Lidar observation of sudden increase of aerosols in the stratosphere caused by volcanic injections-II. Sierra Negra event M. FLJJIWARA, T. SHIBATA and M. HIRONO Department
of Physics, (Received
1 April 1982)
Abstract A striking disturbance in stratospheric aerosols over Fukuoka was observed by Nd-YAG laser radar in December 1979. It began with the appearance of a thin layer of enhanced scattering at an altitude of about 17 km and revealed remarkable variations of the layer in time and height. Measurements at two wavelengths suggest that the aerosols changed in size distribution, and the disturbance is inferred to be due to the Sierra Negra eruption. The integrated aerosol backscattering above the tropopause reached about 8 x 10 ’ sr I ; i.e. some six times that of the Soufritre event when converted to the ruby wavelength. The mean meridional transport speeds ofthe dust clouds were much larger than ever observed previously and this may be due to the activation of meridional transport associated with the Canadian sudden stratospheric warming in November-December 1979. I. INTROIWCTION Preceding the violent disturbance in stratospheric aerosols due to the 1980 eruptions of Mt. St. Helens, which has attracted the interest of many scientists, remarkabledisturbancesweredetectedtwicein 1979,in May and December, by laser radar observation in Fukuoka, Japan (33S”N, 130.4OE). Results observed in the May event which originated from the April 1979 eruption of La Soufrikre were described in an earlier paper (HIRONO et al., 1981 ; hereafter referred to as paper I). The present paper concerns the December event, which is attributed to the November 1979 eruptions of Sierra Negra (0.83”S, 91.17”W). The disturbance in stratospheric aerosols over Fukuoka which began in December 1979 almost terminated before the major eruptions of Mt. St. Helens. From a global point of view, however, the volcanic material from the Sierra Negra may have survived at low and middle latitudes and may have had some role in the radiation budget and chemistry of the stratosphere even after the eruptions of Mt. St. Helens. One must, therefore, be careful in discussing the effect of the Mt. St. Helens eruptions on the stratosphere. The only other laser radar stations to report a positive detection of the Sierra Negra event were those of D’ALTORIO EI ul. (1981) which observed aerosol backscattering somewhat above the values for the unperturbed stratosphere in mid-December 1979 in Italy and MCCORMICK (1982), who recorded the effects in the U.S.A. in December I979 (see also SWISSLERet ul., 1982). But the effect of the Sierra Negra eruptions on stratospheric aerosols has been confirmed by satellite observations (MCCORMICK, 1980) and by balloons (ROSEN and HOFMANN, 1980). 811
The characteristics of the disturbance, which was detected first in December 1979 with the sudden appearance of a strong scattering layer, were quite different from those in the Soufrikre event. The vertical profile of the scattering from aerosols has a sharp maximum at an altitude around 17 km. The maximum value of height integrated aerosol backscattering reaches about 8 x 10e5 sr-l for the Nd-YAG laser wavelength 1.064 pm. This is equivalent to a value of about 1.7 x IO m4sr- ’ for the ruby wavelength 0.694 pm and is about six times that of the Soufriire event. The mean transport speeds of volcanic air parcels giving rise to the initial, intermediate and maximum increase of aerosol concentration (definition of these are given in paper I) are 1.53, 1.30 and 1.10 m s- I, respectively. These values are greater than those for the eruption of Agung (1963), Fuego (1974) and Soufriire (1979), as shown in Table 3 of paper I. It is suggested that such high speeds may be related to a global, transient, dynamical process in the atmosphere which happened to produce the Canadian sudden stratospheric warming in November and December 1979. The center of the warming approached Japan on 10 December and again on 26 December 1979. 2. OBSERVED RESUIATS A ruby laser was replaced by a Nd-YAG laser in our radar system in October 1979 and since then observations have been made on two wavelengths, the fundamental wavelength 1.064 pm and the second harmonic 0.532 pm of the Nd-YAG laser. A nearinfrared sensitive PMT, VPM-164 A, has been used for the detection ofthe fundamental wavelength. Details of the characteristics of the laser radar system and the
M. FUJIWARA,T. SHIBATAand M. HIRONCI
Fig. 1. Vertical profiles of laser radar scattering ratio R for wavelengths 1.064 pm (solid lines) and 0.532 pm (dotted lines) showing the pre and initial stage of the disturbance in stratospheric aerosols over Fukuoka.
method ofdata analysis have been described elsewhere (SHIBATA et al.,1980).The quantity of aerosols measured with laser radar is represented by the scattering ratio R or the backscattering coefficient fl. The latter means the backscattering cross-sections of aerosols per unit volume and the former is defined as the ratio of total backscattering coefficient (aerosols plus air molecules) to that of air molecules. Adopting a statistical method proposed by RUSSELLetal. (1979), the
height of minimum scattering ratio R has been chosen in the height range 13-33 km as the height of normalization where R has been assumed to be unity. This height is found almost always around an altitude of 30 km. Profiles of the aerosol layer obtained in October and November 1979 revealed characteristics ofa quiet, nonvolcanic state, which are shown in Figs 1 and 2. The height of the layer peak was about 21 km and the
Fig. 2. All the profiles of R for 1.064 pm observed
1979 to May 1980.
of sudden increase of aerosols in the stratosphere
profiles of R for the fundamental wavelength which were observed from October 1979 to May 1980. The layer was characterized by its thinness and by the rapid variability of its scattering at the initial stage of the disturbance, in December 1979. Nocturnal variations of the layer were observed a few times in December. On 18 December a sharp layer observed initially with an increased peak at an altitude of 16.5 km decreased in peak value rapidly and disappeared almost completely in a few hours, as shown in Fig. 3. At the 100-mb level over Fukuoka (a height of about 16 km) a westerly zonal wind is always prevailing and the wind speed of 40 m s ’ is estimated from rawinsonde data taken at night. This rapid disappearance of the layer indicates a horizontal inhomogeneity with a characteristic length of about 300 km in the east-west direction. Such a drastic variation was not subsequently observed. The layer was characterized also by its day-to-day variation which may be associated with the variation of the synoptic pattern at the lOO-mb level over the Far East region. In Fig. 4 the lOO-mb synoptic charts for 2100 JST on 22 and 26 December 1979 are shown together with the vertical profile of R on the corresponding days. On 22 December, when the scattering ratio at the peak had its maximum value, a ridge was situated near Fukuoka. The synoptic pattern moved eastward day by day, and on 26 December when the peak disappeared, a trough approached Fukuoka. During the week which included 22 and 26 December, the streamline crossing over Fukuoka was approximated roughly by a sinusoidal wave with an eastward phase velocity of about 4” per day and a longitudinal wavelength of about 60” on the lOO-mb synoptic charts. The mean speeds of zonal winds in the Far East region at 33”N were approximately 45 and 51 m s- ’ on 22 and 26 December, respectively. The amplitude and phase of the trajectories of air parcels
vertical halfwidth of the layer of about 10 km. Values of R at an altitude of 2 1 km were about 1.4 and 1.05 for the wavelength 1.064 and 0.532 pm, respectively. A striking disturbance in stratospheric aerosols was observed first on I1 December, with the sudden appearance of a thin layer of enhanced scattering at an altitude of 16.5 km (Fig. I). Because of the weather change on this night, sufficient data could not be accumulated for the fundamental wavelength. The peak value of R for this wavelength, however, can be presumed to have been more than 2. The half width of the layer was about 1.5 km. Height resolution of the measurement was then 0.75 km. The peak value of R for the second harmonic wavelength was 1.2. At the same altitude mean values of R for the wavelength 1.064 and 0.532 pm were respectively 1.21 f 0.06 and 1.04 + 0.03 throughout October and November 1979. The values of (R-- 1) for both wavelengths on 11 December, therefore, were at least five times the values for the undisturbed stratosphere. The local height of the tropopause was then 15.5 km showing that the layer was located in the bottom part of the stratosphere. Monthly mean values in the height of the tropopause over Fukuoka were shown in Table I of paper I. They were 14.5 + 1.2 km in November and 12.5 km in December 1979. The layer of enhanced scattering could not be found in the next observation on 13 December, but appeared again for a while in the following observation on 18 December 1979, as shown in Fig. 1. The weather permitted frequent observations from that day through the rest of December and the day-to-day variation of the aerosol scattering could be measured. The layer with an apparent enhanced peak persisted at least until March 1980 though it disappeared once again on 26 December 1979. Gradually the layer increased in height to about 18 km in February and spread to a half width of about 3 km in the same month. Figure 2 shows all the
Fig. 3. Nocturnal
15 75 - 16.50
in R - 1 for 1.064 pm at different altitudes
on 18 December
M. FUJIWARA, T. SHIBATA and M. HIRONO
26 Dee 1979
Longitude (“E) 100 mb Synoptic charts
Fig. 4. Profiles of R for 1.064pm observed on 22 and 26 December 1979 with synoptic charts at 100 mb on the corresponding days.
should therefore be neariy equal to those of the streamlines on both days ; the differences being about 10%. Judging from the maps of the streamlines, the air parcel which passes over Fukuoka on 22 December should have been located more than 10” lower in latitude than Fukuoka aday before, whereas the one on 26 December should have moved in from a little higher latitude than Fukuoka. This suggests not only a horizontal inhomogeneity of the dust cloud in the north-south direction but also a meridional transport of the cloud due to a complicated dynamical process induced by the fluctuation of the zonal wind in the stratosphere. The characteristics of the layer discussed above are derived mainly from the results observed with the fundamental wavelength of the Nd-YAC laser. The scattering profiles were also obtained for the second harmonic wavelength to gain information on the size distribution ofaerosols by comparing with those for the fundamental wavelength. The observed values for the second harmonic are less accurate than those for the fundamental because of the small R-l value. Moreover, there remains a possibility that a small amount of aerosols existed even at the height of no~~ization around M km as pointed out by %JSSELL (1980). If this is the case, accurate values of the
backscattering coefficient for both wavelengths, and even more the ratio of these, cannot be determined without knowing the real values of R at the height of normalization. Nevertheless, we tentatively calculated the ratio of fi for the second harmonic to that for a fund~enta~ wavelength to give a rough picture of the variation in aerosol size distribution. The values are averaged over nine observations immediately before and 15 after the first detecting of disturbance. The dependence of the backscattering coefficient &I) on the wavelength 1 can be represented as proportional to A-“. The values of E were also calculated, and are shown in Table 1.They are appreciably different from unity, which is the usual
Table 1. Variation of exponent a in the expression for the backscattering coefficient j?(L)a I-” Period Weight (km) 21 18 15
12 October8 December -1.93
13 December16 January 1.59 2.11 2.33
Average 1.81 2.01 2.01
Lidar observation of sudden increase of aerosols in the stratosphere
Fig. 5. (a) Profiles of the ratio of backscattering coefficient for the wavelength 0.532 pm to that for 1.064 pm before (solid lines) and after (broken lines) the onset of the disturbance.
(b) The same as (a) except that the renorrnalized values are used for 0.532 pm (see the text).
with time at a height of 24 km is based on the observational results obtained by HOFMANNand ROSEN (1981). They found that the variation of the size distribution was not significant in the height range around 15-25 km during the undisturbed period. Consideration of the diffusion velocities (discussed in the next section) indicates that the disturbance would not yet have reached the height of renormalization (24 km) to change averaged aerosol parameters at the level until early January 1980, when perturbations are thought to be localized in the lower region than about 20 km. More careful examination will be needed with respect to the normalization procedure but in spite of this uncertainty about the normalization, the variation in the physical or chemical nature of aerosols is significant. Increase of the ratio fi(O.532)//3(1.064) reflects the change of size distribution from the larger background aerosols to the smaller ones, if the refractive index and the spherical shape of aerosol particles are assumed to be invariable.
value. Profiles of the ratio /?(0.532)//?(1.064)are shown
in Fig. 5(a). The value decreased with decreasing height before the commencement of the disturbance, but afterwards it increased towards the lower region where a sharp scattering layer was found. Considering that any variation in the characteristics of aerosols is expected in this region rather than the higher region, we can safely conclude that the ratio increased appreciably in the lower region, even though the absolute values are not completely reliable. HOFMANNand ROSEN(1981) observed background aerosols by a balloon-borne optical counter and showed that if N(r > x) denotes the number density of aerosol particles the radii of which are larger than x(pm) then the size ratio N(r > O.lS)/N(r > 0.25) was constant at about 5.5 throughout the lower stratosphere (below about 25 km) during 1978 and 1979. These two years were a volcanically quiescent period at Wyoming, where the influence of the Sierra Negraevent was only noticedin December 1979. In Fig. 5(b) we show, for comparison, the same profiles as Fig. 5(a) except that the renormalized values of fi(O.532)are adopted. The values of b(O.532) are renormalized as follows : the size distribution of aerosols at an altitude around 24 km is assumed to be constant throughout the observation. The value of a at this height is assumed to be 1.45 which is consistent with the size ratio 5.5 if appropriate size distribution function (for instance, Zold type) and refractive index (m = 1.42)are assumed. b(O.532) at 24 km is thus estimated from the value of fl(1.064) at the same altitude and the vertical profile of b(0.532) relative to it can be obtained. The assumption that the size distribution is constant
3. DISCUSSION OF THE RESULTS On 13 and 17 November 1979 serious eruptions of Sierra Negra occurred and a maximum elevation of the dust cloud to at least 14 km was reported (SEAN Bulletin, 1979). No other volcanic eruption which could have injected volcanic clouds into the stratosphere had been reported since the April 1979 eruption of La Soufriere. The remarkable features of the enhanced scattering layer initially observed in December 1979 in Fukuoka were its sharpness and rapid variability of scattering. These suggest the possibility that the layer was newly formed and not yet well diffused vertically or horizontally. We shall examine the sharpness of the layer in more detail by the diffusion theory outlined in paper I. First the large sedimentation velocity should be eliminated. If the layer originated in the 13 November injection from the Sierra Negra, the radius of the aerosol particles had to be less than 0.3 pm (Table 2 in paper I) to prevent appreciable settling through the tropopause until at least January 1980. Then the first term on the right hand side of equation (4) in paper I can be neglected and the equation can be written as dn, _ 8
Here n, is the number density of aerosol particles, H the scale height of the atmosphere and D the diffusion coefficient. It follows that H = 6.5 km and is assumed to be nearly constant for the present. When the thickness
M. FUIIWARA,T. SHIBATAand
of the layer is much less than H we have approximately
Here it is assumed that D is also independent of height in the height range greater than the layer width. If the initial distribution is represented by a delta function at the reference level z = 0 (namely, at an altitude of 17 km) then it follows that K
ns= J(lrDt)ev where K is a constant.
The half width at time t is given
hy L, = 3.33 (Dt)“‘. In the lower stratosphere the value of D is most uncertain. If we take a value D = lo3 cm2 s ’ it follows that L, = 1.8 km at a month (t = 3 x lo6 s) and L, = 3 km at three months after the injection, which agree roughly with the results of observation. Particles larger than 1 pm will rapidly settle down as shown in paper I. Gas components and very small particles (r,vc-c I pm) injected as a thin layer in the lower stratosphere will be gradually nucleated and their diffusion will be described by the above equations. On the contrary, if they are injected into the troposphere as thin layers their concentration will decrease ten times as rapidly as calculated for the above case, since D in the troposphere is one hundred times as large as in the stratosphere. Another remarkable feature of the December event which also suggests the layer to be newly formed is the increase of the ratio 8(0.532)/8(1.064) at the commencement of the disturbance. This increase most likely reflects the change in the size distribution from the larger background aerosols to the smaller volcanic ones and is consistent qualitatively with the results of ROSENand HOFMANN( 1980). They observed an unusual layer of aerosols much smaller than usual about a month after the eruptions of Sierra Negra with a balloon-borne optical counter. As the size ratio N(r > O.l5)/N(r > 0.25) they observed was very large, they concluded that the aerosol increase might be difficult to detect by lidar or other optical monitoring systems. The laser radar in Fukuoka, however, could detect a distinct layer of aerosols the size of which was smaller than usual. HOFMANN and ROSEN (1977) discuss a similar situation associated with the eruption of the Fuego volcano 1975. They could detect three layers by their balloon-borne device but only two by laser radar and attributed this to the small size of the aerosols in the third layer. The aerosol layer in the initial stage of volcanic disturbance is, however, remarkably variable as mentioned above and so the difference between
measurements at different stations may be explained rather by the inhomogeneity of the layer in space and time. Though we concluded from the results of local laser radar observation that the December event originated from the Sierra Negra eruptions, there remains another possibility that the ejecta from La Soufriire once confined to lower latitudes moved out towards higher latitudes as a result of dynamical transport processes such as the change of atmospheric circulation incidental to the sudden warming of the stratosphere. Our conclusion is supported by the information obtained by the SAGE satellite which was launched in February 1979 by NASA and which has measured aerosol and gaseous components in the stratosphere. The satellite data obtained during the ten days in December 1979, which included the day of our first detection of the disturbance, were analyzed for comparison with our results by the SAGE Scientific Team in NASA (MCCORWCK, 1981). The preliminary data which we obtained through courtesy of the SAGE Scientific Team covered the latitude range from about 20”s to 25”N with the exception of two aerosol profiles which were observed over Japan at latitudes slightly below and above Fukuoka. The presence of volcanic material was easily seen in the data throughout both the North and South Pacific as reported by MCCORMICK (1980). A dust cloud injected into the stratosphere diffuses to form a zonally uniform veil as it circulates round the earth several times (LAMB, 1970). The enhancement in extinction observed by the SAGE satellite, however, was localized to the Pacific region and was not found outside of these longitudes. This suggests that the dust cloud was caused by the Sierra Negra eruptions. The SAGE data show that the layer of enhanced extinction found at a latitude slightly below Fukuoka agrees well in both height and thickness with the layer ofenhanced scattering observed by laser radar over Fukuoka. The maximum value of the integrated aerosol scattering is 8 x lo- 5 sr ‘. If this value is converted to that of the ruby laser wavelength 0.694 pm by the law that aerosol backscattering is proportional to i-“, and then adopting the value a = 1.8according to Table 1, one obtains 1.7 x 10e4 sr-‘. This value is about six times that observed from the May 1979 eruptions of La Soufriire. The mean meridional velocities of volcanic air particles are u, = 1.53, ul, = 1.30 and uhl = 1.10 m s-l (using the notation introduced in paper I). These velocities are significantly larger than those for the previous cases shown in Table 1 of paper I. In winter a Hadley cell in the northern hemisphere extends to higher latitudes in the lower stratosphere than in other
Lidar observation of sudden increase of aerosols
seasons, and the transport of dust clouds directly from the equatorial region to the latitude of Fukuoka might be made possible by this circulation. The mean meridional wind speeds at 100 mb associated with this circulation, however, are 0.27,0.51 and 0.21 m s- ’ at 5”, 15”, and 25’ N, respectively, according to the table given by NEWELL rf al. (1972), which are significantly smaller than those obtained by laser radar observations. At the end of November 1979 a sudden stratospheric warming occurred, which was classified as a minor Canadian warming (LABITZKEet al., 1980). The center of the warming moved to the region near Japan on 10 December and again on 26 December. The influence of the warming was discernible in the variation of stratospheric temperature observed in Fukuoka around 26 December. The first successful model of stratospheric sudden warming was proposed by MATSUNO (1971). It was subsequently improved by several authors, for example, LORDI et ul. (1980), Hsu (1980, 1981) and KOHNO (1982) by using reasonable dissipation terms and realistic planetary waves of finite amplitude based on the formulation by HOLTON(1976). In their models, a forcing is introduced at a lower base level which is set at IO km (5 km by Kohno) from day zero between latitudes from 30” to 90“N. On day zero the critical surfaces where the zonal wind vanishes, are at their maximum distance from the equator at a latitude of about 18”N and an altitude of 28 km and approaches the equator at higher and lower altitudes formingasomewhat vertical structure. When a forcing with zonal wavenumber two is impressed at the base level, 10 km, the critical surface generally descends and in the range of latitudes lo”30”N it reaches an altitude of about 20 km by day 20 after the start, according to the results of LORDI et ul. (1980) and Hsu (1980). A significant polar warming amounting to about 15°C and a slight equatorial cooling takes place at about the same time, and the former agree well with the observed results at the North Pole on 27 November 1979 (LABITZKEet al., 1980). The amplitude of the planetary wave tends to be a maximum around this day when the volcanic tracer would be on its way from about 20” to 30”N. The forcing by zonal wavenumber one is less effective for the transport of heat and produces less lowering of the critical surface. Based on the above model MATSUNO and NAKAMURA (1979) showed that a stratospheric warming is caused by strong Lagrangian-mean meridional motions of air parcels along a critical surface from low latitudes to the pole with vertical motion needed from continuity relations. KOHNO (1982) shows that the meridional dispersion of the
in the stratosphere
tracer near the critical surface is greatly enhanced in the presence of amplified planetary waves of finite amplitude, because the zonal velocity is nearly equal to the phase velocity of the wave being nearly zero against ground and hence the tracer receives a consistent perturbation from the wave for a sufficiently long time. A preliminary calculation shows that meridional speed induced by the simulated wave would be adequate to explain the observed values. The increased dispersion of a tracer near the critical surface has already been shown by MAHLMAN and MOXIM (1978) based on a general circulation model, but including no mechanism for producing a sudden warming. The stationary planetary wave illustrated here is a first approximation model. In the real stratosphere there are slow variations due to the variations of global tropospheric pressure variation as shown in the preceding section. Some modifications would be necessary to observe the processes in the real atmosphere but judging from the stratospheric weather map (Institute of Meteorology, Freien, University of Berlin, 1979) the essential features will hold, as shown here. Thus the large meridional transport of the volcanic tracer in the present case may be due to the increased meridional dispersion near the critical surface deformed as mentioned above, associated with the Canadian warming. Here it should be noted that no sudden warming is reported in relation to the previous three volcanic events (SCHOEBERL,1978). The greater initial velocity compared with the previous events may be partly due to the improvement in lidar sensitivity and the short time ofthe observation, since most lidar observations were carried out during limited intervals of clear sky in the variable weather. 4. CONCLUDING REMARKS
A remarkable disturbance in stratospheric aerosols was observed by a Nd-YAG laser radar in Fukuoka initially in December 1979 with the sudden appearance of a strong scattering layer at an altitude of around 17 km. The layer was very thin and changed rapidly in scattering in the initial stage of the disturbance. From the results of two wavelength observations, we infer that the aerosols in the layer varied in size distribution at the onset of the disturbance from the usual one to one enriched in smaller particles. These characteristics of the layer indicate that the disturbance may be attributed to the activity of Sierra Negra which erupted in November 1979. Results of the analysis by diffusion theory are consistent with the inference of a Sierra Negra origin. Results of observation by the SAGE satellite which show the extensive spreading of dust
M. FUJIWARA, T. SHIBATAand M. HIRONO
clouds over the Pacific already in mid-December
suDDort our inference. The meridional II
parcels giving rise to the increase of aerosols was larger
than any previously observed. Such high speeds are suggested to be due to transport caused by the Canadian sudden stratospheric warming in November and December 1979.
Acknowledgement-The authors thank Mr J. KOHNOfor his valuable discussions and cooperation and also Mr M. UCHIUMI for his valuable assi&ance in carrying out the observations. Rawinsonde data were supplied through courtesy of the Fukuoka Meteorological Observatory.
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To be published in J. met. Sot. Japan. Beil. Berliner WettKarte, The second winter of MAP-l 1979/80,49/80, SO 9/80,2 April. Second notice of stratospheric penetration of volcanic material from eruptions of Mt. St. Helens. Private communication. Mount St. Helens eruption of 1980: atmospheric effects and potential climatic impact. NASA SP 458. Private communication.
Reference is also made to the following unpublished material: