MICROPHYSICAL ANALYSIS OF SNOWFALL EPISODES THROUGH THE DISPERSION PROFILES

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1 MICROPHYSICAL ANALYSIS OF SNOWFALL EPISODES THROUGH THE DISPERSION PROFILES Laura López (1), José Prieto (2), J.L. Sánchez (1), E. García-Ortega (1), Rafael Posada (1) (1) Group for Atmospheric Physics, IMA, University of León, Spain; (2) EUMETSAT Abstract Different studies (Prieto, 2006; Lenski and Rosenfeld, 2005) have shown how the 3.9μm channel of the Meteosat Second Generation (MSG) satellite has important applications for researching the microphysical characteristics of cloud masses. Its interest lies in the possibility of identifying areas of precipitation over the ground based on the exclusive analysis of the information provided by the MSG satellite. The aim of this study is to identify the vertical profiles that may be used as a template or standard when defining and discriminating areas with precipitation, especially snowfalls. Analysing the dispersion profiles is an objective method that makes it possible to obtain an estimate of the microphysical characteristics of the cloud (size, phase and particle distribution) around a point. In this case, the reference point, in which the ground truth was known precisely, was in the station of Navalmedio, with data from a meteorological station, data from a disdrometer, and data on vertical temperature profiles and liquid water content obtained from a radiometer. Throughout this study, dispersion profiles were calculated which were systematically compared with data provided both by the radiometer (Liquid Water Content, LWC and temperature) and the disdrometer (distribution of hydrometeor sizes), in order to contrast the presence and type of precipitation deduced from the MSG data. Finally, it was possible to identify different precipitating (rain or snow) and non-precipitating standard profiles. As a result, by using the dispersion diagrams it is possible to identify the zones with snowfall, characterised by the high dispersion of the particle sizes throughout the whole of the vertical atmospheric structure. 1. INTRODUCTION Different studies (Prieto, 2006; Lenski and Rosenfeld, 2005) have shown how the 3.9μm channel of the Meteosat Second Generation satellite has important applications for researching the microphysical characteristics of cloud masses. Its interest lies in the possibility of identifying areas of precipitation over the ground based on the exclusive analysis of the information provided by the MSG satellite. The aim of this study is to identify the vertical profiles that may be used as a template or standard when defining and discriminating areas with precipitation, especially snowfalls. In this study, using the Nubes software (PRAPRO, 2009), dispersion profiles were calculated for the area measuring 21 x 21 pixels around a reference point. In these profiles, the Y-axis represents the brightness temperature (Tb) of the pixel in the 10.8 μm channel, and the X-axis represents the albedo percentage in the 3.9 μm channel. As a result, based on the dispersion diagrams, we are able to know the phase and size of the particles (from the albedo values), as well as their vertical distribution (from the temperature values obtained in the 10.8 μm channel). We then see that the dispersion diagrams allow us to characterise the precipitating or nonprecipitating cloud systems. In the latter case, it is even possible to know the characteristics of the type of precipitation (solid or liquid). 2. STUDY AREA AND DATABASE Analysing the dispersion profiles is an objective method that makes it possible to obtain an estimate of the microphysical characteristics of the cloud (size, phase and particle distribution) around a point. In this case, the reference point, in which the ground truth was known precisely, was in the station of Navalmedio, with data from a meteorological station, data from a disdrometer, and data on vertical temperature profiles and liquid water content obtained from a radiometer. Previous studies have used data from SYNOP stations, although the information

2 they provided did not offer information regarding the LWC. It is important to note that although several circumstances can alter the shape of these diagrams, the majority have a linear structure, with their dispersion being a measurement of the characteristics of the precipitation in the study zone. The experimental study was carried out in the region of the Central System whose southern slopes face towards the Community of Madrid. This chain of mountains in the centre of the Iberian Peninsula is some 600 km long, and runs from west to east, starting in the centre of Portugal as far as the Iberian System, and represents the natural boundary between Castile- León to the north, and Extremadura, Madrid and Castile-La Mancha to the south. The chain is also sub-divided into a series of ranges, the most important of which are the Serra da Estela (in Portugal), Sierra de Gredos (which contains the mountain of Almanzor at 2592 m), Sierra de Guadarrama and the Sierra de Ayllón. The Experimental Centre for recording data was installed in the Sierra de Guadarrama, also known as the Sierra de Madrid due to its proximity to the capital. This range divides the provinces of Segovia, Ávila and Madrid, and its highest peak is Peñalara, at 2430 m. In terms of the climatological data from the station in Navacerrada, the mean number of days with precipitation in the form of snow is 55 a year, although this figure varies greatly from year to year, from 42 days to 100 days with snowfall. The annual accumulation level of snow at this station is approximately 50 cm, and probably significantly more on the highest peaks. The experimental campaign for gathering data was carried out between 17 November 2008 and 17 March 2009, over the winter period. Measurements were taken of aerosols (using an aerosol probe), ice nuclei and the wind components (SODAR). The vertical atmospheric profiles of the temperature and LWC were also obtained (radiometer), as well as the environmental conditions on the surface (weather station) and precipitating particles (disdrometer). Using these instruments, a database was created consisting of 23 snowfall episodes. In each of the episodes, dispersion profiles were calculated which were systematically compared with data provided both by the radiometer (Liquid Water Content, LWC and temperature), and the disdrometer (distribution of hydrometeor sizes), in order to contrast the presence and type of precipitation deduced from the MSG data. 3. STANDARD PRECIPITATION PROFILES Finally, it was possible to identify different precipitating (rain or snow) and non-precipitating standard profiles. CLOUD WITHOUT RAIN This type of non-precipitating profile is characterised by the absence of large particles both in freezing and non-freezing zones. Particles of precipitable sizes do not appear in the diagram, which is characterised by the uniform dispersion of sizes. Figure 1: Standard dispersion diagram for cloud without precipitation.

3 RAIN Previous studies revealed an orderly, rectilinear vertical structure marked by condensation and the constant decrease in the radius with height, which explains the slope of the points on the diagram towards the right. The analyses taken with the radiometer show rain profiles with these characteristics, although it is important to note the presence of precipitable (large) particles throughout the profile, which are especially relevant in the initial stages of the precipitation. Also, in these types of profiles, the surface temperatures are relatively high. Figure 2: Standard dispersion diagram for rain precipitation. SNOW The diagram showing snow precipitation is characterised by its high level of dispersion, which indicates that the process is generalised in the region and is disorganised due to a lack of wind at high altitude. Other authors have indicated that in the regions close to 230K, in the first stages of precipitation it is possible to observe the appearance of small ice crystals that weigh down the middle zone and lower the cloud of crystals. The characteristic profile for snow precipitation validated with ground truth data is shown in figure 3. Figure 3: Standard dispersion diagram of snow precipitation.

4 4. STANDARD LIQUID WATER CONTENT PROFILES Thanks to the data provided by the radiometer, we were also able to identify the LWC for each of the profiles. This is very useful, as it is possible to identify zones with a high LWC inside the cloud masses. Basically, the vertical diagrams allowed us to identify two types of profiles with respect to the LWC: profiles with a high LWC, and profiles with a low LWC. PROFILES WITH A HIGH LWC These types of profiles have a high concentration of pixels with a large concentration of small droplets close to the isozero (between 260 and 270K). The temperatures are usually high, and the clouds are compact. The absence of ice crystals in the cold zones of the cloud mean that the LWC is high. This can be seen clearly in the profile from 25 November 2008, where we see the appearance of a high accumulation zone. Figure 4: Type I dispersion diagram of cloud with a high LWC. Figure 5: Type II dispersion diagram of cloud with a high LWC. Here we can see an accumulation zone close to the CCL (25/11/2008 at 12 h.). PROFILES WITH A LOW LWC These types of profiles are characterised by the presence of a large amount of ice crystals, normally of large size and at a high level. Another profile with a low LWC is characterised by the accumulation of crystals in high zones

5 and mist in low zones. The water is in a solid phase at altitude or as vapour on the surface, meaning the LWC is low. Figure 6: Type I dispersion diagram of cloud with a low LWC ( 24/11/2008 at 12:15 h.). 5. CONCLUSIONS 1. Dispersion profile analysis is an objective method which makes it possible to estimate the microphysical characteristics of the cloud (size, phase and particle distribution) around a point. 2. By using the dispersion diagrams it is possible to identify the zones with snow precipitation characterised by the high dispersion of the particle sizes through the vertical atmospheric structure. 3. The dispersion diagrams also provide us with information about the liquid water content of the cloud masses. 4. The profiles with high LWC have dispersion diagrams which slope to the right with concentrations of medium sized or small particles close to the condensation level; on the contrary, the profiles with low LWC have a high concentration of large crystals close to the tropopause. 5. The sudden reduction in the liquid water content within the cloud masses is characterised by profiles in which ice crystals in the growth stage appear. 6. REFERENCES Lenski I.M. and D. Rosenfeld, (2005) The time-space exchangeability of satellite retrieved relations between cloud top temperature and particle effective radius. Atmos. Chem. Phys. Discuss., 5, PRAPRO, (2009) Nubes Software. Prieto, J., (2006) Uso del canal en torno a 3,9μm de MSG para la evaluación del riesgo de precipitación. Jornadas de la Asociación española de Meteorología. Pamplona. 7. ACKNOWLEDGEMENTS We are grateful for the financial support of the Regional Government of Castile-León (LE003B009).