98 lines
5.7 KiB
HTML
98 lines
5.7 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="CONTENT-TYPE" content="text/html; charset=utf-8">
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<title>Extrapolate</title>
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<meta name="GENERATOR" content="OpenOffice.org 1.1.3 (Linux)">
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<meta name="CREATED" content="19951121;16410000">
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<meta name="CHANGED" content="20050706;13040400">
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<style>
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@page { size: 8.5in 11in; margin-right: 1.25in; margin-top: 1in; margin-bottom: 1in }
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P { margin-bottom: 0.08in; direction: ltr; color: #000000; text-align: left; widows: 0; orphans: 0 }
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P.western { font-family: "Bitstream Vera Serif", "Times New Roman", serif; font-size: 12pt; so-language: en-US }
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</style>
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</head>
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<body dir="ltr" lang="de-DE" text="#000000">
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<font face="New Century Schoolbook, Times New Roman, serif"><font
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size="5"><b>Extrapolate</b></font></font>
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<br>
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<br>
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<font face="New Century Schoolbook, Times New Roman, serif"><font
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size="3"><b>Introduction</b></font></font>
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<br>
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<br>
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<font face="New Century Schoolbook, Times New Roman, serif">Extrapolate is a procedure
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that allows one to linearly extrapolate features in time. In combination with tools for
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populating Sky grids from satellite data and PoP grids from radar data, this can be useful
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in extrapolating satellite and radar data. Of course, linear extrapolation is only good
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up to a point, depending upon the weather situation. This may not be useful at all in a
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lot of cases, as precipitation either forms, dissipates, expands, contacts and/or changes
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speed and direction.</font>
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<br>
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<br>
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<br>
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<big><big><font style="font-weight: bold;"
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face="New Century Schoolbook, Times New Roman, serif">How
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the Procedure Works</font><span style="font-weight: bold;">
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</span></big></big><br>
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<br>
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<font face="New Century Schoolbook, Times New Roman, serif">Extrapolate uses movement
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information from a choice of models, the Fcst database, or user-entered sources to
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linearly extrapolate features through time. Given a direction and speed of motion,
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the tool will shift the data on the GFE domain as though moving at that speed in
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that direction for one hour. A screenshot of the Extrapolate tool GUI is shown below.<br><br>
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<img alt="Extrapolate" src="images/Extrapolate.jpg"></font>
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<br>
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<br>
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<font face="New Century Schoolbook, Times New Roman, serif">The most important input in
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the tool GUI is determining which source you wish to use to advect features. For instance,
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if extrapolating satellite or radar data, one can use the distance-speed tool in D2D to arrive
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at the direction and speed of motion, and then enter these values directly into the procedure
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GUI. To use these values, one must use the default “Observed (enter below)” for “Source:”
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<br><br>
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Other movement sources available include model data and forecast surface wind information.
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For the model sources, one can pick one of several levels or layers available under
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“Wind Level if using model:” If a layer is chosen, then the models winds at all available
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model levels within that layer, including both the top and bottom pressure levels indicated,
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are averaged. The surface wind forecast information may not often be that useful, though one
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could always create, say, a temporary Fcst wind grid for use as a steering current.
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<br><br>
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Note that, if using wind information from a model database or the Fcst database, the movement
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values will vary across the grid. This portion of the tool was written to apply the wind
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information from the chosen source at each point, so if the flow varies across the grid domain,
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then this would be reflected in the output.
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<br><br>
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The NAM12 and the RUC13 have data only every three hours; the GFS40 only every six.
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Originally, the code was written to let GFE interpolate model data between its valid times,
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but this did not work in offices whose GFE default for missingDataMode was not set equal to
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‘create,’ i.e., interpolate, and forcing ‘create’ in the smartTool call up command had no
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effect for reasons unknown.
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<br><br>
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To get around this problem, the code was re-written to always look in a three (six) hour window
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around the hour of interest, so model data is always there. It looks 1 (2) hour(s) back and
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2 (4) hours forward, so that the hour half way between valid model times uses forward data,
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which makes more sense when extrapolating data forward. The motion used may not be as ‘precise,’
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but performance is not dependent upon variables outside the control of the Extrapolate procedure.
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<br><br>
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The GUI also provides forward and backward options, so one can also extrapolate backward in time.
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This could be useful in blending a PoP forecast at a future hour with current radar data.
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<br><br>
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The Extrapolate procedure uses a smart tool called “MoveFeatureBySpeed”. The portion of this
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tool that uses an observed speed and direction of movement entered by the forecaster was adapted
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from Les Colins’ EditAreaAdjust tool, and allows one to shift the data in any scalar grid by the
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speed and direction entered. Overall, the procedure and tool are set up to extrapolate an hour
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at a time, using wind information from the hour before. Wind information for hours in between
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model grids are derived via linear interpolation.
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<br><br>
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Used as a stand-alone tool, MoveFeatureBySpeed will shift the data as though moving at that
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speed in that direction for one hour, but only on the selected grid itself rather than the
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grid for the next hour. As such, the forward and backward options are not available from the
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stand-alone tool. A screenshot of the “MoveFeatureBySpeed” tool GUI is shown below.<br>
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<img alt="Extrapolate" src="images/Extrapolate1.jpg"></font>
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</body>
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</html>
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