of convection and water vapor studies.

ラマンライダーによる湿度・後方散乱比・偏光解消度の観測

Simultaneous observation of the vertical profiles of tropospheric water vapor and the density and nonsphericity of aerosol particles was made with a Raman lidar at Nagoya (35°N, 137°E). The data from observations made on April 17, 1994 showed the following features of the water vapor and aerosol particles. Temporal and systematic variations of the parameters were observed at the lower altitude range of 1.0-2.2 km, where the mixing ratio of aerosol particles (backscattering ratio) increased and the nonsphericity of the particles (depolarization ratio) decreased with increasing humidity. There was a positive correlation between the vertical profiles of backscattering ratio and the water vapor mixing ratio. The depolarization ratio was positively correlated with the water vapor mixing ratio above 2.2 km and negatively correlated below this level. The altitudes where the discontinuities of the water vapor and aerosol parameters were found and that where the correlation between the depolarization ratio and the water vapor mixing ratio changed showed the correspondences with the temperature inversion or wind shears observed at Hamamatsu (35°N, 138°E).

Retrieval of 91 and 150 GHz Earth surface emissivities

Surface emissivities at 91 and 150 GHz are retrieved using a microwave radiative transfer model and the atmospheric profiles and brightness temperatures (TBs) for the cloud‐free cases from a global data set of collocated SSM/T‐2 and radiosonde measurements. The retrieved emissivity values for various Earth surface types are consistent with those from other studies. The retrieval scheme displays limited sensitivity to uncertainty in the input data and the retrieved values are found to have good reproducibility for a given region. The possible use of special sensor microwave/imager (SSM/I) TB measurements for estimation of 91‐ and 150‐GHz emissivities is explored using a subset of the retrieved emissivity cases which was augmented with SSM/I data. It is demonstrated that SSM/I TBs are potentially useful for making accurate estimates of surface emissivities at 91 and 150 GHz. This should lead to improved SSM/T‐2 lower tropospheric water vapor profile retrievals.

Moisture Profiling of the Cloudy Winter Atmosphere Using Combined Remote Sensors

Abstract A new method for deriving profiles of tropospheric water vapor and liquid water from a combination of ground-based remote sensors was applied and tested under winter conditions in Colorado. The method is an extension of physical retrieval techniques used to derive coarse profiles from passive microwave radiometer measurements. Unlike an earlier method, it does not depend on climatological data for first-guess profile inputs. Instead, information about current cloud conditions aloft, obtained with active remote sensors, is used to determine physically realistic, first-guess vertical distributions of the radiometer's integrated vapor and liquid measurements. In preliminary tests, the retrieved profiles were compared with in situ measurements by aircraft and radiosondes during the Winter Icing and Storms Project. The shape of the retrieved liquid profiles agreed well with the aircraft measurements, but heights, thicknesses, and amplitudes differed considerably in some cases. The derived vapor profil...

Recent Lidar Technology Developments and Their Influence on Measurements of Tropospheric Water Vapor

In this paper the influences of recent technology developments in the areas of lasers, detectors, and optical filters of a differential absorption lidar (DIAL) system on the measurement of tropospheric water vapor (H20) profiles are discussed. The lidar parameters selected are based upon a diode-seeded Ti:sapphire laser that is locked to an H20 line in the 820- or 930-nm band of H20. To assess the influence of the mode of deployment on the measurement of tropospheric H20, DIAL performance is evaluated for operation from a medium-altitude (12 km) aircraft the ground, and space-based systems. It is found that incorporation of these developments could greatly enhance DIAL measurement capability.

Altitude Profiles of Tropospheric Water Vapor at Low Latitudes

Abstract The altitude profiles of water vapor density, ρ, in the troposphere at low latitudes have been studied using radiosonde observations over nine stations in India. An attempt has been made to evolve a satisfactory model for the altitude variation of ρ in terms of the surface value. It has been found that a simple exponential relation with constant scale height parameter is not always adequate for this purpose. The scale height parameter shows significant variation with altitude. A double exponential profile having two different scale height parameters is found to be more suitable for most of the cases.

ATMOSPHERIC WATER VAPOR PROFILES DERIVED FROM REMOTE-SENSING RADIOMETER MEASUREMENTS

The feasibility and preliminary testing of a low cost, remote-sensing air-borne, double bolometer technique for inferring atmospheric water vapor is illustrated. To deduce the water vapor profile with commercially available equipment, the radiative transfer equation is solved for the water vapor transmissivity employing an input data remote radiometer-measured upward irradiances obtained at aircraft holding levels. Radiometers sensitive in two separate spectral bands are used. The primary radiometer covers the 4.39 to 20.83µ, broad atmospheric radiation band, and the second, for surface temperature deduction, covers the atmospheric window region, 7.35 to 13.16µ. The transfer solution results are acquired from computer programs developed specifically for this purpose. Results indicate an accuracy for inferred total tropospheric water vapor and mixing ratio profiles close to that of the standard sounding electrical hygrometer. The absolute accuracy of the radiosonde hygrometer, considering surface calibration procedures, and for a single ascent, is not better than ±12 percent. The absolute accuracy is greatest for “dry” soundings where the largest changes in irradiance occur for given changes in moisture. Specifically, tests for a vertical profile averaging 6.00 gm./kg. of water vapor produce an average error of 0.70 gm./kg. in the inferred mixing ratio. The average error in mixing ratio obtained by this technique for profiles averaging 2.3 gm./kg. is 0.05 gm./kg. The implications for use on high-flying aircraft or on rockets with highly sensitive radiometers are obvious. The primary purpose in reporting this research is to suggest a technique and illustrate its use. It is clear that with more sensitive bolometer radiometers with selective band pass filters a considerable increase in accuracy can be achieved.