Corrections to the Algorithm Defining the Sample Area of Two-Dimensional Video Disdrometers

The Joanneum 2-Dimensional Video Disdrometer (2DVD) is an instrument frequently used for ground validation of remotely sensed precipitation. The instrument records measured and derived information for each detected hydrometeor including drop equivalent diameter, arrival time, fall speed, and oblateness.

Another variable reported for each drop is the effective sampling area of the detector; the requirement that the entire drop is within the field of view of both embedded cameras necessarily means that the sample area for larger drops is smaller than that for smaller drops. This effective sampling area field is frequently used in estimating bulk rainfall variables like rain rate, radar reflectivity, and liquid water content. Unfortunately, the internal algorithm used by the 2DVD software to compute this effective sample area fails to account for the reduced sample area for potential raindrops falling within the geometric shadow of the detected drop; all reported effective sampling areas from the 2DVD software are falsely high.

Other work has recently revealed that there are periods of time where the 2-Dimensional Video Disdrometer detects spurious drops in part of its field of view. These spurious drops can be removed with a relatively simple flagging algorithm, but the other drops detected during time intervals where the anomaly exists have detector effective sampling area estimates that require an additional correction.

Here, we outline an algorithm that supplies a general correction for the estimated sampling area of each drop including those drops detected during times when the instrument has reported spurious extra drops. In general, the correction of the sampling area field is modest but effects every detected drop in every 2DVD dataset.