Vortex-SE 2017 S-band FMCW Profiler Data


Stephen Frasier, Joseph Waldinger, William Heberling

University of Massachusetts
Microwave Remote Sensing Laboratory
151 Holdsworth Way
Amherst, MA 01003
(413) 545-0779

frasier@umass.edu
jwaldinger@umass.edu


Robin Tanamachi, Dan Dawson

Department of Earth, Atmospheric, and Planetary Sciences (EAPS)
Purdue University
550 Stadium Mall Drive
West Lafayette, IN 47907
(765) 496-2866

rtanamachi@purdue.edu
dandawson@purdue.edu


Data Contact:  frasier@umass.edu



1.0 Data Set Overview

Vertical profiles of radar moments and Doppler spectra over the period 
3/10/2017--4/30/2017 collected by the UMass S-band FMCW profiling radar.  
The radar was located at the Scottsboro,AL Municipal Airport.  



2.0 Instrument Description

The S-band FMCW Radar is principally described in Ince et al.(2003). For 
Vortex-SE 2017 it operated with a substantially higher sampling rate to enable 
a wider range of vertical velocities, with some sacrifice of sensitivity 
to do so.  Basic instrument parameters are as follows

Center Frequency  2.92	  GHz
Bandwidth	  30      MHz
Sweep Interval	  3.48	  ms
Sweep Rate	  287.224 Hz
Transmit Power	  250	  W (continuous)
Antenna Gain	  34	  dB
Compression Gain  48.4	  dB
Noise level	  -97.4   dBm
Beam width	  3	  deg
Range bins	  1024
Range resolution  5	  m
Max/min velocity  +/-7.4  m/s



3.0 Data Collection & Processing

FM sweeps are collected in blocks of 256 sweeps to produce a Doppler
spectrum over a 0.89 s interval.  Fourteen (14) spectra are then
accumulated yielding averaged spectra every 12.5 s.  Moments are
calculated from the spectra.  Reflectivity is estimated from system
SNR using the radar equation and nominal system parameters.  



4.0 Data Format

Data are stored in NetCDF format.  Below is a commented CDL description
of a file.


netcdf S20160307T020003 {

dimensions:
	vels = 256 ;	// number of velocity bins in Doppler spectra
	gate = 1024 ;	// number of 5-m range bins
	time = UNLIMITED ; // (224 currently)

variables:
	float vbins(vels) ;
		vbins:Name = "Spectral Velocity Bins" ;
		vbins:Units = "m/s" ;

	float height(gate) ;
		height:Name = "Height" ;
		height:Units = "m AGL" ;

	int secs(time) ;
		secs:Name = "Secs" ;
		secs:Units = "s since 1/1/1970" ;

	short Zef(time, gate, vels) ;
		Zef:Name = "Spectral Reflectivity Factor" ;
		Zef:Units = "dBZ/bin x10" ;

	short sf(time, gate, vels) ;
		sf:Name = "Raw Spectral Power" ;
		sf:Units = "dB(uncalibrated) x10" ;

	short sn(time, gate) ;
		sn:Name = "Noise Power" ;
		sn:Units = "dB(uncalibrated) x10" ;

	short sff(time, gate, vels) ;
		sff:Name = "Filtered Spectral Power" ;
		sff:Units = "dB(uncalibrated) x10" ;

	short snr(time, gate) ;
		snr:Name = "Signal to Noise Ratio" ;
		snr:Units = "dB x10" ;

	short Ze(time, gate) ;
		Ze:Name = "Reflectivity Factor" ;
		Ze:Units = "dBZ x10" ;

	float vel(time, gate) ;
		vel:Name = "Mean Radial Velocity" ;
		vel:Units = "m/s" ;

	float wid(time, gate) ;
		wid:Name = "Spectrum Width" ;
		wid:Units = "m/s" ;

// global attributes:
		:NetCDFRevision = "UMass MIRSL FMCW V1.0" ;
		:RadarName = "UMa-FMCW" ;
		:PRF = 287.244f ;
		:Frequency = 2.92e+09f ;
		:Latitude = 34.6872f ;
		:Longitude = -86.005f ;
}



5.0 Data Remarks

Remark 1: Intermittent small gaps in the data stream yield some uneven
temporal sampling.  All spectra consist of 14 complete processing
blocks over 12.5 s, and the timestamp corresponds to the middle of the
time interval.  The timestamps themselves are not always evenly
spaced.  A few longer time gaps were due to
power/communications outages.

Remark 2: Raw power spectra (variable "sf") contain spurs/spikes due
to interference from power supplies in the transmitter or elsewhere.
A median filtering method has been used to eliminate most of these
(variable "sff") though some strong spurs may still persist.  In the
presence of strong scattering, these spurs are overwhelmed.

Remark 3: Although the system's equivalent noise floor corresponds to
a constant value reported above, the receiver gain is range dependent
owing to receiver filter characteristics.  Thus, the noise-floor in
raw spectra also varies with range.

Remark 4: The reflectivity units are reflectivity factor (Z), as that
is most familiar to radar meteorologists.  For clear-air scattering
from refractive index turbulence, the more appropriate unit is the
refractive index structure parameter, Cn2.  Both of these are related
to Volume reflectivity, eta (radar cross section per unit volume), as
follows:
	      
      eta = pi^5/lambda^4 |Kw|^2 Z  (Rayleigh Scatter)
      
      eta = 0.38 lambda^(-1/3) Cn2  (Bragg Scatter)

Remark 5: During heavier precipitation episodes, the vertical velocity
is occasionally aliased, yielding positive (upward) velocities.  This
has been left as is, and it is up to the user to de-alias velocities
appropriately.



6.0 References

Ince, T., S. J. Frasier, A. Muschinski, A. L. Pazmany, 2003. "An
S-Band Frequency-Modulated Continuous-Wave Boundary Layer Profiler:
Description and Initial Results," Radio Sci., 38(4), 1072,
doi:10.1029/2002RS002753.