Instruments:arm

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Arecibo ST Radar

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Description of Instrument/Model

Atmospheric radars depend on scattering from refractive index inhomogeneities to obtain information about the medium. In the ionosphere, the irregularities mostly result from the thermal fluctuation of free electrons and the corresponding radar scatter is called incoherent scatter. In the cloud-free neutral atmosphere, VHF and UHF radar scatter mostly results from the structuring of the refractive index by turbulence and sometimes from partial reflection by layers in the refractive index. These types of radars are called coherent scatter radars and they are classified according to the atmospheric regions they can observe such as ST (strato/troposphere) or MST (meso/strato/troposphere).

The ST (strato/troposphere) radar at the National Astronomy and Ionosphere Center (NAIC) at Arecibo, Puerto Rico (18.3N, 66.75W), made measurements during the AIDA (Arecibo Initiative in Dynamics of the Atmosphere) campaign of March-May, 1989, an international multi-instrument campaign which was conducted to compare wind measurements in the mesosphere taken by various radar and optical devices. The tropo- and stratosphere data taken by the Arecibo ST radar during this period were largely decoupled from the main objectives and the observations of the other instruments. Data are available for both day and night times. Integration time per profile varies and is on the order of 30 sec to 1 min. However, because the mesosphere observations were interleaved with this data set, there are many gaps between profiles on the order of a few minutes. During AIDA, the altitude range sampled was from about 5 to 20 km, and the resolution was about 0.3 km. For this data set, the antenna azimuth scan pattern was very roughly as follows: 1.5 hrs at 14 or 32 deg, 10 min at -76 or -58 deg, 10 min at 194 or 212 deg, 30 min at 104 or 122 deg, 10 min at 194 or 212 deg, and 10 min at -76 or -58 deg, then repeat. Beware that the times do appear to vary. Elevation angle was fixed at about 79 degrees. Results from AIDA have been published in J. Atmos. Terr. Phys., 55, 1993, a special issue dedicated to the campaign. H. Mario Ierkic was responsible for the operation of the Arecibo ST radar during AIDA, and John Cho put this data set into the CEDAR Data Base.

This program extracts the following data from the Doppler spectra of the Arecibo ST radar: (1) Line-of-sight velocity of the clear air, (2) signal spectral width, and (3) SNR. Item (1) is self-explanatory. Items (2) and (3) yield information about the actual radar scattering mechanism. If the mechanism is turbulence, then (1) and (3) yield the turbulence intensity. For further information see Gage and Balsley [1980].

Doppler spectra are calculated by (i) subtracting the linear trend from the time-series to be FFTed, (ii) applying a Welch window, (ii) FFTing and squaring, (iv) incoherently averaging with a sliding mean with an overlap equal to half the length of the FFT, and (v) converting from power to SNR spectra. Next, two Gaussians are fitted to each spectrum; the point is to discriminate the ground clutter (a substantial problem for low altitudes at Arecibo) from the atmospheric signal. The program then makes a semi-intelligent choice as to which peak is atmospheric. If it cannot find a peak that seems reasonable, it will mark the data point as missing. Continuity in the velocity profile is checked, a minimum criterium for SNR, a maximum criterium for spectral width and chi-square value must be met. Nevertheless, there may still be bad points. One should be suspicious of a point if its chi-square value is much larger than for the neighboring points. Data points with chi-square larger than 1.0 were rejected. Values greater than 0.5 may be suspect. Although the velocity value is probably okay, the spectral fit may not have been good enough to produce reliable figures for SNR and spectral width. Also, one must be aware that 430-MHz radar data can be contaminated by Rayleigh scattering from large raindrops. Although there was not much rain during the AIDA campaign, it is possible that some data below about 10 km might be contaminated with precipitation echoes. One way to check is to compare the wind profile with another one taken with the azimuth angle 180 deg away but close as possible in time. If the profiles are good mirror images of each other, then there is no rain. If the symmetry breaks down below 10 km, then the data is probably contaminated by the negative fall speed of the rain. One may then devise some clever scheme to subtract out the contamination.

There are 4 areas in the summary plots, which are 2-day plots of the line-of-sight neutral velocity. Bin sizes are 15 min in time and 1 km in height. Since the azimuth look directions were often 10-90 minutes in time, the bins are usually only 1-2 azimuth positions at a time, so there is a checkerboard appearance to the velocities as the azimuth position is changed slowly over a total scan period of about 2 hours and 40 minutes.

References

Results from AIDA have been published in J. Atmos. Terr. Phys., 55, 1993.
Farley, D. T., On-line data processing techniques for MST radars, Radio Sci., 20, 1177, 1985.
Gage, K. S., Radar observations of the free atmosphere: Structure and dynamics, in Radar in Meteorology, D. Atlas, Ed., American Meteorological Society, Boston, 1990.
Gage, K. S. and B. B. Balsley, On the scattering and reflection mechanisms contributing to clear air radar echoes from the troposphere, stratosphere, and mesosphere, Radio Sci., 15, 243, 1980.
Hocking, W. K., Measurement of turbulent energy dissipation rates in the middle atmosphere by radar techniques: A review, Radio Sci., 20, 1403, 1985.
Sato, T. and R. F. Woodman, Spectral estimation of CAT radar echoes in the presence of fading clutter, Radio Sci., 17, 817, 1982.
Woodman, R. F., High-altitude resolution stratospheric measurements with the Arecibo 430-MHz radar, Radio Sci., 15, 417, 1980.

Summary Plots for Arecibo ST Radar


-Revised 19 May 2000 by Barbara Emery