Techniques for Measuring Liquid Water Content Along a Trajectory

Book excerpt: The standard procedure for estimating the point value of liquid water content (M) along the reentry path of a ballistic missile is to use a radar derived value of radar reflectivity factor (Z), with the M to Z relation obtained from aircraft measurements of the particle spectrum. This report contains descriptions of the various techniques used to make the radar and aircraft data compatible. The results of various methods to predict the liquid water content values are compared with aircraft measured values. The extrapolation of instrumentally truncated aircraft measurements to include the entire range of particle sizes is treated in Section 1. Section 2 contains a discussion of the method of deriving nominal M-Z relations, applies the method to data acquired at Kwajalein Missile Range, and provides M-Z relations for rain, small snow, large snow, and bullet rosettes. The introduction of the factor 'k' to convert from aircraft measurements to radar derived values is also discussed in Section 2. The relationship of k with altitude and temperature for data from Kwajalein and Wallops Island is covered in Section 3. Finally, Section 4 contains a determination of the accuracy to which liquid water content can be estimated from climatological storm data for moderate to heavy stratiform winter storms at Wallops Island. The results of Section 4 demonstrate that situations which deviate significantly from climatological averages still require time- specific M-Z relations derived from simultaneous aircraft-radar measurements.