Calculating advected products

When you view nowcasted products by moving the animation slider into the nowcasting region, you see advected products.

IRIS Focus generates advected products using the last motion vector field (MVF) generated for a site along with the last product of the type you are viewing. IRIS Focus generates the advected products on-demand.

Figure 1. Product advection

Calculating advected products

The advection algorithm back traces each pixel's previous positions. To determine the value of one pixel (shown in gray in the previous image), the algorithm performs the following calculations:

  1. Shift the position of the pixel using the MVF point for that pixel, but in the opposite direction.

    The new value is determined by interpolating the raster value at the previous location of the pixel.

  2. To determine value for the pixel N frames in the future, the algorithm performs the shift N times.

  3. The algorithm determines the MVF vector components at each intermediate location using the same interpolation procedure as for the raster value at the previous location. The interpolation computes a weighted average of raster values in four surrounding points.