María Fernández-Raga

Error in the Sampling Area of an Optical Disdrometer: Consequences in Computing Rain Variables

The aim of this study is to improve the estimation of the characteristic uncertainties of optic disdrometers in an attempt to calculate the efficient sampling area according to the size of the drop and to study how this influences the computation of other parameters, taking into account that the real sampling area is always smaller than the nominal area. For large raindrops (a little over 6 mm), the effective sampling area may be half the area indicated by the manufacturer. The error committed in the sampling area is propagated to all the variables depending on this surface, such as the rain intensity and the reflectivity factor. Both variables tend to underestimate the real value if the sampling area is not corrected. For example, the rainfall intensity errors may be up to 50% for large drops, those slightly larger than 6 mm. The same occurs with reflectivity values, which may be up to twice the reflectivity calculated using the uncorrected constant sampling area. The Z-R relationships appear to have little dependence on the sampling area, because both variables depend on it the same way. These results were obtained by studying one particular rain event that occurred on April 16, 2006.

Comparison of precipitation measurements by OTT Parsivel 2 and Thies LPM optical disdrometers

Optical disdrometers are present weather sensors with the ability of providing detailed information of precipitation such as rain intensity, radar reflectivity or kinetic energy, together with discrete information on the particle size and fall velocity distribution (PSVD) of the hydrometeors. Disdrometers constitute a step forward towards a more complete characterization of precipitation, being useful in several research fields and applications. In this article the performance of two extensively used optical disdrometers, the most recent version of OTT Parsivel disdrometer and Thies Clima Laser Precipitation Monitor (LPM), is evaluated. During two years four collocated optical disdrometers, two Thies Clima LPM and two OTT Parsivel, collected up to 100,000 minutes of data and up to 30,000 minutes with rain in more than 200 rainfall events, with intensities peaking at 277 mm h−1 in one minute. The analysis of these records show significant differences between both disdrometer types for all integrated precipitation parameters, which can be explained by differences in the raw particle size and velocity distri∗Correspondence to: santiago.begueria@csic.es Preprint submitted to Elsevier March 6, 2018 bution (PSVD) estimated by the two sensors. Thies LPM recorded a larger number particles than Parsivel and a higher proportion of small particles than OTT Parsivel, resulting in higher rain rates and totals and differences in radar reflectivity and kinetic energy. These differences increased greatly with rainfall intensity. Posible causes of these differences, and their practical consequences, are discussed in order to help researchers and users in the election of the sensor, pointing out at the same time limitations to be addressed in future studies.

Vertical Raindrop Size Distribution in Central Spain: A Case Study

A precipitation event that took place on 12 October 2008 in Madrid, Spain, is analyzed in detail. Three different devices were used to characterize the precipitation: a disdrometer, a rain gauge, and a Micro Rain Radar (MRR). These instruments determine precipitation intensity indirectly, based on measuring different parameters in different sampling points in the atmosphere. A comparative study was carried out based on the data provided by each of these devices, revealing that the disdrometer and the rain gauge measure similar precipitation intensity values, whereas the MRR measures different rain fall volumes. The distributions of drop sizes show that the mean diameter of the particles varied considerably depending on the altitude considered. The level at which saturation occurs in the atmosphere is decisive in the distribution of drop sizes between 2,700 m and 3,000 m. As time passes, the maximum precipitation intensities are registered at a lower height and are less intense. The maximum precipitation intensities occurred at altitudes above 1,000 m, while the maximum fall speeds are typically found at altitudes below 700 m.