TRMM and Lightning Observations of a Low-Pressure System Over the Eastern Mediterranean by K

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TRMM and Lightning Observations of a Low-Pressure System over the Eastern Mediterranean BY K. LAGOUVARDOS AND V. KOTRONI

^ ignificant cyclone activity occurs in the Mediter- \ ranean area, mainly during the cold season. As J 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 low- pressure system moved northeastward, producing significant lightning over the sea, while on 5 Novem- ber, 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 la 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 cut- FIG. I. (a) ECMWF analysis of mean sea level pressure off low at 500 hPa (Fig. lb), while a strong upper-level (solid lines at 3-hPa interval) valid at 0000 UTC 4 Nov 2004. (b) As in (a), except for the 500-hPa geopotential jet streak exceeding 50 m s_1 was also evident at 300 hPa height (solid lines at 40-m interval) and the 300-hPa (blue shading in Fig. lb). The area north from the Gulf wind speed (shaded contours at 10 m s~' interval, only of Sidra and west of Crete was under the left-hand exit values exceeding 30 m s_l are shown). region of the jet streak, an area associated with significant divergence at the higher tropospheric layers. Data from NASA's Quick Scatterometer (QuikSCAT) at approximately 25-km horizontal resolution (avail- AFFILIATIONS: LAGOUVARDOS AND KOTRONI—National Observa- able online at http://podaac.jpl.nasa.gov/quikscat) al- tory of Athens, Institute of Environmental Research and Sustain- lows inspection of the surface wind field. The significant able Development, Athens, Greece sea level pressure gradient at about 0400 UTC 4 Novem- CORRESPONDING AUTHOR: Dr. K. Lagouvardos, National Observatory of Athens, Institute of Environmental Research ber, the time of the satellite passage over the area, was and Sustainable Development, Lofos Koufou, R Penteli, 15236, associated with strong surface winds (see Fig. 2). Figure -1 Athens, Greece 2 shows wind speeds exceeding 17 m s around the low

E-mail: [email protected] center, while very strong easterly winds prevail over the maritime area northeast of the low center. DOI: 10.1175/BAMS-88-9-I363 Figure 3 shows the cloud-to-ground lightning ©2007 American Meteorological Society with data provided by the ZEUS lightning-detection

BAflS* | 1363 AMERICAN METEOROLOGICAL SOCIETY UnauthenticatedSEPTEMBE | DownloadedR 2007 10/01/21 02:07 AM UTC 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 0000- 0100,0600-0700,1200-1300, and 1800-1900 UTC. Indeed, there is significant lightning activity from 0000 to 0600 4 November, progressing slowly eastward. The lightning activity is weaker near FIG. 2. The 10-m wind field provided by QuikSCAT (one barb: 5 M s~'; one 1 the low-pressure center com- half-barb: 2.5 m s ), valid at -0400 UTC 4 Nov 2004. Rain-contaminated QuikSCAT winds have been removed. pared with the maritime area to the northeast, where QuikSCAT shows convergence (Fig. 2). During 1200-1800 UTC Figure 4a presents the VIRS infrared image, as 4 November, the lightning activity decreases while it well as TMI brightness temperature observations progresses farther east. at 19- (vertical polarization, Fig. 4b) and 85.5-GHz The Tropical Rainfall Measurement Mission polarization-corrected temperature (PCT; Fig. 4c) (TRMM) satellite provides additional collocated from the TRMM passage over the study area at 0012 spaceborne observations. TRMM is a low-orbit sat- UTC 4 November 2004. (PCT is defined by Spencer ellite (flying at -400 km since mid-August 2001) on et al. in the April 1989 Journal of Atmospheric and a tropical path that covers (with some of its instru- Oceanic Technology.) VIRS imagery (Fig. 4a) reveals ments) a belt between 38°N and 38°S, providing very the banded structure around the low-pressure center, useful observations (e.g., brightness temperatures, as well as an area with significant convection west of radar reflectivities) from which microphysical char- Crete (35°-36°N, 20°E), while a cloud-free area is evi- acteristics of weather systems over the southern part dent over the maritime area between the low-pressure of the Mediterranean basin can be inferred. center and the convective area to the east. TRMM carries the following three main instru- Images at 19 GHz are extremely valuable for rain- ments: fall mapping, because this frequency is much less susceptible to ice-scattering effects than the higher • Visible and Infrared Scanner (VIRS): A five-chan- frequencies (e.g., 37 and 85.5 GHz) and can be used nel, cross-track-scanning radiometer operating at 0.63,1.6,3.75,10.8, and 12 ^m (the radiances measured by VIRS can be used to infer cloud coverage, cloud type, and cloud-top temperatures); • TRMM Microwave Imager (TMI): A multichan- nel passive microwave radiometer operating at five frequencies (10.65, 19.35, 37.0, and 85.5 GHz at dual polarization, and 22.235 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 FIG. 3. Lightning activity as sensed by ZEUS lightning ocean, from which information about the vertical detection network during l-h periods beginning at structure of precipitation systems is obtained. 0000, 0600, 1200, and 1800 UTC 4 Nov 2004.

1364 I BAflfr SEPTEMBER 2007 Unauthenticated | Downloaded 10/01/21 02:07 AM UTC 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 x 4 km, which is much finer than the -30 km x 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 150-160 K in the area around 35°-36°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 Cen- ter for Atmospheric Research (NCAR) Mesoscale FIG. 4. (a) TRMM VIRS observations and (b) TMI bright- Model (MM5) (version 3.5). MM5 is a nonhydrostatic, ness temperature at 19-GHz vertical polarization channel, at 0012 4 Nov 2004. (c) As in (b), except primitive-equation model using terrain-following UTC for 85.5-GHz PCT. coordinates. We selected parameterizations based on our previous comparative study of convective and microphysical schemes (in Geophysical Research Grid 2 (9-km spacing), covering the central Mediter- Letters, 28,1977-1980), the Kain-Fritsch scheme for ranean. For the vertical dimension, we selected 30 the convective parameterization, and the scheme pro- unevenly spaced full-sigma levels. posed by P. Schultz for the explicit microphysics. The simulations were initialized at 1200 UTC 3 For this study, we defined the following two one- November 2004 and lasted for 36 h. The ECMWF way nested grids as such: Grid 1 (with 36-km spac- gridded analysis fields at 6-h intervals and at 0.5° ing), covering the major part of southern Europe, the latitude x 0.5° longitude horizontal grid increment Mediterranean, and the northern African coasts; and were the initial and boundary conditions. Figure 6a

BAflS* | 1365 AMERICAN METEOROLOGICAL SOCIETY UnauthenticatedSEPTEMBE | DownloadedR 2007 10/01/21 02:07 AM UTC 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 ground- based 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 high- resolution model results, as well as for developing forecaster confidence in utilizing model output in FIG. 5. Horizontal cross section of the TRMM PR reflec- operational forecast procedures for Mediterranean tivity field at 3-km height, at 0012 UTC 4 Nov 2004. storms. Finally, brightness temperature and also precipitation estimates can be used operationally in the assimilation procedure applied in regional models. shows the 700-hPa relative humidity field provided by This application is of great importance, especially MM5's Grid 2, which is valid at 0000 UTC 4 Novem- with the advent of the Global Precipitation Mission, ber 2004 (t + 12). The model is able to reproduce the which in the beginning of the next decade may hope- high-relative humidity area around the low-pressure fully provide spaceborne precipitation measurements center, associated with the rainbands depicted over at higher spatial and temporal resolution than those the same area by VIRS and TMI observations (Fig. currently available. 4). The mode also reproduces a band of high relative humidity over the area of maximum lightning ACKNOWLEDGMENTS. This work has been sup- northeast from the low-pressure center (Fig. 3) as well ported by the Greek Non-EU Countries Cooperation as an area of low relative humidity coinciding with Program, financed by the Greek General Secretariat for the rain- and cloud-free area between the low center Research and Technology. The authors acknowledge Dr. and the major band in the northeast, depicted in the C. Adamo (ISAC/CNR, Italy) for her help on the use of VIRS imagery (Fig. 4a) and the PR horizontal cross TRMM data (through her participation to the Greek-Ital- section (Fig. 5). ian Cooperation Programme, funded by the Greek Gen- Figure 6b shows the MM5 Grid 2 column-integrat- eral Secretariat for Research and Technology). Finally, D. ed ice content (also at 0000 UTC), where significant Katsanos (NOA/IERSD) is acknowledged for his help for ice concentrations can be depicted and again are in producing TRMM figures, as is NASA for the provision of good agreement with the low 85.5-GHz PCT values QuikSCAT and TRMM data. (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 FOR FURTHER READING graupel and ice concentrations aloft. More precisely, Anagnostou, E. N., T. Chronis, and D. P. Lalas, 2002: the narrow vertical stripe of high ice + graupel mix- New receiver network advances long-range lightning ing ratio from 700 to 450 hPa is mainly dominated monitoring. Eos Trans. Amer. Geophys. Union, 83, by the presence of graupel, while the upper part is 594-595. mainly ice. The model also reproduces strong up- Dudhia, J., 1993: A non-hydrostatic version of the Penn drafts (not shown) in the same area. Along with the State/NCAR mesoscale model: Validation tests and high concentration of graupel in the lower part and simulation of an Atlantic cyclone and cold front. of ice in the upper part of the cloud, these updrafts Mon. Wea. Rev., 121, 1493-1513. create the necessary conditions for charge separation Holt, M. A., P. J. Hardaker, and G. P. McClelland, 2001: within the cloud, thus favoring the production of the Lightning climatology for Europe and the UK, observed lightning. 1990-99. Weather, 56, 290-296.

1366 I BAflfr SEPTEMBER 2007 Unauthenticated | Downloaded 10/01/21 02:07 AM UTC Kain, J. S., and J. M. Fritsch, 1993: Convective param- Katsanos, D., K. Lagouvardos, V. Kotroni, and A. Ar- eterization for mesoscale models: The Kain-Fritsch giriou, 2007a: Combined analysis of rainfall and scheme. The Representation of Cumulus in Convec- lightning data produced by mesoscale systems in tion in Numerical Models, Meteor. Monogr. No. 46, the Central and Eastern Mediterranean. Atmos. Amer. Meteor. Soc., 165-177. Res., 83, 55-63. , , , and , 2007b: Relationship of lightning activity with microwave brightness temperature and spaceborne radar reflectivity profiles in the central and eastern Mediterranean. /. 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, 1977-1980. Kummerow, C., W. Barnes, T. Kozu, J. Shiue, and }. Simpson, 1998: The Tropical Rainfall Measuring Mission (TRMM) sensor package. J. Atmos. Oceanic Tech., 15, 809-817. Schultz, P., 1995: An explicit cloud physics parameterization for operational numerical weather prediction. Mon. Wea. Rev., 123, 3331-3343. 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. /. Atmos. Oceanic Tech., 6, 254-273. Toracinta, E. R., and E. J. Zipser, 2001: Lightning and SSM/I-ice-scattering mesoscale convective systems in the global Tropics. /. Appl. Meteor., 40, 983-1002. , 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, 802-824. Uccellini, L. W., 1990: Processes contributing to the rapid development of extratropical cyclones. Extra- tropical Cyclones, The Erik Palmen Memorial Volume, C. W. Newton and E. O. Holopainen, Eds., Amer. Meteor. Soc., 81-105.

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 2004. (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.

BAflS* | 1367 AMERICAN METEOROLOGICAL SOCIETY UnauthenticatedSEPTEMBE | DownloadedR 2007 10/01/21 02:07 AM UTC Radar and Atmospheric Science: A Collection of Essays in Honor of David Atlas

Edited by Roger M. Wakimoto and Ramesh Srivastava

This monograph pays tribute to one of the leading scientists in meteorology, Dr. David Atlas. In addition to profiling the life and work of the acknowledged "Father of Radar Meteorology," this collection highlights many of the unique contributions he made to the understanding of the forcing and organization of convective systems, observation and modeling of atmospheric turbulence and waves, and cloud microphysical properties, among many other topics. It is hoped that this text will inspire the next generation of radar meteorologists, provide an excellent resource for scientists and educators, and serve as a historical record of the gathering of scholarly contributions honoring one of the most important meteorologists of our time.

KADAK AND AI MOSI'HtKIC SCItNCfc A COLLECTION OF ESSAYS IN HONOR OF DAVID AT1.AS

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