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     Vaisala Ceilometer and Visibility Meter Comparison

During SEAS and post SEAS periods (April-October 2000) two Vaisala instruments - a Ceilometer and Weather Sensor (Visibility Meter) were operated at Bellows in continuous mode. The CT25K Ceilometer measures cloud-bottom heights and vertical visibilities based upon pulsed diode laser lidar technology. The volume backscatter coefficient, BSC(z) can be expressed as: BSC(z) = S*E(z) where S is a lidar ratio and E(z) is the extinction coefficient. If visibility is defined according to a 5 % contrast threshold (WMO definition for Meteorological Optical Range (MOR), then E = 3 / V where V is MOR visibility (5 % contrast). In an assumption that Lidar Ratio is constant over the range observed, it is possible to invert the backscatter profile to obtain the extinction coefficient profile.

Comparison of both the CT25K Ceilometer and FD12P Visibility Meter instruments allowed us to do direct ceilometer calibration for near surface data. Figure 17 shows the comparison of ceilometer and visibility meter observations. The bottom figure shows ceilometer visibility at z = 30m. Due to the high noise level o background solar radiation at midday (Fig. 17, middle), ceilometer visibility inversion  was noisy and has been excluded from the comparison.  With this data removed, visibility measured by both instruments revealed high degree of correlation R =0.83 (Fig. 17, top).  As a result, we believe that more extensive analysis of Ceilometer data at other altitudes may allow for its use as a visibility indicator throughout the lower boundary layer. 

Figure 17.  Ceilometer vs. visibility meter data

Last Modified: Feb 27, '01