WindSat Emissivity Sensitivity to Near Surface Wind Field in a Tropical Cyclone Amanda Mims mimsaumi

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WindSat Emissivity Sensitivity to Near Surface
Wind Field in a Tropical CycloneAmanda Mims
and Chris Ruf, University of
MichiganChris Hennon, University of North
Carolina Asheville
Microwave radiometry is a demonstrated means for
remote weather observation ocean surface wind
speed in particular. Being a passive system, it
has many advantages over active radars by
requiring less power to run and being more cost
effective. A radiometer on the WindSat
spacecraft has been extensively tested and
validated at winds speeds of up to 25 m/s. We
are interested in extending this capability to
extreme weather conditions such as tropical
cyclones
Data Analysis Approach
INPUT DATA WindSat TB at 18.7, 23.8, 37.09 GHz
V-pol
WindSats 37 GHz V-pol Channel
Radiative Model Function for ocean surface wind
speed (WS)
Retrieve V and L (primary) and WS (secondary)
using non-linear, successive approximation
inversion of radiative transfer equation
Hypothesis Microwave radiometry can measure
changes in ocean surface wind speed in hurricanes
through extreme precipitation Approach This
study examined the WindSat overpass of a
tropical cyclone, extracting surface emissivity
from its measured brightness temperature data.
The emissivity was matched with a surface wind
field to detect high wind sensitivity. The
overpass in this case study was from hurricane
Katrina in late August, 2005
Radiative Model Function for atmospheric water
vapor (V) and liquid (L)
Estimate atmospheric optical depth at each
WindSat frequency
t c0 c1V c2V2 c3L
Decompose radiative transfer equation and solve
for ocean surface emissivity at each WindSat
channel and polarization TB TBatm eTsfc
TBatm(1-e) Tcose-te-t
INPUT DATA WindSat TB at 6.8, 10.7,18.7, 23.8,
37.09 GHz H- and V-pol
INPUT DATA HWinds ground truth wind fields
Match derived surface emissivity with ground
truth wind speed
Image of retrieved precipitation. Gaps in image
indicate large uncertainty in retrieval (probably
due to extremely high precipitation rates).
However, the cyclones rain bands are still
evident in the image, showing that the model is
behaving correctly.
OUTPUT RESULTS
Feedback radiative model for emissivity
The HWinds analysis provides wind field
groundtruth. It is comprised of raw wind
measurements from various reliable sources
(above) that are transformed into an optimal wind
field (right). All data comes from three hours
before or after the given timestamp, in this case
2349 UTC on the 28th of August, which
corresponds to the time of the WindSat overpass.
It works!
Picture courtesy of NRL
The emissivity is still increasing with wind
speed, even beyond 30 m/s. This indicates that
microwave radiometry is indeed sensitive to the
differences in emissivity caused by winds even
into the tropical cyclone range.