
Level 1c Cloudnet product:Instrument Synergy/Target CategorizationData Home  Overview of Cloudnet products  Product list  Quicklooks  Conditions of useSummary
Dataset contentsSample NetCDF file: 20031012_chilbolton_categorize.nc 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, 20031012This 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 cloudretrieval 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.
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 variablestimeTime UTC
Type:singleprecision floatingpoint vector Height above mean sea level
Type:singleprecision floatingpoint vector Height of model variables above mean sea level
Type:singleprecision floatingpoint vector Radiometer frequency
Type:singleprecision floatingpoint vector VariablesaltitudeHeight of radar above mean sea level
Type:singleprecision floatingpoint scalar Latitude of site
Type:singleprecision floatingpoint scalar Longitude of site
Type:singleprecision floatingpoint scalar 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 rainaffected data that is erroneously flagged as good data. Original comment: Converted to rainrate from raingauge c drop counts Units:mm h^{1}Type:singleprecision floatingpoint vector 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 dualwavelength 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:singleprecision floatingpoint vector Variable containing the random error in lwp:lwp_error Error in liquid water path, one standard deviation This variable is a rough estimate of the onestandarddeviation error in liquid water path, calculated as a combination of a 20 g m2 linear error and a 25% fractional error. Units:g m2Type:singleprecision floatingpoint vector Microwave radiometer brightness temperature
Type:singleprecision floatingpoint array Radar frequency
Type:singleprecision floatingpoint scalar Lidar wavelength
Type:singleprecision floatingpoint scalar 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 meltinglayer attenuation has not been corrected. Calibration convention: in the absence of attenuation, a cloud at 273 K containing one million 100micron 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 100micron droplets per cubic metre will have a reflectivity of 0 dBZ at all frequencies. Noise has been subtracted using the cloudfree gates at the end of each ray to determine the noise level. The presence of a rangedependent 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 cloudfree 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:dBZType:singleprecision floatingpoint array Source:Chilbolton 94GHz 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 Doppler velocity This parameter is the radial component of the velocity, with positive velocities away from the radar. Units:m s^{1}Type:singleprecision floatingpoint array Source:Chilbolton 94GHz Cloud Radar (Galileo) Doppler spectral width This parameter is the standard deviation of the reflectivityweighted velocities in the radar pulse volume. Units:m s^{1}Type:singleprecision floatingpoint array Source:Chilbolton 94GHz Cloud Radar (Galileo) 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:singleprecision floatingpoint array Source:Chilbolton 94GHz Cloud Radar (Galileo) Calibration error in Z, one standard deviation This variable is an estimate of the onestandarddeviation calibration error (i.e. the likely systematic error) in radar reflectivity factor. Units:dBType:singleprecision floatingpoint scalar Random error in Z, one standard deviation This variable is an estimate of the onestandarddeviation 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:dBType:singleprecision floatingpoint array 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:dBZType:singleprecision floatingpoint vector Attenuated backscatter coefficient This variable has not been corrected for attenuation. Units:sr^{1}m^{1}Type:singleprecision floatingpoint array Source:Chilbolton Vaisala 905nm CT75K lidar ceilometer Variable containing the random error in beta:beta_error Variable containing the bias in beta:beta_bias Calibration error in beta, one standard deviation This variable is an estimate of the onestandarddeviation calibration error (i.e. the likely systematic error) in attenuated lidar backscatter coefficient. Units:dBType:singleprecision floatingpoint scalar Random error in beta, one standard deviation This variable is a very approximate esimate of the onestandarddeviation random error in attenuated lidar backscatter coefficient. It should really take account of signaltonoise ratio, number of pulses averaged and so on, but the exact algorithm used to calculate the reported backscatter values is proprietary. Units:dBType:singleprecision floatingpoint scalar Temperature
Type:singleprecision floatingpoint array Source:ECMWF Integrated Forecast System (IFS) Pressure
Type:singleprecision floatingpoint array Source:ECMWF Integrated Forecast System (IFS) 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:dimensionlessType:singleprecision floatingpoint array Source:ECMWF Integrated Forecast System (IFS) Zonal wind
Type:singleprecision floatingpoint array Source:ECMWF Integrated Forecast System (IFS) Meridional wind
Type:singleprecision floatingpoint array Source:ECMWF Integrated Forecast System (IFS) Twoway 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 millimeterwave propagation model of Liebe (1985, Radio Sci. 20(5), 10691089). It has been used to correct Z. Units:dBType:singleprecision floatingpoint array Approximate twoway 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 doubleDebye formulation of Manabe, Liebe and Hufford (1987, Conf. Dig. 12th Int. Conf Infrared & Millimeter Waves, Lake Buena Vista, Dec. 1418); see also Liebe, Manabe and Hufford (1989, IEEE Trans. AP 37(12), 16171623). Units:dBType:singleprecision floatingpoint array Standard deviation of peak lidar return This variable is the standard deviation of the betaweighted 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:mType:singleprecision floatingpoint vector 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:mType:singleprecision floatingpoint vector 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:mType:signed twobyte integer vector 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 arrayDefinition: 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 arrayDefinition: These pages are maintained by . 