LKS DataThe LKS data were derived from data in the Comprehensive Aerological Reference Dataset (CARDS) obtained from NCDC (Eskridge et al. 1995). Monthly means for 87 carefully selected stations were adjusted using a multifactor expert analysis by a team of three climate scientists, without use of satellite data as references and with minimal use of neighbor station comparisons. The team visually examined time series of temperatures at multiple levels, night-day temperature differences, temperatures predicted from regression relationships, and temperatures at other nearby stations. They also considered metadata, statistical change points, the Southern Oscillation Index and the dates of major volcanic eruptions. Using these indicators, they identified artificial change points and remedied them by either adjusting the time series at each affected level or, if adjustment was not feasible, by deleting data. The adjustments were then examined for reasonableness. RATPAC uses the "LIBCON" version of the LKS adjusted data, which includes the most complete set of adjustments and uses the preferred adjustment method. The LKS data consist of monthly temperatures for 16 atmospheric levels from the surface to 10 mb, from 1948 to 1997. Because of previously recognized problems with the data from India (Parker et al. 1997; Free and Angell 2002), the LKS station data for Bombay and Calcutta were not used for RATPAC, leaving 85 stations (see station list). The 10 and 20 mb levels were deleted from the RATPAC products because of the scarcity of data at those levels. LKS found the 1000 mb data to be more erratic and less reliable than other levels in the troposphere, probably due to problems arising from days when the surface pressure is less than 1000 mb (LKS 2003b). The 1000 mb data were therefore also deleted from RATPAC.
IGRATo remedy various recently identified problems in the CARDS database, NCDC has undertaken a wholesale revision of the CARDS quality-control procedures (Durre et al. 2005). The resulting IGRA dataset, rather than the CARDS dataset, has been used to extend the station data past 1997. The global mean time series from the (CARDS-based) unadjusted LKS and IGRA differ most notably before 1965.
Combining LKS and IGRABecause of the careful scrutiny used by the LKS team to create the adjusted LKS data, LKS is likely to be more reliable than a dataset derived by applying the first difference method to the IGRA data before 1995. The RATPAC team therefore uses LKS instead of IGRA before 1995 to reap the substantial benefits of the LKS homogeneity adjustments. However, because of the differences between the datasets before 1965, RATPAC data from that period should be viewed with caution.
ReferencesDurre, I., R. Vose and D. Wuertz, (2004), Overview of the Integrated Global Radiosonde Archive, J. Climate, in press.
Eskridge, R. E., O.A. Alduchov, I.V. Chernykh, P. Zhai, A.C. Polansky, and S.R. Doty, (1995), A comprehensive aerological reference data set (CARDS): Rough and systematic errors, Bull. Am. Meteor. Soc., 76, 1759-1775.
Free, M., and J. K. Angell (2002), Effect of volcanoes on the vertical temperature profile in radiosonde data, J. Geophys. Res., 10.1029/2001JD001128.
Lanzante, J.R., S.A. Klein, and D.J. Seidel (2003a), Temporal homogenization of monthly radiosonde temperature data. Part I: Methodology, J. Climate, 16, 224-240.
Lanzante, J.R., S.A. Klein, and D.J. Seidel (2003b), Temporal homogenization of monthly radiosonde temperature data. Part II: Trends, sensitivities, and MSU comparison, J. Climate, 16, 241-262.
Parker, D.E., M. Gordon, D.P.N. Cullum, D.M.H. Sexton, C.K. Folland and N. Rayner (1997), A new gridded radiosonde temperature data base and recent temperature trends, Geophys. Res. Lett., 24, 1499-1502.