Significant cyclone activity occurs in the Mediterranean

Transcription

1 TRMM and Lightning Observations of a Low-Pressure System over the Eastern Mediterranean BY K. LAGOUVARDOS AND V. KOTRONI Significant cyclone activity occurs in the Mediterranean area, mainly during the cold season. As most of these cyclones form over the sea, spaceborne platforms are especially useful for observing these systems. Moreover, during the cold season, lightning usually occurs over the relatively warm surface waters, and thus lightning detection data can also help us study the evolution of convective systems. OBSERVATIONS. During 4 6 November 2004, a low-pressure system formed over the southern part of the central Mediterranean, namely, over the Gulf of Sidra. During the two following days, the lowpressure system moved northeastward, producing significant lightning over the sea, while on 5 November, heavy precipitation fell in Crete; two stations in western Crete accumulated more than 145 mm of rain during the 36 h ending at 0600 UTC 6 November. Figure 1a shows the mean sea level pressure at 0000 UTC 4 November [as given by the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses] with a low-pressure center of 1006 hpa northwest of the Gulf of Sidra. The surface low was associated with a cutoff low at 500 hpa (Fig. 1b), while a strong upper-level jet streak exceeding 50 m s 1 was also evident at 300 hpa (blue shading in Fig. 1b). The area north from the Gulf of Sidra and west of Crete was under the left-hand exit region of the jet streak, an area associated with significant divergence at the higher tropospheric layers. AFFILIATIONS: LAGOUVARDOS AND KOTRONI National Observatory of Athens, Institute of Environmental Research and Sustainable Development, Athens, Greece CORRESPONDING AUTHOR: Dr. K. Lagouvardos, National Observatory of Athens, Institute of Environmental Research and Sustainable Development, Lofos Koufou, P. Penteli, 15236, Athens, Greece lagouvar@meteo.noa.gr DOI: /BAMS American Meteorological Society FIG. 1. (a) ECMWF analysis of mean sea level pressure (solid lines at 3-hPa interval) valid at 0000 UTC 4 Nov (b) As in (a), except for the 500-hPa geopotential height (solid lines at 40-m interval) and the 300-hPa wind speed (shaded contours at 10 m s 1 interval, only values exceeding 30 m s 1 are shown). Data from NASA s Quick Scatterometer (QuikSCAT) at approximately 25-km horizontal resolution (available online at allows inspection of the surface wind field. The significant sea level pressure gradient at about 0400 UTC 4 November, the time of the satellite passage over the area, was associated with strong surface winds (see Fig. 2). Figure 2 shows wind speeds exceeding 17 m s 1 around the low center, while very strong easterly winds prevail over the maritime area northeast of the low center. Figure 3 shows the cloud-to-ground lightning with data provided by the ZEUS lightning-detection AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER

2 network operated by the National Observatory of Athens during 2004 (Anagnostou et al. 2002). The ZEUS network consists of five very low-frequency (VLF) sensors installed across Europe. Figure 3 shows the lightning detected during four 1-h periods at , , , and UTC. Indeed, there is significant lightning activity from 0000 to November, progressing slowly eastward. The lightning activity is weaker near the low-pressure center compared with the maritime area to the northeast, where QuikSCAT shows convergence (Fig. 2). During UTC 4 November, the lightning activity decreases while it progresses farther east. The Tropical Rainfall Measurement Mission (TRMM) satellite provides additional collocated spaceborne observations. TRMM is a low-orbit satellite (flying at ~400 km since mid-august 2001) on a tropical path that covers (with some of its instruments) a belt between 38 N and 38 S, providing very useful observations (e.g., brightness temperatures, radar reflectivities) from which microphysical characteristics of weather systems over the southern part of the Mediterranean basin can be inferred. TRMM carries the following three main instruments: Visible and Infrared Scanner (VIRS): A five-channel, cross-track-scanning radiometer operating at 0.63, 1.6, 3.75, 10.8, and 12 μm (the radiances measured by VIRS can be used to infer cloud coverage, cloud type, and cloud-top temperatures); TRMM Microwave Imager (TMI): A multichannel passive microwave radiometer operating at five frequencies (10.65, 19.35, 37.0, and 85.5 GHz at dual polarization, and GHz at single polarization) that provides information on the integrated column precipitation content, cloud liquid water, ice water path, rain intensity, and rainfall types; Precipitation Radar (PR): An electronically scanning radar, operating at 13.8 GHz, which provides the 3D structure of reflectivity over both land and ocean, from which information about the vertical structure of precipitation systems is obtained. FIG. 2. The 10-m wind field provided by QuikSCAT (one barb: 5 m s 1 ; one half-barb: 2.5 m s 1 ), valid at ~0400 UTC 4 Nov Rain-contaminated QuikSCAT winds have been removed. Figure 4a presents the VIRS infrared image, as well as TMI brightness temperature observations at 19- (vertical polarization, Fig. 4b) and 85.5-GHz polarization-corrected temperature (PCT; Fig. 4c) from the TRMM passage over the study area at 0012 UTC 4 November (PCT is defined by Spencer et al. in the April 1989 Journal of Atmospheric and Oceanic Technology.) VIRS imagery (Fig. 4a) reveals the banded structure around the low-pressure center, as well as an area with significant convection west of Crete (35 36 N, 20 E), while a cloud-free area is evident over the maritime area between the low-pressure center and the convective area to the east. Images at 19 GHz are extremely valuable for rainfall mapping, because this frequency is much less susceptible to ice-scattering effects than the higher frequencies (e.g., 37 and 85.5 GHz) and can be used FIG. 3. Lightning activity as sensed by ZEUS lightning detection network during 1-h periods beginning at 0000, 0600, 1200, and 1800 UTC 4 Nov SEPTEMBER 2007

3 as an estimate of the vertically integrated rainwater content. The rainbands around the low-pressure system are evident, with a rain-free area over the storm center. A major rainband is also evident over the maritime area northeast of the low center, coinciding with the area of convection shown in Fig. 4a and the area of significant lightning depicted in Fig. 3. Figure 4c presents the corresponding TMI PCT image at the 85.5-GHz channel. The resolution at this frequency is ~6.7 km 4 km, which is much finer than the ~30 km 18 km resolution at 19 GHz. Scattering from precipitation-sized ice is the dominant process at that frequency. Low PCT values at that frequency are an indication of significant scattering resulting from the large ice-water path of precipitation-sized ice particles. The band northeast of the low center contains very low brightness temperatures, down to K in the area around N, 20 E. The presence of ice and mixed-phase hydrometeors is related to lightning, an idea that has been pointed out in some of the recent literature by Toracinta, Katsanos, and others that we recommend to the reader below. The comparison of Fig. 4c with Fig. 3 provides additional evidence of the good correlation between ice within clouds and lightning. Finally, Fig. 5 shows the 3-km AGL reflectivity field over the area, as measured by TRMM PR, and indicates the highest reflectivity cores within the rainbands also depicted in Fig. 4 (note that PR swath is ~3 times narrower than the corresponding TMI swath). Reflectivity values at that level reach 45 dbz in the area of the lowest 85.5-GHz PCT values, while a rain-free area is evident between the low-pressure center and the convective area to the east. RESULTS OF MODEL SIMULATIONS. We simulated this event with the fifth-generation Pennsylvania State University (PSU) National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) (version 3.5). MM5 is a nonhydrostatic, primitive-equation model using terrain-following coordinates. We selected parameterizations based on our previous comparative study of convective and microphysical schemes (in Geophysical Research Letters, 28, ), the Kain Fritsch scheme for the convective parameterization, and the scheme proposed by P. Schultz for the explicit microphysics. For this study, we defined the following two oneway nested grids as such: Grid 1 (with 36-km spacing), covering the major part of southern Europe, the Mediterranean, and the northern African coasts; and FIG. 4. (a) TRMM VIRS observations and (b) TMI brightness temperature at 19-GHz vertical polarization channel, at 0012 UTC 4 Nov (c) As in (b), except for 85.5-GHz PCT. Grid 2 (9-km spacing), covering the central Mediterranean. For the vertical dimension, we selected 30 unevenly spaced full-sigma levels. The simulations were initialized at 1200 UTC 3 November 2004 and lasted for 36 h. The ECMWF gridded analysis fields at 6-h intervals and at 0.5 latitude 0.5 longitude horizontal grid increment were the initial and boundary conditions. Figure 6a AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER

4 FIG. 5. Horizontal cross section of the TRMM PR reflectivity field at 3-km height, at 0012 UTC 4 Nov shows the 700-hPa relative humidity field provided by MM5's Grid 2, which is valid at 0000 UTC 4 November 2004 (t + 12). The model is able to reproduce the high relative humidity area around the low-pressure center, associated with the rainbands depicted over the same area by VIRS and TMI observations (Fig. 4). The mode also reproduces a band of high relative humidity over the area of maximum lightning northeast from the low-pressure center (Fig. 3) as well as an area of low relative humidity coinciding with the rain- and cloud-free area between the low center and the major band in the northeast, depicted in the VIRS imagery (Fig. 4a) and the PR horizontal cross section (Fig. 5). Figure 6b shows the MM5 Grid 2 column-integrated ice content (also at 0000 UTC), where significant ice concentrations can be depicted and again are in good agreement with the low 85.5-GHz PCT values (Fig. 4c) and the significant lightning (Fig. 3). A vertical cross section along 36 N latitude (Fig. 6c) shows that across the area of frequent lightning, the model reproduces high relative humidity and significant graupel and ice concentrations aloft. More precisely, the narrow vertical stripe of high ice + graupel mixing ratio from 700 to 450 hpa is mainly dominated by the presence of graupel, while the upper part is mainly ice. The model also reproduces strong updrafts (not shown) in the same area. Along with the high concentration of graupel in the lower part and of ice in the upper part of the cloud, these updrafts create the necessary conditions for charge separation within the cloud, thus favoring the production of the observed lightning. CONCLUSIONS. During fall and winter, midlatitude low-pressure systems form over the warm Mediterranean waters, and during their evolution can affect land areas, especially isolated islands in the open sea, such as Crete in southern Greece. This study showed how the synergistic use of various spaceborne (low-orbiting satellites) and groundbased instruments (lightning-detection networks) could be particularly useful for the observation of such midlatitude weather systems. In addition, these datasets are important for the validation of highresolution model results, as well as for developing forecaster confidence in utilizing model output in operational forecast procedures for Mediterranean storms. Finally, brightness temperature and also precipitation estimates can be used operationally in the assimilation procedure applied in regional models. This application is of great importance, especially with the advent of the Global Precipitation Mission, which in the beginning of the next decade may hopefully provide spaceborne precipitation measurements at higher spatial and temporal resolution than those currently available. ACKNOWLEDGMENTS. This work has been supported by the Greek Non-EU Countries Cooperation Program, financed by the Greek General Secretariat for Research and Technology. The authors acknowledge Dr. C. Adamo (ISAC/CNR, Italy) for her help on the use of TRMM data (through her participation to the Greek Italian Cooperation Programme, funded by the Greek General Secretariat for Research and Technology). Finally, D. Katsanos (NOA/IERSD) is acknowledged for his help for producing TRMM figures, as is NASA for the provision of QuikSCAT and TRMM data. FOR FURTHER READING Anagnostou, E. N., T. Chronis, and D. P. Lalas, 2002: New receiver network advances long-range lightning monitoring. Eos Trans. Amer. Geophys. Union, 83, Dudhia, J., 1993: A non-hydrostatic version of the Penn State/NCAR mesoscale model: Validation tests and simulation of an Atlantic cyclone and cold front. Mon. Wea. Rev., 121, Holt, M. A., P. J. Hardaker, and G. P. McClelland, 2001: Lightning climatology for Europe and the UK, Weather, 56, SEPTEMBER 2007

5 Kain, J. S., and J. M. Fritsch, 1993: Convective parameterization for mesoscale models: The Kain Fritsch scheme. The Representation of Cumulus in Convection in Numerical Models, Meteor. Monogr. No. 46, Amer. Meteor. Soc., Katsanos, D., K. Lagouvardos, V. Kotroni, and A. Argiriou, 2007a: Combined analysis of rainfall and lightning data produced by mesoscale systems in the Central and Eastern Mediterranean. Atmos. Res., 83, ,,, and, 2007b: Relationship of lightning activity with microwave brightness temperature and spaceborne radar reflectivity profiles in the central and eastern Mediterranean. J. Appl. Meteor., submitted. Kotroni, V., and K. Lagouvardos, 2001: Precipitation forecast skill of different convective parameterization and microphysical schemes: Application for the cold season over Greece. Geophys. Res. Lett., 28, Kummerow, C., W. Barnes, T. Kozu, J. Shiue, and J. Simpson, 1998: The Tropical Rainfall Measuring Mission (TRMM) sensor package. J. Atmos. Oceanic Tech., 15, Schultz, P., 1995: An explicit cloud physics parameterization for operational numerical weather prediction. Mon. Wea. Rev., 123, Spencer, R., H. M. Goodman, and R. E. Hood, 1989: Precipitation retrieval over land and ocean with the SSM/I: Identification and characteristics of the scattering signal. J. Atmos. Oceanic Tech., 6, Toracinta, E. R., and E. J. Zipser, 2001: Lightning and SSM/I-ice-scattering mesoscale convective systems in the global Tropics. J. Appl. Meteor., 40, , D. J. Cecil, E. J. Zipser, and S. W. Nesbitt, 2002: Radar, passive microwave, and lightning characteristics of precipitating systems in the Tropics. Mon. Wea. Rev., 130, Uccellini, L. W., 1990: Processes contributing to the rapid development of extratropical cyclones. Extratropical Cyclones, The Erik Palmen Memorial Volume, C. W. Newton and E. O. Holopainen, Eds., Amer. Meteor. Soc., FIG. 6. (a) MM5 Grid 2 map of 700-hPa relative humidity field (at 10% interval, only values exceeding 40% are shown) at 0000 UTC 4 Nov (b) As in (a), except for column-integrated ice mixing ratio (shaded contours at 0.3 g kg 1 interval). (c) Vertical cross section of graupel and ice mixing ratio (shaded contours at 0.1 g kg 1 intervals) and of relative humidity (solid lines at 10% intervals) along the white line shown in Fig. 6a. AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER