Comparing NUCAPS and radiosonde profiles
The radiosonde profile from the balloon launch just before 0000 UTC 7 September, above, (imagery from the Wyoming Sounding site), shows a very moist airmass, with saturation from the surface up to 400 hPa, and relative dryess above that level. Total Precipitable Water was 63 mm.
NOAA-20, NOAA-21 and Metop-C all provide NUCAPS (NOAA-Unique Combined Atmospheric Processing System) profiles. The Metop-C overpasses near American Samoa at 1842 UTC on 6 September, shown below, include a series of four green points (that is, points where the infrared retrieval converged to a solution) in the westernmost column of profiles just east of the Samoan Islands. The NUCAPS profile below is similar to the observation from Tutuila: very moist (this one had a Precipitable Water value of more than 2″), and a dry region above 400 hPa. NUCAPS profiles can give useful thermodynamic information in regions where conventional data are missing.
At 0000 UTC on 9 September, the upper-air sounding, below (source), shows a much drier atmosphere. Note, for example, that Total Precipitable Water is 45.7mm on 9 September compared to 63mm on the 7th.
The NUCAPS profile (from Metop-C), at 2054 UTC on 8 September, below, centered on Tutuila (the island on which Pago Pago sits) shows structures that are similar to the rawindsonde above, namely relatively moist from 500-300 mb with dryer air above and below. It is always important to remember that NUCAPS moisture resolution is about 4-6 layers in the troposphere.
