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M. K. Rama Varma Raja, Seth I. Gutman, James G. Yoe, Larry M. McMillin, and Jiang Zhao

1. Introduction Atmospheric water vapor is an important parameter to be considered for a wide range of applications, including studies of the earth’s radiation budget, hydrological cycle, atmospheric chemistry, and global warming, and therefore its accurate measurement is of great interest. The task of accurately measuring atmospheric water vapor is challenging, given that moisture field variations are more sporadic in nature than variations in temperature or pressure. Conventional in situ

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R. Rosolem, W. J. Shuttleworth, M. Zreda, T. E. Franz, X. Zeng, and S. A. Kurc

: Terrestrial Hydrometeorology . John Wiley & Sons, 448 pp. Shuttleworth, W. J. , Rosolem R. , Zreda M. , and Franz T. , 2013 : The Cosmic-Ray Soil Moisture Interaction Code (COSMIC) for use in data assimilation . Hydrol. Earth Syst. Sci. Discuss. , 10 , 1097 – 1125 , doi:10.5194/hessd-10-1097-2013 . Tomasi, C. , 1977 : Precipitable water vapor in atmospheres characterized by temperature inversions . J. Appl. Meteor. , 16 , 237 – 243 . Tomasi, C. , 1978 : On the water vapour absorption

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B. H. Kahn, J. Teixeira, E. J. Fetzer, A. Gettelman, S. M. Hristova-Veleva, X. Huang, A. K. Kochanski, M. Köhler, S. K. Krueger, R. Wood, and M. Zhao

literature to date focuses on comparisons of KE spectra between observations and models ( Hamilton et al. 2008 ). The scale dependences of the variability of temperature T and especially water vapor q are not well characterized in space and time on a global basis despite both variables strongly controlling cloud processes at the subgrid scale in NWP and climate models ( Cusack et al. 1999 ; Tompkins 2002 ; Seiffert and von Storch 2008 ; Wang et al. 2010 ) and therefore helping to establish the

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Zhengzhao Luo, Dieter Kley, Richard H. Johnson, and Herman Smit

1. Introduction Water vapor is the key atmospheric constituent for the earth’s hydrological cycle and the most important greenhouse gas in the climate system. It is highly variable on multiple spatial and temporal scales and is usually expected to be increasing under a global warming scenario resulting in an amplification of the CO 2 greenhouse effect (e.g., Cess et al. 1990 ). Unfortunately, water vapor in the atmosphere is not measured with a high degree of accuracy (compared to the

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Siebren de Haan, Iwan Holleman, and Albert A. M. Holtslag

1. Introduction At present, radiosonde observations are the most important operational source for upper-air water vapor data. These observations are expensive and thus are sparse in time and space. Global positioning system (GPS) zenith total delay (ZTD) observations contain integrated water vapor path information, which can be used in numerical weather prediction (NWP) models and for nowcasting severe weather. Assimilation of GPS observations has a positive impact on the quality of an NWP

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A. M. Chiodi and D. E. Harrison

meridional advection of water vapor, (iii) the formation of the SST anomalies is characterized by abrupt austral summer warming that occurs when moist near-tropical air is advected southward along the western flank of the subtropical anticyclone, and (iv) the position of the subtropical atmospheric anticyclone largely determines the character of these SST anomalies. SST variability in the subtropical southwestern Indian Ocean (the region south of Madagascar, 30°–45°S, 45°–75°E) is known to be positively

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Daniel M. Gilford, Susan Solomon, and Robert W. Portmann

1. Introduction Transport of air from the troposphere to the stratosphere largely occurs across the tropical tropopause layer (TTL), typically located between 20°S and 20°N and from 150 to 70 hPa ( Fueglistaler et al. 2009 ). Water vapor and ozone concentrations vary in the TTL as air parcels cross the cold-point tropopause (CPT; ~90 hPa) into the stratosphere, and have been shown to have substantial radiative impacts within the lower stratosphere and on the troposphere below (e.g., Forster

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Tobias Selz, Lucas Fischer, and George C. Craig

1. Introduction The complex interaction of water vapor with atmospheric motion and mixing processes over a wide range of spatial scales, together with various sinks and sources, leads to a very heterogeneous humidity distribution in the troposphere. A better understanding of the spatial variability of water vapor in the free troposphere is essential for the representation of clouds, including fractional cloud cover, in numerical weather prediction models (NWP) and global circulation models (GCM

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D. D. Turner, E. J. Mlawer, and H. E. Revercomb

1. Introduction From the earliest days of the ARM Program, water vapor and temperature measurements were considered among the highest priority measurements made at the ARM sites. The program founders recommended that these observations “be performed on a continuing, real-time basis throughout the experiment” so that the radiation models and cloud parameterizations could be evaluated over a wide range of atmospheric conditions ( DOE 1990 ; ARM 2016 , appendix A). Furthermore, the program

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R. K. Newsom, D. D. Turner, R. Lehtinen, C. Münkel, J. Kallio, and R. Roininen

1. Introduction The U.S. National Research Council ( NRC 2009 , 2010 , 2012 ) has identified the need for a national network of ground-based boundary layer thermodynamic profilers to fill a critical measurement gap that currently impedes the skill of weather prediction model forecasts. In particular, the variability of humidity in the lower troposphere is not adequately sampled at the mesoscale ( NRC 2009 ). Improved water vapor measurements are also required for better understanding of the

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