Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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5 Observational Challenges of Low-Level Radar Coverage in Complex Terrain and Coastal Areas
5 Observational Challenges of Low-Level Radar Coverage in Complex Terrain and Coastal Areas
Pages 59-73
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From page 59... ...
This is particularly true for the west coast of the United States, where weather systems approach predominantly from the west. However, the observational challenges posed by complex terrain are compounded for coastal regions because of the presence of a large water body that affects the atmospheric conditions.
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From page 60... ...
If the radar beam is only partially blocked, it may still be useful for observing weather echoes if the amount of power remaining in the beam is large enough for the radar return to be above the signal detection level. However, precipitation observations collected by partially blocked radar beams may lead to biased results if left uncorrected.
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From page 61... ...
. For the United States, digital elevation data are available with a spatial resolution of about 30 m and in some locations even 10 m.
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. Coastal areas, including southern California, are particularly prone to this problem.
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. Only those radar beams launched nearly parallel to the duct will be trapped.
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SOURCE: Richard Paulus, Space and Naval Warfare Systems.
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must decrease with height. Temperature inversions must be very pronounced to produce superrefraction, while water vapor gradients are more effective than temperature gradients alone (Fabry et al., 1997)
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From page 66... ...
These values were then compiled into a climatology of average refractivity gradients for each operational sounding station and used to study the likeli hood of atmospheric conditions across the United States that are susceptible to anomalous propagation of radar signals. The sounding-based climatology highlights large-scale temperature inversions and moisture gradients, yet only by chance does it capture conditions favorable to anomalous propaga tion produced by thunderstorm outflow boundaries.
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From page 67... ...
, when chances for such conditions to occur exceed 20 percent throughout most of the United States and are greater than 30 percent for Southern California and most of the eastern seaboard states (see Figures 1 and 2 of Steiner and Smith, 2002)
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From page 68... ...
or black (trapping) indicates the percentage of soundings of that particular station exhibiting the respective atmospheric conditions.
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OBSERVATIONAL CHALLENGES OF LOW-LEVEL RADAR COVERAGE 69 FIGURE 5.4 Same as Figure 5.3, but for soundings taken at 00 UTC. SOURCE: Adapted from Steiner and Smith (2002)
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From page 70... ...
Because anomalous propagation conditions appear to be a legitimate basis of con cern for radars near the Southern California coastline, minimizing this problem is a valid consideration for siting NEXRADs. In the case of the Sulphur Mountain radar, its higher elevation is necessary for mitigating problems with anomalous propagation.
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Superrefractive propagation conditions are indicated by the gray-shaded area. SOURCE: Matthias Steiner, Princeton University.
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792 (600) NOTE: The data are grouped into statistics based on soundings collected from midnight to early morning (0008 LST)
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top heights based on soundings collected at San Nicolas Island, California, from July 1975 through August 1993. Shown are statistics for elevated inversions only (i.e., excludes surface-based inversions)
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