International Sub-Millimetre Airborne Radiometer

The integrated column ice mass is an important parameter that can be used to constrain simulations of ice clouds in General Circulation Models. It is currently measured using a combination of infrared and microwave techniques, but indirectly and by using many assumptions.

The figure above compares brightness temperatures depressions relative to a clear sky for frequencies between 240 and 1000 GHz that would be observed by a nadir-viewing instrument at high altitude over an ice cloud with ice water path between 1.5 and 6000 gm-2. The occurence of It may be seen that throughout this frequency range, there are large brightness temperature depressions over cloud due to the scattering of radiation emitted from warmer layers below the cloud. The magnitude of these signals and their frequency-dependence can be exploited to give a direct estimate of the column ice mass, its vertical distribution and the mean ice particle size.

ISMAR (the International Sub-Millimetre Airborne Radiometer) is a new passive remote sensing instrument with polarisation and multiple channels (118, 243 (V/H), 325, 424, 448, 664 (V/H) and 874 GHz (V/H)), designed to operate on an aircraft platform. The development of ISMAR is a broad international collaboration involving the Met Office and Rutherford-Appleton Laboratory in the UK, with the space agencies in Europe and Japan and other research institutes in Sweden, Germany and France.

Together with existing aircraft instruments, ISMAR can potentially be used to determine column ice mass, particle size and some shape information in ice clouds, mixed phase cloud properties, an indirect measurement of precipitation, snow surface emissivity and near-source volcanic ash loading. The interpretation of ISMAR measurements will be heavily dependent on the ability to measure independently the column ice mass.

Range

Channels

(GHz)
BW (GHz) Polarisation Feature
118.75

±1.1
±1.5
±2.1
±3.0
±5.0

0.4
0.4
0.8
1
2

V Oxygen line
243.2 ±2.5 3 V&H Quasi-window, cloud ice retrieval, cirrus clouds
325.15

±1.5
±3.5
±9.5

1.6
2.4
3.0

V

Water vapour profile,

Cloud ice effective radius
424.7 ±1.0
±1.5
±4.0

0.4
0.6
1

V Oxygen line

ISMAR:

Awaiting funding
448 ±0.8
±2.0
±4.5 ±11.5
1.2
2
3
3
V

Water vapour profile,

Cloud ice effective radius
664 ±4.2 3 V&H Quasi-window, cirrus clouds, cloud ice water path
874.4 ±6.0 3 V&H Quasi-window, cirrus clouds, cloud ice water path

ISMAR:

ESA receiver development

Images

Contacts

Rob King, MetOffice

Clare Lee, MetOffice

Stuart Fox, MetOffice

Ian Rule, MetOffice

Additional information