Global thunderstorm tracking via measurements of radio atmospherics ...
Global thunderstorm tracking via measurements of radio atmospherics at Palmer Station U.S. INAN and S.C. REISING, Space, Telecommunications, and Radioscience Laboratory, Stanford University, Stanford,
California 94305
at two distant sites are then used to determine the source location via spherical triangulation. This capability was demonstrated recently with recordings made at Stanford, California, and at Palmer Station. The recordings allowed the measurement of the location of individual lightning discharges in the Gulf of Mexico with an accuracy to less than 100 km (Reising et al. 1993). In this article, we illustrate two examples of the use of the VLF Fourier goniometry technique with Palmer data. In one case, illustrated in figure 1, we compare results with SSM/I data from the DMSP satellite for 30 March 1989. SSM/I data are a composite of several microwave radiometer frequencies and polarizations, optimized to locate and estimate rainfall. Figure 1A shows the areas (marked A and B) of cold cloud tops detected over South America, along with the bearings of great-circle paths arriving at Palmer Station, Antarctica. Fig-
tanford University has maintained an extremely-low-freS quency/very-low-frequency (ELF/VLF) observation program at Palmer Station for many years, primarily for the purpose of measuring electromagnetic waves from the Earth's magnetosphere, especially the type known as "whistlers," which originate in lightning discharges in the Northern Hemisphere (Burgess and man 1993, and references therein). The VLF measurements at Palmer Station are conducted using highly sensitive VLF antennas and receivers. The relatively quiet (electromagnetically speaking) nature of the site, its geomagnetic conjugacy to major thunderstorm centers in the east coast of the United States, and its proximity to the South Atlantic magnetic anomaly make Palmer Station a unique site for these studies. During the past 2 years, an additional potential use of the Palmer VLF data has emerged. It now appears that thunderstorm activity on a global scale can be tracked with the help of VLF measurements at Palmer Station. For this purpose, researchers use directional wideband measurements of the socalled radio atmospherics, impulsive electromagnetic signals, which originate in lightning discharges and which propagate to very large distances along the surface of the Earth within the Earth-ionosphere waveguide. Wideband direction-finding techniques developed at Stanford for whistler measurements are directly applicable for this purpose, as illustrated below. Tracking lightning activity on a global scale is important in view of the possible connections between frequency of lightning occurrence (i.e., flash rate) and convective rain flux (Workman and Reynolds 1949; Goodman 1990). Measurement of the rainfall over the oceans, particularly over the Pacific Ocean, is of critical importance in determining the variability of global circulations and the short-term changes in climate and, thus, constitutes a primary objective of the Climate and Global Change Program. Global lightning activity provides one signature of tropical moisture and energetics (Goodman and Christian 1993), and quantitative measurement of lightning on a global scale is needed as an input and a verification of global climate change models. The new method of lightning location is known as VLF Fourier goniometry and was developed at Stanford as part of a recently completed Ph.D. dissertation (Burgess 1993). The method relies on the measurement of the arrival direction of radio atmospherics (spherics) from distant [more than 1,000kilometer (km)] source regions. The arrival direction is measured using two magnetic loop antennas, one oriented northsouth and one oriented east-west. Digital wideband recordings of the data from two antennas are postprocessed to determine arrival azimuth with approximately 10 accuracy (Reising et al. in preparation). Simultaneous recordings made
Satellite Microwave Radiometry Data (SSM/I image) indicates active storm centers A and B. .' 0' 10' 20'
DJ
Spheric histogram for PA. 30 March 1989 0701-0730 UT
I
Ii (b)
-80-60 -40 -20 0 20 40 60 80 Magnitude Arrival Azimuth (deg)
Figure 1. Comparison of SSM/I image sensing cold cloud tops indicative of rainfall with VLF spheric detection and direction-finding data. A. Locations of two storms observed on 30 March 1989, with great-circle bearings arriving at Palmer Station, Antarctica. B. Histogram of Palmer data for 30 March 1989, with respect to arrival azimuth and magnitude (uncalibrated) of received spherics. (UT denotes universal time. deg denotes degree.)
ANTARCTIC JOURNAL 319
- REVIEW 1994
ure lB shows spheric measurements at Palmer, presented in the form of a two-dimensional histogram of a number of spherics observed in a given period sorted in terms of intensity and arrival azimuth. We note that two peaks are clearly observed at 00 and 100 bearing corresponding to the two storms marked A and B. In addition, we note that other storm centers are evident (e.g., one near approximately 85 0 and another near approximately 550 azimuth) that are active at the same time. Figure 2 illustrates another application of the Fourier goniometry method to assess the location of active thunderstorm centers at a time when an intense gamma-ray burst ("flash") was observed to emerge from the Earth's upper atmosphere (Fishman et al. 1994). One of about 12 such cases observed on the Compton Gamma Ray Observatory (CGRO), the point of origin of the particular flash observed on 20 May 1993 was deduced to be the point F shown in figure 2 (Fishman personal communication). In their paper published
kHz 0
4.0
-10
-20 2.0
-30 0.0 0 1 4 6 0,OdB At 31
Time (sec)
recently in Science, Fishman et al. (1994) indicated that the observed gamma-ray bursts were probably caused by lightning discharges. The VLF Fourier goniometry method, when applied to the wideband VLF data recorded at Palmer Station during this period, clearly reveals a very active storm center as shown in figure 2C, strongly corroborating the association of the observed gamma-ray flash with lightning. The arrival azimuth of the center of this storm center is within approximately 0.50 of the great-circle bearing between point F in figure 2B and Palmer Station. Figure 2A shows an isolated and very intense spheric recorded at Palmer within 200 milliseconds of the observed gamma-ray flash (Fishman et al. 1994). It is interesting to note that the arrival bearing at Palmer of this intense spheric is 35.5 0 west of north, again within approximately 0.50 of the bearing of the CGRO-deduced location of the gamma-ray flash (i.e., point F). The above cases simply represent examples in which the location of lightning activity was known by other measurements and was accurately confirmed by the VLF Fourier goniometry method. In the coming year, Palmer VLF measurements will be carried out in coordination with those at other sites within the United States, and combined data sets will be used to track thunderstorms in oceanic regions, where other data on lightning location are only sparsely available. We acknowledge the enthusiastic support of our equipment by John Gress of Antarctic Support Associates and Antarctic Support Associates Science Technician at Palmer Station, John Booth. We are grateful to W. Burgess for his development of the VLF Fourier goniometry method and to D. Hakeman for his help in developing the spheric identification software. This work is supported under National Science Foundation grant OPP 90-20687 to Stanford University.
References Burgess, W.C. 1993. Lightning-induced coupling of the radiation belts to geomagnetically conjugate ionospheric regions. (Ph.D. Thesis, Stanford University Department of Electrical Engineering, Stanford, California). Burgess, W.C., and U.S. man. 1993. The role of ducted whistlers in the precipitation loss and equilibrium flux of radiation belt electrons. Journal of Geophysical Research, 98(A9), 15643. Fishman, G.J. Personal communication. Fishman, G.J., P.N. Bhat, R. Mallozzi, J.M. Horack, T. Koshut, C. Kouveliotou, G.N. Pendleton, C.A. Meegan, R.B. Wilson, W.S. Paciesas, S.J. Goodman, and H.J. Christian. 1994. Discovery of intense gamma-ray flashes of atmospheric origin. Science, 264, 1225.
Spheric histogram for Palmer 20 May 1993 0150-0220 UT
Goodman, S.J. 1990. Predicting thunderstorm evolution using groundbased lightning detection networks (National Aeronautics and
2%8
Space Administration technical memo TM-103521). Goodman, S.J., and H.J. Christian. 1993. Global observations of lightning. In Gurney, Foster, and Parkinson (Eds.), Atlas of satellite observations related to global change. Cambridge, U.K.: Cambridge Press. Reising, S.C. U.S. man, J.L. Schols, and J.A. Weinman. In preparation. Long range lightning measurement using VLF Fourier goniometry. American Geophysical Union Fall Meeting 1993. [Abstract] Workman, E.J., and S.E. Reynolds. 1949. Electrical activity as related to thunderstorm cell growth. Bulletin of American Meteorological Society, 30, 142-144.
-80 -60 -40 -20 0 20 40 60 80
Magnitude
Arrival Azimuth (deg)
Figure 2. A. Spectrogram of VLF wideband data from Palmer showing an intense spheric determined to be arriving from 35.5 0 west of north. B. The location (point F) of the gamma-ray flash as deduced from GRO data (Fishman et al. 1994), with respect to Palmer Station. C. Spheric histogram for a 30-minute period, clearly indicating the active storm center at bearings between -30 0 and -400 . (kHz denotes kilohertz. Hz denotes hertz. UT denotes universal time. dB denotes decibel. deg denotes degree. "Sferics" and "spherics" are alternate spellings.)
ANTARCTIC JOURNAL - REVIEW 1994 320
California 94305
at two distant sites are then used to determine the source location via spherical triangulation. This capability was demonstrated recently with recordings made at Stanford, California, and at Palmer Station. The recordings allowed the measurement of the location of individual lightning discharges in the Gulf of Mexico with an accuracy to less than 100 km (Reising et al. 1993). In this article, we illustrate two examples of the use of the VLF Fourier goniometry technique with Palmer data. In one case, illustrated in figure 1, we compare results with SSM/I data from the DMSP satellite for 30 March 1989. SSM/I data are a composite of several microwave radiometer frequencies and polarizations, optimized to locate and estimate rainfall. Figure 1A shows the areas (marked A and B) of cold cloud tops detected over South America, along with the bearings of great-circle paths arriving at Palmer Station, Antarctica. Fig-
tanford University has maintained an extremely-low-freS quency/very-low-frequency (ELF/VLF) observation program at Palmer Station for many years, primarily for the purpose of measuring electromagnetic waves from the Earth's magnetosphere, especially the type known as "whistlers," which originate in lightning discharges in the Northern Hemisphere (Burgess and man 1993, and references therein). The VLF measurements at Palmer Station are conducted using highly sensitive VLF antennas and receivers. The relatively quiet (electromagnetically speaking) nature of the site, its geomagnetic conjugacy to major thunderstorm centers in the east coast of the United States, and its proximity to the South Atlantic magnetic anomaly make Palmer Station a unique site for these studies. During the past 2 years, an additional potential use of the Palmer VLF data has emerged. It now appears that thunderstorm activity on a global scale can be tracked with the help of VLF measurements at Palmer Station. For this purpose, researchers use directional wideband measurements of the socalled radio atmospherics, impulsive electromagnetic signals, which originate in lightning discharges and which propagate to very large distances along the surface of the Earth within the Earth-ionosphere waveguide. Wideband direction-finding techniques developed at Stanford for whistler measurements are directly applicable for this purpose, as illustrated below. Tracking lightning activity on a global scale is important in view of the possible connections between frequency of lightning occurrence (i.e., flash rate) and convective rain flux (Workman and Reynolds 1949; Goodman 1990). Measurement of the rainfall over the oceans, particularly over the Pacific Ocean, is of critical importance in determining the variability of global circulations and the short-term changes in climate and, thus, constitutes a primary objective of the Climate and Global Change Program. Global lightning activity provides one signature of tropical moisture and energetics (Goodman and Christian 1993), and quantitative measurement of lightning on a global scale is needed as an input and a verification of global climate change models. The new method of lightning location is known as VLF Fourier goniometry and was developed at Stanford as part of a recently completed Ph.D. dissertation (Burgess 1993). The method relies on the measurement of the arrival direction of radio atmospherics (spherics) from distant [more than 1,000kilometer (km)] source regions. The arrival direction is measured using two magnetic loop antennas, one oriented northsouth and one oriented east-west. Digital wideband recordings of the data from two antennas are postprocessed to determine arrival azimuth with approximately 10 accuracy (Reising et al. in preparation). Simultaneous recordings made
Satellite Microwave Radiometry Data (SSM/I image) indicates active storm centers A and B. .' 0' 10' 20'
DJ
Spheric histogram for PA. 30 March 1989 0701-0730 UT
I
Ii (b)
-80-60 -40 -20 0 20 40 60 80 Magnitude Arrival Azimuth (deg)
Figure 1. Comparison of SSM/I image sensing cold cloud tops indicative of rainfall with VLF spheric detection and direction-finding data. A. Locations of two storms observed on 30 March 1989, with great-circle bearings arriving at Palmer Station, Antarctica. B. Histogram of Palmer data for 30 March 1989, with respect to arrival azimuth and magnitude (uncalibrated) of received spherics. (UT denotes universal time. deg denotes degree.)
ANTARCTIC JOURNAL 319
- REVIEW 1994
ure lB shows spheric measurements at Palmer, presented in the form of a two-dimensional histogram of a number of spherics observed in a given period sorted in terms of intensity and arrival azimuth. We note that two peaks are clearly observed at 00 and 100 bearing corresponding to the two storms marked A and B. In addition, we note that other storm centers are evident (e.g., one near approximately 85 0 and another near approximately 550 azimuth) that are active at the same time. Figure 2 illustrates another application of the Fourier goniometry method to assess the location of active thunderstorm centers at a time when an intense gamma-ray burst ("flash") was observed to emerge from the Earth's upper atmosphere (Fishman et al. 1994). One of about 12 such cases observed on the Compton Gamma Ray Observatory (CGRO), the point of origin of the particular flash observed on 20 May 1993 was deduced to be the point F shown in figure 2 (Fishman personal communication). In their paper published
kHz 0
4.0
-10
-20 2.0
-30 0.0 0 1 4 6 0,OdB At 31
Time (sec)
recently in Science, Fishman et al. (1994) indicated that the observed gamma-ray bursts were probably caused by lightning discharges. The VLF Fourier goniometry method, when applied to the wideband VLF data recorded at Palmer Station during this period, clearly reveals a very active storm center as shown in figure 2C, strongly corroborating the association of the observed gamma-ray flash with lightning. The arrival azimuth of the center of this storm center is within approximately 0.50 of the great-circle bearing between point F in figure 2B and Palmer Station. Figure 2A shows an isolated and very intense spheric recorded at Palmer within 200 milliseconds of the observed gamma-ray flash (Fishman et al. 1994). It is interesting to note that the arrival bearing at Palmer of this intense spheric is 35.5 0 west of north, again within approximately 0.50 of the bearing of the CGRO-deduced location of the gamma-ray flash (i.e., point F). The above cases simply represent examples in which the location of lightning activity was known by other measurements and was accurately confirmed by the VLF Fourier goniometry method. In the coming year, Palmer VLF measurements will be carried out in coordination with those at other sites within the United States, and combined data sets will be used to track thunderstorms in oceanic regions, where other data on lightning location are only sparsely available. We acknowledge the enthusiastic support of our equipment by John Gress of Antarctic Support Associates and Antarctic Support Associates Science Technician at Palmer Station, John Booth. We are grateful to W. Burgess for his development of the VLF Fourier goniometry method and to D. Hakeman for his help in developing the spheric identification software. This work is supported under National Science Foundation grant OPP 90-20687 to Stanford University.
References Burgess, W.C. 1993. Lightning-induced coupling of the radiation belts to geomagnetically conjugate ionospheric regions. (Ph.D. Thesis, Stanford University Department of Electrical Engineering, Stanford, California). Burgess, W.C., and U.S. man. 1993. The role of ducted whistlers in the precipitation loss and equilibrium flux of radiation belt electrons. Journal of Geophysical Research, 98(A9), 15643. Fishman, G.J. Personal communication. Fishman, G.J., P.N. Bhat, R. Mallozzi, J.M. Horack, T. Koshut, C. Kouveliotou, G.N. Pendleton, C.A. Meegan, R.B. Wilson, W.S. Paciesas, S.J. Goodman, and H.J. Christian. 1994. Discovery of intense gamma-ray flashes of atmospheric origin. Science, 264, 1225.
Spheric histogram for Palmer 20 May 1993 0150-0220 UT
Goodman, S.J. 1990. Predicting thunderstorm evolution using groundbased lightning detection networks (National Aeronautics and
2%8
Space Administration technical memo TM-103521). Goodman, S.J., and H.J. Christian. 1993. Global observations of lightning. In Gurney, Foster, and Parkinson (Eds.), Atlas of satellite observations related to global change. Cambridge, U.K.: Cambridge Press. Reising, S.C. U.S. man, J.L. Schols, and J.A. Weinman. In preparation. Long range lightning measurement using VLF Fourier goniometry. American Geophysical Union Fall Meeting 1993. [Abstract] Workman, E.J., and S.E. Reynolds. 1949. Electrical activity as related to thunderstorm cell growth. Bulletin of American Meteorological Society, 30, 142-144.
-80 -60 -40 -20 0 20 40 60 80
Magnitude
Arrival Azimuth (deg)
Figure 2. A. Spectrogram of VLF wideband data from Palmer showing an intense spheric determined to be arriving from 35.5 0 west of north. B. The location (point F) of the gamma-ray flash as deduced from GRO data (Fishman et al. 1994), with respect to Palmer Station. C. Spheric histogram for a 30-minute period, clearly indicating the active storm center at bearings between -30 0 and -400 . (kHz denotes kilohertz. Hz denotes hertz. UT denotes universal time. dB denotes decibel. deg denotes degree. "Sferics" and "spherics" are alternate spellings.)
ANTARCTIC JOURNAL - REVIEW 1994 320
