Synthetic Aperture Radar Observations of Mesoscale Atmospheric Phenomena

1
Synthetic Aperture Radar Observations of
Mesoscale Atmospheric Phenomena

• George Young, Nathaniel Winstead, Frank
  Monaldo, Don Thompson, Todd Sikora
• The Pennsylvania State University
• Johns Hopkins University Applied Physics
  Laboratory
• Millersville University

2
Synthetic Aperture Radar - Physics

• Senses surface roughness at Bragg scales
• Obtains sea surface wind from backscatter
• Much as is done with scatterometers
• Pixel size
• lt 1 km for SAR
• gt 10 km for scatterometer
• Look directions
• 1 for SAR
• 2 for scatterometer

3
Fronts Lobe and Cleft

• Cold fronts and other gravity currents
• Lobes bow towards the warm air
• Scale 10 km

4
Fronts Vortices

• Shearing instability
• All frontal types
• Varies with frontal type
• Cold fronts
• 10 km
• Tightly wrapped
• Other Fronts
• 100 km
• Open wave

5
Fronts Gravity Waves

• Shear driven waves on frontal inversion
• Waves modulate surface wind near front
• Often observed in packets spaced along front

6
Deep Convection

• Deep convections precipitation enhances
  downdrafts
• Downdrafts spread at surface creating gust and
  lull pattern
• Gust scale larger and more varied than for
  shallow convection

7
Shallow Convection

• Surface manifestation of boundary layer thermals
• Gusts and lulls scale with boundary layer depth
• Cells align if wind is strong enough

8
Mountain Lee Waves

• Large variation in surface wind speed
• Gusts and lulls parallel terrain crest
• Wavelength 10 km
• Downwind extent gt 100 km
• Interference patterns sometimes observed

9
Lee Waves from Peaks

• Wave pattern differs for isolated peaks
• Gusts and lulls form chevron pattern
• Spreading angle often acute
• Downwind extent gt 100 km

10
Obstacle Wakes

• Breaking waves and other momentum sinks cause
  slow wakes behind coastal peaks and islands
• Wakes are narrower than the peaks responsible
• Wakes often persist for gt 100 km downstream
• Wakes can be modulated by other phenomena

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Conclusions

• SAR measures sea surface wind at sub-kilometer
  resolution
• Maps detailed surface wind patterns within
  mesoscale phenomena
• Detects phenomena not readily observed by other
  remote sensors