Characterizing the diurnal cycle of precipitation over complex Alpine orography using fourdimensional

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1 Characterizing the diurnal cycle of precipitation over complex Alpine orography using fourdimensional radar observations Pradeep V. Mandapaka 1, Urs Germann 1, Luca Panziera 1 1 MeteoSwiss, Locarno Monti, Switzerland, urs.germann@meteoswiss.ch Urs Germann 1. Introduction It is a challenging task for the numerical weather prediction models to accurately reproduce the diurnal cycle of precipitation, particularly in regions with complex local phenomena such as orographic lifting and coastline circulations (e.g., Yang and Slingo 2001; Lee et al. 2007). The degree of representation of precipitation diurnal cycle is often used as a test for physical parameterization schemes in numerical weather prediction and climate models (e.g., Vasic et al. 2007; Surcel et al. 2010). Several studies have examined the regional, seasonal, and annual variations in precipitation diurnal cycle over land (e.g., Wallace 1975; Twardosz 2007; Carbone and Tuttle 2008), open oceans (e.g., Cifelli et al. 2008), and in the maritime continental region (e.g., Kerns et al. 2010). The advances in radar and satellite remote sensing along with the availability of continental to global scale datasets has contributed greatly towards better characterization of the diurnal cycle of precipitation (e.g., Nesbitt and Zipser 2003; Biasutti et al. 2011). In general, it can be concluded from the literature that precipitation diurnal signal displays an afternoon-evening peak over land, a midnight-predawn peak over the open oceans, and that the diurnal cycle is stronger and more variable over the land than over ocean (e.g., Nesbitt and Zipser 2003; Dai et al. 2007; Biasutti et al. 2011). However, significant regional and seasonal differences in the above general behavior exist. Examples include nocturnal rainfall maximum over the U.S. Great Plains (e.g., Wallace 1975; Jiang et al. 2006) and an afternoon peak in cloudiness over south Pacific convergence zone (e.g., Nesbitt and Zipser 2003). The diurnal cycle of vertical structure of precipitation has received little attention in the literature (e.g., Geerts and Dejene 2005; Xu and Zipser 2011). Geerts and Dejene (2005) analyzed TRMM spaceborne radar observations and reported that for most climatic regions of Africa, the storm echo tops are highest and their intensities are maximum between 1500 and 1800 local time. Xu and Zipser (2011) described the diurnal behavior of surface and vertical precipitation structure, deep convection, and lighting during pre-mei-yu, mei-yu, and midsummer seasons in eastern Tibetan plateau. 1.1 Studies related to Europe Unlike U.S. and the tropics, most studies related to Europe primarily used rain gauge dataset to examine the diurnal cycle of precipitation (e.g., Svensson et al. 2002; Twardosz 2007; Yaqub et al. 2011). These studies highlighted the role of orography and the atmospheric fronts in modulating the diurnal cycle of precipitation. However, a thorough analysis of the diurnal cycle in complex topography requires high-resolution information on the horizontal and vertical distribution of precipitation, which is not possible to obtain from the rain gauge networks. Very few studies employed radar and satellite derived information to examine precipitation diurnal cycle over the European Alps and surrounding regions (Dai et al. 2007; Paulat et al. 2008; Wüest et al. 2010). Dai et al. (2007) characterized the spatial behavior of precipitation amount, frequency, intensity and diurnal cycle between 60 N and 60 S latitudes with a grid resolution of 2. However, the spatial resolution of 2 2 is too coarse to characterize the regional differences within the Alps and quantify the effects of complex orography on the diurnal cycle. Paulat et al. (2008) used 4 years of radar-gauge merged precipitation dataset (hourly accumulations with a spatial resolution of 7 km) and reported that the diurnal maximum occurs between 1700 and 2400 local time. The study also reported that the amplitude is largest in the mountainous regions along the German-Czech border and near the Alpine Foreland. Wüest et al. (2010) developed gridded hourly precipitation dataset for Switzerland by disaggregating daily rain gauge data using radar-rainfall information. The study obtained summertime diurnal cycle of precipitation amount from the gridded dataset and compared it against the diurnal cycle computed from rain gauges. The diurnal maximum in precipitation amount was found to be in between 1500 and 1800 UTC. 1.2 Objective of the study The objective of this study is to characterize the diurnal cycle in horizontal and vertical structure of precipitation for the region, which includes Jura Mountains and Swiss flatlands in the northwest, Alps running from southwest to east, and Po River plain and part of Appennine mountain range in the south (Figure 1). The domain is broadly centered on Switzerland, and includes parts of Germany, France, Italy, and Austria. The study region lies in higher latitudes, which are not covered by the TRMM satellite. Also, since the region is characterized by complex orography, the precipitation vertical structure is highly variable. Therefore, one would require higher resolution data than that typically provided by satellites. We utilize volumetric data from ground-based radars in this study to analyze the diurnal behavior of precipitation.

Fig. 1. Map showing the area covered by the Swiss radar composite along with the orography, the location of three radars, and the regions of interest.

2 Fig. 1. Map showing the area covered by the Swiss radar composite along with the orography, the location of three radars, and the regions of interest. The Figure also shows the sections along which the vertical structure of precipitation is studied. 2. Study area and the dataset The region is covered by the MeteoSwiss network of three single-polarimetric Doppler C-band radars (Albis, La Dole and Lema; Figure 1). One of the unique features of the network is the high-resolution scan strategy adopted to obtain radar measurements in the mountainous region. The scan strategy, which comprises of 20 beam elevations per 5 min volume scan, is particularly useful for characterizing the precipitation vertical structure. Figure 1 also shows the cross sections along which the vertical structure of precipitation is analyzed. The characteristics of the cross sections will be described in detail later in the paper. We focused on three seasons: 1) Spring (March, April, May), 2) Summer (June, July, August), and 3) Autumn (September, October, November). We employed six years ( ) of high-resolution radar observations in this study. Data prior to the year 2005 had only the global bias adjustment whereas data from 2005 were adjusted for both local and global bias. To avoid any effects of different bias adjustments on the diurnal analysis, we did not consider data prior to the year Beginning June 2011, MeteoSwiss started replacing single-polarization radars with the state-of-the- art dual-polarization radars. During the upgrade, there were frequent interruptions and changes in data processing system. Therefore, we did not include data from the year Methodology We divided the study into two parts. In the first part of the study, we focused on the diurnal cycle of the surface rain rate fields, also referred to as Niederschlags Alarm Signal System (NASS) composites (e.g., Germann et al. 2006). In the second part, we analyzed the diurnal cycle in vertical structure along four strategically selected sections, which will be described later in the paper. 3.1 Spatial structure The NASS rainfall composite is the final product from the MeteoSwiss data processing system for quantitative precipitation estimation, containing algorithms such as clutter mitigation, correction for the beam shielding and vertical variability of the radar-reflectivity, and polar-cartesian grid transformation (e.g., Germann et al. 2006). The spatial resolution of the composite is 1 km and time resolution is 5 min. Although data were available for the 280,000 km 2 domain, we focused only on the 198,200 km 2 region to mitigate the effects of radar visibility on the characterization of diurnal cycle. The study region was then divided into three regions, referred to hereafter as Jura, Alps, and Po (Figure 1). The regions were delineated mainly to focus on the orographic controls on the diurnal cycle. There are still some locations within these regions, which suffer from poor radar coverage (for example towards the southwest and eastern parts of the domain), but we believe that the study region selected is a fair trade-off between the radar visibility and sample size (i.e. robustness of statistics). For each season and time of the day, average rainfall intensities and frequency of precipitation exceeding 0.63 mm/h (reflectivity threshold of 22 dbz with Z = 316 R 1.5 ) were computed. The threshold of 22 dbz was chosen after repeated trials to minimize

any effects of residual ground clutter on the diurnal analysis. 3.2 Vertical structure The vertical structure of precipitation was analyzed for four cross sections within the study region (Figure 1).

3 any effects of residual ground clutter on the diurnal analysis. 3.2 Vertical structure The vertical structure of precipitation was analyzed for four cross sections within the study region (Figure 1). The sections were chosen such that one of them (AlpsPer) is in the direction perpendicular to the Alpine barrier, and the remaining three sections (JuraPar, AlpsPar, and TiciPar) roughly parallel to the Alpine barrier (Figure 1). The JuraPar section extends from Jura mountains near Swiss-French border to Black Forests in southwest Germany, AlpsPar is the section in central Switzerland near the northern slopes of the Alps, and TiciPar is the section near the southern foothills of Alps between Piemonte in Italy and Ticino in Switzerland. Except for the TiciPar section, the dimensions of all sections are 120 km in length, 15 km in width, and 10 km in height (Figure 1). The length of TiciPar section is restricted to 90 km due to limited visibility from the Lema radar. For each section, the three-dimensional structure of precipitation was obtained using volumetric data from single radar. The data from Albis radar was used for JuraPar, AlpsPar, and AlpsPer cross sections, whereas the data from the Lema radar was used for the TiciPar cross section. For each section, radar observations between the beam elevation angles of -0.3 and 11 were rebinned onto the threedimensional grid with a horizontal resolution of 1 km and vertical resolution of 100 m. A total of 12 beam elevation angles with a 3 db beam-width of 1 were used in the rebinning procedure (See Germann et al., 2006 for details on the radar scan geometry). The frequency of occurrence within predefined classes of reflectivity (or rainfall intensity) and altitude were computed. The frequency distribution of reflectivity plotted against the altitude is widely referred to as frequency-altitude diagram (FAD), and is used to characterize the vertical structure of storms (e.g., Yuter and Houze Jr 1995). We then averaged the precipitation frequencies and intensities in width and length dimensions, and plotted against the time of the day to obtain Hovmoller diagrams of precipitation vertical structure. 4. Results 4.1 Spatial structure Figure 2 shows the spatial distribution of frequencies at 8, 16, and 24 UTC for spring, summer and autumn seasons. Some residual instrumental and visibility issues can be noticed in Figure 2, particularly due to the partial beam blockage towards southwest and southeast directions. However, the regional boundaries (black lines in Figure 2) were selected such that the visibility effects on the diurnal analysis are minimal. Fig. 2. Spatial distribution of frequency of precipitation exceeding 0.63 mm/h (22 dbz) at three time instants (8, 16, and 24 UTC) and for three seasons (spring, summer, and autumn).

4 Significant seasonal and regional variations in diurnal behavior of frequencies can be noticed from Figure 2. At 8 UTC and 24 UTC, spring season has largest frequencies, whereas at 16 UTC, summer season registered largest frequencies. The diurnal cycle of frequencies is strongest during summer, moderate during spring, and weakest during autumn. In addition, the diurnal cycle is stronger over the Jura Mountains and over the northern slopes of Alps, and very weak over the Po River plain. Independent of the season and time of the day, Ticino region near the southern foothills of the Alps recorded large rainfall frequencies (Figure 2). Similar patterns were noticed in the diurnal behavior of average rainfall intensities (See Mandapaka et al. 2012). In the Figure 3, we show regional averages of precipitation intensity and frequency as a function of daytime for three regions discussed in section 3.1 and three seasons. It should be noted that only pixels within regional boundaries (Figures 1 and 2) were considered when computing regional averages. As expected the small-scale effects seen in Figure 2 are filtered out in the diurnal behavior of regional time series. The diurnal signal is strongest during summer and weakest during autumn, and it is much stronger over the Alps compared to other regions. The timing of maximum is in between 16 and 17 UTC for the spring and summer seasons, and around 19 UTC for the autumn season. The regions north of the Alps are frequently subject to westerly and northwesterly atmospheric fronts. While the frontal precipitation contributes substantially to the diurnal cycle of rainfall frequency and intensity, convective activity driven by local forcing (e.g., orography) plays a key role in modulating the amplitude of the diurnal signal. Fig. 3. Diurnal behavior of average precipitation intensity (top panels), and frequency (bottom panels) for each region of interest and each season. 4.2 Vertical structure For each section, we obtained frequency-altitude diagrams (FAD) by counting the number of occurrences within each predefined class of reflectivities and altitudes. The reflectivity range selected was 22 dbz to 54 dbz with a class interval of 1 dbz, and the altitude range varied from 0 to 10 km with an interval of 100 m. The occurrences in each 1 dbz 100 m bin were then normalized with total number of occurrences to obtain the normalized FADs (integral over the FAD is equal to 100%). Figure 4 shows the normalized FADs in the form of percentages for each vertical section discussed in section 3.2. The total number of occurrences is also shown on each panel of the Figure 4. The FADs for all sections show nose shaped pattern with a narrow range of reflectivies at high altitudes, and broad spectrum of reflectivities between 2 and 4 km. The 16 UTC FADs exhibited broader spectrum of reflectivities at all altitudes compared to 8 UTC and 24 UTC FADs. It can be seen from the Figure 4 that the diurnal variation is stronger for the sections north of the Alps and weaker for the TiciPar section near the southern foothills of Alps. Figure 5 shows the Hovmoller diagrams of frequencies and intensities for four vertical cross sections discussed in section 3.2. Throughout the day, larger precipitation intensities and occurrence frequencies were observed between the altitudes of 3-4 km for the JuraPar, AlpsPer, and AlpsPar sections, and between 2-3 km for the TiciPar section (Figure 5). In the morning (until 12 UTC), the time-altitude plots showed similar pattern for all the sections, although the vertical extent for the TiciPar section is slightly lower than those observed for the sections north of the Alps. In the afternoon, precipitation intensities and frequencies at all altitudes increased rapidly in response to the solar heating, and the time-altitude plots displayed significant regional differences in vertical structure and timing of maximum. For example, large intensities and frequencies were observed for the AlpsPar section at the altitudes of 5-6 km.

5 Fig. 4. Frequency-Altitude distribution at three time instants and for each section showed in Figure 1. Each panel shows the normalized probabilities expressed as percentage. Fig. 5. Time-height variation of average precipitation intensity and frequency for each vertical section shown in Figure 1.

6 The diurnal signal reached maximum between UTC for JuraPar section, UTC for AlpsPer section, and UTC for AlpsPar section. That is, the phase of the diurnal signal shifted by approximately 2 h as we moved from the JuraPar section over the Jura Mountains to the AlpsPar section near the northern slopes of the Alps. 5. Conclusions Diurnal behavior of precipitation structure over the Alps and surrounding forelands was characterized using six years of high-resolution radar observations. Using the surface rain rate maps, the diurnal cycles of average precipitation intensity and 22 dbz exceedence frequency at the surface were described. Significant regional and seasonal variability was observed in the diurnal behavior of precipitation intensity and frequency. The diurnal cycles of precipitation intensities and 22 dbz occurrence frequencies are strongest during the summer, moderate during spring, and weakest during autumn. Furthermore, the diurnal cycle is strongest over the northern slopes of the Alps and Jura Mountains and weakest in the Po River plain south of the Alps. The study then focused on the summertime precipitation variability within selected three-dimensional sections in the region. The volumetric radar observations within the 3D sections were used to obtain the frequency- altitude and Hovmoller diagrams of precipitation vertical structure. The frequency-altitude diagrams for the sections north of the Alps were found to be remarkably different from those for the TiciPar section south of the Alps. The high-altitude precipitation frequencies were found to be larger for the sections north of the Alps. The diurnal variability of vertical precipitation structure was found to be strongest for the AlpsPar section along the northern slopes of the Alps and weakest for the TiciPar section. In addition the phase of the diurnal signal shifted by 2 h from the JuraPar section to the AlpsPar section. The results from this study form an excellent basis for verification of numerical weather prediction models, improvement of the underlying physical parameterization schemes, and for incorporating orographic forcing into heuristic precipitation nowcasting schemes. Future studies will focus on understanding the physical mechanisms responsible for the variability in the diurnal cycle in the study region. Acknowledgment We acknowledge the support from the IMPRINTS project under the European Union FP7 framework. References Biasutti M, Yuter S, Burleyson C, Sobel A Very high resolution rainfall patterns measured by TRMM precipitation radar: Seasonal and diurnal cycles. Climate Dynamics. Carbone R, Tuttle J Rainfall occurrence in the us warm season: The diurnal cycle*. Journal of Climate 21(16): Dai A, Lin X, Hsu K The frequency, intensity, and diurnal cycle of precipitation in surface and satellite observations over low-and mid-latitudes. 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Diurnal cycle of precipitation over complex Alpine orography: inferences from high-resolution radar observations

Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 139: 1025 1046, April 2013 B Diurnal cycle of precipitation over complex Alpine orography: inferences from high-resolution