Geoscience Reference
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strong updraft. The estimated northwesterly winds are much stronger in this spurious
downdraft region than in the original model output (Fig. 4). This stronger northwesterly wind
causes stronger divergence, which in turn induces the spurious strong downdraft. The strength
of the enhanced northwesterly winds at 2 km ASL is similar to the strong northwesterly winds
at 3-4 km ASL (cf. Fig. 2). Vertical smoothing of the radial velocity field by the Cressman
interpolation procedure likely plays a major role in the erroneous vertical velocity field. Errors
in radial velocity from vertical interpolation tend to occur near the boundaries of the storm, as
shown in Fig. 7. This concentration of errors near the storm boundaries occurs because the
number of radial velocity samples measured at neighboring grid cells is limited, so that the
relative influence of radial velocities measured at distant grid cells grows.
Fig. 7. Horizontal (vectors) and vertical wind velocity (shading) deduced by variational
multiple Doppler radar analysis using 30 PPIs (EL30) at 2 km ASL. Solid contours indicate
updrafts at 3 m s
-1
contour intervals and downdrafts at 1 m s
-1
contour intervals. The blue
contour line outlines the region where the mixing ratio of rain exceeds 0.1 g kg
-1
. A vertical
cross-section along the thick horizontal line is shown in Fig. 8.
Figure 8 shows a vertical cross-section of the wind fields retrieved by EL30 along the y = 26
km transect in Fig. 7. Compared with the original model output (Fig. 5), the strong updraft
and downdraft cores in the heavy precipitation region (at 8 km ASL) and the downdraft
core associated with gravity wave dynamics (at 11 km ASL) are well retrieved. The
maximum retrieved updraft speed is 21 m s
-1
, and occurred at approximately 4 km ASL. The
maximum retrieved downdraft speed is 7 m s
-1
. The maximum updraft speed was
underestimated by 3 m s
-1
, while the maximum downdraft speed was overestimated by 1 m
s
-1
. The speed of the downdraft associated with gravity wave dynamics was underestimated
by 1 m s
-1
relative to the original model output (Fig. 5). Several spurious updrafts and
downdrafts can be identified in the EL30 retrieval, especially at 7-12 km ASL. Figure 7 and
Fig. 8 indicate that the EL30 pseudo-radar configuration provides a good estimation of
vertical velocity. Errors in the retrieved vertical velocity are uniformly less than 3 m s
-1
.
