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).
Although the MWR data will be reprocessed to apply the new monortm-based retrievals, for
most purposes it will be sufficient to correct the data using the following factors:
PWV_MONORTM = 0.9695 * PWV_ROSENKRANZ
LWP_MONORTM = 1.026 * LWP_ROSENKRANZ
The Rosenkranz-based retrieval coefficients became active as follows (BCR 456):
SGP/C1 (Lamont) 4/16/2002, 2000
SGP/B1 (Hillsboro) 4/12/2002, 1600
SGP/B4 (Vici) 4/15/2002, 2300
SGP/B5 (Morris) 4/15/2002, 2300
SGP/B6 (Purcell) 4/16/2002, 2200
SGP/E14(Lamont) 4/16/2002, 0000
NSA/C1 (Barrow) 4/25/2002, 1900
NSA/C2 (Atqasuk) 4/18/2002, 1700
TWP/C1 (Manus) 5/04/2002, 0200
TWP/C2 (Nauru) 4/27/2002, 0600
TWP/C3 (Darwin) inception
The MONORTM-based retrieval coefficients became active as follows (BCR 984):
SGP/C1 (Lamont) 6/28/2005, 2300
SGP/B1 (Hillsboro) 6/24/2005, 2100
SGP/B4 (Vici) 6/24/2005, 2100
SGP/B5 (Morris) 6/24/2005, 2100
SGP/B6 (Purcell) 6/24/2005, 1942
SGP/E14(Lamont) 6/28/2005, 2300
NSA/C1 (Barrow) 6/29/2005, 0000
NSA/C2 (Atqasuk) 6/29/2005, 0000
TWP/C1 (Manus) 6/30/2005, 2100
TWP/C2 (Nauru) 6/30/2005, 2100
TWP/C3 (Darwin) 6/30/2005, 2100
PYE/M1 (Pt. Reyes) 4/08/2005, 1900**
** At Pt. Reyes, the original retrieval coefficients implemented in March 2005 were based
on a version of the Rosenkranz model that had been modified to use the HITRAN half-width
at 22 GHz and to be consistent with the water vapor continuum in MONORTM. These
retrievals yield nearly identical results to the MONORTM retrievals. Therefore the Pt. Reyes
data prior to 4/08/2005 may not require reprocessing. |