On polarimetric characteristics of mesoscale cellular convection in the marine atmospheric

boundary layer

Haiyan Li, William Perrie, Lanli Guo ， Biao Zhang

2011 IEEE International Geoscience and Remote Sensing Symposium

24-29 July Vacouver, Canada

Outline

Motivation

Dataset

WRF

Polarimetric Characters of Convection

Polarimetric Characters of open ocean surface

Summary

Motivation

‘Blister’ like provided inFig 7 in Babin et al.(2003)

Linked the imaging pattern with convective processGerling 1986; Alpers and Brummer 1994; Beal et al. 1994; Nilsson and Tildesley 1995; Sikora et al. 1995…

Estimated the parameter: depth of a convective MABl (Sikora et al. 1997) Obukhov lengths and drag coefficinets (Sikora et al. 2001)

Study dynamic and structure Babin et al. 2003; Young et al. (2005,2007) ; Fisher (2007) ;Sikora et at (2009)

Motivation (continuous)

Before above work, firstly,we should distinguish convective process SAR images from open ocean surface SAR images.

RS-2 PolSAR (HH+HV+VH+VV)

Modis

NARR (North American Regional Reanalysis)

data

QuikSCAT L3 data

SSM/I

Dataset

RS-2 Quad-Pol

UTC 05:05:35 on Feb. 2, 2009, RADARSAT-2 Data and Products© MacDonald, Dettwiler and Associates Ltd. (2008- 2009) - All Rights Reserved.

AB C

MODIS data

at 22:10:00 UTC on 1 Feb. and 7 hours before SAR; ‘’ represents SAR image.

The MODIS imagesuggests that convectiondid occur from the morphology and that the SAR image area appears to be a transition zone between open and closed convection.

Air pressure field

from NARR data

Dataset (continuous)

SST vertical profile

Dataset (continuous)

The atmosphere is instability

on 2 February 2009 at (49.67oN 170oW),

Air-Sea difference : -4oC

Wind field from QuikScat L3

Dataset (continuous)

UTC 06:22:45, 77 minutes after the SAR image

Rain rate from F15 SSM/I

Dataset (continuous)

on 2 February 2009, 29 minutes after the SAR imaging time

Dataset summary

provide an overview of the meteorological conditions；

no precipitation

Cyclone trajectory in space

WRF Results

WRF Results （ continuous）

horizontal distribution of vertical velocity

field at 1000hPa

A

BC

WRF Results （ continuous）

Vertical profile of the vertical velocity

 along 170.01oW

along 49.65oN

WRF Results （ continuous）

accumulated precipitation

from UTC 18:00 on 1 February to UTC 05:05 on 2 February 2009

WRF Summary

verify that the SAR imaging area was influenced by cyclonic processes. After the passage of the vigorous cyclone, a high pressure ridge gradually developed and dominated this area;

upward and downward air motions at the bottom of atmosphere；

exclude rain effects.

Polarimetric Characters of convection

Variations of backscatter intensity cross section

AB

C

Polarimetric Characters of convection (continuous)

Area A Area B Area C

ϕhhvv 20° 43° 70°

H 0.57 0.60 0.68

α 38° 40° 51°

A 0.34 0.32 0.37

Range  Meaning

ϕhhvv 0-180° Odd, even bounce

H 0-1 randomness

α 0-90° type

A 0-1 roughness

MCC Polarimetric Characters summary

mesoscale cellular convection(MCC) is with higher

modulation depths of HH and VV polarizations, the co-

polarization phase difference, average alpha angle,

and entropy. However, the contrast in anisotropy is

not as obvious as that of the other three parameters.

• What is the difference in the polarimetric characteristics between MCC and open ocean surface?

• To compare the polarimetric characteristics of MCC with those of the ocean surface, 641 open ocean surface quad-polarization RS-2 SAR images collocated with 52 NDBC buoys were analyzed.

Polarimetric charateristics of open ocean

The distributions do not show any obvious regular pattern and the actual variations are small, resulting from variations in these four parameters. The range is limited between 0o to 6 o.

Area A Area B AreaC

ϕhhvv 20° 43° 70°

Polarimetric charateristics of open ocean

H clearly tends to increase with increasing incidence angles, for each of the three sea state parameters. There are a few exceptional cases, however.When wind speeds are larger than 2 m/s, the largest H value is less than 0.55; the maximum value for H is ~ 0.68.

Area A Area B AreaC

H 0.57 0.60 0.68

Polarimetric charateristics of open ocean

increases with increasing incidence angles, for each of the three sea state parameters. There are a few exceptions to this trend, however. Except for the maximum in , when wind speed is about 2 m/s, the variation in is from 3o to 25o, and taking its standard deviation into account, the variation of is from 2o to 27o , which is smaller than that of MCC.

Area A Area B AreaC

α 38° 40° 51°

Polarimetric charateristics of open ocean

A generally tends to decrease with increasing incidence angles.

Area A Area B AreaC

A 0.34 0.32 0.37

• MCC typical areas A, B and C tend to show higher co-polarization phase difference, H and , compared to typical polarimetric characteristics of the ocean surface.

Comparison Summary

Conclusions

The collated data and WRF model results verified that the signals in SAR images are MCC;

The polarimetric characteristics of MCC are different with those of open ocean surface;

Polarimetric parameters show good potential to identify atmospheric convection phenomena from open ocean surface SAR images.

lihaiyan.flying@gmail.com