On the Climatology of Precipitable Water and Water Vapor Flux in the Mid-Atlantic Region of the United StatesJ. Hydrometeor

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Authors
Young-Hee Ryu, James A. Smith, Elie Bou-Zeid
Year
2015
DOI
10.1175/jhm-d-14-0030.1
Subject
Atmospheric Science

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On the Climatology of Precipitable Water and Water Vapor Flux in the

Mid-Atlantic Region of the United States

YOUNG-HEE RYU, JAMES A. SMITH, AND ELIE BOU-ZEID

Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey (Manuscript received 19 February 2014, in final form 12 July 2014)

ABSTRACT

The seasonal and diurnal climatologies of precipitable water and water vapor flux in the mid-Atlantic region of the United States are examined. A new method of computing water vapor flux at high temporal resolution in an atmospheric column using global positioning system (GPS) precipitable water, radiosonde data, and velocity–azimuth display (VAD)wind profiles is presented. It is shown that water vapor flux exhibits striking seasonal and diurnal cycles and that the diurnal cycles exhibit rapid transitions over the course of the year. A particularly large change in the diurnal cycle of meridional water vapor flux between spring and summer seasons is found. These features of the water cycle cannot be resolved by twice-a-day radiosonde observations. It is also shown that precipitable water exhibits a pronounced seasonal cycle and a less pronounced diurnal cycle. There are large contrasts in the climatology of water vapor flux between precipitation and nonprecipitation conditions in the mid-Atlantic region. It is hypothesized that the seasonal transition of large-scale flow environments and the change in the degree of differential heating in the mountainous and coastal areas are responsible for the contrasting diurnal cycle between spring and summer seasons. 1. Introduction

Water vapor in the atmosphere plays critical roles in cloud formation, precipitation, and the atmospheric radiation budget. The correct initialization of atmospheric water vapor, for example, directly affects the forecast accuracy of precipitation in terms of occurrence and amount (Hanesiak et al. 2010). Water vapor also plays a central role in analyses of Earth’s climate, especially in connection with the response of the climate system to warming (Held and Soden 2006). In this study, we examine the climatology of atmospheric water vapor in the mid-Atlantic region of the United States, with a special focus on the diurnal and seasonal cycles of precipitable water and vertically integrated water vapor flux. The mid-Atlantic region of the United States is adjacent to the Atlantic Ocean to the east in midlatitudes.

Since the 1990s, the total mass of water vapor in an atmospheric column, that is, the precipitable water, has been derived at high temporal resolution (30min) from groundbased global positioning system (GPS) measurements (e.g., Bevis et al. 1994). Radiosonde observations that are traditionally used for atmospheric water vapor analyses have poor temporal resolution (12 h) for resolving diurnal variations in atmospheric water vapor.

The GPS-derived precipitable water data have the advantages of continuousmeasurementswith high temporal resolution, availability under all weather conditions, and high accuracy (Wang et al. 2007). The high temporal resolution of GPS observations provides an opportunity to improve our understanding of the diurnal cycle in atmospheric water vapor as well as its seasonal cycle.

Analyses of GPS precipitable water data have provided important new insights into the climatology of precipitable water over the world (e.g., Dai and Wang 2002;

Wu et al. 2003; Wang et al. 2007; Hanesiak et al. 2010).

The amount of water vapor at a location is greatly affected by atmospheric transport as well as local evaporation and condensation. To understand the atmospheric water cycle, it is critical to understand climatological properties of water vapor flux, which depend on both the vertical profile of the horizontal wind and the vertical profile of humidity. Because of the concentration of water vapor in the lower troposphere, climatological properties of the low-level winds are most important for assessing water vapor transport. The monthly and seasonal variations inwater vapor flux overNorthAmerica (Benton and

Corresponding author address: Young-Hee Ryu, Department of

Civil and Environmental Engineering, Princeton University,

Princeton, NJ 08544.

E-mail: younghee@princeton.edu 70 JOURNAL OF HYDROMETEOROLOGY VOLUME 16

DOI: 10.1175/JHM-D-14-0030.1  2015 American Meteorological Society

Estoque 1954; Rasmusson 1967) have been examined using observations from the radiosonde network. The diurnal variation in water vapor flux over North America (Rasmusson 1967) has been examined using radiosonde observations for special observation periods. Radiosonde observations have the advantage of combined measurements of water vapor and horizontal winds, providing the foundation for assessingwater vapor flux.As noted above, however, radiosonde observations have the drawback of poor temporal resolution, which is particularly important for water vapor flux (as detailed below). In this study, we present a new method of computing water vapor flux at high temporal resolution using GPS precipitable water observations, velocity–azimuth display (VAD) wind profiles derived from a Doppler radar, and radiosonde observations.

The objectives of this study are to present a new method for computing water vapor flux and to analyze the climatology of precipitable water and of vertically integrated water vapor flux. The study area is the midAtlantic region of the United States, and analyses are designed to provide insights into the water cycle over the eastern megalopolis of the United States.

The mid-Atlantic region (see Fig. 1 for a geographical map) is an interesting and complex setting for examining the atmosphericwater cycle. The geographical features of the study region can be characterized by sloping terrain from the Appalachian Mountains in the west and by land–ocean contrast due to the Atlantic Ocean in the east. The study region is subject to local circulations such as mountain–valley circulation and land–sea breeze circulation and also low-level jets. Because the diurnal variation in water vapor transport can be associated with the diurnal variation in wind (Rasmusson 1967), it is important to understand local wind systems and their