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maps of water vapor over North America from MLS and GMAO in 1993.  In addition there is a map showing pressure over the same region

The top panel shows MLS 215 hPa water vapor on 14 March 1993 when the eastern coast of the United States was experiencing a severe blizzard. The center panel shows a humidity map from the NASA Goddard Earth Observing System Version 1 (GEOS-1) data assimilation system [Schubert et al. "A multiyear assimilation with the GEOS-1 system: Overview and results", NASA Tech. Memo. 104606, 207 pp, 1995] output that has been sampled similar to the MLS measurements. The bottom panel shows the tropopause pressure for the same day from the GSFC GEOS-1 model. The GSFC GEOS-1 data, analyses of the MLS upper tropospheric water vapor and interpretation given below, are from Dr. Richard Rood and colleagues.

The main features associated with the east coast US storm are in good agreement between the MLS measurements and the assimilation model. The clash between the tropospheric and stratospheric air along the east coast of the United States, as shown in both GEOS-1 fields, is clearly indicated in the MLS map by a sharp water vapor gradient. Other features also correlate well between the tropopause height and the MLS maps, but not so well with the GEOS-1 humidity map. Notably, an arm of high-tropopause pressure associated with dry air extending northwest from Hawaii and an unusually low pressure tropopause and a wet region occurring north of the Chukchi Sea (70oN, 170oW) are present. A significant difference is the existence of quite moist air in the assimilation result as far north as Greenland where MLS indicates dry air. An interesting moist feature at (25oN, 140oW) correlates very well between MLS and GEOS-1 and does not have an associated feature in the tropopause pressure.

The tropopause pressure map shows whether air at 215 hPa is in the stratosphere and hence should be dry, or is in the troposphere and should be moist. The East Coast blizzard is a very large event that occurred over a region where the radiosonde network is dense and the GEOS-1 humidity map should be of high quality. This distinct synoptic event, coupled with the dense radiosonde observations, provides good validation that the MLS can detect synoptic variability. Over the Pacific, radiosonde moisture data are sparse. The tropopause pressure from the assimilation is derived from satellite temperature measurements. The MLS detects a signature in the moisture field, which is verified by the pattern of the assimilation tropopause pressure. This shows that in the absence of moisture observations being directly asssimilated into GEOS-1, the moisture fields from GEOS-1 contain significant errors, and demonstrates that future assimilations using MLS can change even the qualitative representations of upper tropospheric humidity.

For further discussing of these data and analyses see Read et al. Bull. Amer. Meteor. Soc. vol. 76, 2381-2389, 1995.

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