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Steven M. Babin

Abstract

Microwave propagation conditions in the lower marine troposphere are evaluated using gradients of radio refractivity profiles. These profiles can be characterized as normal refraction, superrefraction, and subrefaction. Ducting of the microwave energy occurs in extremely superrefractive cases and leads to better than normal microwave propagation within the duct. Subrefractive conditions are observed the least frequently. Since subrefractive conditions are characterized by poor microwave propagation, it would be useful to be able to forecast these conditions. Subrefractive conditions may occur in the proximity of strong temperature and dewpoint inversions. Such conditions may be found with the overrunning that occurs in the proximity of a warm front or a quasi-stationary front. A case study is presented in which high-resolution atmospheric measurements made from a specially instrumented helicopter are compared with the associated synoptic surface charts. For the presented, subrefractive conditions were observed in association with warm and quasi-stationary fronts.

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Steven M. Babin

Abstract

This paper examines the sensitivity of annual area mean runoff calculations to the effects of spatial variability and temporal correlation of rainfall. The model used is based upon the hypothesis that the annual water balance is determined only by rainfall, potential evapotranspiration, and soil water storage. A simple bucket hydrology model with a seasonally varying potential evapotranspiration is used with rainfall data measured at several sites on the Delmarva Peninsula. Annual area mean runoffs are calculated for three cases: 1) actual spatial variability among the rain gauge sites and temporal correlation between consecutive 1-min rainfall amounts are maintained (the actual case); 2) actual spatial variability among the sites is maintained but temporal correlation between the consecutive 1-min rainfall amounts is minimized (the site-shuffled case); and 3) both spatial variability and temporal correlation are ignored (the area-averaged case). The actual case represents the baseline for comparison with the other two cases. The annual a” mean runoffs show little sensitivity to spatial variability and temporal correlation for this model. Therefore, if finite soil permeability effects are ignored in favor of simple water storage capacity, then spatial variability and temporal correlation of rainfall appear to have little impact on the annual area mean runoff for the data considered in this study.

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Steven M. Babin

Abstract

A surface duct is defined as a layer of air adjacent to the earth's surface, in which temperature and humidity gradients cause microwave energy originating within the layer to be sufficiently refracted so that it becomes trapped into propagating along the surface. This layer then acts as a waveguide for microwave propagation and results in propagation beyond the horizon. Failure to consider such conditions may result in erroneous radar meteorological measurements. These ducts can be located by examining refractivity profiles derived from atmospheric measurements. Since 1985, over 3900 profiles of microwave refractivity have been measured using an instrumented helicopter over the Atlantic Ocean off the coast of Wallops Island, Virginia. This helicopter data acquisition system provides higher-resolution measurements than those obtained from radiosondes. This paper presents the heights and associated frequency distributions of surface ducts as determined from these profiles. The year is divided into four quarters to examine seasonal variations, and surface duct statistics are determined for each quarter. These statistics are useful for characterizing the climatology of radar and communication ducting phenomena for this region. Such statistics have been used in microwave propagation prediction models and are useful in radar design. Surface ducts were most commonly observed and had the largest mean heights in the April–June and July–September quarters.

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Steven M. Babin and G. Daniel Dockery

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A wave-riding catamaran with a mast-traveling sensor package (profiling buoy) was developed to make fine-scale atmospheric measurements within the first meter above the ocean surface. These measurements are used to generate time-averaged modified refractivity (M) profiles that are then compared with those determined from four evaporation duct models based on the surface layer theory of Liu, Katsaros, and Businger (LKB). Model inputs are derived from measurements from masts on the R/V Chessie and from a tethered sea surface temperature buoy. Because electromagnetic propagation is critically dependent on the M-profile slopes, different analytical techniques are employed to compare the curvature of the model profiles with that of the profiles measured by the profiling buoy. One comparison criterion was to use the rms M slope difference between the model and a curve fit to the buoy profile data. Another analytical technique was to use the rms M difference after mean M removal between the model and the buoy profiles. Using these criteria for comparison of these models with the data seems to indicate that the model-derived profiles may be missing some phenomena in the surface layer such as wave effects. Overall, however, the shapes of the measured M profiles showed log-linear characteristics near the surface. One interesting result is that each model was better at approximating the M-profile curvature for stable than for unstable conditions.

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Steven M. Babin, Robert E. Miller, and John R. Rowland

Abstract

Microwave propagation conditions in the lower marine troposphere are evaluated using gradients of radio refractivity profiles. An inexpensive, weather-resistant system for Continuous monitoring of radio refractivity conditions in the lower marine troposphere would be useful for deciding when more detailed measurements should be made. Radio refractivity is largely dependent on vertical profiles of water vapor pressure. A high-power, dual-frequency, monostatic acoustic sounder was constructed to investigate the possibility of measuring water vapor pressure profiles in the marine boundary layer by acoustic means. These water vapor pressure profiles may be combined with surface measurements of atmospheric temperature and pressure to obtain estimated radio refractivity profiles. A fundamental assumption for this technique is that the pair of frequencies used should observe the same atmospheric backscatter. That is, the scattering coefficients of the two frequencies should remain a constant ratio. Measurements made with this acoustic sounder at the Boulder Atmospheric Observatory demonstrate that the 6- and 10-kHz frequencies used do not always observe the same atmospheric phenomena. Therefore, a different pair of frequencies should be sought to derive water vapor pressure profiles.

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Steven M. Babin, George S. Young, and James A. Carton

Abstract

Failure to consider anomalous propagation of microwave radiation in the troposphere may result in erroneous meteorological radar measurements. The most commonly occurring anomalous propagation phenomenon over the ocean is the evaporation duct. The height of this duct is dependent on atmospheric variables and is a major input to microwave propagation prediction models. This evaporation duct height is determined from an evaporation duct model using bulk measurements. Two current evaporation duct models in widespread operational use are examined. We propose and test a new model that addresses deficiencies in these two models. The new model uses recently refined bulk similarity expressions developed for the determination of the ocean surface energy budget in the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. Comparison of these models is made using data collected from a boat off Wallops Island, Virginia, during a range of seasons and weather conditions and from the tidal Potomac River during June and August. Independent evaporation duct height determinations are made using profile measurements from the same boat and are corroborated with fade measurements made with a nearby microwave link whenever possible. The proposed model performs better than the other (operational) models for the cases examined and has advantages of internal consistency.

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Frank M. Monaldo, Todd D. Sikora, Steven M. Babin, and Raymond E. Sterner

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No abstract available.

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