| 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.C1
20020504.0200. The MONORTM-based retrieval coefficients became active
at TWP.C1 20050630.2100.
Note: The TWP.C1 data for 19961011-20050630 have been reprocessed to apply the |