Estimation of missing values in climatological time series is an important task. In order to ®nd an appropriate method, we examined six methods for estimating missing climatological data (daily maximum temperature, minimum temperature, air temperature, water vapour pressure, wind speed and precipitation) for different time scales at six German weather stations and three Bavarian forest climate stations. The multiple regression analysis (using the ®ve closest weather stations) with least absolute deviations criteria (REG) predominantly gave the best estimation for daily, weekly, biweekly, and monthly maximum temperature, minimum temperature, mean temperature, water vapour pressure, wind speed, under different topographical conditions (valley, alpine foothills and mountain sites). The six methods gave similar estimates for the averaged precipitation amount. The mean absolute errors (MAE) of estimating climatological data using different techniques are of similar magnitude at the weather stations, but they are signi®cantly different at the forest climate stations. For the same climatological variable (i.e., air temperature) for different time scales, mean absolute errors of estimated data are larger for shorter time scales (e.g., a day) than for longer ones (e.g., a month). For the different climatological variables, the most accurately estimated variables are maximum temperatures, mean temperatures and water vapour pressure, followed by minimum temperature and wind speed. The poorest results were obtained for precipitation data. #

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Simultaneous measurements of radiation, photolysis frequencies, O 3 , CO, OH, PAN and NO x species were carried out in the boundary layer, along with pertinent meteorological parameters, under total solar eclipse conditions. This experiment performed at about 34 solar zenith angle and noontime conditions thus provided a case study about the interactions between radiation and photochemistry under fast "day-night" and "night-day" transitions, at high solar elevation. The results reveal a close correlation of photolysis frequencies j O 1 D and j NO2 with the UV radiation ux. All three parameters show, due to the decreasing fraction of direct radiation at shorter wavelengths, much weaker cloud shading effects than global solar radiation. NO and OH concentrations decrease to essentially zero during totality. Subsequently, NO and OH concentrations increased almost symmetrically to their decrease preceding totality. The NO/NO 2 ratio was proportiona l to j NO2 over 30 min before and after totality indicating that the partitioning of NO x species is determined by j NO2. Simple box model simulations show the effect of reduced solar radiation on the photochemical production of O 3 and PAN. Zusammenfassung Während der totalen Sonnen nsternis am 11. August 1999 wurden simultane und kontinuierliche Messungen von O 3 , CO, OH, PAN and NO x , Strahlung, Photolysefrequenzen und relevanten meteorologischen Parametern durchgeführt. Dieses Experiment, durchgeführt etwa am Mittag, bei 34 Zenithwinkel der Sonne, ermöglichte die Untersuchung der Interaktion von Strahlung und Photochemie für schnelle Tag-Nacht und Nacht-Tag-Übergänge bei hohem Sonnenstand. Die Ergebnisse zeigen eine enge Korrelation der Photolysefrequenzen j O 1 D und j NO2 mit dem UV-Strahlungs uss. Alle drei Parameter zeigen, wegen des abnehmenden Anteils direkter Sonnenstrahlung bei kurzen Wellenlängen, erheblich geringere Abschwächung durch Wolken als die Globalstrahlung. NO und OH gehen während der Verdunkelungsphase auf Null herunter. Danach steigen NO und OH nahezu symmetrisch zum vorherigen Abfall wieder an. Das NO/NO 2-Verhältnis war 30 min vor bis 30 min nach der Totalität proportiona l zu j NO2 und zeigte damit, dass die Partitionierung von NO x durch j NO2 bestimmt wurde. Mit einem einfachen Boxmodell wurde der Effekt der reduzierten Sonnenstrahlung auf die photochemische Produktion von O 3 und PAN berechnet.

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