Covadonga Palencia

Radiative forcing of haze during a forest fire in Spain

Intense fires occurred in northwestern Spain on 6 September 2000, filling a valley with smoke haze. Aerosol size distribution measurements were performed during 1 day with a thermal inversion, so the aging process of the smoke aerosol could be closely monitored. In 3.5 h, the fine aerosol increased up to 0.06 μm in the geometric median diameter of the fine mode. This aging process enhanced the scattering ability of aerosols. On the basis of several hypotheses on the data obtained, shortwave radiative forcing at surface level, at top level, and in the atmosphere was estimated: instantaneous surface forcing reached up to between −80.4 and −67.4 W/m2, top of the atmosphere (TOA) instantaneous forcing reached up to between −23.4 and +4.9 W/m2, and instantaneous atmosphere forcing reached up to between +44.2 and +85.3 W/m2. The study reveals not only the absorption of solar radiation in the atmosphere by smoke aerosols but also an aerosol-induced case study, where TOA cooling forcing shifts to warming for specific aerosol single scattering albedo. The daily mean heating rate of the smoke haze was estimated at 5.9 ± 0.6 K/d.

The influence of melting on hailstone size distribution

AbstractThe physical properties of hailstones registered by a hailpad network (size distribution, mass,kinetic energy) are essential data for the establishment of a regional hail climatology. Nevertheless,when comparing these data to the same properties of hailstones inside the cloud, the melting processmust be taken into account. This paper presents a brief theoretical study of the changes effected onhailstone size distribution due to the melting process. The paper is based on previous studies dealingwith the melting of hailstones before they reach the ground. The aim is to analyze the influence ofthis melting process on hailstone size distribution. An initial melting simulation was carried out inorder to achieve this aim. Despite the common assumption that hailstone size distribution on theground is exponential, it was found that when the in-cloud size distribution is exponential, on theground, there are fewer small hailstones than what would be expected in an exponential distribution.The data registered by the hailpad network in Leo´n (Spain) for 1 year were used to estimate thesize of every hailstone before the melting process. The results show that the hailstone sizedistribution simulated inside the cloud resembles more closely an exponential distribution than thehailstones on the ground.The type of hailstone size distribution inside the cloud will be the starting point for calculating thehailstone size distribution on the ground. Several equations describing the melting processes are usedtocalculate anew probability densityfunction thatinitiallycorresponds to anexponential distributionthat undergoes a partial melting process. The result is a function that is not monotonously decreasinglike the exponential function, but rather a function that has a peak for a given size. This new functionfits better the data found than the exponential function and actually resembles the gamma function.D 2003 Elsevier B.V. All rights reserved.

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.

Dent Overlap in Hailpads: Error Estimation and Measurement Correction

The measurement of the physical characteristics of hailstones reaching the ground is usually carried out by means of hailpads, on which the impact of hailstones leaves dents. Hailstone dents provide information about parameters, such as the number N of hailstones, their size M, and their kinetic energy E. In the case of intense hailfalls, however, the dents often overlap and the final measurement may not be totally reliable. This paper presentsacomputerizedsimulationwiththeaimofassessingmeasurementerrorscausedbydentoverlap.The simulated dents represent several random hailfalls with both exponential size distributions and monodispersed size distributions. The simulated hailpads were measured following the procedure employed in the case of hailpads exposed to authentic hailfalls, and it was thus possible to assess the error due to dent overlap. The results show that dent overlap makes it impossible to measure all the dents, which means that in a real hailfall the number of hailstones registered will often be lower than the number of hailstones that actually hit the ground (up to 25% may go undetected). Consequently, the energy and mass of the hailstones are also underestimated (they may be up to 50% higher than the values registered on a hailpad). The maximum size registered, however, does not depend on the degree of overlapping and neither does the slope parameter l of the exponential distribution, except when l takes higher values. Finally, the authors suggest a heuristic correction of the data obtained by real hailpads based on the results of the simulations. An example is provided that applies these corrections to the 228 hailfalls registered by the Italian hailpad network over a period of 10 yr. The results show that, on average, the correction applied because of overlapping increases the number of hailstones in 3.2%, the mass in 1.9%, and the energy in 5.4%. However, there are cases in which these corrections reached much higher values of up to 6.9% in N and M, and up to 25.2% in E. It is therefore advisable to correct dent overlap before carrying out a regional climatic study of hail, since this study would certainly be affected by the errors accumulated by all the hailpads.

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.