Gerber PVM-100 Probe Corrections for SHEBA

The corrections that need to be made to the Gerber PVM-100 data are fairly straightforward and are outlined below for the Gerber variables XGLWC (LWC), XGSFC (particle surface area), and XGREFF (effective radius):

1. XGLWC -- The XGLWC output values presently contained in the final SHEBA C-130 data files need to be scaled by the ratio of the new calibration voltage (0.233) to the old calibration voltage (0.196).

   Thus, XGLWC should be corrected as follows:

   XGLWC = XGLWC * (0.233/0.196)

2. XGSFC -- The XGSFC values output in the final netCDF files need to be scaled by a factor equal to the published XGSFC scaling factor (2980) multiplied by the ratio of the new calibration voltage (1.213) to the old calibration voltage (1.286). Thus, the correction is:

   XGSFC = XGSFC * [(1.213/1.286) * 2980]

   or

   XGSFC = XGSFC * 2811.0

3. XGREFF -- You will recall that I didn't output XGREFF values in any of the final netCDF data files. Hermann Gerber informed me several weeks ago that, once the XGLWC and XGSFC data have been properly corrected, it is possible to calculate XGREFF values from those two variables. If you wish to do this, the algorithm is as follows:

   XGREFF = (XGLWC/XGSFC) * 30,000.0

   Note that the XGLWC and XGSFC values in the above equation MUST be the corrected values determined from 1) and 2) above.

In terms of the accuracy of the XGREFF values obtained from the above equation, I have so far seen mixed results. While I have occasionally seen XGREFF values that agree with measurements of mean diameter obtained from the FSSP-300 probe, I have also seen differences between the two of anywhere from 50-100%. So, my advice would be to use the XGREFF values you might determine with caution, unless you come up with a means to independently confirm their accuracy.

One final note regarding the Gerber probe XGLWC and XGSFC values. You will note that, during almost every SHEBA C-130 flight, there are out-of-cloud offsets apparent in the XGLWC and XGSFC data. In order to get the most accurate XGLWC and XGSFC data, these offsets must be subtracted out. There are a couple of ways to do this. First, you can remove the offset prior to applying the corrections given in 1) and 2) above. Or, you can apply the above correction factors and then remove the resulting offsets. Either method will work. But I do recommend that the out-of-cloud offsets be taken out before doing detailed analyses involving the Gerber probe data. In looking at RF08 and RF12, a quick perusal of two flights, so I would recommend that you and others working with the data examine the Gerber probe data carefully for any offsets and remove them accordingly.

Some quick comparisons were done between the King probe LWC values and the corrected Gerber PVM-100 LWC values. In general, they seem to compare favorably, and both also seem to compare somewhat favorably with the LWC values derived from the FSSP-100.

Contact Krista Laursen krista@ucar.edu if you need any further assistance or if you have any additional questions on the above.