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SMILES Ice Cloud Products

Contact: Luis Millán or William Read

Basic Information

Upper tropospheric water vapor and clouds play an important role in Earth's climate but knowledge of them, in particular diurnal variation in deep convective clouds, is limited. An essential variable to understand them is cloud ice water content. The Japanese Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on board the International Space Station (ISS) samples the atmosphere at different local times allowing the study of diurnal variability of atmospheric parameters.

Here, we describe cloud ice water content (IWC), partial Ice Water Path (pIWP) and relative humidity with respect to ice (Rhi) datasets derived by the MLS team. IWC is the density of ice (cloud ice mass in unit volume) at the measured altitude and pIWP is the integrated cloud ice mass along the line of sight.

SMILES Instrument

SMILES was launched in September 2009 and successfully attached to the front-side of the ISS. From October 2009 to April 2010, SMILES measured, on a time sharing basis two out of three frequency bands at any given time: 624.32-625.52 GHz (band A), 625.12-626.32 GHz (band B), and 649.12-650.32 GHz (band C).

During this period, SMILES measured the Earth's limb between 10 and 60 km about 1600 times per day. SMILES data has a latitudinal coverage from 38S to 65N covering all the local solar times (occasionally, the coverage will be 65S to 38N during periods when the ISS yaws to accommodate manned space flight vehicle dockings).
Simulated limb radiances at 32, 10 and 1hPa for the SMILES bands A, B, C. The molecules responsible for most prominent emissions are labeled. Gray lines show the window channels used in this study, each of them being 2 MHz wide.

How SMILES derives IWC and pIWP

The fundamental quantity to retrieved IWC and pIWP is the cloud induced radiances (Tcir). This quantity is defined as the difference between the measured radiance and the expected clear-sky radiance.

These Tcir were mapped to either IWC or pIWP using Tcir-IWC and Tcir-pIWP relations calculated using a 2D radiative transfer model, where ice particle size distribution is prescribed based on in-situ data.

As part of the computation of the Tcir, relative humidity with respect to ice (RHi) was retrieved directly from the water vapour continuum, modifying the water vapour Jacobians using the Goff-Gratch function, an empirical function that estimates the saturation water vapour pressure as a function of temperature.
Measured radiance from SMILES band A and retrieved Tcir. Points outside the blue lines are cloud detections while points between them are clear sky or undetectable clouds.

Data Characteristics

Data availability: 21 October 2009 to 21 April 2010
Data coverage: 38S to 65N (occasionally, the coverage will be 65S to 38N during periods when the ISS yaws to accommodate manned space flight vehicle dockings).

Pressure (hPa) Resolution Sensitivity Systematic Errors
IWC 80 Measurement box of 275 km along track and 3.3 km vertical. 0.02 mg/m3 77%
90 0.04 mg/m3 66%
100 0.07 mg/m3 60%
110 0.13 mg/m3 54%
pIWP 180 Ice water column above 12.5 km 0.22 g/m2 83%
220 0.07 g/m2 73%
600 0.13 g/m2 61%
1000 0.21 g/m2 57%

RHi Characteristics
at the tropics
at high latitudes
Pressure (hPa) Resolution (km) Precision (%) Resolution (km) Precision (%)
83 6.3 25 1.6 10
100 4.3 23 1.6 10
121 3.4 18 1.6 10
147 3.0 17 1.6 10
178 1.8 12 1.5 9
215 1.9 20 1.4 6

Where do I get the SMILES Ice Cloud Data?

The data are available at

JEM/SMILES mission is a joint project of Japan Aerospace Exploration Agency (JAXA) and National Institute of Information and Communications Technology (NICT).The provision of data and support is gratefully acknowledged.

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