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É. Gerard, D. G. H. Tan, L. Garand, V. Wulfmeyer, G. Ehret, and P. Di Girolamo

The need for an absolute standard for water vapor observations, in the form of a global dataset with high accuracy and good spatial resolution, has long been recognized. The European Space Agency's Water Vapour Lidar Experiment in Space (WALES) mission aims to meet this need by providing high-quality water vapor profiles, globally and with good vertical resolution, using a differential absorption lidar (DIAL) system in a low earth-orbit satellite. WALES will be the first active system to measure humidity from space routinely. With launch envisaged in the 2008–2010 time frame and a minimum duration of two years, the primary mission goals are to (a) contribute to scientific research and (b) demonstrate the feasibility of longer-term operational missions. This paper assesses the benefits of the anticipated data to NWP through quantitative analysis of information content. Good vertical resolution and low random errors are shown to give substantial improvements in analysis error in one-dimensional variational data assimilation (1DVAR) comparisons with advanced infrared sounders. In addition, the vertical extent of the profiles is shown to reach 16.5 km or ~100 hPa, well above the limit of radiance assimilation (13 km or 200 hPa). Also highlighted are important applications in atmospheric sciences and climate research that would benefit from the low bias promised by spaceborne DIAL data and their complementarity to other types of humidity observations.

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David K. Adams, Rui M. S. Fernandes, Kirk L. Holub, Seth I. Gutman, Henrique M. J. Barbosa, Luiz A. T. Machado, Alan J. P. Calheiros, Richard A. Bennett, E. Robert Kursinski, Luiz F. Sapucci, Charles DeMets, Glayson F. B. Chagas, Ave Arellano, Naziano Filizola, Alciélio A. Amorim Rocha, Rosimeire Araújo Silva, Lilia M. F. Assunção, Glauber G. Cirino, Theotonio Pauliquevis, Bruno T. T. Portela, André Sá, Jeanne M. de Sousa, and Ludmila M. S. Tanaka

The Amazon Dense GNSS Meteorological Network provides high spatiotemporal resolution, all-weather precipitable water vapor for studying the evolution of continental tropical and sea-breeze convective regimes of Amazonia. The meteorology and climate of the equatorial tropics are dominated by atmospheric convection, which presents a rather challenging range of spatial and temporal scales to capture with present-day observational platforms ( Mapes and Neale 2011 ; Moncrieff et al. 2012 ; Zhang

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David L. Randel, Thomas H. Vonder Haar, Mark A. Ringerud, Graeme L. Stephens, Thomas J. Greenwald, and Cynthia L. Combs

A comprehensive and accurate global water vapor dataset is critical to the adequate understanding of water vapor's role in the earth's climate system. To begin to satisfy this need, the authors have produced a blended dataset made up of global, 5-yr (1988–92), l°x 1° spatial resolution, atmospheric water vapor (WV) and liquid water path products. These new products consist of both the daily total column-integrated composites and a multilayered WV product at three layers (1000–700, 700–500, 500–300 mb). The analyses combine WV retrievals from the Television and Infrared Operational Satellite (TIROS) Operational Vertical Sounder (TOVS), the Special Sensor Microwave/Imager, and radiosonde observations. The global, vertical-layered water vapor dataset was developed by slicing the blended total column water vapor using layer information from TOVS and radiosonde. Also produced was a companion, over oceans only, liquid water path dataset. Satellite observations of liquid water path are growing in importance since many of the global climate models are now either incorporating or contain liquid water as an explicit variable. The complete dataset (all three products) has been named NVAP, an acronym for National Aeronautics and Space Administration Water Vapor Project.

This paper provides examples of the new dataset as well as scientific analysis of the observed annual cycle and the interannual variability of water vapor at global, hemispheric, and regional scales. A distinct global annual cycle is shown to be dominated by the Northern Hemisphere observations. Planetary-scale variations are found to relate well to recent independent estimates of tropospheric temperature variations. Maps of regional interannual variability in the 5-yr period show the effect of the 1992 ENSO and other features.

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Luca Palchetti, Giovanni Bianchini, Gianluca Di Natale, and Massimo Del Guasta

Two years of spectral infrared measurements of downwelling radiation between 7 and 100 µ m in wavelength at high altitude over the Antarctic Plateau are presented. Water vapor and clouds are among the most important components of the atmosphere that modulate the Earth radiation budget by means of the strong contributions to the greenhouse effect and the planetary albedo. They trap a significant amount of the longwave thermal infrared radiation emitted by the underlying atmosphere and the

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W. G. Read, J. W. Waters, D. A. Flower, L. Froidevaux, R. F. Jarnot, D. L. Hartmann, R. S. Harwood, and R. B. Rood

Initial results of upper-tropospheric water vapor obtained from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are presented. MLS is less affected by clouds than infrared or visible techniques, and the UARS orbit provides daily humidity monitoring for approximately two-thirds of the earth. Best results are currently obtained when water vapor abundances are approximately 100–300 ppmv, corresponding to approximately 12-km height in the Tropics and 7 km at high latitudes. The observed latitude variation of water vapor at 215 hPa is in good agreement with the U.K. Universities's Global Atmospheric Modelling Project model. The ability to observe synoptic-scale features associated with tropopause height variations is clearly illustrated by comparison with the National Aeronautics and Space Administration Goddard Space Flight Center assimilation model. Humidity detrainment streams extending from tropical convective regions are also observed.

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Ralph Alvin Petersen, Lee Cronce, Richard Mamrosh, Randy Baker, and Patricia Pauley

Automated aircraft water vapor reports, provided by 148 aircraft worldwide through the WMO’s AMDAR program, are at least as accurate as rawinsonde observations and have greater influence on 1–2 day NWP forecasts than all other in-situ moisture data over the United States Currently, 148 aircraft-based Water Vapor Sensing Systems (WVSS) deliver observations operation-ally across the globe daily, principally across the United States. Although previous studies [summarized by Petersen (2016) ] have

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The Arm Program's Water Vapor Intensive Observation Periods

Overview, Initial Accomplishments, and Future Challenges

H. E. Revercomb, D. D. Turner, D. C. Tobin, R. O. Knuteson, W. F. Feltz, J. Barnard, J. Bösenberg, S. Clough, D. Cook, R. Ferrare, J. Goldsmith, S. Gutman, R. Halthore, B. Lesht, J. Liljegren, H. Linné, J. Michalsky, V. Morris, W. Porch, S. Richardson, B. Schmid, M. Splitt, T. Van Hove, E. Westwater, and D. Whiteman

A series of water vapor intensive observation periods (WVIOPs) were conducted at the Atmospheric Radiation Measurement (ARM) site in Oklahoma between 1996 and 2000. The goals of these WVIOPs are to characterize the accuracy of the operational water vapor observations and to develop techniques to improve the accuracy of these measurements.

The initial focus of these experiments was on the lower atmosphere, for which the goal is an absolute accuracy of better than 2% in total column water vapor, corresponding to ~1 W m−2 of infrared radiation at the surface. To complement the operational water vapor instruments during the WVIOPs, additional instrumentation including a scanning Raman lidar, microwave radiometers, chilled-mirror hygrometers, a differential absorption lidar, and ground-based solar radiometers were deployed at the ARM site. The unique datasets from the 1996, 1997, and 1999 experiments have led to many results, including the discovery and characterization of a large (> 25%) sonde-to-sonde variability in the water vapor profiles from Vaisala RS-80H radiosondes that acts like a height-independent calibration factor error. However, the microwave observations provide a stable reference that can be used to remove a large part of the sonde-to-sonde calibration variability. In situ capacitive water vapor sensors demonstrated agreement within 2% of chilled-mirror hygrometers at the surface and on an instrumented tower. Water vapor profiles retrieved from two Raman lidars, which have both been calibrated to the ARM microwave radiometer, showed agreement to within 5% for all altitudes below 8 km during two WVIOPs. The mean agreement of the total precipitable water vapor from different techniques has converged significantly from early analysis that originally showed differences up to 15%. Retrievals of total precipitable water vapor (PWV) from the ARM microwave radiometer are now found to be only 3% moister than PWV derived from new GPS results, and about 2% drier than the mean of radiosonde data after a recently defined sonde dry-bias correction is applied. Raman lidar profiles calibrated using tower-mounted chilled-mirror hygrometers confirm the expected sensitivity of microwave radiometer data to water vapor changes, but it is drier than the microwave radiometer (MWR) by 0.95 mm for all PWV amounts. However, observations from different collocated microwave radiometers have shown larger differences than expected and attempts to resolve the remaining inconsistencies (in both calibration and forward modeling) are continuing.

The paper concludes by outlining the objectives of the recent 2000 WVIOP and the ARM–First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Water Vapor Experiment (AFWEX), the latter of which switched the focus to characterizing upper-tropospheric humidity measurements.

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Angelyn W. Moore, Ivory J. Small, Seth I. Gutman, Yehuda Bock, John L. Dumas, Peng Fang, Jennifer S. Haase, Mark E. Jackson, and Jayme L. Laber

ionospheric delay to be determined, and the remaining tropospheric effects estimated from the residual delay are the foundation of GPS meteorology. The total tropospheric delay (TD) observed by GPS is the integrated refractivity of the atmosphere, N , over the signal ray path where P is the atmospheric pressure, T is temperature, e is water vapor partial pressure, and the k’s are empirically determined physical constants in an expression for N ( Bevis et al. 1994 ). Therefore, this estimated

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Christopher S. Velden, Christopher M. Hayden, Steven J W. Nieman, W. Paul Menzel, Steven Wanzong, and James S. Goerss

The coverage and quality of remotely sensed upper-tropospheric moisture parameters have improved considerably with the deployment of a new generation of operational geostationary meteorological satellites: GOES-8/9 and GMS-5. The GOES-8/9 water vapor imaging capabilities have increased as a result of improved radiometric sensitivity and higher spatial resolution. The addition of a water vapor sensing channel on the latest GMS permits nearly global viewing of upper-tropospheric water vapor (when joined with GOES and Meteosat) and enhances the commonality of geostationary meteorological satellite observing capabilities. Upper-tropospheric motions derived from sequential water vapor imagery provided by these satellites can be objectively extracted by automated techniques. Wind fields can be deduced in both cloudy and cloud-free environments. In addition to the spatially coherent nature of these vector fields, the GOES-8/9 multispectral water vapor sensing capabilities allow for determination of wind fields over multiple tropospheric layers in cloud-free environments. This article provides an update on the latest efforts to extract water vapor motion displacements over meteorological scales ranging from subsynoptic to global. The potential applications of these data to impact operations, numerical assimilation and prediction, and research studies are discussed.

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D. B. O'Sullivan, B. M. Herman, D. Feng, D. E. Flittner, and D. M. Ward

Present Global Positioning System Meteorology (GPS/MET) refractivity profiles cannot distinguish between refractivity effects due to water vapor and those due to air density. Current methods of resolving the ambiguity rely heavily on ancillary upper-air data, such as National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. However, the accuracy of these ancillary sources suffers in regions where upper-air data are sparse. A method of separating the water vapor and temperature effects in GPS/MET-derived refractivity profiles with the addition of only ancillary surface pressure and temperature data and the hydrostatic assumption is discussed. Water vapor and temperature data derived from this method are presented and compared with accepted values. This method allows for the construction of temperature profiles with a mean bias of 0.33 K and a mean standard deviation of 1.86 K when compared with ECMWF data from 30 to 1000 mb. Height fields can also be corrected to within an average bias of 6 m and a standard deviation of 31 m. These corrected profiles result in retrieved water vapor pressure profiles with an average bias of 0.19 mb and a standard deviation of 0.53 mb.

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