The Ice Cloud Imager (ICI) instrument is a conically scanning millimetre/sub-millimetre wave radiometer providing information on ice clouds.
ICI will measure ice clouds, especially cirrus clouds, cloud ice water path, cloud ice effective radius and cloud altitude. It will also provide vertical humidity profile and vertical profiles of hydrometeors (cloud ice, graupel and snow), as well as water vapour.
The primary objective of the Ice Cloud Imaging (ICI) mission is to support climate monitoring and the validation of the presentation of ice clouds in weather and climate models through the provision of ice cloud products, including bulk microphysical variables.
Further objectives include the measurement snowfall distributions in support of numerical weather prediction and nowcasting.
ICI data will enhance the ability of NWP centres to initialise global and regional models with information on clouds. Although their treatment in NWP is important for accurate weather forecasts, clouds are currently not well represented in models.
The ICI frequency coverage is from 183 GHz up to 664 GHz, with two window channels (243 GHz and 664 GHz) measured at both *V and **H polarisation*. The table shows the spectral characteristics of ICI channels. The 183 GHz channel selection overlaps with the one of MWI, allowing a cross-calibration between the instruments.
|Channel||Frequency||Bandwidth||***NEΔT (K)||Bias (K)||Polarisation||Footprint Size at 3dB|
|ICI-1||183.31±7.0 GHz||2x2000 MHz||0.8||1.0||V||16 km|
|ICI-2||183.31±3.4 GHz||2x1500 MHz||0.8||1.0||V||16 km|
|ICI-3||183.31±2.0 GHz||2x1500 MHz||0.8||1.0||V||16 km|
|ICI-4||243.2±2.5 GHz||2x3000 MHz||0.7||1.5||V, H||16 km|
|ICI-5||325.15±9.5 GHz||2x3000 MHz||1.2||1.5||V||16 km|
|ICI-6||325.15±3.5 GHz||2x2400 MHz||1.3||1.5||V||16 km|
|ICI-7||325.15±1.5 GHz||2x1600 MHz||1.5||1.5||V||16 km|
|ICI-8||448±7.2 GHz||2x3000 MHz||14||1.5||V||16 km|
|ICI-9||448±3.0 GHz||2x2000 MHz||1.6||1.5||V||16 km|
|ICI-10||448±1.4 GHz||2x1200 MHz||2.0||1.5||V||16 km|
|ICI-11||664±4.2 GHz||2x5000 MHz||1.6||1.5||V, H||16 km|
*V — Vertical Polarisation
**H — Horizontal Polarisation
***NEΔT — Noise Equivalent Temperature change
The ICI collects radiation coming from the Earth by means of a rotating antenna, composed by an offset parabolic reflector and a feed-horns cluster rotating together. The rotation of the slanted antenna allows the conical scan to be performed. The ICI shall provide measurements over a conical scan with an observation zenith angle of 53° ±2°. The basic scan geometry is depicted in Figure 2. ICI scans in a clockwise rotation when viewed from the nadir side of the platform, where it is located.
The observations are acquired within an angle of ±65° in azimuth for the fore view, equivalent to a swath of about 1700 km at nominal orbit altitude of 817 km.