Windspeed

Windspeed is a measure of how fast air is moving past a certain point. Windspeed is impacted by Pressure Gradients and Air Temperature. Windspeed also plays an integral role in other meteorologic processes available to be modeled in HEC-HMS including Evapotranspiration and Longwave Radiation.

The Windspeed Method included in the Meteorologic Model is required when certain types of Evapotranspiration Methods or Longwave Radiation Methods are used. Three methods, Gridded Windspeed, Interpolated Windspeed, and Specified Thermograph are available for adding windspeed data to the Meteorologic Model. More detail about each method is provided in the following sections.

Gridded Windspeed

The most common use of the Gridded Windspeed Method is to utilize radar-based windspeed estimates. Gridded windspeed data may come from other software or be the output from atmospheric models. If Gridded Windspeed is used with a transform method other than ModClark, an area-weighted average of the grid cells in the subbasin is used to compute the thermograph for each subbasin.

The Gridded Windspeed Method includes a Component Editor with parameter data for all subbasins in the Meteorologic Model. The Watershed Explorer provides access to the Windspeed Component Editor.

The Component Editor for all subbasins in the Meteorologic Model includes the selection of a datasource. Gridded windspeed data must be stored as a windspeed grid before it can be used in the Meteorologic Model. The data may be from radar sources or could be the result of complex calculations exterior to the program. Regardless, the grid data must be stored as a windspeed grid. Only windspeed grids already defined will be shown in the selection list. If there are many different gridsets available, you may wish to choose a gridset from the selector accessed with the Grid button next to the selection list. The selector displays the description for each gridset, making it easier to select the correct one.

The Time Shift can be used to correct for windspeed grids stored with a time zone offset. All calculations during a simulation are computed assuming an arbitrary local time zone that does not observe summer time (daylight savings in the United States). Set the shift to zero if all the time-series and grid data is referenced in same local time zone. If other data sources such as observed discharge or precipitation are referenced in local time and the windspeed grid is in referenced to a different time zone, select the correct shift so that the windspeed data will match the rest of the data.

Interpolated Windspeed

Gridded data better captures the meteorology temporally and spatially across a watershed when compared to gage measurements at a single point. However, hourly or sub-hourly gridded meteorology products are not always available, particularly for historic events of interest to the modeler. In these cases, a gridded dataset can be created by interpolating point observations recorded at weather stations. The Interpolated Windspeed Method allows the user to develop an interpolated windspeed grid based on point windspeed gage data.

The Interpolated Windspeed Method includes a Component Editor with parameter data for all subbasins in the Meteorologic Model. The Watershed Explorer provides access to the Windspeed Component Editor. 

The Component Editor for each subbasin in the Meteorologic Model is used to enter parameter data that defines the Interpolation Method and to select windspeed gages to use in the interpolation.

The Interpolation Method can be selected from the dropdown menu. An interpolation method must be selected. The user has four interpolation options briefly summarized below.

The Inverse Distance interpolation method assumes the weight, or influence, of a gage is equal to the inverse of its distance from the interpolated cell. The Inverse Distance Squared interpolation method assumes the weight of a gage is equal to the inverse of the square of its distance from the interpolated cell. The Nearest Neighbor interpolation method simply assigns the nearest value to the cell center of interest without considering values of other nearby points. Bilinear interpolation within HEC-HMS relies on triangulation of the irregularly spaced gage locations. Based on the gage coordinates, a Triangulated Irregular Network (TIN) is created to represent the gage network in the basin model coordinate system. This TIN defines triangles, where each gage is a corner of one or more triangles. Given this TIN, a value at any given point is computed by first identifying the triangle in which that point falls, then interpolating within that triangle using Barycentric Coordinates. You must use three or more gages and the gages need to bound all grid cells.

Select Windspeed Gages in the dropdown rows under the Gage column. The Radius of Influence is an optional parameter and represents the maximum interpolation distance. Beyond its radius of influence, the gage will not affect the cell values of the interpolated grid. Windspeed Gages used in this method must be loaded in as Time-Series of Windspeed Data with defined latitude and longitude information.

An interpolated windspeed grid will be created once the simulation has been computed. Once the compute is complete, the interpolated grid will be saved with the same name as the Meteorologic Model in DSS format in the HEC-HMS project directory.

Specified Anemograph

The Specified Anemograph Method includes a Component Editor with parameter data for all subbasins in the Meteorologic Model. The Watershed Explorer provides access to the Windspeed Component Editors.

The Component Editor for all subbasins in the Meteorologic Model includes the Time-Series Gage of Windspeed for each subbasin. A Windspeed Gage must be selected for a subbasin. The current gages are shown in the selection list.