Resource: object_param

The filter parameter is used to determine how smooth the resulting grid will be. In general, values around 1 are used for high detail and values nearing 10 for smoother grids. Only those stations within the radius of influence (initially set to 3 grid spacings) are included in the weighted average. If no stations fall within this radius, the gridpoint value is set to missing and the resulting grid box is not contoured. Data void regions will show up as missing areas on the contour plot. The number of stations that must fall within the radius of influence for the gridpoint to be valid is defined by min_stats. This defaults to 1.

Subsequent passes through the objective analysis scheme can be performed which attempts to converge the gridpoint value to the value of the nearest station. The second pass is done by bilinearly interpolating station values from the derived grid and computing residuals. Then these station residuals are fit to the same grid and resulting values used to correct the gridpoint values. On the second pass, a convergence factor (default is .2) is applied to the filter parameter to speed up convergence. A drawback of a one pass Barnes scheme is often that maximum and minimum values are smoothed over. The benefit of the second pass is to reestablish the maximum and minimum points in the grid but this is often at the expense of creating a noisy gridded field.