Ensemble Adaptive Data Assimilation Techniques Applied to Land-Falling North American Cyclones - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 560

Fig. 23.8 Value of maximum variance reduction of the response function averaged over all vertical
model levels for all cyclones for all observation types

tropopause involved with the general temperature minimum at that level. It is not
obvious why the 600-hPa level is important, or why wind targeting locations are
important near the surface and at 400 hPa. Either large ensemble-based sensitivity
or analysis variance values would contribute to large estimated variance reduction
values, and it is thus likely one of these two quantities is consistently larger
at the levels where the peaks are evident in Fig. 23.8 . In any case, these levels
indicated preferred locations where on average, supplemental temperature and wind
observations would be most beneficial.
Figure 23.9 shows the horizontal locations of the maximum estimated variance
reduction for temperature observations organized by cyclones that approach the
coastal zone from the northwest, west, southwest, and south. Cyclones along these
tracks have been binned over a 45 ı swath centered on each direction listed. The

locations are shown relative to the 24-h forecast position of the ensemble mean
cyclone. It is clear that for all cyclone tracks there is significant variability in
the horizontal location of the maximum variance reduction values, varying up
to about

40 ı both longitudinally and latitudinally. Interestingly, a number of the
maximum variance reduction locations occur at or downstream of the 24-h forecast
position of the cyclone, indicating the ability of the EnKF to spread observational
information upstream into regions where large dynamical sensitivity is likely to
exist. Nonetheless, it seems the highest priority targeting regions rarely exist in the
same location relative to the forecast position of the cyclone. Although there is some
clustering of maximum variance values to the southwest of cyclones that approach
from the southwest, there is still a large spread in the location of maximum variance
reduction such that the chance that a single location would provide consistently large