Geoscience Reference
In-Depth Information
23.3.5
Summary and Concluding Remarks
Observation targets of pressure, winds, and temperature within an EnKF for
land-falling Pacific cyclones on the west coast of North America were examined
for a 6-month wintertime synoptic period in 2009/2010. These targets represented
estimates of where assimilated hypothetical observations beyond assimilated routine
observations would produce the largest reduction in the uncertainty of 24-h cyclone
forecasts around the time of landfall. It was found that temperature and wind
targets were mesoscale in nature, whereas pressure targets were more prominent
on the synoptic scale. Furthermore, pressure observations produced the largest
positive impacts on the uncertainty of cyclone forecasts of the four observation
types examined. The most important targeting regions in the vertical for winds and
temperature varied substantially throughout the troposphere when considering all
cyclones, but there was an indication of preferred regions in the mid- and upper-
troposphere for temperature and the upper-troposphere and near the surface for
winds. Although the largest benefits from pressure observations existed near the
surface, similar benefits existed throughout the troposphere with no clear preferred
level. In the horizontal, there was significant variability in the most important
targeting areas, showing no clear location where a routine observation would be
consistently beneficial to land-falling cyclone forecasts. Lastly, it was found that
targeted observations are more beneficial to forecasts of deepening cyclones than
to decaying systems as they approach the coast. This result was not found when
considering the directions along which the cyclones track as cyclones from all
directions showed similar benefits from targeted observations.
It is important to note that the best way to view the results presented here is
in a relative sense. Specifically, these experiments have provided an understanding
of how impacts vary within an EnKF among the different observation types of
pressure, winds, and temperature for land-falling mid-latitude cyclones. Whether
these results extend to other assimilation systems and different high-impact events
is unclear. Furthermore, the estimated variance reductions in this study are based
on ensemble sensitivity, and thus the particular variance reduction values would
need to be compared with experiments that actually assimilate targeted observations
to understand the relationship between estimated and actual forecast impacts.
The effects of nonlinearity, inflation, and localization may all play a role in any
discrepancy. Nonetheless, this study has provided a unique perspective on how
targeting techniques might be designed to best benefit forecasts of land-falling
Pacific cyclones. Lastly, as assimilation and forecasting systems at higher and higher
resolution become more feasible in the coming years, gaining an understanding
of the effects of targeted observations across multiple scales will be an intriguing
endeavor.
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