Alessandro Battaglia,

Simone Tanelli, Gerry Heymsfield and Lin Tian

The Dual Wavelength Ratio knee: a signature of multiple scattering

in airborne Ku-Ka observations

Paper now out in J. Appl. Meteorol. Climatol., early on line release

Focus and motivation• Convective towers heavy precipitation events

• CloudSat and TRMM providing single frequency observations of convective towers. GPM-DPR: first dual wavelength observations from space.

• CloudSat observations certainly affected by multiple scattering. For TRMM hints that rain rates were underestimated in deep convection (maximum not occurring under the convective core, Liu Chuntao)

CloudSat W-band

TRMM Ku-band

GPM proxy: NASA-HIWRAP Ku-Ka MC3E observations

Region of strong attenuationVery likely

presence of hail (ground reports+

Ground-based S-pol radars)

Heymsfield et al., 2013: Airborne Radar Observations of Severe Hail Storms: Implications for Future Spaceborne Radar, J. Appl. Meteorol. Climatol.

3dB/km

0.5dB/km1.6dB/km one way

High-density ice layer

S-band Z above 60 dBZ

only in the last 4 km

Vertical reflectivity profiles in sector II

iAnomalous sloping

DWR knee

Absence of surface return peak

Ka Ku

Inflexion point

This feature was observed during MC3E in

severalconvective cells

The Dual Wavelength Ratio knee

In a singlescattering world …..

>10 dB

>12 dB

Single scattering explanation?DWR sources from hail/graupel/rain

Only hail particles in this range of size But almost monodisperse can produce a

decrease in DWR. Rain and wet hail cannot!!

However quite hard to produce a 12 dB effect! Vertical microphysical processes not supported by S-band observations.

ways how to decrease DWR

Even if DWR can be explained there is no explanation of the disappearance of the surface with realistic s0 values

Differential attenuation

Mie effects

The multiple scattering explanation

Anomalous sloping and surface peak disappearing have been already documented for CloudSat, but quite astonishing to see this features in Ku-Ka airborne observations. Ka is expected to be more heavily affected by multiple scattering than Ku.

Dense ice layer high SS albedo

Simulation framework: single ice layer

Two features must be carefully treated:1) Antenna pattern2) Size of the convective cell

Pulse stretching effect generated by an hail layer

The MS strength is a strong function of the horizontal extent of the system lateral diffusion allows radiation to re-enter the small footprint (whose 3 dB radius is of the order of 500 and 200 m for HIWRAP at 10 km distance for Ku and Ka, respectively) after being scattered away from it.

Ku profile

DWR profile

Pulse stretching

Ku profile

Ka profile

Profile with hail explaining sector II HIWRAP+S-band observations

DWR profile Radiationheight

The radarIs loosing its

ranging capabilities

Grey-shaded envelopes=observed HIWRAP profiles within 1, 3, 5 km from the cluster center

Heuristic top-bottom

approach

GPM configuration

3km10km

SS attenuatedSS effective

MDT MDT5km

• Maximum in DWR strongly suppressed• Appearance of a knee• Even negative DWR possible!

ConclusionsA Ka-Ku DualWavelengthRatio knee was frequently observed during MC3E.

This signature is straightforwardly explained with the help of multiple scattering theory in presence of hail-bearing highly-diffusive convective cores with large horizontal extents Ka pulse stretching typically exceeding that occurring in the Ku channel anomalous sloping knee, visible thanks to the profile large attenuation and to the good HIWRAP sensitivity (10 and 0 dBZ at 20 km for Ku and Ka, respectively).

Multiple scattering effects are likely to be more pronounced in the upcoming space-borne GPM-DPR observations. Our forecast: DWR knees will be observed by the GPM radar when overflying high-density ice shafts embedded in large convective systems their explanation must not be sought in differential attenuation or differential Mie but via multiple scattering but interplay witn non uniform beam filling still under study. With real GPM-DPR data, we will be able to confirm or refute our conjecture.