Variation Of Precipitable Water Vapor Research Articles

Using a global positioning system to estimate precipitable water vapor in Antarctica

The Global Positioning System (GPS) technology has become an essential tool in retrieving atmospheric precipitable water vapor (PWV) around the globe. This paper addressed the determination of GPS PWV in Antarctica and its response to atmospheric events. The GPS PWV result is then compared to the PWV determined from radiosonde (RS) and NCEP/NCAR Reanalysis. Time series comparisons of PWV between GPS and RS show strong linear correlation (R 2=0.70, p<1%) with their different mean values bounded at ±3 mm and with an RMS of 1.41 mm. Moreover, the total annual GPS PWV content at Scott Base, Casey, and Syowa stations averaged over the period from 2006 to 2008 has been quantified below 30 mm during summer polar days (wet summer) and of about 15 mm in winter polar night (dry winter), and their variability has shown a seasonal cycle dependence. One of the most remarkable features of the PWV variability from these regions carried by transient weather systems was that they closely followed the temperature patterns and revealed U-distribution. The temperature and PWV patterns characterize a coreless winter phenomenon.

A Study of Near Real-time Water Vapor Analysis Using a Nationwide Dense GPS Network of Japan

In order to use the nationwide dense receiver network of the Global Positioning System (GPS) in Japan to contribute to water vapor monitoring and numerical weather prediction, a near real-time (NRT) analysis trial was performed at the Meteorological Research Institute. In this paper, the NRT analysis procedure is described along with some validation results.In view of the computational load involved in analyzing more than 1,300 GPS stations in Japan, the precise point positioning (PPP) procedure was adopted. The PPP procedure requires accurate information of GPS satellites’ positions and clock offsets. International GNSS Service (IGS) has been routinely providing ultra rapid ephemeris (IGU) that includes satellite orbits and clock offsets with latency of about 3 hours. We found the accuracy of satellite clock offsets in IGU was insufficient for the retrieval of precipitable water vapor (PWV) through the PPP procedure. Therefore, we applied correction to the IGU clock using the predicted clock offset at an IGS station “USUD”. The hydrogen maser atomic clock at USUD also had some differences with GPS time. However, we found it could be fitted and predicted by a linear equation for a period of several days.The resulting satellite clock offsets exhibited some biases toward the IGS final ephemeris, but the time constant biases of satellite clock offsets did not affect the PWV retrieval at all. The retrieved PWV data agreed well with those obtained from radio-sonde observations. The root mean square differences in summer and in winter were around 3.4 mm and 1.6 mm, respectively. The results were comparable with those obtained by preceding studies using the final ephemeris. The Retrieved spatial and temporal variation of PWV in a heavy rainfall case demonstrated the usefulness of the NRT PWV retrieval for weather monitoring. We could capture the water vapor increase that preceded torrential rain.

Systematic errors between VLBI and GPS precipitable water vapor estimations from 5-year co-located measurements

Space geodetic techniques (e.g., Global Positioning System, GPS and very long baseline interference, VLBI) have been widely used to determine the precipitable water vapor (PWV) for meteorology and climatology, which was verified by comparing with co-located independent technique observations. However, most of these comparisons have been conducted using only short-time spanning observations at several stations. The goal of this study is to identify and quantify the systematic errors between VLBI and GPS PWV estimates using a 5-year (2002–2007), PWV data set constructed from co-located measurements and radiosonde data as well. It has found systematic errors between VLBI and GPS PWV estimations from comparisons with long-term co-located GPS measurements. The total mean VLBI PWVs are systematically smaller than GPS estimates with 0.8–2.2 mm for all sites, but can be as much as 15–30%. The subdiurnal PWV variation magnitudes and long-term trends between VLBI and GPS are nearly similar, but the VLBI-derived PWV trends are systematically smaller than GPS estimates with about 0.1±0.02 mm/year. These systematic errors in PWV estimates between VLBI and GPS are probably due to technique own problems, different used elevation angles and co-location separation.

Analysis of GPS-sensed atmospheric water vapour variability and its response to the terrestrial winds over Antarctica

Recent advances in GPS sensing technology and the availability of low-cost GPS receivers have allowed the continuous measurements of atmospheric water vapour in all weather conditions and on a global scale. This paper aims to analyze the variability of the Antarctic precipitable water vapour (PWV) observed by GPS receivers. In this contribution, the influence of terrestrial winds on the PWV variations is investigated where the climatology is extremely different from that in the Equatorial region. For this purpose, GPS and surface meteorological data for the period of 2003–2005 are analyzed. A significant diurnal variation in PWV was found over most of the sites with PWV difference increasing more than 0.50 mm during summer compared to the winter times on a monthly analysis. Increase in Antarctic PWV evidently follows the variations of the surface temperature. In winter, the temperature increase will be lower by about 1 °C and values of water vapour are 13% smaller than during summer months. The characteristics of Antarctic climate represented by the PWV variation show a seasonal dependence, higher (active) in summer and lower (inactive) in winter. It was found that the large decreases in the monthly mean PWV distribution were accompanied by strong winds above 10 m s −1.

Observations of Antarctic precipitable water vapor and its response to the solar activity based on GPS sensing

Predicting global climate change is a great challenge and must be based on a thorough understanding of how the climate system components behave. Precipitable water vapor (PWV) is one of the key components in determining and predicting the global climate system. It is well known that the local surface temperature and pressure have a direct influence on the production of PWV. However, the influence of solar activity on atmospheric dynamics and their physical mechanisms is still an open debate, where past studies are focused at mid-latitude regions. A new method of determining and quantifying the solar influence on PWV based on GPS observations to correlate the GPS PWV and total electron content (TEC) variations is proposed. Observed data from Scott Base (SBA) and McMurdo (MCM) stations from 2003 to 2005 have been used to study the response of PWV to solar activity. In the analysis, the effects of local conditions (wind speed and relative humidity) on the distribution of PWV are investigated. Results show significant correlation between PWV and solar activity for four geomagnetic storms, with correlation coefficients of 0.74, 0.77, 0.64 and 0.69, which are all significant at the 95% confidence level. There was no significant correlation between TEC and PWV changes during the absence of storms. On a monthly analysis, a strong relationship exists between PWV and TEC during storm-affected days, with correlation coefficients of 0.83 and 0.89 (99% confidence level) for SBA and MCM respectively. These indicate a statistically significant seasonal signal in the Antarctic region, which is very active (higher) during the summer and inactive (lower) for the winter periods.

GPS‐based atmospheric precipitable water vapor estimation using meteorological parameters interpolated from NCEP global reanalysis data

This study for the first time used the Indian GPS network data along with the interpolated National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis meteorological data to estimate the precipitable water vapor (PWV) over the Indian atmosphere for a 4 year period (2001–2004) at 21 Indian GPS and 7 International GNSS Service stations. The site‐specific daily average meteorological data required for water vapor estimation at these sites is obtained by performing vertical interpolation on 2.5 × 2.5° gridded NCEP/NCAR global reanalysis data set available at 17 pressure levels and horizontal interpolation on similarly gridded NCEP/NCAR data set available at the surface. The comparison of the site‐specific interpolated surface pressure and temperature thus obtained from NCEP/NCAR data with the measured pressure and temperature profiles available at six GPS sites gives biases of &lt;1 mbar and &lt;3°C which exhibits the robustness of the interpolation schemes. The water vapor estimated at these six sites located in different geographical regions using interpolated surface pressure and weighted mean temperature values from NCEP data compare well (bias &lt; 0.5 mm and standard deviation &lt; 0.6 mm) with water vapor estimated using measured surface pressure and temperature values. GPS PWV estimates also compare well (bias &lt; 3 mm) with the PWV estimates from the nearby radiosonde sites. GPS PWV values at all the sites compare well (bias &lt; 5 mm) with the horizontally interpolated NCEP PWV values except for few sites located in the highly undulated terrain. These GPS PWV values of the Indian network (4–35°N) are then used to model the spatial variability of PWV over the Indian subcontinent as a function of zenith wet delay (ZWD). The modeled spatial variability function gives a quick and reliable estimate of near real‐time GPS PWV in Indian subcontinent directly from ZWD thus eliminating the need of site‐specific weighted mean temperature values.

Multiscale analysis of precipitable water vapor over Africa from GPS data and ECMWF analyses

This is the first climatological analysis of precipitable water vapor (PWV) from GPS data over Africa. The data reveal significant modulations and variability in PWV over a broad range of temporal scales. GPS PWV estimates are compared to ECMWF reanalysis ERA40. Both datasets show good agreement at the larger scales (seasonal cycle and inter‐annual variability), driven by large scale moisture transport. At intra‐seasonal (15–40 days) and synoptic (3–10 days) scales, strong PWV modulations are observed from GPS, consistently with ECMWF analysis. They are shown to be correlated with convection and the passage of equatorial waves and African Easterly waves. The high‐frequency GPS observations also reveal a significant diurnal cycle in PWV, which magnitude and spectral content depends strongly on geographic location and shows a seasonal modulation. The diurnal cycle of PWV is poorly represented in ERA40 reflecting weaknesses in the water cycle of global circulation models at this timescale.

Moisture analysis of a squall line case based on precipitable water vapor data from a ground-based GPS network in the Yangtze River Delta

A squall line swept eastward across the area of the Yangte River Delta and produced gusty winds and heavy rain from the afternoon to the evening of 24 August 2002. In this paper, the roles of moisture in the genesis and development of the squall line were studied. Based on the precipitable water vapor (PWV) data from a ground-based GPS network over the Yangtze River Delta in China, plus data from a Pennsylvania State University/National Atmospheric Center (PSU/NCAR) mesoscale model (MM5) simulation, initialized by three-dimensional variational (3D-VAR) assimilation of the PWV data, some interesting features are revealed. During the 12 hours prior to the squall line arriving in the Shanghai area, a significant increase in PWV indicates a favorable moist environment for a squall line to develop. The vertical profile of the moisture illustrates that it mainly increased in the middle levels of the troposphere, and not at the surface. Temporal variation in PWV is a better precursor for squall line development than other surface meteorological parameters. The characteristics of the horizontal distribution of PWV not only indicated a favorable moist environment, but also evolved a cyclonic wind field for a squall line genesis and development. The “+2 mm” contours of the three-hourly PWV variation can be used successfully to predict the location of the squall line two hours later.

Precipitable Water Vapor around Orographically Induced Convergence Line

During the winter monsoon, a convergence line called the Boso Front often appears in the area between the Kanto Plain and the Izu Islands, Japan. Two typical cases of high-level Precipitable Water Vapor (PWV) are observed along the convergence line in the lee of Chubu Mountains by the aid of latest techniques of Global Positioning System (GPS).Statistic analysis of GPS derived Precipitable Water Vapor (GPS-PWV) indicates that the prevailing wind direction controls the position of the high-level PWV in the lee of Chubu Mountains. A numerical model simulates the behavior of water vapor, namely, the temporal variation of simulated PWV and surface wind, which suggest the reasons for the high PWV near the convergence line. The convergence of surface wind gathers moisture, and trapped moisture by reduced wind velocity may also contribute the high-level PWV.

Aerosol variability in the Po Valley analyzed from automated optical measurements

Aerosol characteristics for the Po Valley (northern Italy) are analyzed using 7‐year time series of automated optical measurements recorded at two sites located at the extreme ends of the valley, one in the northwest area and the other in the northern Adriatic coastal region (Acqua Alta Oceanographic Tower). The annual cycles of aerosol optical thickness τa at 440 nm are very similar, with an overall daily average of 0.35–0.39, minima in winter and maxima in spring or summer. The Ångström exponent is remarkably constant in time and across sites, averaging 1.55 ± 0.35. Conversely, the diurnal variability is very different. The site of Ispra shows an increase in aerosol load during the day (amplitude of ∼10%), typical of an environment near urban/industrial sources, whereas the coastal Adriatic site exhibits a strong decrease from morning to afternoon (amplitude of ∼20%). Both trends are mirrored by the variability in precipitable water vapor.

Detection of water vapor variations driven by thermally‐induced local circulations using the Japanese Continuous GPS Array

I present the detailed variations of precipitable water vapor (PWV) for a summer day using the Japanese continuous Global Positioning System (GPS) array. Spatial and temporal PWV variations show good coherence with the development of thermally‐induced local circulations, which reflect the movement of water vapor over a spatial scale of a few tens of kilometers.

Effect of coarse biogenic aerosol on downwelling infrared flux at the surface

The effect of coarse biogenic aerosol on the downwelling infrared flux at the surface was estimated from the enhancement of spectral IR zenith radiances measured by the ground‐based Fourier transform spectrometer Emission‐Infrared‐Spectrometer for Atmospheric Research (EISAR) at Potsdam, Germany. The enhancement in IR downwelling flux was found to 10.40 W m−2 for a pine pollen concentration of ∼2000 pollens per m3 per day, which is ∼8 times the monthly mean at the site in May where pine pollens are present in the air. The enhancement is ∼2 times larger than caused by a 10% change in precipitable water vapor (PWV) at midlatitudes in summer and also roughly 2 times the amount that would be caused by conventional aerosol models at 5 km visibility. The enhancement at average pine pollen abundance is estimated to ∼1–2 W m−2 and is not negligible if the effect found is not pine pollen specific. The effect of pollens on the downwelling radiance is a spectrally smooth curve with a maximum of 11 mW (m2 sr cm−1)−1 at 1000 cm −1. A broad dip in the radiance enhancement was found between 850 and 1000 cm−1 that could not be assigned to known atmospheric absorbers. It is thus most likely pollen specific and is probably caused by pollen exines. The spectral enhancement in IR zenith radiance caused by the pollens was compared with changes induced by PWV variations and the abundance of aerosol types. A distinct radiance enhancement by pine pollens of up to 80% in maximum and 20% on average was found around the 4.67 μm CO fundamental band used for remote sounding of CO from ground and satellites.

Temporal variability of precipitable water vapor in the North Atlantic

Radiometric measurements of precipitable water vapor were made at Porto Santo Island, Madeira, Portugal, over a one-year period. Temporal variability of the precipitable water vapor is estimated using structure functions. The power law index of the structure functions is compared to values predicted by a statistical model of water vapor fluctuations.

Results of Year-Round Remotely Sensed Integrated Water Vapor by Ground-Based Microwave Radiometry

Based on two years of measurements with a time resolution of 1 min, some climatological findings on precipitable water vapor (PWV) and cloud liquid water (CLW) in central Europe are given. A weak diurnal cycle is apparent. The mean overall diurnal variation was about 0.15 cm in summer and 0.05 cm in winter, equivalent to 8% and 5%, respectively. The PWV increase starts in summer at about 0800 local time and has a maximum of about 0.02 cm h21 between 1000 and 1500 local time, equivalent to about 1% PWV h21. There was, on average, no PWV variation during the night in winter. PWV decreased in winter during the morning with a maximum of 0.01 cm h21 and increased in the afternoon with a maximum of 0.01 cm h21. Thus, the accuracy of the monthly means of PWV based on monitoring systems with low-time resolution (satellites, radiosondes) is only slightly affected by the diurnal course of PWV in central Europe. On average an increase in PWV and CLW was found in the 30-min interval before precipitation in summer. The PWV increase was, however, only about 0.1 cm or 5% of PWV within the last 2 h before rain. The corresponding CLW increase was 0.1 mm, which is considerable as precipitation was observed when CLW reached 0.3‐0.4 mm.

Possible Near-IR Channels for Remote Sensing Precipitable Water Vapor from Geostationary Satellite Platforms

Remote sensing of troposheric water vapor profiles from current geostationary weather satellites is made using a few broadband infrared (IR) channels in the 6-13 micron region. Uncertainties greater than 20% exist in derived water vapor values just above the surface from the IR emission measurements. In this paper, we propose three near-IR channels, one within the 0.94-micron water vapor band absorption region, and the other two in nearby atmospheric windows, for remote sensing of precipitable water vapor over land areas, excluding lakes and rivers, during daytime from future geostationary satellite platforms. The physical principles are as follows. The reflectance of most surface targets varies approximately linearly with wavelength near 1 micron. The solar radiation on the sun-surface-sensor ray path is attenuated by atmospheric water vapor. The ratio of the radiance from the absorption channel with the radiances from the two window channels removes the surface reflectance effects and yields approximately the mean atmospheric water vapor transmittance of the absorption channel. The integrated water vapor amount from ground to space can be obtained with a precision of better than 5% from the mean transmittance. Because surface reflectances vary slowly with time, temporal variation of precipitable water vapor can be determined reliably. High spatial resolution, precipitable water vapor images are derived from spectral data collected by the Airborne Visable-Infrared Imaging Spectrometer, which measures solar radiation reflected by the surface in the 0.4-2.5 micron region in 10-nm channels and has a ground instantaneous field of view of 20 m from its platform on an ER-2 aircraft at 20 km. The proposed near-IR reflectance technique would complement the IR emission techniques for remote sensing of water vapor profiles from geostationary satellite platforms, especially in the boundary layer where most of the water vapor is located.

Atmospheric effects on SMMR and SSM/1 37 GHz polarization difference over the Sahel

The atmospheric effects on the difference of vertically and horizontally polarized brightness temperatures, Delta(T) observed at 37 GHz frequency of the SMMR on board the Nimbus-7 satellite and SSM/I on board the DMSP-F8 satellite are studied over two 2.5 by 2.5 deg regions within the Sahel and Sudan zones of Africa from January 1985 to December 1986 through radiative transfer analysis using surface temperature, atmospheric water vapor, and cloud optical thickness. It is found that atmospheric effects alone cannot explain the observed temporal variation of Delta(T), although the atmosphere introduces important modulations on the observed seasonal variations of Delta(T) due to rather significant seasonal variation of precipitable water vapor. These Delta(T) data should be corrected for atmospheric effects before any quantitative analysis of land surface change over the Sahel and Sudan zones.

The measurement of atmospheric water vapor: radiometer comparison and spatial variations

Two water vapor radiometer (WVR) experiments were conducted to evaluate whether such instruments are both suitable and necessary to correct for propagation effects that are induced by precipitable water vapor (PWV) on signals from the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI). WVRs are suitable for these corrections if they provide wet path delays to better than 0.5 cm. They are needed if spatial variations of PWV result in complicated, direction-dependent propagation effects that are too complex to be parameterized in the GPS or VLBI geodetic solution. In the first experiment, the suitability of radiometers were addressed by comparing six WVRs at Stapleton International Airport in Denver, Colorado, for two weeks. The second experiment addressed the question whether radiometers are needed for the detection of inhomogeneities in the wet delay. Three JPL D-series radiometers were operated at three sites in Colorado approximately 50 km apart. The WVRs simultaneously sampled PWV at different azimuths and elevations in search of spatial variations of PWV. >

Mean precipitable water vapour and rainfall liability index for the Indian area and neighbourhood

Mean monthly perceptible water vapour at 19 radiosonde stations in India, based on data for all years for which upper air ascents are available upto 1965 and 15 radiorisonde stations in the neighbouring countries have been calculated. The seasonal variations of precipitable water vapour and the mean monthly rainfall over the country are discussed. It is seen that the diversity in the magnitude of rainfall is not reflected by the values of the prccipitable water. There may be enough precipitable water at a place but dynamic factors like ascending currents etc may not be generally present to cause rain. As a rough indicator of the liability for rainfall at a place, the ratio of the mean monthly rainfall and the monthly mean precipitable water have been worked out for all the stations. Isopleths of this ratio-Rainfall Liability Index (RLI)-have been drawn for various months. It is felt that the maps of RLI over the country could be more meaningful and of greater practical utility for hydrological purposes than those of mean precipitable water alone.

Seasonal variation of precipitable water vapour in the atmosphere over India

Based on the upper air data for the six-year period, 1956 to 1961, the mean monthly precipitable water vapour at 12 Indian radiosonde stations have been calculated. The seasonal and diurnal variations of precipitable water vapour and. its relation to mean monthly rainfall have been discussed.