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Some characteristics of cirrus clouds deduced from laser-radar observations at different elevation angles
Journat of Atmospheric and Terrestriil Physics, Vol. 39, pp. 65’ _ 660. Pergamon Press, 1977. Printed in Northern Ireland
SOME CHARACTERISTICSOF CIRRUS CLOUDS DEDUCED FROM LASER-RADAR OBSERVATIONS AT DIFFERENT ELEVATION ANGLES
A. J. Gibson, L. Thomas and S. K. Bhattacharyya S.R.C., Appleton Laboratory, Ditton Park, Slough, SL3 9JX, England (Received14Febtmiry1977)
Abstract Measurements with a laser-radar system at Winkfield (51.4"N, 0.7'W) during AugustOctober 1976 have shown cirrus clouds to be present at heights of lo-12 km on about 5(p/. of otherwise clear nights. The measured backscatter and extinction coefficients of cirrus for different angles of elevation of the laser beam have been found to be inconsistentwith models consisting of spherical or randomly-orientatedirregular-shapedparticles. The results have been interpreted on the basis of a model of long cylindrical crystals orientated in a horizontal plane with arbitrary azimuthal angle, and show the importance of taking into account the directional dependence of the ratio of backscatter to extinction coefficient in the interpretationand exploitation of laser-radar data. 1.
INTRODUCTION
It is well known that the Presence of cirrus clouds can have a significant influence on the earth's radiation balance (e.g. Hall, 1968). However, attempts to deduce the visible and infra-red transmissionand reflection properties of such clouds suffer from lack of information on their height and thickness. Furthermore, the theoretical treatment of scattering from ice crystals typical of those in cirrus clouds is complicated by their non-spherical shape and non-random orientation (Heymsfield and Knollenberg, 1972). This has been illustrated by Liou (1972) who has considered a cirrus-cloudmodel composed of horizontallyorientated ice cylinders. Laser-probing experiments carried out by Evans (1965), Davis (1969) and others have provided information on the location and thickness of cirrus clouds. Furthermore, Davis (1971) and Platt (1973) have shown that laser-radar data can represent,avaluable complement to radiometer measurements in deducing the optical parameters of the cloud particles. However, the possible non-random orientation of ice crystals in cirrus clouds implies a possible dependence of the parameters measured upon the direction of the laser beam. Qualitative evidence for such a directional dependence has been mentioned by Uthe and Russell (1976) for the case of high-altitude tropical cirrus clouds. The purpose of this study is to investigate these effects by means of laser measurements of backscatter coefficient (&SC) and extinction coefficient (Pext) using different beam elevation angles (6). 2.
OBSERVATIONS
A laser radar incorporatinga steerable plane mirror for transmissionand reception has been us'rti at Winkfield (51.k0N, 0.7'W) to measure the backscattered signal from heights of 6-20 km. The laser operated at a wavelength of 0.6 pm and produced 1 flspulses at a repetition frequency of 2 s"l. The receiver (0.3 m diameter) operated in the photon-countingmode, the return signals being recorded in channels corresponding to 1 km range intervals. The system had been designed for other atmospheric-probingexperiments and in the present application the laser energy was adjusted to a very low value to avoid overloading the receiver. The observations were confined to nights on which no clouds at lower heights were detectable during the period 20 August-8 October 1976, cirrus clouds being recorded on about 50% of these nights. A sample of the results obtained at 90" elevation angle is shown in Fig. 1. This presents the variation with height, h, of the range-correctedreturn signal and standard 651
6.58
Short Paper
in each case is shown the corresponding variation expected for the error on 5 nights; Rayleigh-scattered signal, as derived for an atmospheric model relating to middle-latitude summer conditions (Cole, et al., 1965). The presence of cirrus clouds near lo-12 km is The data for 25 August and shown distinctly in the data for 23 August, 3 and 7 September. 6 September are reasonably consistent with the theoretically derived Rayleigh-scatter profiles over most of the 6-20 km height range.
00.40 25/8/76
21.07 3(9/i%
I
23.40 6/9/i-6
21.26 7/9/-Z
1
IO 3 sigml ccmcted for rarQe
FIGURE 1
Variations of the range-corrected signal deduced from soundings at The smooth cumes represent 90° elevation angles during five nights. the corresponding variation expected for Rayleigh scattering from an atmospheric model for middle-latitude summer conditions.
The first set of data shown in Fig. 1 formed part of an extensive series of measurements taken during the period 2310-0130 on the night of 22/23 August. Strong scattering from heights of about 11.5 km was recorded throughout the observing period. Figure 2 shows the height variation of the average signals observed for seven runs at 90’ elevation angle and three interspersed runs at 30’ elevation angle; a total of about 250 laser firings were used in each run. As in Fig. 1, both sets of results have been fitted to the height variation expected for Rayleigh scattering from the model atmosphere at heights below the cirrus cloud. The observed variations with height above 12.5 km are also consistent with Rayleigh scattering. The interpretation of the signals observed from heights of 6-9 km and 13-18 km in terms of Rayleigh scattering permits the apparent optical thickness (r(e) = jpext cosec 8. dh) and integrated backscattering coefficient ( i&g0 cosec 6. dhf of the cirrus cloud to be deduced independently for the two elevation angles. The values of optical thickness could have been slightly underestimated because of near-forward , multiple-scattered radiation from the laser pulse, but the corresponding small corrections would be in the same sense for the two elevation angles. The values obtained are shown in Table 1. It is to be noted in comparing these results with those in Fig. 2, that plotting the results as a function of height rather than range removes the geometrical factor (cosec 6) from the integrated backscatter but not from the attenuation. The bracketed values shown in the column for 90° elevation angle in Table 1 are those to be expected from the 30° elevation angle results, on the assumption that the cloud consisted of spherical or randomly-orientated irregulardshaped particles, for which the variation with elevation angle involves only the change in path length. It is seen that
659
Short Paper
0 0
9o~elevatim 3CYelevotion
.
51 I
I
I
I
I
I,,,,
3
IO
Signalcorrected for range
Variations of the mean signals from soundings at 90° and 30’ elevation angles on the night of 22/23 August 1976, after correction for range. The smooth curve represents the variation expected for Rayleigh scattgring from an atmospheric model for middle-latitude summer conditions.
FIGURE 2
whereas the observed value of ~(90’) is slightly less than that predicted on this basis, the integrated backscatter is greater than expected by a factor of 2.7. It is evident that the two sets of experimental results cannot be reconciled by a horizontal variation of the concentration of the particles considered above. TABLE 1
Elevation
Optical thickness and apparent integrated backscatter coeificient for elevation angles of 30° and 90°. angle,
0
300
Apparent optical thickness s(0) = JPext cosece .dh
Integrated coefficient,
backscatter Jblso cosec
0.115
0.dh
2’o
900
+ 0.019
’
lo
-3
0.047
2.7
_+ 0.009
x 1O-3 (1.0
(0.058
2 0.010)
x 10-3)
The values in brackets have been deduced for 90’ from the 30’ results on the assumption of spherical or randomly-orientated particles. 3.
DISCUSSION
Liou (1972) has considered the scattering of radiations by cirrus clouds consisting of long circular cylinders orientated in a horizontal plane with arbitrary azimuthal angle. For such a model it is predicted that the backscatter coefficient per unit volume for 90’ elevation angle will be significantly larger than for other elevation angles. This can account for the factor of 2.7 noted above. In addition, it can be shown that for this model the ratio of optical thicknesses at 90° and 30’ elevation angles should be 0.65; furthermore, this value applies to crystals of any shape but small vertical extent randomly orientated in a horizontal
660 Short Papet
plane, and having horizontal dimensions large compared with the wavelength. The corresponding value observed, Table 1, is 0.41 f 0.10. In practice, it is unlikely that all crystals in the cirrus would be perfectly horizontally orientated so that the optical-thicknessratio would be expected to take some value between 0.65 and 0.50, the value expected for random orientation. The possible small discrepancy with the value of 0.41 can probably be accounted for by assuming a contribution of about 10% to the observed backscatter from the 13-18 km region due to aerosols in that height range. However, in general it may not be assumed that the optical thickness of cirrus is proportional to cosec 8, and this has implications for the measurement of atmospheric transmissioncoefficients as described by Ottway et a1.(1972). The results in Table 1 yield values for the ratio of backscatter coefficient to extinction coefficient of 0.057 and 0.017 for elevation angles 90' and 30°, respectively. If 10% excess backscatteringat heights above the cirrus cloud is assumed, the value of this ratio is reduced to 0.027 and 0.012 for the two elevation angles. The observed directional dependence of laser measurements is of considerable consequence in the application of this type of measurement to the interpretationof radiometer data. In this application, the ratio of integrated backscatter coefficient to optical thickness is of particular significance (Davis,1971;Platt,1973). The factor of about 3 between the values of this ratio found for the 90° and 30' elevation angles illustrates the importance of the non-spherical nature of cirrus-cloud particles.
The work described above was carried out at the Science Research Council's Appleton Laboratory and is published by the permission of the Director. REFERENCES Cole, A. E., Court, A. and Kantor, A. .I.,in Handbook of Geophysics and Space Environments (edit. by Valley, S. L.), Ch. 2 (McGraw-Hill,New York, 1965). Davis, P. A., Appl. Opt. 8, 2099, 1969. Davis, P. A., J. Appl. Meteor. lo, 1314, 1971. Hall, F. F., Appl. Opt. 7, 2264, 1968. Heymsfield, A. J. and Knollenberg, R. G., J. Atmos. Sci. 2, Liou, K., J. Atmos. Sci. 2,
1358, 1972.
524, 1972.
Ottway, M. T., Wright, R. W. H. and Kent, G. S., J. Atmos. Terr. Phys. 2,
1337, 1971.
Platt, C. M. R., J. Atmos. Sci. 3Q, 1191, 1973.
Reference is also made to the following unpublished material : Evans, D. E., Remote probing of high-cloud cover via satellite-bornelidar, Final Rept. Contract NASA-49 (27), NASA, Washington D.C. 1965. Uthe, E. E. and Russell, P. B., Lidar observations of tropical high-altitude cirrus clouds. Preprint,IAMAPSymposium on Radiation in the Atmosphere, Garmisch-Partenkirchen, Germany, August, 1976.
SOME CHARACTERISTICSOF CIRRUS CLOUDS DEDUCED FROM LASER-RADAR OBSERVATIONS AT DIFFERENT ELEVATION ANGLES
A. J. Gibson, L. Thomas and S. K. Bhattacharyya S.R.C., Appleton Laboratory, Ditton Park, Slough, SL3 9JX, England (Received14Febtmiry1977)
Abstract Measurements with a laser-radar system at Winkfield (51.4"N, 0.7'W) during AugustOctober 1976 have shown cirrus clouds to be present at heights of lo-12 km on about 5(p/. of otherwise clear nights. The measured backscatter and extinction coefficients of cirrus for different angles of elevation of the laser beam have been found to be inconsistentwith models consisting of spherical or randomly-orientatedirregular-shapedparticles. The results have been interpreted on the basis of a model of long cylindrical crystals orientated in a horizontal plane with arbitrary azimuthal angle, and show the importance of taking into account the directional dependence of the ratio of backscatter to extinction coefficient in the interpretationand exploitation of laser-radar data. 1.
INTRODUCTION
It is well known that the Presence of cirrus clouds can have a significant influence on the earth's radiation balance (e.g. Hall, 1968). However, attempts to deduce the visible and infra-red transmissionand reflection properties of such clouds suffer from lack of information on their height and thickness. Furthermore, the theoretical treatment of scattering from ice crystals typical of those in cirrus clouds is complicated by their non-spherical shape and non-random orientation (Heymsfield and Knollenberg, 1972). This has been illustrated by Liou (1972) who has considered a cirrus-cloudmodel composed of horizontallyorientated ice cylinders. Laser-probing experiments carried out by Evans (1965), Davis (1969) and others have provided information on the location and thickness of cirrus clouds. Furthermore, Davis (1971) and Platt (1973) have shown that laser-radar data can represent,avaluable complement to radiometer measurements in deducing the optical parameters of the cloud particles. However, the possible non-random orientation of ice crystals in cirrus clouds implies a possible dependence of the parameters measured upon the direction of the laser beam. Qualitative evidence for such a directional dependence has been mentioned by Uthe and Russell (1976) for the case of high-altitude tropical cirrus clouds. The purpose of this study is to investigate these effects by means of laser measurements of backscatter coefficient (&SC) and extinction coefficient (Pext) using different beam elevation angles (6). 2.
OBSERVATIONS
A laser radar incorporatinga steerable plane mirror for transmissionand reception has been us'rti at Winkfield (51.k0N, 0.7'W) to measure the backscattered signal from heights of 6-20 km. The laser operated at a wavelength of 0.6 pm and produced 1 flspulses at a repetition frequency of 2 s"l. The receiver (0.3 m diameter) operated in the photon-countingmode, the return signals being recorded in channels corresponding to 1 km range intervals. The system had been designed for other atmospheric-probingexperiments and in the present application the laser energy was adjusted to a very low value to avoid overloading the receiver. The observations were confined to nights on which no clouds at lower heights were detectable during the period 20 August-8 October 1976, cirrus clouds being recorded on about 50% of these nights. A sample of the results obtained at 90" elevation angle is shown in Fig. 1. This presents the variation with height, h, of the range-correctedreturn signal and standard 651
6.58
Short Paper
in each case is shown the corresponding variation expected for the error on 5 nights; Rayleigh-scattered signal, as derived for an atmospheric model relating to middle-latitude summer conditions (Cole, et al., 1965). The presence of cirrus clouds near lo-12 km is The data for 25 August and shown distinctly in the data for 23 August, 3 and 7 September. 6 September are reasonably consistent with the theoretically derived Rayleigh-scatter profiles over most of the 6-20 km height range.
00.40 25/8/76
21.07 3(9/i%
I
23.40 6/9/i-6
21.26 7/9/-Z
1
IO 3 sigml ccmcted for rarQe
FIGURE 1
Variations of the range-corrected signal deduced from soundings at The smooth cumes represent 90° elevation angles during five nights. the corresponding variation expected for Rayleigh scattering from an atmospheric model for middle-latitude summer conditions.
The first set of data shown in Fig. 1 formed part of an extensive series of measurements taken during the period 2310-0130 on the night of 22/23 August. Strong scattering from heights of about 11.5 km was recorded throughout the observing period. Figure 2 shows the height variation of the average signals observed for seven runs at 90’ elevation angle and three interspersed runs at 30’ elevation angle; a total of about 250 laser firings were used in each run. As in Fig. 1, both sets of results have been fitted to the height variation expected for Rayleigh scattering from the model atmosphere at heights below the cirrus cloud. The observed variations with height above 12.5 km are also consistent with Rayleigh scattering. The interpretation of the signals observed from heights of 6-9 km and 13-18 km in terms of Rayleigh scattering permits the apparent optical thickness (r(e) = jpext cosec 8. dh) and integrated backscattering coefficient ( i&g0 cosec 6. dhf of the cirrus cloud to be deduced independently for the two elevation angles. The values of optical thickness could have been slightly underestimated because of near-forward , multiple-scattered radiation from the laser pulse, but the corresponding small corrections would be in the same sense for the two elevation angles. The values obtained are shown in Table 1. It is to be noted in comparing these results with those in Fig. 2, that plotting the results as a function of height rather than range removes the geometrical factor (cosec 6) from the integrated backscatter but not from the attenuation. The bracketed values shown in the column for 90° elevation angle in Table 1 are those to be expected from the 30° elevation angle results, on the assumption that the cloud consisted of spherical or randomly-orientated irregulardshaped particles, for which the variation with elevation angle involves only the change in path length. It is seen that
659
Short Paper
0 0
9o~elevatim 3CYelevotion
.
51 I
I
I
I
I
I,,,,
3
IO
Signalcorrected for range
Variations of the mean signals from soundings at 90° and 30’ elevation angles on the night of 22/23 August 1976, after correction for range. The smooth curve represents the variation expected for Rayleigh scattgring from an atmospheric model for middle-latitude summer conditions.
FIGURE 2
whereas the observed value of ~(90’) is slightly less than that predicted on this basis, the integrated backscatter is greater than expected by a factor of 2.7. It is evident that the two sets of experimental results cannot be reconciled by a horizontal variation of the concentration of the particles considered above. TABLE 1
Elevation
Optical thickness and apparent integrated backscatter coeificient for elevation angles of 30° and 90°. angle,
0
300
Apparent optical thickness s(0) = JPext cosece .dh
Integrated coefficient,
backscatter Jblso cosec
0.115
0.dh
2’o
900
+ 0.019
’
lo
-3
0.047
2.7
_+ 0.009
x 1O-3 (1.0
(0.058
2 0.010)
x 10-3)
The values in brackets have been deduced for 90’ from the 30’ results on the assumption of spherical or randomly-orientated particles. 3.
DISCUSSION
Liou (1972) has considered the scattering of radiations by cirrus clouds consisting of long circular cylinders orientated in a horizontal plane with arbitrary azimuthal angle. For such a model it is predicted that the backscatter coefficient per unit volume for 90’ elevation angle will be significantly larger than for other elevation angles. This can account for the factor of 2.7 noted above. In addition, it can be shown that for this model the ratio of optical thicknesses at 90° and 30’ elevation angles should be 0.65; furthermore, this value applies to crystals of any shape but small vertical extent randomly orientated in a horizontal
660 Short Papet
plane, and having horizontal dimensions large compared with the wavelength. The corresponding value observed, Table 1, is 0.41 f 0.10. In practice, it is unlikely that all crystals in the cirrus would be perfectly horizontally orientated so that the optical-thicknessratio would be expected to take some value between 0.65 and 0.50, the value expected for random orientation. The possible small discrepancy with the value of 0.41 can probably be accounted for by assuming a contribution of about 10% to the observed backscatter from the 13-18 km region due to aerosols in that height range. However, in general it may not be assumed that the optical thickness of cirrus is proportional to cosec 8, and this has implications for the measurement of atmospheric transmissioncoefficients as described by Ottway et a1.(1972). The results in Table 1 yield values for the ratio of backscatter coefficient to extinction coefficient of 0.057 and 0.017 for elevation angles 90' and 30°, respectively. If 10% excess backscatteringat heights above the cirrus cloud is assumed, the value of this ratio is reduced to 0.027 and 0.012 for the two elevation angles. The observed directional dependence of laser measurements is of considerable consequence in the application of this type of measurement to the interpretationof radiometer data. In this application, the ratio of integrated backscatter coefficient to optical thickness is of particular significance (Davis,1971;Platt,1973). The factor of about 3 between the values of this ratio found for the 90° and 30' elevation angles illustrates the importance of the non-spherical nature of cirrus-cloud particles.
The work described above was carried out at the Science Research Council's Appleton Laboratory and is published by the permission of the Director. REFERENCES Cole, A. E., Court, A. and Kantor, A. .I.,in Handbook of Geophysics and Space Environments (edit. by Valley, S. L.), Ch. 2 (McGraw-Hill,New York, 1965). Davis, P. A., Appl. Opt. 8, 2099, 1969. Davis, P. A., J. Appl. Meteor. lo, 1314, 1971. Hall, F. F., Appl. Opt. 7, 2264, 1968. Heymsfield, A. J. and Knollenberg, R. G., J. Atmos. Sci. 2, Liou, K., J. Atmos. Sci. 2,
1358, 1972.
524, 1972.
Ottway, M. T., Wright, R. W. H. and Kent, G. S., J. Atmos. Terr. Phys. 2,
1337, 1971.
Platt, C. M. R., J. Atmos. Sci. 3Q, 1191, 1973.
Reference is also made to the following unpublished material : Evans, D. E., Remote probing of high-cloud cover via satellite-bornelidar, Final Rept. Contract NASA-49 (27), NASA, Washington D.C. 1965. Uthe, E. E. and Russell, P. B., Lidar observations of tropical high-altitude cirrus clouds. Preprint,IAMAPSymposium on Radiation in the Atmosphere, Garmisch-Partenkirchen, Germany, August, 1976.