Simulation of the Impact of New Aircraft- and Satellite-Based Ocean Surface Wind

Measurements on Estimates of Hurricane Intensity

Eric Uhlhorn (NOAA/AOML)

R. Atlas (NOAA/AOML), P. Black (SAIC/NRL), C. Buckley (NASA/MSFC), S. Chen (UMiami/RSMAS), S. El-Nimri (UCF), R. Hood (NOAA), J. Johnson

(UCF), L. Jones (UCF), T. Miller (NASA/MSFC), C. Ruf (UMich.)

Objective: Improving Surface Wind Measurement Accuracy

• A TC’s intensity is determined by the peak sustained surface wind anywhere in the storm.

• Current observational practices limit the probability of directly measuring this quantity.– Radial resolution is very high (in situ flight-level and

SFMR).– Azimuthal resolution above BL is moderate (Doppler

radar).– Azimuthal resolution at surface is poor.

• If we fill this data gap, can intensity estimates be significantly improved?

Hurricane Imaging Radiometer (HIRad): A multi-agency partnership to extend SFMR

capability to wide-swath (±60°) imaging

From SFMR...

...to HIRad

±60°

Measuring hurricane winds at large incidence angles

• SFMR has proven the capability at nadir (i.e. zero) incidence angle, but what about at large off nadir angles?

Exc

ess

Bri

gh

tnes

s T

emp

erat

ure

(K

)

Current SFMR

model function for

nadir incidence

SFMR measurements in Hurricane Gustav (2008)

“Nature” Model Run• MM5 1.67 km grid• Frances (2004)• Provides synthetic

“observations”• Aircraft flies a typical

“alpha” pattern and samples the simulated wind field based on instrument characteristics

• “Full-up” experiment (OSSE) considers all potentially available data

• Here, we concentrate on the extension of SFMR to HIRad

70 km wide HIRad swath 20 km alt.

SFMR

Surface (10 m) Wind Speed (m/s)

• Radial structure is captured well.• Azimuthal structure is unknown, since there are few

continuous surface observations available at high wavenumber.

• A limitation of current sampling strategy (“alpha” pattern).

Model Wind Field “Representativeness”• How well does the model capture surface wind

structure?

Vast data void

Error Modeling• Consider two sources of error

– Instrument noise (1 K)– Model function accuracy (5 K)

• Express errors over a range of expected wind speeds and rain rates• Extend SFMR model function error to large incidence angles • Consider a four-channel system (4, 5, 6, 6.6 GHz)

Instrument Noise SFMR Model Function Accuracy

0

30

60

90

120

150

HIRad Surface Wind Speed Errors (knots)

Instrument noise-induced error

Model function -induced error

HIRad Surface Footprint (20 km alt.)

• SFMR resolution is maintained to angles ±40° off nadir.• Variability in footprint size is taken into account when

sampling model wind field.

Improving Peak Wind “Observation”Spatial Variability

• Find peak wind using standard alpha (Fig.4) pattern • Initiate pattern at same time (17Z 08/31/2004), but at

different initial points (IP)• Compare “observed” peak wind to actual (model) peak

wind over time of flight pattern0

45

90

135

180225

270

315

RMW

IP

Increasing spatial coverage may decrease the magnitude of underestimate, as well as limit variations.

Improving Peak Wind “Observation” Temporal Variability

• Find peak wind using alpha pattern• Initiate pattern at same location (105 nm 225°/SW)

hourly over six-hour period• Compare observed peak wind with model peak wind

over flight period

Increasing spatial coverage may improve the observation of intensity change.

Summary• Hurricane surface winds can be retrieved from brightness

temperature measurements at large off-nadir incidence angles.• High-resolution simulations of a hurricane can provide adequate

synthetic “observations” for testing new instrument capabilities and sampling strategies.

• HIRad extends SFMR surface wind measurement capability to a wide swath.

• By increasing azimuthal resolution of the surface wind field, HIRad could potentially:– Reduce the uncertainty in the TC’s intensity estimate– Capture fine-scale details, including Rapid Intensification,

with greater accuracy.

See poster this evening by Miller et al. for more details/discussion/questions!