(BRAVO) Study: Final Report. - Desert Research Institute

Final Report — September 2004
averaged over the comparatively very coarse FNL vertical layers. Perhaps averaging the
RWP data within each FNL layer would give better results, but this was not tried.
Table 9-4 provides the same comparisons for October. (There are no EDAS entries
for October because EDAS data were not available then.) Comparing Tables 9-3 and 9-4, it
can be seen that MM5 performance in October was much worse than for the July to
September period, while the FNL performance was slightly poorer in October than for July-
September. MM5 bias in wind directions increased substantially, to 16-23 degrees counterclockwise.
The fraction of wind directions within ± 20 degrees and ± 30 degrees decreased
substantially.
Table 9-4. Comparison of modeled winds to radar wind profiler measurements in October
1999. Data are averaged over all vertical levels and over all observation times.
Location
Big Bend
Llano
Brownsville
Eagle Pass
Model
Average
Model WS,
m/s
Average
RWP WS,
m/s
Average
WD
Difference,
deg.
% Within
±20 deg
% Within
±30 deg
MM5 5.20 6.15 -21.30 22 31
FNL 6.12 6.67 -12.00 22 32
MM5 6.07 7.00 -16.31 33 45
FNL 6.46 7.73 2.22 36 46
MM5 6.77 6.47 -16.51 35 47
FNL 6.59 6.68 6.99 31 43
MM5 6.12 7.60 -22.86 34 46
FNL 6.86 8.28 4.55 33 42
Even though the FNL performance degraded during October, the FNL performed
about as well as MM5 did in October. Wind speed differences were somewhat higher for
FNL than MM5, but wind direction bias was less for FNL. The fractions of wind directions
within 20 and 30 degrees were very similar for the models, with a slight edge to MM5.
The percentages of modeled and RWP wind directions from 8 general directions (N,
NE, E, SE, S, SW, W, NW) were then compared by layers — 0-500 m, 500-1500 m, and
>1500 m. The comparisons were done for MM5 and EDAS the period July-September 1999.
No comparison was done for October because EDAS data do not exist for October.
Figure 9-1 portrays the comparison at Big Bend. The RWP winds at the lowest level
are mainly from the south, while the models show winds predominantly from the SE and E.
The low level winds at Big Bend are most likely due to channeling by local terrain; the
modeled winds would not be expected to resolve this channeling. At the 500-1500 m level,
there is better agreement as the RWP winds shift more toward the southeast and east. The
best agreement is for the layer above 1500 m, although both models (especially MM5) are a
bit low on the frequency of northerly winds (i.e., winds from the north) and high for easterly
and southeasterly winds.
9-7