This paper presents a parametric automatic procedure to calibrate the multichannel Rayleigh-Mie-Raman lidar at the Institute for Atmospheric Science and Climate of the Italian National Research Council (ISAC-CNR) in Tor Vergata, Rome, Italy, using as a reference the operational 0000 UTC soundings at the WMO station 16245 (Pratica di Mare) located about 25 km southwest of the lidar site. The procedure, which is applied to both channels of the system, first identifies portions of the lidar and radiosonde profiles that are assumed to sample the same features of the water vapor profile, taking into account the different time and space sampling. Then, it computes the calibration coefficient with a best-fit procedure, weighted by the instrumental errors of both radiosounding and lidar. The parameters to be set in the procedure are described, and values adopted are discussed. The procedure was applied to a set of 57 sessions of nighttime 1-minsampling lidar profiles (roughly about 300 h of measurements) covering the whole annual cycle (February 2007-September 2008. A calibration coefficient is computed for each measurement session. The variability of the calibration coefficients (;10%) over periods with the same instrumental setting is reduced compared to the values obtained with the previously adopted, operator-assisted, and time-consuming calibration procedure. Reduction of variability, as well as the absence of evident trends, gives confidence both on system stability as well as on the developed procedure. Because of the definition of the calibration coefficient and of the different sampling between lidar and radiosonde, a contribution to the variability resulting from aerosol extinction and to the spatial and temporal variability of the water vapor mixing ratio is expected. A preliminary analysis aimed at identifying the contribution to the variability from these factors is presented. The parametric nature of the procedure makes it suitable for application to similar Raman lidar systems.
The Pinatubo eruptions of June 1991 introduced large plumes into the local stratosphere. On several occasions, volcanic gases and particles reached altitudes of about 30 km, quickly spreading to the west. Twenty days after the first eruption, the volcanic aerosol cloud was detected by lidar 14 km over Frascati, Italy. The upper portion of the cloud was observed for the first time on September 4, 1991, at an altitude of 23 km. Vertical and temporal evolution of the cloud, as observed from Frascati, are in agreement with trajectories estimated by means of northern hemisphere, stratospheric wind maps. Temperature records and characteristics of the cloud during the first 6 months following the eruption are reported. These characteristics are also compared to the ones of El Chichon, whose eruption, in 1982, rated amongst the largest in the century. This first analysis shows that, three and a half months after the eruption, the aerosol perturbation generated by Pinatubo reached and exceeded the maximum loads, recorded 11 months after the El Chichon event. However, by the end of the year, the aerosol columns of the two events tend towards comparable magnitudes.
Stratospheric temperatures derived from five different lidars are compared. Although the lidars are in five separate geographic locations, the evaluation is accomplished by comparing each of the sets of lidar data taken over the course of a year (1991–1992) with temperatures interpolated to each location from daily global temperature analyses from the National Meteorological Center (NMC). Average differences between the lidars and NMC temperatures vary for the different lidars by up to 6.7 K. Part of this large average temperature difference is shown to be due to the real temperature variation throughout the day, and the different times of observation of the NMC data and each of the lidar systems. Microwave limb sounder (MLS) data from the upper atmosphere research satellite are used to model the diurnal and semidiurnal variations in temperature for each lidar location, for each season. After adjusting for the temperature changes caused by variations in observation time, average temperature differences are reduced among four of the five lidars, compared with the NMC temperatures, but still vary by as much as 3.9 K at stratospheric altitudes between 30 and 45 km. Results of direct comparisons at two permanent lidar sites with a mobile lidar show that sometimes agreement within 1 to 2 K is achieved, but for other cases, larger average differences are seen. Since the precision of lidar temperatures has been estimated to be better than 1 K, further research is needed to reconcile this small expected error with the larger average differences deduced here using measurements made under operational conditions.
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