Home
News
Publications
Meeting Talks
Reports
Data
Quicklooks
Instruments
Links
Contacts

    

Level 1c Cloudnet product:

Instrument Synergy/Target Categorization

Data Home | Overview of Cloudnet products | Product list | Quicklooks | Conditions of use

Summary

Designation
categorize
Level
1c
Algorithm design/Product maintainer
Institute
University of Reading and [institute providing the model data used]
Description
This product is intended to facilitate the application of multi-sensor algorithms by performing much of the required preprocessing. It includes radar, lidar, microwave radiometer, rain gauge and model data with regridding, correction for attenuation, reporting of measurement errors, data quality flags and categorization of targets.
Documentation
categorization.pdf
Relevant publication(s)
None yet available
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 Instrument Synergy/Target Categorization data, which was produced by the University of Reading and [institute providing the model data used] using measurements from [SITE].

Dataset contents

Sample NetCDF file: 20031012_chilbolton_categorize.nc
Header of sample NetCDF file in ASCII: 20031012_chilbolton_categorize.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: Cloud categorization products from Chilbolton, 2003-10-12

This dataset is an aggregation of data from cloud radar, lidar, a numerical forecast model and optionally a rain gauge and microwave radiometer. It is intended to facilitate the application of synergistic cloud-retrieval algorithms by performing a number of the preprocessing tasks that are common to these algorithms. Each of the observational datasets has been interpolated on to the same grid, although the model data are provided on a reduced height grid. Radar reflectivity has been corrected for attenuation, where possible, and two additional fields have been added: "category_bits" contains a categorization of the targets in each pixel and "quality_bits" indicates the quality of the data at each pixel. Finally, estimates of the random and systematic errors in reflectivity factor and attenuated backscatter are provided.

    Institution:Data produced at Department of Meteorology, University of Reading, UK
    Microwave radiometer institution:
    Data processed at the University of Reading,UK : http://www.met.reading.ac.uk
    Reference:Documentation may be found at http://www.met.rdg.ac.uk/radar/doc/categorization.html
    Software version:0.9.2

Dimensions and coordinate variables

time
Time UTC
    Units:hours since 2003-10-12 00:00:00 +0:00
    Type:single-precision floating-point vector
height
Height above mean sea level
    Units:m
    Type:single-precision floating-point vector
model_height
Height of model variables above mean sea level
    Units:m
    Type:single-precision floating-point vector
radiometer_frequency
Radiometer frequency
    Units:GHz
    Type:single-precision floating-point vector

Variables

altitude
Height of radar above mean sea level
    Units:m
    Type:single-precision floating-point scalar
latitude
Latitude of site
    Units:degrees_north
    Type:single-precision floating-point scalar
longitude
Longitude of site
    Units:degrees_east
    Type:single-precision floating-point scalar
rainrate(time)
Rain rate

    There were gaps in the rain gauge dataset on this day. Where this occurs we revert to the radar for rain detection: when Z at gate 3 is below 0 dBZ, the rain rate is assumed to be zero, when it exceeds this threshold there is a significant probability of rain and this variable contains "missing_value". This is then used in assigning the elements of the "quality_bits" variable. Note that it likely to overestimate the occurrence of but that is in order to minimize the amount of rain-affected data that is erroneously flagged as good data. Original comment: Converted to rainrate from raingauge c drop counts

    Units:mm h-1
    Type:single-precision floating-point vector
lwp(time)
Liquid water path

    This variable is the vertically integrated liquid water directly over the site. The temporal correlation of errors in liquid water path means that it is not really meaningful to distinguish bias from random error, so only an error variable is provided. Original comment: This variable is the vertically integrated liquid water content, or liquid water path (lwp), obtained using dual-wavelength microwave radiometer, lidar and information from a forecast model. The lwp is assumed to be proportional to microwave optical depth, and the coefficients used to derive it are obtained as follows. For the liquid water coefficients, the lidar is used to locate the cloud and the model to diagnose cloud base temperature. The water vapour coefficients are derived from model temperature and vertical humidity distribution. Note that humidity is not used in an absolute sense but only in the sense of determining the effective emission temperature of a given water vapour path. Finally, periods of clear sky identified by lidar are used to estimate calibration errors in the radiometers using the fact that lwp retrieved in these regions should be zero.

    Units:g m-2
    Type:single-precision floating-point vector
    Variable containing the random error in lwp:lwp_error
lwp_error(time)
Error in liquid water path, one standard deviation

    This variable is a rough estimate of the one-standard-deviation error in liquid water path, calculated as a combination of a 20 g m-2 linear error and a 25% fractional error.

    Units:g m-2
    Type:single-precision floating-point vector
brightness_temperature(time, radiometer_frequency)
Microwave radiometer brightness temperature
    Units:K
    Type:single-precision floating-point array
radar_frequency
Radar frequency
    Units:GHz
    Type:single-precision floating-point scalar
lidar_wavelength
Lidar wavelength
    Units:nm
    Type:single-precision floating-point scalar
Z(time, height)
Radar reflectivity factor

    This variable has been corrected for attenuation by gaseous attenuation (using the thermodynamic variables from a forecast model; see the radar_gas_atten variable) and liquid attenuation (using liquid water path from a microwave radiometer; see the radar_liquid_atten variable), but rain and melting-layer attenuation has not been corrected. Calibration convention: in the absence of attenuation, a cloud at 273 K containing one million 100-micron droplets per cubic metre will have a reflectivity of 0 dBZ at all frequencies. Original comment: Calibration convention: in the absence of attenuation, a cloud at 273 K containing one million 100-micron droplets per cubic metre will have a reflectivity of 0 dBZ at all frequencies. Noise has been subtracted using the cloud-free gates at the end of each ray to determine the noise level. The presence of a range-dependent interference in the noise level was taken into account, but there may still be occasional anomalous horizontal lines in the clouds observed one or two pixels thick. To reduce speckle noise, any cloudy pixel or pair of cloudy pixels that had cloud-free pixels to each side of them in range were removed. Note that this parameter was calculated using the incoherent processing system, while the others were calculated using coherent processing.

    Units:dBZ
    Type:single-precision floating-point array
    Source:Chilbolton 94-GHz Cloud Radar (Galileo)
    Frequency: 94.00 GHz
    Antenna diameter: 0.46 m
    Axis separation of transmit and receive antennas: 0.66 m
    Peak power: 1.6 kW
    Pulse width: 0.5 us
    Pulse repetition frequency: 6250 Hz
    Beam width: 0.5 degrees
    Variable containing the random error in Z:Z_error
    Variable containing the bias in Z:Z_bias
    Variable containing the minimum detectable Z:Z_sensitivity
v(time, height)
Doppler velocity

    This parameter is the radial component of the velocity, with positive velocities away from the radar.

    Units:m s-1
    Type:single-precision floating-point array
    Source:Chilbolton 94-GHz Cloud Radar (Galileo)
width(time, height)
Doppler spectral width

    This parameter is the standard deviation of the reflectivity-weighted velocities in the radar pulse volume.

    Units:m s-1
    Type:single-precision floating-point array
    Source:Chilbolton 94-GHz Cloud Radar (Galileo)
sigma_v(time, height)
Standard deviation of mean velocity

    The data in this file are at a lower resolution than the raw data, and this parameter is the standard deviation of the 30 raw Doppler velocities measured during in each output gate and ray.

    Units:m s-1
    Type:single-precision floating-point array
    Source:Chilbolton 94-GHz Cloud Radar (Galileo)
Z_bias
Calibration error in Z, one standard deviation

    This variable is an estimate of the one-standard-deviation calibration error (i.e. the likely systematic error) in radar reflectivity factor.

    Units:dB
    Type:single-precision floating-point scalar
Z_error(time, height)
Random error in Z, one standard deviation

    This variable is an estimate of the one-standard-deviation random error in radar reflectivity factor. It originates from the following independent sources of error: 1) Precision in reflectivity estimate due to finite signal to noise and finite number of pulses 2) 10% uncertainty in gaseous attenuation correction (mainly due to error in model humidity field) 3) Error in liquid water path (given by the variable lwp_error) and its partitioning with height).

    Units:dB
    Type:single-precision floating-point array
Z_sensitivity(height)
Minimum detectable radar reflectivity

    This variable is an estimate of the radar sensitivity, i.e. the minimum detectable radar reflectivity as a function of height. It includes the effect of ground clutter and gas attenuation but not liquid attenuation.

    Units:dBZ
    Type:single-precision floating-point vector
beta(time, height)
Attenuated backscatter coefficient

    This variable has not been corrected for attenuation.

    Units:sr-1m-1
    Type:single-precision floating-point array
    Source:Chilbolton Vaisala 905-nm CT75K lidar ceilometer
    Variable containing the random error in beta:beta_error
    Variable containing the bias in beta:beta_bias
beta_bias
Calibration error in beta, one standard deviation

    This variable is an estimate of the one-standard-deviation calibration error (i.e. the likely systematic error) in attenuated lidar backscatter coefficient.

    Units:dB
    Type:single-precision floating-point scalar
beta_error
Random error in beta, one standard deviation

    This variable is a very approximate esimate of the one-standard-deviation random error in attenuated lidar backscatter coefficient. It should really take account of signal-to-noise ratio, number of pulses averaged and so on, but the exact algorithm used to calculate the reported backscatter values is proprietary.

    Units:dB
    Type:single-precision floating-point scalar
temperature(time, model_height)
Temperature
    Units:K
    Type:single-precision floating-point array
    Source:ECMWF Integrated Forecast System (IFS)
pressure(time, model_height)
Pressure
    Units:Pa
    Type:single-precision floating-point array
    Source:ECMWF Integrated Forecast System (IFS)
specific_humidity(time, model_height)
Model specific humidity

    This variable is the specific humidity from the model. Note that it has not been modified according to where the category_bits variable diagnoses the presence of liquid cloud.

    Units:dimensionless
    Type:single-precision floating-point array
    Source:ECMWF Integrated Forecast System (IFS)
uwind(time, model_height)
Zonal wind
    Units:m s-1
    Type:single-precision floating-point array
    Source:ECMWF Integrated Forecast System (IFS)
vwind(time, model_height)
Meridional wind
    Units:m s-1
    Type:single-precision floating-point array
    Source:ECMWF Integrated Forecast System (IFS)
radar_gas_atten(time, height)
Two-way radar attenuation due to atmospheric gases

    This variable was calculated from the model temperature, pressure and humidity, but forcing pixels containing liquid cloud to saturation with respect to liquid water. It was calculated using the millimeter-wave propagation model of Liebe (1985, Radio Sci. 20(5), 1069-1089). It has been used to correct Z.

    Units:dB
    Type:single-precision floating-point array
radar_liquid_atten(time, height)
Approximate two-way radar attenuation due to liquid water

    This variable was calculated from the liquid water path measured by microwave radiometer, using the lidar and radar returns to perform an approximate partitioning of the liquid water content with height. Bit 5 of the quality_bits variable indicates where a correction for liquid water attenuation has been performed. The dielectric parameters of liquid water were calculated using model temperature and the double-Debye formulation of Manabe, Liebe and Hufford (1987, Conf. Dig. 12th Int. Conf Infrared & Millimeter Waves, Lake Buena Vista, Dec. 14-18); see also Liebe, Manabe and Hufford (1989, IEEE Trans. AP 37(12), 1617-1623).

    Units:dB
    Type:single-precision floating-point array
sigma_zbeta(time)
Standard deviation of peak lidar return

    This variable is the standard deviation of the beta-weighted heights in the vicinity of the highest attenuated lidar backscatter (beta) return in each profile containing liquid water droplets. It therefore provides a measure of the width of the "spike" measured by lidar at the base of liquid water clouds. It can be used to estimate cloud droplet number concentration. It is calculated from the original (high vertical) resolution lidar data so should be more accurate than would be possible using the vertically averaged beta recorded in this file. The resolution of the original lidar data is given in metres by the "lidar_vertical_resolution" attribute. It should be used in conjunction with the variables "mean_zbeta", which provides the corresponding mean height of the peak in beta, and "numgates_zbeta", which provides the number of high resolution range gates used in computing this variable. If no liquid cloud is present then "missing_value" is reported.

    Units:m
    Type:single-precision floating-point vector
mean_zbeta(time)
Mean height of peak lidar return

    This variable is the mean height of the peak in attenuated lidar backscatter (beta) for each profile containing liquid water droplets. The variable "numgates_zbeta" provides the number of high resolution range gates used in computing this variable. If no liquid cloud is present then "missing_value" is reported. It may be used in conjunction with the variable "sigma_zbeta".

    Units:m
    Type:single-precision floating-point vector
numgates_zbeta(time)
Raw gates used to calculate std. dev. of peak lidar return

    This variable is the number of high vertical resolution lidar range gates that were used to calculate the variables "sigma_zbeta" and "mean_zbeta". It may be used to estimate the error in sigma_zbeta and parameters derived from it. The original resolution of the lidar is indicated by the "lidar_vertical_resolution" attribute of sigma_zbeta.

    Units:m
    Type:signed two-byte integer vector
category_bits(time, height)
Target categorization bits

    This variable contains information on the nature of the targets at each pixel, thereby facilitating the application of algorithms that work with only one type of target. The information is in the form of an array of bits, each of which states either whether a certain type of particle is present (e.g. aerosols), or the whether some of the target particles have a particular property. The definitions of each bit are given in the definition attribute. Bit 0 is the least significant.

    Type:byte array
    Definition:
    • Bit 0: Small liquid droplets are present.
    • Bit 1: Falling hydrometeors are present; if Bit 2 is set then these are most likely to be ice particles, otherwise they are drizzle or rain drops.
    • Bit 2: Wet-bulb temperature is less than 0 degrees C, implying the phase of Bit-1 particles.
    • Bit 3: Melting ice particles are present.
    • Bit 4: Aerosol particles are present and visible to the lidar.
    • Bit 5: Insects are present and visible to the radar.
quality_bits(time, height)
Data quality bits

    This variable contains information on the quality of the data at each pixel. The information is in the form of an array of bits, and the definitions of each bit are given in the definition attribute. Bit 0 is the least significant.

    Type:byte array
    Definition:
    • Bit 0: An echo is detected by the radar.
    • Bit 1: An echo is detected by the lidar.
    • Bit 2: The apparent echo detected by the radar is ground clutter or some other non-atmospheric artifact.
    • Bit 3: The echo detected by the lidar is due to clear-air molecular scattering.
    • Bit 4: Liquid water cloud, rainfall or melting ice below this pixel will have caused radar and lidar attenuation; if bit 5 is set then a correction for the radar attenuation has been performed; otherwise do not trust the absolute values of reflectivity factor. No correction is performed for lidar attenuation.
    • Bit 5: Radar reflectivity has been corrected for liquid-water attenuation using the microwave radiometer measurements of liquid water path and the lidar estimation of the location of liquid water cloud; be aware that errors in reflectivity may result.

These pages are maintained by .