Level 2a Cloudnet product:

Ice water content (radar/temperature method)

Summary

Designation

iwc-Z-T-method

Level

2a

Algorithm design/Product maintainer

Robin Hogan

Institute

University of Reading

Description

Ice water content derived from radar reflectivity factor and temperature using an empirical formula derived using aircraft data.

Documentation

See relevant publication

Relevant publication(s)

Hogan, R. J., M. P. Mittermaier and A. J. Illingworth, 2004: The retrieval of ice water content from radar reflectivity factor and temperature and its use in the evaluation of a mesoscale model. Submitted to J. Appl. Meteorol., September 2004. PDF

Quicklooks

http://www.met.rdg.ac.uk/radar/cloudnet/quicklooks/

Suggested acknowledgement

We acknowledge the Cloudnet project (European Union contract EVK2-2000-00611) for providing the ice water content data, which was produced by the University of Reading using measurements from [SITE].

Dataset contents

Sample NetCDF file: 20030701_chilbolton_iwc-Z-T-method.nc
Header of sample NetCDF file in ASCII: 20030701_chilbolton_iwc-Z-T-method.cdl

The following is a description of the variables in this product, produced automatically from the metadata in the sample file (using the nc2html unix utility):

Dataset title: Ice water content from Chilbolton

Institution: Data processed at the Department of Meteorology, University of Reading.

Dimensions and coordinate variables

Units: hours since 2003-07-01 00:00:00 +0:00
Type: single-precision floating-point vector

Units: m
Type: single-precision floating-point vector

Variables

Units: degrees_north
Type: single-precision floating-point scalar

Units: degrees_east
Type: single-precision floating-point scalar

Units: m
Type: single-precision floating-point scalar

Units: GHz
Type: single-precision floating-point scalar

This variable was calculated from the 94-GHz radar reflectivity factor after correction for gaseous attenuation, and temperature taken from a forecast model, using the following empirical formula: log10(iwc[g m-3]) = 0.00058Z[dBZ]T[degC] + 0.0923Z[dBZ] + -0.00706T[degC] + -0.992. In this formula Z is taken to be defined such that all frequencies of radar would measure the same Z in Rayleigh scattering ice. However, the radar is more likely to have been calibrated such that all frequencies would measure the same Z in Rayleigh scattering liquid cloud at 0 degrees C. The measured Z is therefore multiplied by |K(liquid,0degC,94GHz)|^2/0.93 = 0.7194 before applying this formula. The formula has been used where the "categorization" data has diagnosed that the radar echo is due to ice, but note that in some cases supercooled drizzle will erroneously be identified as ice. Missing data indicates either that ice cloud was present but it was only detected by the lidar so its ice water content could not be estimated, or that there was rain below the ice associated with uncertain attenuation of the reflectivities in the ice. Note that where microwave radiometer liquid water path was available it was used to correct the radar for liquid attenuation when liquid cloud occurred below the ice; this is indicated a value of 3 in the iwc_retrieval_status variable. There is some uncertainty in this prodedure which is reflected by an increase in the associated values in the iwc_error variable. When microwave radiometer data were not available and liquid cloud occurred below the ice, the retrieval was still performed but its reliability is questionable due to the uncorrected liquid water attenuation. This is indicated by a value of 2 in the iwc_retrieval_status variable, and an increase in the value of the iwc_error variable

Units: kg m^-3
Type: single-precision floating-point array
Variable containing the random error in iwc: iwc_error
Variable containing the bias in iwc: iwc_bias
Variable containing the minimum detectable iwc: iwc_sensitivity

This variable is the same as iwc, except that values of iwc in ice above rain have been included. This variable contains values which have been severely affected by attenuation and should only be used when the effect of attenuation is being studied

Units: kg m^-3
Type: single-precision floating-point array
Variable containing the random error in iwc_inc_rain: iwc_error
Variable containing the bias in iwc_inc_rain: iwc_bias
Variable containing the minimum detectable iwc_inc_rain: iwc_sensitivity

This variable is an estimate of the possible systematic error (one-standard-deviation) in ice water content due to the calibration error of the radar reflectivity factor from which it was calculated.

Units: dB
Type: single-precision floating-point scalar

This variable is an estimate of the one-standard-deviation random error in ice water content due to both the uncertainty of the retrieval (about +50%/-33%, or 1.7 dB), and the random error in radar reflectivity factor from which ice water content was calculated. When liquid water is present beneath the ice but no microwave radiometer data were available to correct for the associated attenuation, the error also includes a contribution equivalent to approximately 250 g m-2 of liquid water path being uncorrected for. As uncorrected liquid attenuation actually results in a systematic underestimate of ice water content, users may wish to reject affected data; these pixels may be identified by a value of 2 in the iwc_retrieval_status variable. Typical errors in temperature contribute much less to the overall uncertainty in retrieved ice water content so are not considered. Missing data in iwc_error indicates either zero ice water content (for which an error in dB would be meaningless), or no ice water content value being reported. Note that when zero ice water content is reported, it is possible that ice cloud was present but was just not detected by any of the instruments.

Units: dB
Type: single-precision floating-point array

This variable is an estimate of the minimum detectable ice water content as a function of height.

Units: kg m^-3
Type: single-precision floating-point vector

This variable describes whether a retrieval was performed for each pixel, and its associated quality, in the form of 8 different classes. The classes are defined in the definition and long_definition attributes. The most reliable retrieval is that without any rain or liquid cloud beneath, indicated by the value 1, then the next most reliable is when liquid water attenuation has been corrected using a microwave radiometer, indicated by the value 3, while a value 2 indicates that liquid water cloud was present but microwave radiometer data were not available so no correction was performed. No attempt is made to retrieve ice water content when rain is present below the ice; this is indicated by the value 5.

Type: byte array
Definition:
• 0: No ice present
• 1: Reliable retrieval
• 2: Unreliable retrieval due to uncorrected attenuation from liquid water below the ice (no liquid water path measurement available)
• 3: Retrieval performed but radar corrected for liquid attenuation using radiometer liquid water path which is not always accurate
• 4: Ice detected only by the lidar
• 5: Ice detected by radar but rain below so no retrieval performed due to very uncertain attenuation
• 6: Clear sky above rain, wet-bulb temperature less than 0degC: if rain attenuation were strong then ice could be present but undetected
• 7: Drizzle or rain that would have been classified as ice if the wet-bulb temperature were less than 0degC: may be ice if temperature is in error