reprinted from: Proceedings, EUMETSAT
Meteorological Satellite Data Users' Conference, 16 - 20 September
1996, Vienna, Austria, pp. __.
Spatial and temporal characteristics of convective storm tops in the 3.7/3.9 micron spectral bands are examined from meteorological satellite imagery. Previous observations from Europe, based on NOAA/AVHRR data, have shown that some of the storms exhibit a significant increase of cloud top 3.7 micron reflectivity. Recent observations have shown that similar cloud top phenomena exist in the NOAA/AVHRR data for convective storms over the U.S. Great Plains. Launch of the GOES-8 and GOES-9 geostationary satellites has enabled a study of development of storm cloud top structures in a similar band (3.9 micron) with high temporal resolution. This imagery shows that the smallest areas of increased 3.9 micron reflectivity appear and fade on the scale of few minutes, while the larger ones may persist for tens of minutes to several hours.
Selected cases of convective storms exhibiting an increase in the 3.9 micron reflectivity have been studied with respect to internal storm structure as observed by NEXRAD Doppler radars. This has revealed that majority of spots of increased 3.9 micron reflectivity can be found above no significant (or weak) radar echo region, though close to storm's core. However, some of these spots have appeared above a mesocyclone at the time (or very close to it) of associated tornado touchdown, which suggests that these spots might be of different origin. One case of high 3.9 micron reflectivity over entire storm tops has been recorded on 22 May 1996 simultaneously from GOES-8 and GOES-9. Some aspects of bidirectional scattering are evident from the differences in reflectivity observed from these two satellites.
1. INTRODUCTION
The GOES-8/9 channel 2 (3.78 - 4.03 micron) and the NOAA/AVHRR channel 3 (3.55 - 3.93 micron), all located in a spectral band that includes both emitted and reflected solar radiation during the daytime, represent a unique observational tool for studies of convective storms' cloud top microphysics. Given the very low temperatures of anvil tops, the emitted component in these channels is almost negligible when observing storm tops. Hence, the reflected component plays a major role in appearance of convective storms in these channels in daytime hours (Setvak, 1989).
Earlier NOAA/AVHRR observations of convective storms over Europe have shown that some of these exhibit a significant increase of the 3.7 micron cloud top reflectivity and that the observed features fall into two broad classes (Setvak and Doswell, 1991):
Present work extends the research to U.S. Great Plains storms, which offers several significant advantages. First, comparison of the U.S. and European storms can generalize the observations from Europe (and inferences made from these). Second, the broader range of observational tools available in the U.S. can help link the observed cloud top features to storm internal processes and to accompanying weather phenomena. Finally, introduction of the GOES-8/9 satellites (Menzel and Purdom, 1994) enables determination of the evolution of these cloud top phenomena with temporal sampling from 15 to 1 minute.
2. DATA SOURCES AND PROCESSING
2.1. NOAA/AVHRR
The AVHRR/HRPT data from NOAA-11 and -12 for 1994, NOAA-12 and -14 for 1995 and 1996 have been obtained from the NOAA/NESDIS Satellite Active Archive in level-1B format. The AVHRR data sets were processed (calibrated and georeferenced) by software written at the Czech Hydrometeorological Institute (CHMI) for MS-DOS platforms. Daytime data from the AVHRR channel 3 have been converted into 3.7 micron reflectivity by an algorithm developed at CHMI (Setvak and Doswell, 1991). An advantage of using 3.7 micron reflectivity instead of the more commonly used channel 3/4 brightness temperature difference as a characteristic of a cloud top is that the reflectivity is normalized by incident solar radiation, which is not true for the ch3/ch4 difference.
2.2. GOES-8
Most of the GOES-8/9 imagery were obtained from the NOAA/NESDIS archive. In addition, data was supplied from the NESDIS Regional and Mesoscale Meteorological Branch (RAMM) and from the NCAR Cooperative Program for Operational Meteorology, Education, and Training (COMET). Data were processed using the University of Wisconsin Man Computer Interactive Data System (McIDAS).
Daytime reflectivity at 3.9 micron (GOES-8/9 channel 2) was computed from measured radiance at 3.9 micron and 11 micron (channel 4) following the same methodology used for the AVHRR data. Visible (channel 1) data have been used for verification of optical thickness of the anvils to exclude "false spots" where gaps in cloud top might appear.
2.3. OTHER DATA
Radar data (WSR-88d) were obtained from the National Climatic Data Center (NCDC) archive for individual NEXRAD radar sites which recorded data in level-II format. Reflectivity and radial velocity data were displayed using the Radar and Algorithm Display System (RADS) developed at the NSSL.
Supplementary surface and rawinsonde observations obtained during the 1995 VORTEX field experiment in the southern U.S. Plains (see Rasmussen et al.1994) augmented routine meteorological data during this period. Surface observations of hail, high winds, and tornados were obtained from the log of severe weather maintained by the NOAA Storm Prediction Center and displayed using "Severe Plot" software on a PC.
3.OBSERVATIONS
3.1. NOAA/AVHRR OBSERVATIONS
Examination of about 30 AVHRR data sets from 1994 and 1995 has shown that 3.7 micron features, observed for European storms, can be found also over some of the U.S. Great Plains convective storms. Though the number of investigated cases is much smaller to date compared to the European observations, preliminary results indicate significantly higher frequency of plumes than in Europe, though majority of these is detectable rather in visible and near infrared channels only (showing no increase of the 3.7 micron reflectivity). However, this may result from the fact that most of these plumes were found on early evening NOAA 12 images when low elevation of the Sun does not provide enough 3.7 micron radiance to illuminate the scene sufficiently. Nevertheless, in general these observations have proved similar appearance of storm cloud tops in the AVHRR 3.7 micron channel for both continents.
3.2. COMPARISON OF THE AVHRR AND GOES 8 DATA CALIBRATION
As a first step, brief cross-calibration tests have been performed for processed AVHRR/HRPT NOAA-14 and GOES-8 data. Comparison of thermal IR channels (AVHRR channel 4 and GOES-8 channel 4) has shown close agreement between these. The brightness temperatures from both instruments are within about a half degree Kelvin at both ends of temperature range (warm ground surface and cold tops of storms). This is in much better agreement than found previously from AVHRR/NOAA versus Meteosat cross-calibration tests (Levizzani et al, 1992) where the differences were as high as 7 K in the low temperature range (~ 200-220 K) due to resolution differences and calibration uncertainty at low temperatures on Meteosat.
Comparison of the 3.7/3.9 micron reflectivities of various objects shows significant differences between the two satellites. Highest differences were observed for storm cloud tops, while for surface targets the differences were somewhat smaller. These differences likely result from variations in viewing geometry and relative solar angle, although differing resolution and center wavelength of these channels can also be factors. This subject is planned for future study utilizing simultaneous observations from GOES-8 and GOES-9 to provide some insight on magnitude of bidirectional scatter effects at cloud tops.
3.3. STORM CLOUD TOPS IN THE 3.9 MICRON GOES-8 CHANNEL 2
Seven GOES-8 data sequences, showing severe storm development, have been examined for the presence of spots or plumes of increased 3.9 micron cloud top reflectivity. As expected, spots or larger areas of increased 3.9 micron reflectivity were found over some of these storms; however, not so clearly as in AVHRR images for the same time periods. Since most of the investigated cases involved storms that developed on VORTEX operations days, ground observations were available for some of these storms. Based on these observations, one of the first results of this study was rejection of speculations based on earlier European observations, which indicated a possible direct link between hail and increased 3.7 micron reflectivity (Setvak, 1989). Some of the storms which are known to have produced significant hail on VORTEX days, have shown NO significant increase of 3.9 micron reflectivity at all. However, many of the storms did produce spots of varying size, persistence and magnitude. Life time of these spots ranged from few minutes (as determined from 1 minute data scans) to about two hours. The size of these varied from one single GOES-8 channel 2 pixel (4x4km) up to about 20 - 30 km across. Highest recorded 3.9 micron reflectivity was about 0.20, while typical "background" of the anvils was about 0.02 - 0.04 (all at brightness temperatures below 215K). No direct link between the 3.9 micron reflectivity and brightness temperature minima was found, which indicates that overshooting tops can be excluded as preferred areas where the spots of increased 3.9 micron reflectivity occur.
3.4. COMPARISON OF THE 3.9 MICRON OBSERVATIONS WITH NEXRAD DATA
Given the observation that spots of increased 3.9 micron reflectivity do not develop at any specific location with respect to brightness temperature field and therefore are not linked exclusively to overshooting tops, an attempt to determine their possible sources was performed using NEXRAD radar reflectivity and radial velocity data. Though only four data sets (07 May 1995, 23 May 1995, 02 June 1995 and 08 June 1995) have been examined so far (combining radar and 3.9 micron data), it is obvious now that number of categories of increased 3.9 micron reflectivity is much broader than was suspected before.
Most of the smaller scale spots (up to a few GOES 8 channel 2 pixels) appear above areas with relatively weak radar reflectivity, lasting from a few minutes up to somewhat less than one hour. In cases when spots appear above storms organized in lines, they typically develop on westward side of a ridge of overshooting tops and later drift westward (storm-relative) into the "stratiform" part of anvil. Nevertheless, even such spots may have a well pronounced "core" from which the material seems to spread out into the surrounding area. The generating mechanism of such spots remains uncertain.
Another category of spots appears to be linked to mesocyclones. For example, on 07 May 1995 a spot of higher 3.9 micron reflectivity (between 0.78 to 0.87, over "background" values at about 0.35 to 0.40) appeared above a mesocyclone at 2145 UTC, which was also the time of first touchdown of an associated tornado. Since the previous 3.9 micron image that is available from 2130 UTC shows no trace of this spot, an uncertainty up to 15 minutes remains for the time of its onset. Nevertheless, the spot persisted in the anvil for next almost 2 hours, disappearing after 2330 UTC. In this case, the spot began instantly to drift away from its "parent" cell as determined by radar observations. Three other cases were recorded when a spot of increased 3.9 micron reflectivity is clearly linked to underlying mesocyclone, however the behaviour and appearence of spots differed from case to case.
Thus, on the basis of satellite imagery alone it is impossible to distinguish between different types of spots of increased 3.9 micron reflectivity. However, it should be stressed that examination of a significantly larger sample of cases is required to determine the fraction of 3.9 micron spots associated with strong updrafts and/or rotating features.
There might be a possible link between 3.9 micron spots and "stratospheric" cirrus as observed from aircraft flying at anvil top levels (Fujita, 1982). This cirrus, "jumping up" above anvil tops downwind of overshooting towers as these collapse (Fujita's proposed explanation of stratospheric cirrus generation), is likely to be composed of smaller particles than was their original distribution within the anvil top (due to gravitational settling). Though (Fujita, 1982) has reported the stratospheric cirrus to extend at great distances from its source (25 km or more), one of authors of this paper (Setvak) has observed (on 24 May 1996, Alabama, Georgia) similar "jumping cirrus" to be a quite frequent phenomenon on somewhat smaller scale. Many storm tops on that day in that area displayed a "crest" of cirrus, with horizontal cross-section much smaller (about 1/5) than was the size of nearby overshooting tops. Persistence of these "crests" was few minutes and more (the upper limit was above the time of plane's fly-by). Location of these and their typical life time are close to those of some of the 3.9 micron spots. Perhaps some of the 3.9 micron spots could alternatively be attributed to pileus clouds.
3.5. PLUMES ABOVE STORM TOPS
So far the best pronounced plume of increased 3.7 micron reflectivity, found in the U.S. AVHRR data sets, is shown on the Figure 1. The plume exceeds well over the edge of storm's anvil, reaching highest reflectivity values of around 0.05 to 0.07, while anvil's mean "background" is around 0.03 - 0.04. Notice the almost "point like" source of this plume - highest 3.7 micron reflectivity within it reaches 0.106.
Figure 1. Convective storms over south-east Oklahoma, west Arkansas and north Texas on 26 April 1994, 22:50 UTC, NOAA-11. This image is a composite of the AVHRR channel 4 (shown in red) and channel 3 reflectivity (blue, cyan and green). The plume of increased 3.7 micron reflectivity (almost white) emanates from a weakening cell in middle of the image, while upper-level winds carry the particles of the plume from its source to north- east, well beyond the anvil margins.
Several plume like structures were observed from GOES-8, GOES-9 and NOAA-12 satellite data on 22-23 May 1996. By mid-afternoon, several isolated storms were developing in the High Plains. Two of these storms (northeastern Colorado and southwest Nebraska) had notably higher 3.9 micron reflectivity than other storms within the region and soon began to generate narrow, long plumes that could be seen in visible as well as in the 3.9 micron channel imagery. The 3.9 micron reflectivity was locally higher in the plumes than over the surrounding anvil cloud. This case also demonstrates high sensitivity of the 3.9 micron reflectivity on scattering conditions (Figure 2). The 3.9 micron reflectivity begins to be noticably higher about 2 hours before sunset for GOES-8 compared to GOES-9 (due to more favorable forward scattering angles), while earlier on that afternoon the storm top reflectivity was almost the same from both satellites.
Figure 2. Evolution of the 3.9 micron reflectivity of cloud tops of convective storms over northeastern Colorado and southwest Nebraska on 22 May 1996 (late afternoon - sunset), as observed from GOES-8 (top) and GOES-9 (bottom). Individual curves in both graphs represent the evolution of 3.9 micron reflectivity of the same object: top (yellow) lines depict the mean reflectivity of one of the plumes; middle (red) curve corresponds to anvil top of the storm that produced the plume, and the bottom (blue) line stands for a cloud top of another nearby storm. NOAA-12 3.7 micron reflectivity at 00:55 UTC was for these objects 0.08, 0.07 and 0.03 respectively (the storms were almost at nadir of the satellite).
4. SCATTERING COMPUTATIONS AND MICROPHYSICAL MODELING
The potential of theoretical and experimental studies on the optical properties of non-spherical atmospheric ice crystals of the last two decades is yet far from being fully exploited. Theoretical solutions and numerical computations of the scattering by arbitrarily oriented non-spherical particles were proposed, among others, by Liou (1972a,b) and Asano and Sato (1980) for ice cylinders and spheroidal particles, respectively. Satellite multispectral techniques for the analysis of cloud microphysics were developed: interesting examples are the works by Ou et al. (1993) and Rao et al. (1995) using either the 3.7 micron channel alone or its combination with the 10.9 micron one. Radiative transfer models were coupled to radiometric observations for the identification of cirrus and stratiform cloud optical properties, which are of great relevance for the radiation budget and global warming issues. We will only references the works of Stephens (1980) on cirrus cloud properties in the infrared, Stone et. al (1990) on thin cirrus clouds in the near infrared and infrared from satellite with simultaneous observations with lidar and lirad, and Kleespies (1995) on marine stratiform cloud in the 3.9 micron channel.
Note that little of the above mentioned activity has been devoted to convective storms cloud top studies. Scattering computations and radiative transfer theory represent at present the only available "probe" for the identification of the microphysical composition of the 3.7 micron channnel plumes. A radiative model of plumes is presently under construction for the simulation of the 3.7 micron channel response to varying sun-satellite-cloud geometries. Numerical modeling of the storm's structure is needed to investigate the dynamical and microphysical origin of the plumes and their evolution in time. The Wisconsin Dynamical Microphysical Model (WISC-DYMM) (Johnson et al., 1993, 1994) will be used, given its very detailed microphysical parameterization.
5. CONCLUSIONS
Preliminary studies have revealed a veritable "bestiary" of phenomena at the tops of deep convective clouds, including plumes and various types of spots of increased 3.7/3.9 micron reflectivity. These features do not seem to be tied to any particular form of severe weather. At this point, we do not have any definitive explanations for the observations, either. Any effort to understand the meaning of such signatures must include: 1) a study of the radiative transfer properties of storm tops, 2) a multisensor look at the storms that exhibit these features and those which do not, to look for clues about what distinguishes signature producing storms from those that do not, and 3) an accurate knowledge of the weather events the storms have produced.
ACKNOWLEDGEMENTS
This research was supported by the U.S./Czechoslovak Science and Technology Program, grant #94067, the Italian Space Agency (ASI) and the Italian National Council of Research under the Progetto Strategico for the Mesoscale Alpine Programme (MAP). The authors wish to thank Pavel Hampl from CHMI for his software support when processing AVHRR data, Julie Adolphson and Patrick Dills of UCAR/COMET, Jim Purdom and John Weaver OF NESDIS/RAMM for providing archived and real-time GOES-8 data.
Asano, S. , and M. Sato, 1980: Light scattering by randomly oriented spheroidal particles. Appl. Opt., 19, 962-974.
Fujita, T.T., 1982: Principles of stereographic height computations and their applications to stratospheric cirrus over severe thunderstorms. J. Meteor. Soc. of Japan, 60, 355-368.
Johnson, D.E., P.K. Wang, and J.M. Straka, 1993: Numerical simulations of the 2 August 1981 CCOPE supercell storm with and without ice microphysics. J. Appl. Meteor,, 32, 745-759.
______, ______, and ______ 1995: A study of microphysical processes in the 2 August 1981 CCOPE supercell storm. Atmos.Res., 33, 92-123.
Kleespies, T.J., 1995: The retrieval of marine stratiform cloud properties from multiple observations in the 3.9-micron window under conditions of varying solar illumination. J. Appl. Meteor., 34, 1512-1524.
Levizzani, V., M. Setvak, J. Kracmar, F. Porcu, and F. Prodi, 1992: Multisensor remote sensing analysis of deep convective storms' structure over continental Europe and Mediterranean. Proc., 11th Int. Conf. on Clouds and Precipitation, IAMAP - Montreal, 1075 - 1078.
______, and ______, 1996: Multispectral, high-resolution satellite observations of plumes on top of convective storms. J. Atmos. Sci., 53, 361-369.
Liou, K.-N., 1972a: Electromagnetic scattering by arbitrarily oriented ice cylinders. Appl. Opt., 11, 667-674.
______, 1972b: Light scattering by ice clouds in the visible and infrared: A theoretical study. J. Atmos. Sci., 29, 524-536.
Menzel, W.P., and J.F.W. Purdom, 1994: Introducing GOES-I: the first of new generation of Geostationary Operational Environmental Satellites. Bull. Amer. Meteor. Soc., 75, 757-781.
Ou, S.C., K.-N. Liou, W.M. Gooch, and Y. Takano, 1993: Remote sensing of cirrus cloud parameters using advanced very-high-resolution radiometer 3.7- and 10.9-micron channels. Appl. Opt., 32, 2171-2180.
Rao, N.X., S.C. Ou and K.N. Liou , 1995: Removal of the solar component in AVHRR 3.7-micron radiances for the retrieval of cirrus cloud parameters. J. Appl. Meteor., 34, 482-499.
Rasmussen, E.N., J.M. Straka, R. Davies-Jones, C.A. Doswell III, F.H. Carr, M.D. Eilts, and D.R. MacGorman, 1994: Verification of the Origins of Rotation in Tornadoes EXperiment (VORTEX). Bull. Amer. Meteor. Soc., 75, 995-1006.
Setvak, M., 1989: Convective storms - the AVHRR channel 3 cloud top reflectivity as a consequence of internal processes. Proc., 5th WMO Conf. on Weather Modification and Applied Cloud Physics, Beijing, WMO TD - No.269, 109-112.
______ and C.A. Doswell III, 1991: The AVHRR channel 3 cloud top reflectivity of convective storms. Mon. Wea. Rev., 119, 841-847.
Stephens, G.L., 1980: Radiative properties of cirrus clouds in the infrared region. J. Atmos. Sci., 37, 435-446.
Stone, R.S., G.L. Stephens, C.M.R. Platt , and S. Banks, 1990: The remote sensing of thin cirrus cloud using satellites, lidar and radiative transfer theory. J. Appl. Meteor., 29, 353-366.