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L. J. Bruce McArthur and John E. Hay

Abstract

A technique to map the distribution of diffuse solar radiation over the sky hemisphere is described. The method is based on an analysis of all-sky, visible photographs and concurrent actinometric measurements of diffuse solar radiance. The photographs were digitized and the resulting relative density values correlated with directly measured radiances. The resulting relationship was then used to determine the radiance for each density value, enabling a map of diffuse solar radiation for the celestial dome to be constructed.

The validity and utility of the approach are assessed by several tests. In the first test, the estimated radiances were integrated over the hemisphere and compared with measured diffuse irradiances for a horizontal surface. These were found to be within ±10% for the variety of sky conditions examined. A second test, under clear sky conditions, was performed to estimate the shortwave irradiance on several south-facing inclined surfaces. The results were found to be within ±5% of the measured irradiances. In a third test, comparisons with the normalized radiance distributions of Steven (1977) indicated good qualitative agreement.

Finally, problems and deficiencies in the technique are reviewed and possible means of surmounting them are discussed.

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R. L. Johnson, D. E. Janota, and J. E. Hay

Abstract

During the spring-summer of 1979, six lightning warning devices were evaluated in a side-by-side comparison study at three test sites. Stock commercial devices were selected based upon distinct concepts of operation. The devices tested included a sferics counter, a corona point, a radioactive probe, a field mill, an azimuth/range locator and a triangulation locator. The test sites were chosen to provide varied thunderstorm conditions: 1) San Antonio, Texas (cold air advection), 2) Kennedy Space Center, Florida (localized surface heating) and 3) Langmuir Laboratory, New Mexico (orographic effects). The evaluation parameters were advance warning time, time to clear after hazard, alarm reliability, and false alarm and failure to alarm probabilities. The triangulation locator provided the best overall performance; however, all systems indicated a need for improvement in the failure to alarm rate.

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H. Hukuda, A. E. Hay, and R. J. Greatbatch

Abstract

The role played by bottom friction, bottom slope, β effect, and density mixing in exchange between the shelf and a frontal jet is examined in the light of a two-layer model. The model uses linear dynamics and bottom and interfacial friction to parameterize vertical momentum mixing, and Newtonian cooling to parameterize density mixing. It is assumed that the dynamics approximately obey Csanady’s diffusion equations in both shelf and frontal regions. Assuming a velocity profile appropriate to a shelfbreak jet, the authors examine how the various dynamical factors can affect cross-shelf exchange in the frontal region and derive some front solutions by assuming a simple torque balance. It is shown that the β effect and density mixing both produce onshore geostrophic fluxes and counteract the offshore flux due to the frictional torque. Consideration of the conditions at the intersection point of the front with the bottom shows an important role of the JEBAR effect in changing the cross-shelf circulations obtained from barotropic numerical models. It is also found that only bottom slope can provide the alongshelf structure of the front, whereas other factors such as β and density mixing determine only the cross-shelf structure.

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Liang Wang, Jinyu Sheng, Alex E. Hay, and Douglas J. Schillinger

Abstract

An extreme weather event (Hurricane Juan) made landfall in Nova Scotia, Canada, in September 2003. The storm produced an ∼70-cm storm surge and ∼40 cm s−1 coastal currents in Lunenburg Bay, registered by a coastal observing system. A fine-resolution (60 m) coastal circulation model is used to examine the response of Lunenburg Bay to Hurricane Juan. The model is forced by local wind stress at the sea surface, and tides and remotely generated waves specified at model open boundaries. The model performance is assessed in terms of γ 2, the variance of the model errors normalized by the observed variance. The model reproduces very well the observed surface elevations with γ 2 values of less than 0.05, and reasonably well the observed currents with γ 2 values between 0.2 and 1.1 in the bay during Hurricane Juan. The model–data comparisons demonstrate that the coastal circulation in the bay is significantly affected by local wind associated with the storm. The model results are also used to demonstrate the importance of nonlinear dynamics in the barotropic response of the bay to the storm.

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Gregory J. McCabe, Martyn P. Clark, and Lauren E. Hay

Rain-on-snow events pose a significant flood hazard in the western United States. This study provides a description of the spatial and temporal variability of the frequency of rain-on-snow events for 4318 sites in the western United States during water years (October through September) 1949–2003. Rain-on-snow events are found to be most common during the months of October through May; however, at sites in the interior western United States, rain-on-snow events can occur in substantial numbers as late as June and as early as September. An examination of the temporal variability of October through May rain-on-snow events indicates a mixture of increasing and decreasing trends in rain-on-snow events across the western United States. Decreasing trends in rain-on-snow events are most pronounced at lower elevations and are associated with trends toward fewer snowfall days and fewer precipitation days with snow on the ground. Rain-on-snow events are more (less) frequent in the northwestern (southwestern) United States during La Niña (El Niño) conditions. Additionally, increases in temperature in the western United States appear to be contributing to decreases in the number of rain-on-snow events for many sites through effects on the number of days with snowfall and the number of days with snow on the ground.

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J. D. Irish, J. F. Lynch, P. A. Traykovski, A. E. Newhall, K. Prada, and A. E. Hay

Abstract

Studies and models of sediment transport in the bottom boundary layer require knowledge of the bottom roughness as a parameter affecting the suspension and transport of sediment. Knowledge of this has often been quite imprecise since measurements could only be made from diver observations or camera pictures at times when the water was clear. A self-contained, tripod-mounted rotating-beam sonar system has been developed, which allows bottom topography to be imaged on scales of a few square centimeters out to 4-m radius at regular intervals in time. Most importantly, this system produces images during high suspended sediment concentration transport events, when knowledge of the bottom structure and movement is most crucial. To accomplish this, a Simrad/Mesotech sonar head was adapted as the sensing element for remote use. A separate self-contained controller/recorder was constructed and housed in its own pressure case with sufficient battery and storage capacity for up to six months deployment with hourly imaging. The controller/recorder was based on the PC/104 family of components. It digitized the acoustic signal to 12 bits at 75 kHz and stored the 450-kbyte images on 2 Gbytes of hard disk. After recovery, the data were retrieved from the instrument via an ethernet link for analysis.

Three deployments were made with the system on the east and west coasts of the United States, and three distinctly different types of bottom topography were observed. STRESS III on the northern California shelf in 90 m of water showed random, small amplitude features due to biologically formed structures in the bottom. The STRATAFORM experiment, farther north by the mouth of the Eel River in 50 m of water, showed 10-cm wavelength anorbital ripples. A deployment at the LEO-15 site off the New Jersey coast in 12 m of water showed large orbital ripples, which were well correlated with wave direction and wave particle excursions. The acoustic system was able to image these ripples as they moved and changed direction during storm events. This unique view of how the bottom feature evolution relates to the forcing will enable improvements to be made in modeling and sediment transport predictions.

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J. E. Frederick, P. B. Hays, B. W. Guenther, and D. F. Heath

Abstract

Backscatter ultraviolet data obtained by the Explorer E satellite imply very large ozone column abundances above 56 km in the tropics during mid-day. The number of molecules in a vertical column decays by a factor of 2–3 after the solar zenith angle exceeds 75° in the evening. An increase of similar magnitude occurs after sunrise. Such behavior implies the presence of a greater source of odd oxygen than is included in current photochemical theories. Ozone profiles deduced between altitudes of 50 and 62 km when the solar zenith angle exceeds 80° are in reasonable agreement with past rocket results.

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Roland J. Viger, Lauren E. Hay, Steven L. Markstrom, John W. Jones, and Gary R. Buell

Abstract

The potential effects of long-term urbanization and climate change on the freshwater resources of the Flint River basin were examined by using the Precipitation-Runoff Modeling System (PRMS). PRMS is a deterministic, distributed-parameter watershed model developed to evaluate the effects of various combinations of precipitation, temperature, and land cover on streamflow and multiple intermediate hydrologic states. Precipitation and temperature output from five general circulation models (GCMs) using one current and three future climate-change scenarios were statistically downscaled for input into PRMS. Projections of urbanization through 2050 derived for the Flint River basin by the Forecasting Scenarios of Future Land-Cover (FORE-SCE) land-cover change model were also used as input to PRMS. Comparison of the central tendency of streamflow simulated based on the three climate-change scenarios showed a slight decrease in overall streamflow relative to simulations under current conditions, mostly caused by decreases in the surface-runoff and groundwater components. The addition of information about forecasted urbanization of land surfaces to the hydrologic simulation mitigated the decreases in streamflow, mainly by increasing surface runoff.

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L. E. Hay, M. P. Clark, M. Pagowski, G. H. Leavesley, and W. J. Gutowski Jr.

Abstract

This paper examines the accuracy of high-resolution nested mesoscale model simulations of surface climate. The nesting capabilities of the atmospheric fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) were used to create high-resolution, 5-yr climate simulations (from 1 October 1994 through 30 September 1999), starting with a coarse nest of 20 km for the western United States. During this 5-yr period, two finer-resolution nests (5 and 1.7 km) were run over the Yampa River basin in northwestern Colorado. Raw and bias-corrected daily precipitation and maximum and minimum temperature time series from the three MM5 nests were used as input to the U.S. Geological Survey’s distributed hydrologic model [the Precipitation Runoff Modeling System (PRMS)] and were compared with PRMS results using measured climate station data.

The distributed capabilities of PRMS were provided by partitioning the Yampa River basin into hydrologic response units (HRUs). In addition to the classic polygon method of HRU definition, HRUs for PRMS were defined based on the three MM5 nests. This resulted in 16 datasets being tested using PRMS. The input datasets were derived using measured station data and raw and bias-corrected MM5 20-, 5-, and 1.7-km output distributed to 1) polygon HRUs and 2) 20-, 5-, and 1.7-km-gridded HRUs, respectively. Each dataset was calibrated independently, using a multiobjective, stepwise automated procedure. Final results showed a general increase in the accuracy of simulated runoff with an increase in HRU resolution. In all steps of the calibration procedure, the station-based simulations of runoff showed higher accuracy than the MM5-based simulations, although the accuracy of MM5 simulations was close to station data for the high-resolution nests. Further work is warranted in identifying the causes of the biases in MM5 local climate simulations and developing methods to remove them.

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D. G. Steyn, J. E. Hay, Ian D. Watson, and Glenn T. Johnson

Abstract

A technique is described whereby sky view-factors may be determined using a video camera equipped with a fish-eye lens. The video image is digitized and then analyzed to distinguish between “sky” and “non-sky” pixels. View-factors are calculated for each pixel and then summed for all “sky” pixels to yield a composite sky view-factor for the image. The technique is illustrated by applying it in three urban locations, all of which are characterized by high building densities (and hence complex skylines). The three images processed have sky, view-factors in the range 0.15 to 0.46 (as independently determined). It is shown that the present technique produces values in close agreement with these and appears quite robust when compared with calculations based on the work of Johnson and Watson.

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