Petr Pesice

First Results of DSD Measurement by Videodistrometer in the Czech Republic in 1998-1999

The first results of the almost one year drop size distribution (DSD) measurement in the Czech Republic are summarised in this study. The ESA-ESTEC 2D videodistrometer was used to measure the rain drop parameters. The average DSD is shown to be of the gamma type. One minute DSDs were evaluated to test the accuracy of analytical DSD models. Parameters of gamma distribution and exponential distribution functions were evaluated for the whole data set as well as for the various rain rate intervals. Regression technique and the method of moments were applied to estimate the parameters of DSD. It is shown that the μ parameter value strongly depends on the method of computation as well as on the rain type. Its average value is about 0.59 for the average (smooth) one minute DSD while an average value of un-smoothed DSD is 11.0 (moment method) or 5.4 (regression technique). The Josss shape parameter and the Tokay-Shorts parameter CS estimating roughly the rain type are also discussed (if CS>1, the event should be convective). The tendency of increasing numerical value of the CS parameter with the increasing rain rate was observed (the DSDs were distributed into classes respecting the rain rate value) and thus the idea of the convectivity occurrence bounded with the higher CS parameter value was supported. The study also compares the parameters of the average DSD with the averages of parameter values of all 4 183 one minute DSDs.

Convection precursors in the Czech territory

Abstract The goal of this study was to statistically assess the set of thermodynamic and dynamic precursors of significant convection, and above all the occurrence of convective precipitation. The radar maximum column reflectivity was used as a predictand characterizing the convective rainfall. The conditional probabilities for the occurrence of radar maximum reflectivity categories given precursor values were evaluated. This study also analyzes 73 significant convective phenomena from the years 1998–2001. The results indicate that most of deep convection was developed in an environment with unidirectional or clockwise-turning hodograph containing moderate or strong vertical wind shear. In contrast, the majority of downbursts or hailstorms were connected with unidirectional hodographs.

Attenuation measurement on dual-wavelength optical free-space link

Institute of Atmospheric Physics measures atmospheric attenuation on experimental FSO link on 850 and 1550 nm for more than one year. Experimental site is located at meteorological observatory on the isolated mountain with frequent fog, low clouds and strong wind occurrence. Measured attenuation is correlated with the wind turbulence intensity, visibility and LWC. Simple empirical models estimating attenuation on FSO link from meteorological parameters are formulated and verified through the experiment described. The paper shows also basic statistical behavior of the longterm FSO signal level in extreme conditions.

The inclusion of surface synoptic data in very short range predictions of convective events

SummaryPredictor vectors, including upper air as well as surface data, were used for categorical forecasting convective events over a subregion of the Czech territory, and the effect of including surface variables in the predictor vector was examined. While upper air data were considered as Perfect Prognosis, the surface data were successively included according to the time of their origin. The forecasting technique was based on linear multiple regression with learning, and the accuracy of the forecast was measured by the Critical Success Index. The input data from the three May-September periods in 1989–91 were used, and the first year served as the learning set. The aerological data from TEMP 12 UTC, simulating Perfect Prognosis, were the source of the upper air predictors. The performance of all, upper air, surface and combined, predictors were evaluated and compared. It turned out that the improvement of prediction accuracy due to the inclusion of surface variables was not negligible. Significant improvements were made in the forecasts of thunderstorm occurrence between 18 and 24 UTC.

Cloud microphysics from the free space optical link point of view — Preliminary experimental results

Our experimental site is located on the top of the Milesovka hill in the Czech Republic. Low clouds reach the top of the hill very often. Using the sensor which measures the liquid water content and the particle surface area, we are able to derive the parameters of the Khrgian-Mazin model of the drop size distribution. Results of processing a few selected cloud events are shown considering the free space optical link attenuation.