DIAGNOSING THE PRECIPITATION CYCLES OVER AFRICA AND EUROPE FROM SATELLITE DATA

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1 DIAGNOSING THE PRECIPITATION CYCLES OVER AFRICA AND EUROPE FROM SATELLITE DATA Arlene Laing 1, Vincenzo Levizzani 2, Richard Carbone 1, Roberto Ginnetti 2 1 National Center for Atmospheric Research (NCAR), Boulder, CO, USA 2 CNR-Institute of Atmospheric Sciences and Climate, Bologna, Italy Abstract Long-term statistics of organized convection are vital to improved understanding of the hydrologic cycle at various scales. Warm season precipitation prediction remains one of the important forecasting challenges. Numerical weather prediction models exhibit poor skill in forecasting convective precipitation partly because of problems with the representatino of mesoscale convection. Satellite and radar observations are being exploited to understand the timing and duration of precipitation events. With satellite data, diurnal, sub-seasonal, and seasonal cycles of precipitation can be examined, systematically, on continental scales. Warm season precipitation comprises of coherent precipitation structures that are characteristic of systems propagating under a broad range of atmospheric conditions. They are frequent in weakly forced conditions in summer and are strongly modulated by the diurnal heating. The World Weather Research Programme (WWRP) of the World Meteorological Organization (WMO) has endorsed a global study to achieve better knowledge of the physical characteristics of precipitating systems in the warm season. This paper reports results of an analysis of five years of data from the Meteosat satellite over sub-saharan Africa, Europe, and the Mediterranean Sea from 1999 to INTRODUCTION The prediction of convective precipitation remains a great challenge. Our ability to predict warm season precipitation depends on our understanding of the development and environment of organized mesoscale convective systems (MCSs). Numerical models have low predictive skill for convective precipitation sequence, intensity, frequency, and timing. Models need to be able to reproduce these statistics, because of the implications for the hydrologic cycle and feedbacks, such as surface latent heat exchange and radiative effects of large cloud systems that persist during nighttime. In recent decades, analysis of satellite and radar observations has increased our understanding of convection and precipitation. Using data from the United States (US) Weather Surveillance Radar-88 Doppler (WSR-88D), Carbone et al. (2002) found that heavy precipitation often occurs in organized episodes or streaks that originate in the lee of the Rocky Mountains and propagate eastward. Episodes display coherent patterns of propagation across the continent with some systems lasting up to 60h. For other continents, geostationary IR brightness temperatures (T bb ) are used to decipher patterns of cold-cloud clusters, which serve as a proxy for convective precipitation. Wang et al. (2004) and Keenan and Carbone (2007) found that cold-cloud episodes in East Asia and Australia, respectively, have zonal spans on the order of 1000km and duration on the order of one day. Laing et al. (2007) found that episodes in northern, tropical Africa, were longer lasting and encompassed a longer span than other continental regions, with a few episodes undergoing four cycles of regeneration over 100h and 5000km. Given the similarity in the properties of MCSs globally (Laing and Fritsch 1997), coherence in propagating characteristics in these continental regions is not unexpected. Few studies exist on the span and the duration of convective precipitation systems in Europe although several studies have tracked individual storm systems over specific regions of Europe. For example, Schiesser et al. (1995) found that MCSs over Switzerland were not as well organized as those in the US and Hernandez et al. (1998) summarized the characteristics and environments of mesoscale convective complexes over (MCCs) over the Iberian Peninsula and western Mediterranean during

2 Chaboureau and Claud (2006) used the temperature of the low stratosphere, pressure of the top of the cloud, and a precipitation index from the TIROS-N Operational Vertical Sounder (TOVS), to characterize Mediterranean cyclones and associated cloud system types for each season. This study examines the systematic propagation and evolution of cold-cloud clusters across continents. Patterns of warm-season convective precipitation in Africa and Europe are summarized and compared with those of other continents. 2. DATA AND METHODS Primary data are IR ( µm) brightness temperatures from the European geostationary satellite (Meteosat-7). The images are available at 30 minute intervals and sampled to 0.2degree grids. For Europe, the study domain is N, 15 W-40 E; for Africa, domains are, 0 to 20 N and 20 W to 40 E; 15 S to 15 N and 20 W to 45 E; 35 S to 15 S and 10 E to 45 E (Fig. 1). Figure 1: Study domains for (a) Europe and (b) Africa Threshold T bb are used to identify cloud systems that are most likely to be precipitating. Choices for Africa were based on satellite and gauge comparative studies over West Africa (Arkin 1979, Arnaud et al. 1992; Mathon et al. 2002). Threshold T bb for propagation statistics over tropical Africa is 233K; for subtropical-to-midlatitude southern Africa, it is 235K; for Europe, it is 265K. Reduced-dimension (Hovmöller) techniques determine the propagation of cold clouds. Each pixel colder than the threshold T bb constitutes an event at a given distance time coordinate. A 2-D auto-correlation function is stepped through all points in the distance-time space and rotated until the correlation coefficient is maximized. Contiguous fits to the function define coherent patterns or streaks (Fig. 2). The primary statistics described herein are the zonal span, duration and phase speed retrieved from Hovmöller longitude-time diagrams. Analysis is performed on sub-domains to better capture the zonal progression. Mean diurnal cycles are determined by computing the frequency of convection at a particular longitude at the same time of day. A Fourier power spectrum analysis is used to identify daily cycles and propagation features of systems over Europe. Large-scale environments are diagnosed from the operational National Center for Environmental Prediction (NCEP) Global Final Analyses, which are on a 1º grid at 6hourly synoptic intervals.

3 Figure 2: Domain for Central Africa Hovmoller longitude-time analysis, example of objectively analyzed streaks, and conceptual model of auto-correlation function 3. AFRICA 3.1 Zonal propagation statistics Deep convection in tropical Africa (northern and central Africa) is organized as coherent episodes that occur on an almost daily basis and propagate westward (Fig. 3). For northern tropical Africa, an average of about 5 episodes occurs daily while, for central Africa, the average is about 3 episodes per day. A large fraction of the episodes initiate in the lee of high terrain (e.g., the mountains of East Africa, Darfur Mountains, Jos Plateau, Guinea Highlands). A major generating factor is thermal forcing associated with large elevated heat sources. Figure 3: Longitude time plot of cold cloud episodes during Jul 2002, March 2003, and 1-15 Jan 2000.

4 South of 20ºS, episodes propagate eastward, in the direction of the prevailing westerly flow (Fig. 3). They are coherent in phase with mean zonal phase speeds that are similar to those of other continental domains. However, periods of propagating convection are much less frequent than in tropical Africa (Fig. 3). The phase speed for most cold-cloud streaks in Africa is m s -1 ; similar to phase speeds of episodes in the USA, East Asia, and Europe (Carbone et al. 2002, Wang et al. 2004). Although the phase speeds are similar, southern Africa results are preliminary as only three seasons have been fully analysed. Analysis of the remaining seasons is in progress. 3.2 Average daily zonal progression of convection Examples of bi-weekly average diurnal cycle of deep convection are shown in Figure 4. A domainwide maximum occurs between 1800 and 2200 LT and the minimum occurs one to two hours before noon. The diurnal cycle is complicated by the superposition of remotely-forced convective systems onto the convection due to the local diurnal heating maximum. That is because convection in tropical Africa is triggered along several mountains ranges (cf. Figs 1, 3) compared with episodes in the USA mainland and East Asia that are triggered along one large and high mountain range. Therefore, the typical pattern for tropical Africa shows several maxima connected by axes of higher frequency associated with propagating systems (Fig. 4). For subtropical-to-mid-latitude southern Africa, convection propagates east from high terrain (Fig. 4), except during the passage of tropical cyclones, such as Tropical Cyclone Cela in December Figure 4: Average percentage of time with cold cloud at a given longitude at the same time of day for: northern tropical Africa, central Africa, and subtropical-midlatitude southern Africa. Shaded contours are average percentage of occurrence. Time is given in UTC. Axes of maximum frequency are marked, subjectively, by black dashed lines. Time zones are noted as hours relative to GMT. The diurnal cycle is repeated to show the 24h cycle. Cross-sections of average elevation are shown for each domain. 3.3 Large-scale influences Moderate vertical shear of the horizontal wind helps to organize ordinary convection into MCS. In northern, tropical Africa, this is a common condition associated with the migration of the African Easterly Jet, while in subtropical-mid-latitude southern Africa it is associated with the deep westerlies (Fig. 5). Well-organized convection develops and propagates where low-level, northerly winds bring warm, moist air from the equatorial zone, e.g. after 5 December 2003 (Fig. 5a). Without westerly wind shear very little propagation occurs.

5 Figure 5: (a) Daily averaged meridional wind at 850 hpa (m s -1 ), (b) cold-cloud episodes averaged between 35 and 20 S, and (c) daily averaged zonal wind at 500 hpa (m s -1 ) for 1 15 December Dashed arrows mark eastward propagation of the zonal wind at 500 hpa. The propagation of organized convection in central Africa is most affected by variations in the largescale wind velocity in the lower-troposphere and near the tropopause (near the level of the Tropical Easterly Jet), such as occurs with the passage of Kelvin waves (Straub and Kiladis 2003). For example, during the first half of April 2003, episodes are propagating across the continent almost daily until 18 April when the large-scale circulation changes (Fig. 6) and westward propagation ceases. The cessation occurs in the presence of strong easterly winds at 200hPa and weak southerly winds at 700hPa. Figure 6: (a) Daily averaged meridional wind at 850 hpa (m s -1 ), (b) cold-cloud episodes averaged between 35 and 20 S, and (c) daily averaged zonal wind at 500 hpa (m s -1 ) for 1 15 December Dashed arrows mark eastward propagation of the zonal wind at 500 hpa.

6 4. EUROPE 4.1 Zonal propagation statistics Organized deep convection over Europe exists often as coherent, eastward propagating streaks. However, there are periods during which non-propagation or westward propagation dominates. For example, during 1-15 May 2000 (Fig. 7), organized eastward propagation is common. Meanwhile in the north, the first period is marked by little or propagation and the second period by westward propagation. The median phase speeds remains consistent throughout the entire period. Table 1 gives the statistics of zonal span, duration and zonal propagation speed of all events longer than 3 h. Figure 7: Longitude time plot of cold cloud episodes over Europe during 1-15 May 2000 Year Span (km) Duration (h) Phase Speed(m s -1 ) All Events median Largest 50% median Table 1: Mean zonal span, duration and zonal phase speed for all systems, May August, , that last longer than 3 h, and same quantities for those belonging to the top 50%.

7 4.2 Cold cloud persistence and daily cycle Cold cloud persistence was examined for i) May August of each year, ii) each month for all five years, and ii) the overall average of the whole period. The latter is shown in Figure 8. Figure 8: Average cloud top temperature over the whole period May August The May August average temperature fields for each year show that a clear signal of persistence of clouds with a cold top corresponds with the lee of the Alps, the Pyrenees, the Balkans and the Carpathians, where the average cloud top temperature is quite low over the whole period. Upon examining the fields of each month for all years, the signal appears stronger over the Alps and the Pyrenees and perhaps less evident over the Balkans and Carpathians (Fig. 9). This fact holds also in the whole period average cloud top temperature field (Fig. 8) although smoothed by the averaging process. The strength of the cycles varies considerably from year to year, being stronger on wetter years (2002) and weaker on drier years (2003). Figure 9: Average diurnal cycle for May, June, July, and August for the years 1999 to 2003.

8 4.3 Intraseasonal variations Table 3 provides a summary of the intraseasonal variations in the zonal statistics of organized convection. Episodes that occur in June travel farthest and endure longer than other months; August has the episodes with the lowest span and duration. May has the maximum frequency of streaks and July has the minimum. The propagation velocity is greatest in June and then decreases through August. May June July August No. of streaks Span (km) Duration (h) Phase speed (m s -1 ) Table 3: Intraseasonal variation in the zonal statistics of cold clouds in Europe for 1999 to CONTINENTAL COMPARISONS The observed properties over Europe and Africa are in good agreement with the observations of Carbone et al. (2002) over the US, those of Wang et al. (2004) over East Asia (Table 4). The duration span relationship is very highly correlated over tropical Africa (Fig. 10), indicating the greater impact of diurnal, thermal forcing and cumulus convection. Over Europe and subtropical Africa, the correlation is weaker because some cold clouds are associated with midlatitude cyclones especially during the early periods of May. Region (zonal domain longitude) Zonal phase speed (m s -1 ) Contiguous US (37º) Median 13.6 East Asia (50º) Mean 12.4 Europe (50º) Mean Median 13.6 Northern Tropical Africa (60º) Mean 12.0 Median 11.2 Central Africa (50º) Mean 11.8 Median 10.7 Mid-latitude southern Africa (35º) Mean 12.1 Median 11.2 Table 4 Mean and median zonal phase speed of convection and precipitation episodes over continents

9 Figure 10 Zonal span(km) versus duration(h) for episodes over N. Tropical Africa, Europe, Central Africa, and S. Subtropical Africa. Black dotted and dashed lines represent the phase speeds of 7 and 30ms 1, respectively, which encompass most streaks with span > 1000 km and duration > 20 h. For Europe, the other lines refer to median velocities for: all systems (red), systems occurring between 1 per day and 2 per week (blue), and those occurring between 1 per week and 1 per month (green). For the African domains, the solid black line represents the regression model whose equation is listed in the lower right. 6. SUMMARY Organized convection appears as coherent sequences or episodes that propagate on regional and continental scales over sub-saharan Africa and Europe. The zonal statistics, such as propagation speeds, are similar across continental regions, which indicate common environmental conditions that favor increasingly organized precipitation regimes. A large fraction of episodes have their origin in the lee of major mountain ranges such as the Alps, the Carpathians, East Africa Mountains, Guinea Highlands of West Africa, and the South African escarpment. The triggering of convection is largely due to the thermal forcing associated with large scale elevated heat sources. The diurnal cycles of convective precipitation over Europe and Africa display similarities with warmseason precipitation in North America, East Asia, and Australia. Propagating convective systems modulate the diurnal cycle so that instead of a single period of maximum precipitation in the late afternoon, precipitation maxima occur as axes across the domain.

10 Convective episodes propagate across the continent when steering winds are present in the mid- to upper troposphere. In the absence of steering winds, the longevity, and spatial span of events are greatly diminished, and significant propagation ceases. When steering winds cease, the dominant regime of convection is simple diurnal cycling over elevated terrain. Episodes occur in the presence of moderate vertical shear of the horizontal wind. This is a common condition associated with the deep westerlies in Europe and subtropical southern Africa. In tropical Africa, vertical shear is most commonly associated with the mid-level African Easterly Jet, the West African monsoon and, close to the equator, the Tropical Easterly Jet. These results show a potential for the improvement of NWP models and for a regional climate characterization in Africa, Europe, and the Mediterranean. Acknowledgements Thanks to John Tuttle for assistance with software. The second author wishes to acknowledge CNR for sponsoring the research and the Italian Space Agency (ASI) for partial support under the projects GOMAS and LAMPOS. Meteosat imagery is copyright of EUMETSAT and was made available by the EUMETSAT Archives. NOAA Climate Diagnostic Center provided NCEP Analyses on their website. REFERENCES Arkin, P. A. (1979) The relationship between fractional coverage of high cloud and rainfall accumulation during GATE over the B-scale array. Mon. Weath. Rev. 107, pp Arnaud, Y., Desbois, M. and Maizi, J. (1992) Automatic tracking and characterization of African convective systems on Meteosat pictures. J. Appl. Met. 31, pp Carbone, R. E., Tuttle, J. D., Ahijevych, D., & Trier, S. B. (2002) Inferences of predictability associated with warm season precipitation episodes. J. Atmos. Sci. 59, Chaboureau J.P., Claud C. (2006) Satellite-based climatology of Mediterranean cloud systems and their association with large-scale circulation. J. Geophys. Res., 111: D Hernandez, E. ; Canal, L.; Diaz, J.; Garcia, R., Gimeno, L. ( (1998) Mesoscale Convective Complexes over the Western Mediterranean area during , Meteorol. atmos. phys., 68, pp.1-12 Keenan, T. and Carbone, R. E., (2007) Propagation and Diurnal Evolution of Warm Season Cloudiness in the Australian and Maritime Continent Region. In press, Mon. Weath. Rev. Laing, A. G. and Fritsch, J. M. (1997) The global population of mesoscale convective complexes. Quart. J. Royal Met. Soc. B, 123, pp Laing, A. G., Carbone, R.E., Levizzani, V., and Tuttle, J. (2007) The propagation and diurnal cycles of deep convection in northern tropical Africa. In review, Quart. J. Roy. Meteor. Soc. Linder, W., W. Schmid, and H.H. Schiesser, 1999: Surface Winds and Development of Thunderstorms along Southwest Northeast Oriented Mountain Chains. Wea. Forecasting, 14, pp Mathon, V. and Laurent, H. and Lebel, T. (2002) Mesoscale convective system rainfall in the Sahel. J. Appl. Met. 41, pp Schiesser, H., R. Houze, and H. Huntrieser, (1995) The Mesoscale Structure of Severe Precipitation Systems in Switzerland. Mon. Wea. Rev., 123, pp Straub, K. H. and G. N. Kiladis (2003) The observed structure of the coupled Kelvin waves: Comparison with simple models of coupled wave instability. J. Atmos. Sci., 60, Wang, C.-C., Chen, G. T.-J. and Carbone, R. E. (2004) A climatology of warm season cloud patterns over East Asia based on GMS infrared brightness temperature observations. Mon. Weath. Rev