• Ewan O’Connor, Robin Hogan, Anthony Illingworth, Nicolas Gaussiat

Liquid water path frommicrowave radiometers

– Take radiative transfer model (RTM)• Choose from Liebe, Rosenkranz etc.

– Model atmospheric attenuation at surface

LWP Retrieval

Attenuation due to oxygen

Attenuation due to water

vapour: typical values

Niamey – humidChilbolton - dry

– Note that water vapour path varies from site to site. Shape of peak near 20 GHz varies with amount of saturation.

– No liquid clouds yet!

LWP Retrieval

Attenuation due to oxygen

Attenuation due to water vapour: fully

saturated

Niamey – humidChilbolton - dry

– Add liquid water layer 1 km deep with LWC = 1 g m-3. – Attenuation depends on temperature (dielectric constant)

LWP Retrieval

20 C

0 C

Chilbolton – typical atmosphere, no cloud

-20 C

Note: dielectric constant not well known for supercooled liquid water!

Dual wavelength microwave radiometer

– Brightness temperatures -> Liquid water path– Improved technique – Nicolas Gaussiat

• Use lidar to determine whether clear sky or not• Adjust coefficients to account for instrument drift• Removes offset for low LWP

Dual wavelength microwave radiometer

– Brightness temperatures -> Liquid water path– Improved technique – Nicolas Gaussiat

• Use lidar to determine whether clear sky or not• Adjust coefficients to account for instrument drift• Removes offset for low LWP

LWP - initialLWP - lidar corrected

Radiometrics MP1516A• Water vapor profiler

– 21 frequencies from 22-30 GHz• Possible to determine profile of humidity

– Half-power beamwidth 6.3-4.9 degrees

Brightness temperatures

Interested in LWP• Column integrated liquid water

– Warm liquid layers usually have 100-500 g m-2

– Supercooled layers may have as little as 10-20g m-2 • This is close to the uncertainty in the radiative transfer models

Case: 6 October 2008

Case: 10 October 2008

Case: 5 October 2008

Conclusions• Radiometrics algorithm misses low values

– Any values close to or less than 0 set to 0• Assumed to be noise• Can be caused by a slight offset • Problematic for supercooled clouds

• Noise is useful check!– Can see how algorithm is performing– Spot drifts in

• Instrument• Synoptic conditions

– Can average to retrieve low LWP in certain situations