Bernat Codina

The Sensitivity of Single Polarization Weather Radar Beam Blockage Correction to Variability in the Vertical Refractivity Gradient

Radars operating in complex orographic areas usually suffer from partial or total beam blockage by surrounding targets at their lowest elevation scans. The need for radar quantitative precipitation estimates in such environments led to the development of beam blockage corrections. This paper aims at evaluating the performance of beam blockage corrections under different electromagnetic propagation conditions with particular interest in anaprop situations. Three years of radiosonde data collected at Barcelona, Spain, a typical Mediterranean coastal site, are used to characterize the behavior of the vertical refractivity gradient near a weather radar. Three different targets surrounding the radar have been chosen and used to evaluate the different beam shielding simulated under different propagation conditions. A simple interception function between the radar beam and the topography is proposed and used for the different targets and propagation conditions considered. Results show that beam blockage correction is generally robust, with departures of 1 dB from the standard propagation conditions correction less than 10% of the time. However, as the presence of extreme anaprop cases would lead to higher differences, the monitoring of the propagation conditions is suggested as a criteria to be considered, among others such as the analysis of the echo structure, as a quality control of the radar quantitative precipitation estimate.

Modeling Raindrop Size Distribution and Z(R) Relations in the Western Mediterranean Area

This study shows the results of the modeling of drop size distributions (DSD) observed during a 2-yr study in Barcelona. Thirty-second individual sample collections of drop sizes and velocities were measured with an optical disdrometer and grouped into different classes according to their rain rate. Using the moments method, the entire experimental dataset was fitted to three standard distribution functions: exponential, gamma, and lognormal. Relationships were found between rain rate R and other moments of the DSD, such as optical attenuation S, liquid water content W, and reflectivity Z. Although gamma distribution generally reproduced experimental measurements more accurately, the Z(R) relationship, which is particularly relevant in radar meteorology, yielded the best results when calculated from fitted exponential distribution.

Weather radar anaprop conditions at a mediterranean coastal site

Abstract Many weather radars are affected by some type of anomalous propagation (AP) with a wide variety of frequency and intensity. Some geographical areas have been described to be particularly prone to AP. Even in some sites, AP is not statistically abnormal at all but dominant. Severe AP cases have been reported in coastal areas affected by strong temperature inversions and humidity contrasts such as the Baltic Sea, the Indic Ocean or the Mediterranean. In this paper we present average and extreme propagation conditions in the coastal area of Barcelona (NE Spain) calculated using three years of radiosonde data. Occurrence of different propagation conditions is discussed and related to local prevailing circulations such as the sea breeze with particular interest in the nearby Vallirana weather radar. The effect of some extreme ducting episodes seems to be remarkable in beam blocking correction procedures leading to wrong precipitation estimates when anaprop and rainfall occur at the same time. The use of radiosonde and mesoscale NWP data to derive operationally propagation conditions jointly with other existing techniques based in the analysis of radar data is overviewed.

Prediction of a mesoscale convective system over Catalonia (Northeastern Spain) with a nested numerical model

SummaryA mesoscale convective system that affected Northeastern Spain on October 10, 1994, with rainfall amounts up to 400 mm, is simulated reasonably well by a nested 3-dimensional hydrostatic mesoscale model. Previous studies carried out in this region had already portrayed the main synoptic patterns that give rise to these devastating episodes. The present contribution takes a further step since it goes down to the mesoscale by means of a numerical model providing a more detailed representation not otherwise achieved by earlier analysis methods. Although the model was unable to forecast accurately the precipitation fields, it captured satisfactorily the framework in which the convective system originated and evolved.

Basic statistical characteristics of hourly rainfall amounts in Barcelona (Spain)

SummaryIn order to derive some statistical rainfall characteristics applicable to hydrology, data of continuous rainfall rate recordings of a Jardí gauge installed in Barcelona (Spain) have been converted to an hourly precipitation series. From these data, four useful distributions have been obtained and further compared with some theoretical models. It has been found that the duration of events is distributed exponentially. The duration of rainless intervals follow a generalized Pareto distribution, and the cumulative rainfall in the cumulative rain duration is beta distributed. Concerning the distribution of rain amounts, two models can be accepted, depending on the duration of the events. Comparison with a similar study carried out in Farnborough (United Kingdom) indicates that the events are shorter and that the amounts of rain collected in short events are larger in Barcelona.

Evaluation of atmospheric anomalous propagation conditions: an application for weather radars

Several meteorological conditions are known to cause anomalous propagation (AP) of microwave radiation. The effect of AP in weather radar measurements my be important as spurious echoes from distant ground targets may appear as precipitation leading to wrong rainfall estimations. AP may also affect dramatically the quality of clear air radar observations. In this study, more than one hundred radiosonde ascents are examined to evaluate the occurrence of AP at the coastal site of Barcelona (Spain). Temperature and humidity profiles are used to calculate refractivity gradients and to estimate the existence of ducting layers. Ducts represent the worst case of super refraction and within them microwaves travel trapped like in a waveguide. To detect thin AP features a vertical resolution higher than that given by standard operational radiosonde data is desirable. For this reason, radiosonde data recorded every 10 s have been used. Results are compared against standard operational radiosonde analysis revealing a significantly higher number of AP layers. The output of a mesoscale numerical weather prediction model is also used to derive refractivity gradients. The ability of the model to simulate the propagation conditions is overviewed in order to assess the feasibility of an operational diagnostic AP product.

Effects of Anomalous Propagation Conditions on Weather Radar Observations

The effect of atmospheric propagation on radar observations is an important topic both for radar application developers and end-users of radar products, particularly of weather radar systems. An excellent review of this subject is given by Patterson (2008), and most general books about weather radars have a chapter on the topic –see for example Battan (1973), Collier (1996), Doviak and Zrnic (2006), Rinehart (2001) or Sauvageot (1991).