Height-correlation analysis of data from an S-band zenith-pointing radar in Singapore

This paper presents the results of height-correlation analysis using 8-months of vertically-pointing (zenith) S-band radar data in Singapore. The radar recorded co-polar reflectivity, linear depolarization ratio, together with the Doppler spectrum; typically 37 profiles were recorded in one minute. Details of the radar and the recorded data have been published previously (Thurai et al., 2003). From the recorded events, several examples of stratiform and convective events have been chosen to determine the height correlations of radar reflectivity. For these chosen events, reflectivity-time series were extracted at 1 km above ground level and compared with reflectivity calculations using 1-minuted drop size distributions measured by a co-located disdrometer. This was performed to establish the reflectivity calibration of the S band radar.

Once the reflectivity calibration is established, the time series of the reflectivity at a given height is extracted. A Lee-type filtering is used for smoothing the high time-resolution data. From the smoothed time series, the Pearson correlation coefficient is evaluated using the corresponding time series at the lowest height with non-clutter-contaminated data as the reference. The correlation coefficients are calculated at various heights above ground level, and the whole process is repeated for all the chosen events, both straiform and convective.

Typically, the correlation coefficients decrease with height - as expected - but the stratiform and convective events show different behavior. Below 4 km, the correlation coefficient for stratiform rain shows a slower decrease with height than for convective rain. Above 4 km, the correlation coefficient for stratiform rain decreases at a much sharper rate, the rate being much higher than for convective case. Note, the freezing height in Singapore is typically 4.5 km, and has relatively little variability.

Using the ground-based disdrometer measurements, reflectivity-rainfall rate relationships were determined for stratiform and convective rain separately. The relationships were used to convert the ‘smoothed' radar reflectivity time series to the corresponding rainfall rate time series, for all heights up to 4 km. As with the reflectivity time series, the calculated rain-rate time series were used to determine the corresponding height correlations.

We will present our results in a form which will be helpful to the development and/or improvement of satellite radar/radiometer retrieval algorithms.

Reference:

Thurai, M., T. Iguchi, T. Kozu, J. D. Eastment, C. L. Wilson, and J. T. Ong, 2003: Radar observations in Singapore and their implications for TRMM Precipitation radar retrieval algorithms, Radio Science, 38 (5), doi:10.1029/2002RS002855.