| Description: | IN THE BEGINNING (June 1992), the retrieval coefficients used to derive the precipitable
water vapor (PWV) and liquid water path (LWP) from the MWR brightness temperatures were
based on the Liebe and Layton (1987) water vapor and oxygen absorption model and the Grant
(1957) liquid water absorption model.
Following the SHEBA experience, revised retrievals based on the more recent Rosenkranz
(1998) water vapor and oxygen absorption models and the Liebe (1991) liquid waer absorption
model were developed. The Rosenkranz water vapor absorption model resulted a 2 percent
increase in PWV relative to the earlier Liebe and Layton model. The Liebe liquid water
absorption model decreased the LWP by 10% relative to the Grant model. However, the
increased oxygen absorption caused a 0.02-0.03 mm (20-30 g/m2) reduction in LWP, which was
particularly significant for low LWP conditions (i.e. thin clouds encountered at SHEBA).
Recently, it has been shown (Liljegren, Boukabara, Cady-Pereira, and Clough, TGARS v. 43,
pp 1102-1108, 2005) that the half-width of the 22 GHz water vapor line from the HITRAN
compilation, which is 5 percent smaller than the Liebe and Dillon (1969) half-width used in
Rosenkranz (1998), provided a better fit to the microwave brightness temperature
measurements at 5 frequencies in the range 22-30 GHz, and yielded more accurate retrievals.
Accordingly, revised MWR retrieval coefficients have been developed using MONORTM, which
utilizes the HITRAN compilation for its spectroscopic parameters. These new retrievals
provide 3 percent less PWV and 2.6 percent greater LWP than the previous
retrievals based on Rosenkranz (1998).
The Rosenkranz-based retrieval coefficients became active at TWP.C2 20020427.0600. The
MONORTM-based retrieval coefficients became active at TWP.C2 20050630.2100.
Note: The TWP.C2 data for 19981028-20050630 have been reprocessed to apply the
MONORTM-based retrievals for all time. The reprocessed data were archived 20061003. |