Moisture Soundings Research Articles

Three-Dimensional Water Vapor and Cloud Variations Associated with the Madden–Julian Oscillation during Northern Hemisphere Winter

Abstract The Madden–Julian oscillation (MJO) is the dominant form of intraseasonal variability in the Tropics. In previous studies, intraseasonal variability has usually been characterized in terms of wind or convection anomalies, while the structure of MJO-related moisture variations has been greatly unexplored. This work focuses on the behavior of moisture and related hydrological fields associated with MJO events during Northern Hemisphere winter. Five-day averaged (1979–99) Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) moisture soundings from the NASA Pathfinder data were used, providing global coverage at specific pressure levels. The TOVS moisture, as well as the International Satellite Cloud Climatology Project (ISCCP) cloud fraction anomaly data, were composited based on MJO events selected via an index constructed from Xie–Arkin bandpassed pentad rainfall data. Analysis of the three-dimensional structure and evolution of precipitation, water vapor, and cl...

Retrieval of atmospheric-temperature and water-vapor profiles by use of combined satellite and ground-based infrared spectral-radiance measurements.

A nonlinear sounding retrieval algorithm is used to produce vertical-temperature and water-vapor profiles from coincident observations taken by the airborne High-resolution Interferometer Sounder (HIS) and the ground-based Atmospheric Emitted Radiance Interferometer (AERI) during the SUbsonic Contrails and Clouds Effects Special Study (SUCCESS). Also, clear sky Geostationary Operational Environmental Satellite (GOES) and AERI radiance measurements, achieved on a daily real-time basis at the Department of Energy's Oklahoma CART (Cloud and Radiation Testbed) site, are used to demonstrate the current profiling capability by use of simultaneous geostationary satellite and ground-based remote sensing observations under clear-sky conditions. The discrepancy principle, a method to find the proper smoothing parameters from the minimum value between the normalized spectral residual norm and the a priori upper bound, is used to demonstrate the feasibility and effectiveness of on-line simultaneous tuning of the multiple weighting and smoothing parameters from the combined satellite/airborne and ground-based measurements for the temperature and water-vapor retrieval in this nonlinear-retrieval process. An objective method to determine the degrees of freedom (d.f.) of the observation signal is derived. The d.f. of the radiance signal for the combined GOES and AERI measurements is larger than that for either instrument alone; while the d.f. of the observation signal for the combined GOES and AERI measurements is larger than that for either instrument alone and of the combined GOES and AERI measurements. The use of simultaneous clear-sky AERI and GOES data now provides improved vertical temperature and moisture soundings on an hourly basis for use in the Atmospheric Radiation Measurement program [J. Appl. Meteorol. 37, 875 (1998)].

Diagnosis of Three-Dimensional Water Vapor Using a GPS Network

Abstract In recent years techniques have been developed to obtain integrated water vapor along slant paths between ground-based Global Positioning System (GPS) receivers and the GPS satellites. Results are presented of an observing system simulation (OSS) to determine whether three-dimensional water vapor fields could be recovered from a high-resolution network (e.g., with 40-km spacing) of GPS receivers, in combination with surface moisture observations and a limited number of moisture soundings. The paper describes a three-dimensional variational analysis (3DVAR) that recovers the moisture field from the slant integrated water vapor and other observations. Comparisons between “nature” moisture fields taken from mesoscale models and fields recovered using 3DVAR are presented. It is concluded that a high-resolution network of GPS receivers may allow diagnosis of three-dimensional water vapor, with applications for both positioning and mesoscale weather prediction.

Joint variations of temperature and water vapor over the midlatitude continents

We have used warm‐season data from the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains site to investigate the joint variability of the temperature and moisture soundings, and their relationship to the variability of the generalized convective available potential energy (GCAPE). The actual temperature and humidity soundings vary together in such a way as to produce variations of the GCAPE which are far smaller than those which would occur if the relative humidity varied while the temperature sounding was fixed, or vice versa.

Assimilation of DMSP and TOVS Satellite Soundings in a Mesoscale Model

Abstract A modeling investigation explores the impacts of the assimilation of satellite-retrieved soundings on forecast error in the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Simulations of the period of the U.S. Air Force’s Contrail Experiment (18–29 September 1995) vary the initialization method and datasets assimilated, the performance of first-guess reanalysis, the imposition of quality control (QC) on the satellite data, and the frequency of the model update cycle. MM5 experiments employing four-dimensional data assimilation (FDDA) are compared with a control experiment without FDDA. In the former, combinations of conventional surface and radiosonde observations and retrieved temperature and moisture soundings from the Defense Meteorological Satellite Program (DMSP) and Television and Infrared Observation Satellite Operational Vertical Sounder (TOVS) satellite instruments are assimilated. Forecast error statistics for the experiments are computed and analyzed. It is found that for retrieved temperatures the DMSP and TOVS sounding datasets used have similar, reasonable accuracy, but for retrieved dewpoints they display significant, and more differing, errors. Overall, the TOVS retrievals obtained are of poorer quality than are the DMSP retrievals. Sensitivity tests reveal that imposing a QC filter on the satellite data prior to assimilation does improve the resultant MM5 simulations. With such QC, it is found that assimilating DMSP and TOVS soundings with the methods used can significantly improve the forecasts of both temperature and moisture variables in the MM5. Model performance, however, can still reflect the relative quality of the satellite retrievals assimilated, with the lower-error DMSP data yielding better simulations than do the TOVS data. Tests exploring the reanalysis of first-guess fields obtained from FDDA show that it does benefit the short-term (0–12 h) forecast but that significant gains diminish thereafter.

Effects of Possible Scan Geometries on the Accuracy of Satellite Measurements of Water Vapor

Abstract For future satellite instruments, two scan geometries have been proposed: cross-track and conical scanning. With the mixtures of possible instruments, a future sounding suite may consist of all cross track, all conical, or a combination with temperature channels on one and moisture channels on the other. This paper evaluates the effect of scan angle on the accuracy of moisture soundings. It is found that, as the scan angle increases from nadir, the accuracy of the moisture soundings near the surface decreases because less of the surface signal reaches the satellite. At the same time, the accuracy of the upper-level sounding increases because the weighting functions become sharper as the angle increases. If a mixed system is required where the temperature and moisture channels have different scan geometries, the best accuracy is obtained if the temperature channels are on a cross-track scanner. The scan angles also affect the number of measurements that are too cloud contaminated to provide accura...

Assessing the impact of cloud morphology on infrared sounder scan geometry

Next generation infrared sounders from satellites are expected to provide temperature and moisture soundings with an improved vertical resolution. However, this goal could be severely limited by clouds, since infrared sounders are not able to sense through dense clouds. This paper examines the impact of cloud morphology on the scan geometry. More specifically, the problem of how many clear soundings can be achieved, in cloudy areas, as a function of the scan geometry has been addressed. The analysis is particularly devoted to frontal cloud systems which are typically associated with active weather phenomena. These systems are characterized by many spatial scales and result in fractal structures. This particular geometry has been exploited to formulate and check a power law for the dependence of the probability of achieving a clear sounding on the size of the field of view of the sounder. Six Advanced Very High Resolution Radiometer (AVHRR) scenes over north Europe form the basis of our database and various scan geometries, including mono-pixel and multi-pixel configurations, have been analysed.

Application ofGOES-8/9Soundings to Weather Forecasting and Nowcasting

Since April 1994 a new generation of geostationary sounders has been measuring atmospheric radiances in 18 infrared spectral bands and thus providing the capability for investigating oceanographic and meteorological phenomena that far exceed those available from the previous generation of Geostationary Operational Environmental Satellites (GOES). Menzel and Purdom foreshadowed many of the anticipated improvements from the GOES-8/9 sounders. This article presents some of the realizations; it details the in-flight performance of the sounder, presents both validated operational as well as routinely available experimental products, and shows the impact on nowcasting and forecasting activities. For the first time operational hourly sounding products over North America and adjacent oceans are now possible with the GOES-8/9 sounders. The GOES-8/9 sounders are making significant contributions by depicting moisture changes for numerical weather prediction models over the continental United States, monitoring winds over oceans, and supplementing the National Weather Service's Automated Surface Observing System with upper-level cloud information. Validation of many sounding products has been accomplished by comparison with radiosondes and aircraft measurements. Considerable progress has been made toward assimilation of soundings from clear skies and cloud properties in cloudy regions in operational as well as research forecast models; GOES-8/9 moisture soundings are now being used in the operational Eta regional forecast model.

Retrieval of Precipitation from Microwave Airborne Sensors during TOGA COARE

Abstract The aim of this paper is to report on the retrieval of the vertically averaged liquid cloud water content and vertically averaged precipitation rates (rain and ice) from microwave airborne radiometric observations in a two-plane parallel layer atmosphere. The approach is based on the inversion of a simple radiative transfer model in which a raindrop size distribution derived from microphysical measurements is introduced. The microwave data (18.7, 21, 37, and 92 GHz) used were acquired by the Airborne Multichannel Microwave Radiometer and Advanced Microwave Moisture Sounder on board NASA DC8 within a mesoscale convective system on 6 February 1993 during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. Before interpreting the results, the quality of the inversion is checked. The fit between the measured and the model-retrieved brightness temperatures is good when compared to the model and measurements uncertainties. Doppler radar data from three other aircraft hel...

Lidar-Measured Winds from Space: A Key Component for Weather and Climate Prediction

The deployment of a space-based Doppler lidar would provide information that is fundamental to advancing the understanding and prediction of weather and climate. This paper reviews the concepts of wind measurement by Doppler lidar, highlights the results of some observing system simulation experiments with lidar winds, and discusses the important advances in earth system science anticipated with lidar winds. Observing system simulation experiments, conducted using two different general circulation models, have shown 1) that there is a significant improvement in the forecast accuracy over the Southern Hemisphere and tropical oceans resulting from the assimilation of simulated satellite wind data, and 2) that wind data are significantly more effective than temperature or moisture data in controlling analysis error. Because accurate wind observations are currently almost entirely unavailable for the vast majority of tropical cyclones worldwide, lidar winds have the potential to substantially improve tropical cyclone forecasts. Similarly, to improve water vapor flux divergence calculations, a direct measure of the ageostrophic wind is needed since the present level of uncertainty cannot be reduced with better temperature and moisture soundings alone.

Simulations of the Effects of Water Vapor, Cloud Liquid Water, and Ice on AMSU Moisture Channel Brightness Temperatures

Radiative transfer simulations are performed to determine how water vapor and nonprecipitating cloud liquid water and ice particles within typical midlatitude atmospheres affect brightness temperatures TB's of moisture sounding channels used in the Advanced Microwave Sounding Unit (AMSU) and AMSU-like instruments. The purpose is to promote a general understanding of passive top-of-atmosphere TB's for window frequencies at 23.8, 89.0, and 157.0 GHz, and water vapor frequencies at 176.31, 180.3 1, and 182.31 GHz by documenting specific examples. This is accomplished through detailed analyses of TB's for idealized atmospheres, mostly representing temperate conditions over land. Cloud effects are considered in terms of five basic properties: droplet size distribution, phase, liquid or ice water content, altitude, and thickness. Effects on TB of changing surface emissivity also are addressed. The brightness temperature contribution functions are presented as an aid to physically interpreting AMSU TB's. Both liquid and ice clouds impact the TB's in a variety of ways. The TB's at 23.8 and 89 GHZ are more strongly affected by altostratus liquid clouds than by cirrus clouds for equivalent water paths. In contrast, channels near 157 and 183 GHz are more strongly affected by ice clouds. Higher clouds have a water impact on 157- and 183-GHz TB's than do lower clouds. Clouds depress TB's of the higher-frequency channels by suppressing, but not necessarily obscuring, radiance contributions from below. Thus, TB's are less closely associated with cloud-top temperatures than are IR radiometric temperatures. Water vapor alone accounts for up to 89% of the total attenuation by a midtropospheric liquid cloud for channels near 183 GHz. The Rayleigh approximation is found to be adequate for typical droplet size distributions; however, Mie scattering effects from liquid droplets become important for droplet size distribution functions with modal radii greater than 20 µm near 157 and 183 GHz, and greater than 30–40 µm at 89 GHz. This is due mainly to the relatively small concentrations of droplets much larger than the mode radius. Orographic clouds and tropical cumuli have been observed to contain droplet size distributions with mode radii in the 30–40-µm range. Thus, as new instruments bridge the gap between microwave and infrared to frequencies even higher than 183 GHz, radiative transfer modelers are cautioned to explicitly address scattering characteristics of such clouds.

Water vapor profiling over ocean surface from airborne 90 and 183 GHz radiometric measurements under clear and cloudy conditions

Radiometric measurements at 90 GHz and three sideband frequencies near the water vapor absorption line at 183.3 GHz were made with the Advanced Microwave Moisture Sounder (AMMS) aboard the NASA DC-8 aircraft over some regions of the Pacific Ocean during November 1989. These measurements were used to retrieve atmospheric water vapor profiles over ocean surface using the algorithm developed by T.T. Wilheit (1979). The algorithm incorporates a mechanism to estimate cloud liquid water when the estimated relative humidity is greater than 95%. The results are compared with the estimated values from the measurements of Special Sensor Microwave Imager (SSMI) and TIROS Operational Vertical Sounder (TOVS). The water vapor profiles estimated from AMMS are generally higher at low altitudes and lower at high altitudes compared to those from the TOVS measurements. Values of total precipitable water estimated from the AMMS and SSM/I are in general agreement. Cloud liquid water vapor profiles retrieved from the AMMS show more fluctuations than those from SSM/I.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Retrieval of Total Precipitable Water over High-Latitude Regions Using Radiometric Measurements near 90 and 183 GHz

Radiometric measurements at 90 GHz and three sideband frequencies near the peak water vapor absorption line of 183.3 GHz were made with Advanced Microwave Moisture Sounder (AMMS) aboard the NASA DC-8 aircraft during the Global Aerosol Backscatter Experiment (GLOBE) mission over the Pacific Ocean in November 1989. Some of the measurements over the high-latitude regions (greater than 50 deg N or 50 deg S) were analyzed for the retrieval of total precipitable water less than 0.5 g/sq cm both over land and ocean surfaces. The results show that total precipitable water from a relatively dry atmosphere could be estimated with high sensitivity from these radiometric measurements. The retrieved values over ocean surface show a decrease toward the polar region as expected. The retrieved total precipitable water over land correlates positively with the aircraft radar altitude. This positive correlation is expected because the aircraft radar altitude provides a measure of atmospheric water vapor burden above the surface. Retrieved high reflectivities over land surface at 90 GHz and 183 GHz are presumably related to snow cover on the ground. This suggests that radiometric measurements at these frequencies could be used to map snow at high-latitude regions.

Precipitation observed by the airborne advanced microwave moisture sounder (AMMS)

Airborne radiometric measurements at frequencies near 92 GHz and 183 GHz were conducted over two precipitation events near Wallops Island, Virginia during February, 1986. The measured brightness temperatures are compared with those from calculations to estimate the snowfall and rainfall rates for both events. The estimated rates over water surface are within a factor of two of those derived from the concurrent measurements by the SPANDAR radar at the NASA Wallops Flight Facility. These estimated rates, however, suggest that both snowfall events are light and close to the threshold of radiometric detection especially over land surface. Observations of additional snowfall events with lower frequency channels are needed to demonstrate the approach and to account for cloud effects.

Ground-Based Microwave Radiometric Observations of Precipitable Water Vapor: A Comparison with Ground Truth from Two Radiosonde Observing Systems

Abstract Dual-channel microwave radiometric measurements of precipitable water vapor are compared with values determined from two types of radiosondes. The first type is used in conventional soundings taken by the National Weather Service. The second is used by the CLASS system, as operated by the National Center for Atmospheric Research. The standard deviations of the two comparisons are nearly equal, being about 0.1 cm, but statistically significant biases occur between the radiometer and the radiosondes. A bias of 0.162 cm is present between radiometer and NWS values during the day and 0.075 cm during the night. The comparison shows that significant differences exist between the radiometer and the NWS moisture soundings when the relative humidity drops below 20 percent for pressures greater than 500 hPa. When this situation occurs, the NWS soundings contain a default dewpoint depression value of 30°C. After such data are removed from the comparisons, agreement between radiometer and NWS radiosonde data...

Retrieval of Total Precipitable Water Using Radiometric Measurements Near 92 and 183 GHz

The strong water vapor absorption line at 183 GHz is explored in this paper for retrieval of total precipitable water in the atmosphere. This strong line has generally been utilized in the past for the profiling of the atmospheric water vapor. It is shown from radiative transfer calculations that, under very dry atmospheric conditions, the radiometric response near this frequency behaves much like that near the 22 GHz absorption line but, with the advantages of an increase in sensitivity and potentially an improvement in spatial resolution. Total precipitable water can be retrieval almost independent of atmospheric temperature profiles under these conditions. The technique is demonstrated with the airborne Advanced Microwave Moisture Sounder (AMMS) which has four channels, three of them centered around 183 GHz (183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz) and another at 92 GHz. The calculated sensitivities of radiometric response to total precipitable water are approximately 410, 230, and 130 K (cm)2 g−1 for total precipitable water less than 0.2, 0.3, and 0.5 g (cm)−2 at 183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz respectively. The inclusion of the 92 GHz channel extends the range of the retrieval in excess of 1 g (cm)−2 total precipitable water. However, the effect of cloud cover proves to be strong at this frequency and the retrieval has to be applied with care. Two AMMS observations of dry atmosphere following the cold air outbreaks are analyzed to demonstrate the technique.

Determination of Cloud Ice Water Content and Geometrical Thickness Using Microwave and Infrared Radiometric Measurements

Cloud ice water content and cloud geometrical thickness have been determined using a combination of near-infrared, thermal infrared and thermal microwave radiometric measurements. The radiometric measurements are from a Multispectral Cloud Radiometer, which has seven channels ranging from visible to thermal infrared, and an Advanced Microwave Moisture Sounder, which has four channels ranging from 90 to 183 GHz. Studies indicate that the microwave brightness temperatures depend not only on the amount of ice water content but also on the vertical distribution of ice water content. Studies also show that the low brightness temperature at 92 GHz for large ice water content is due to cloud reflection which reflects most of the irradiance incident at the cloud base downward. Therefore the 92 GHz channel detects a low brightness temperature at the cloud top.

Millimeter wave moisture sounding: The effect of clouds

The effect of water clouds on the retrieval of atmospheric water vapor profiles using simulated millimeter wave sounder data is investigated. Synthetic nadir brightness temperatures are evaluated for a five‐channel hypothetical millimeter wave moisture sounder (91.655 ± 1.25, 150.0 ± 1.25, 183.31 ± 1, 183.31 ± 3, and 183.31 ± 7 GHz) using an appropriate radiative transfer model. Clouds filling the field of view are shown to significantly distort the weighting functions for each channel. The effect of cloud on simulated brightness temperatures over ocean and dry land backgrounds is investigated by para‐metrically varying cloud liquid water content for two cloud types. In general, typical clouds increase brightness temperatures over the ocean and decrease them over land. By applying a simple statistical inversion method to obtain moisture profiles, it is demonstrated that cloud presence degrades the accuracy of water vapor profile retrievals performed in simulation over both land and ocean backgrounds.

Effects of Enhanced Initial Moisture Fields on Simulated Rainfall over West Africa and the East Atlantic

Numerical simulations of a 24-h period during GATE are performed to evaluate the impact of enhanced initial moisture fields on short range forecasts of tropical rainfall. The domain covers West Africa and most of the eastern half of the Atlantic Ocean. A version of the Pennsylvania State University Mesoscale Model is used, and results are verified against satellite rainfall estimates. The initial moisture fields are enhanced subjectively using visible satellite imagery, available rawinsonde soundings and monthly mean soundings. The rainfall forecasts with initial moistures enhanced throughout the analysis region clearly outperform forecasts which use either the unaltered initial moisture analysis or an analysis performed with selectively enhanced moisture soundings. The former produce better zonal and meridional distributions of rain, narrower and more intense intertropical convergence zone (ITCZ) regions and stronger local maxima in agreement with verification. The success of the relatively simple moisture enhancement used in this study suggests that satellite-enhanced moisture analyses will have a major impact on short-range numerical forecasts of tropical rainfall.

Hurricane Debby—An Illustration of the Complementary Nature of VAS Soundings and Cloud and Water Vapor Motion Winds

The utility of VISSR Atmospheric Sounder (VAS) temperature and moisture soundings and cloud and water vapor motion winds in defining a storm and its surroundings at subsynoptic scales has been examined using a numerical analysis and prognosis system. It is shown that the VAS temperature and moisture data, which specify temperature and moisture well in cloud-free areas, are complemented by cloud and water vapor motion data generated in the cloudy areas. The cloud and water vapor 'winds' provide thermal gradient information for interpolating the soundings across cloudy regions. The loss of analysis integrity due to the reduction of VAS sounding density in the cloudy regions associated with synoptic activity is ameliorated by using cloud and water vapor motion winds. The improvement in numerical forecasts resulting from the addition of these data to the numerical analysis is recorded.