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, space- borne platforms are especially useful for observing these systems. Moreover, during the cold season, lightning usually occurs over the relatively warm sur- face 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 signifi- cant 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 mea- sured 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 scan- ning 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 sys- tem 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.