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S. Nicholls
,
W. Shaw
, and
T. Hauf

Abstract

During the Joint Air-Sea Interaction (JASIN) experiment over the North Atlantic, three aircraft equipped to measure turbulent fluctuations of wind, temperature and humidity flew together in close formation, in order to compare results. These aircraft were the MRF C130, the NCAR Electra and the DFVLR Falcon. Most runs were made in the atmospheric boundary layer. This paper presents the results of this intercomparison exercise. Results are presented in terms of comparisons between variances and covariances which are further investigated by comparing spectra and co-spectra.

Overall, very good agreement is found between the C130 and the Electra, although small differences can be detected. However, these are negligible compared to the scatter usually observed when making measurements in the turbulent atmospheric boundary layer. The Falcon, at an earlier stage of development, also shows reasonable agreement although the amount of available data was much more limited.

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S. Nicholls
,
J. Leighton
, and
R. Barker

Abstract

A device for measuring the total water content of a parcel of air from an aircraft has been developed. The total water of a parcel of air is a conserved quantity, independent of phase changes, provided there is no transport of water through the parcel boundaries. Current airborne hygrometers normally attempt to measure the water content in individual phases and the presence of other phases invariably influences the quality of the data. However, any liquid water or ice entering this new probe is efficiently evaporated and the resultant water vapor measured using a Lyman-alpha hygrometer.

In airborne trials the device was calibrated against a cooled-mirror dewpoint device. Runs were conducted in warm stratocumulus tops, through small cumulus, in mixed-phase precipitation and cirrus cloud. In all cases the device was found to produce high quality, fast response data.

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Carsten S. Frederiksen
,
Huqiang Zhang
,
Ramesh C. Balgovind
,
Neville Nicholls
,
Wasyl Drosdowsky
, and
Lynda Chambers

Abstract

An evaluation of trial seasonal forecasts during the 1997/98 El Niño, using an atmospheric GCM forced by persisted sea surface temperature and sea-ice anomalies, is presented. Generally, forecasts of seasonal anomalies of precipitation, surface air temperature, 200-hPa geopotential height, and mean sea level pressure (MSLP) are shown to have statistically significant skill in the Tropics and subtropics, but predominantly over the oceans. Surface air temperature and 200-hPa height anomalies are also skillfully forecast over land in the 30°S–30°N latitudinal band, and, in contrast to precipitation and MSLP, also show significant skill in the extratropics. The global pattern of significant skill seems not to be oversensitive to the use of a Kuo or a mass-flux convection scheme (Tiedtke), although the global root-mean-square errors are consistently larger, in the latter case.

Results from multidecadal simulations of the model, when forced by observed sea surface temperature and sea-ice, show that the model reproduces quite well the observed global Southern Oscillation index relationships and that these go some way to explaining the skill in the model forecasts. In addition, the global patterns of skill are consistent with those seen in the model forecasts. An estimate of the role of sea surface temperature and sea-ice in forcing interseasonal climate variations, suggests that the model displays forecasts skill in those areas where this forcing plays a large, if not dominant, role. In areas where internal, or chaotic, variability plays a dominant role, the model shows little statistically significant skill.

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