of Cirrus Clouds Observed by Laser Radar (Lidar) during the Spring of 1987 and the Winter of 1987/88

Transcription

1 June 1991 R. Imasu and Y. Iwasaka 401 Characteristics of Cirrus Clouds Observed by Laser Radar (Lidar) during the Spring of 1987 and the Winter of 1987/88 By Ryoichi Imasu National Research Institute for Pollution and Resources, 16-3, Onogawa, Tsukuba, Ibaraki 305, Japan and Yasunobu Iwasaka Solar Terrestrial Environment Laboratory, Nagoya University, 3-13, Honohara, Toyokawa, Aichi 442, Japan (Manuscript received 20 April 1990, in revised form 11 April Abstract Laser radar observations of cirrus clouds were carried out during the spring of 1987 and the winter of at Nagoya, Japan. In most cases the integrated backscattering coefficient of the clouds ranged from 0.001(/strad) to 0.1(/strad). The maximum value of the backscattering coefficient of the clouds smoothly decreased with temperature below -40* and -20* in the spring and winter respectively, and the value of the winter coefficient was smaller than that of the spring in the temperature range below -40*. The temperature dependence of cloud depth was different in the two seasons, and the minimum depth was observed near -40*. The lowest cloud temperature was about -75* in the spring, whereas it was limited to near -50* in the winter. Double-layered cirrus clouds were frequently seen during the observation period. The upper layers were located near the tropopause, and the height differences of the two layers were about 2-3 km. Most of them were observed in the spring, and they sometimes continued for longer than a few hours. The relation between cirrus clouds and jet streams was studied. The results showed that the lower limitation of the winter cloud temperature was due to the facts that a jet stream was usually located over Japan and only the clouds below the jet stream could be seen in the winter; the clouds were frequently far below the tropopause with high wind speed area between; and the tropopause on several occasions was located below the jet stream. The winter cirrus clouds were observed around two types of jet stream. One was in the anticyclonically curved area of a meandering jet stream, and the other was in the vicinity of a strong and straight jet stream. On the basis of these results, we discuss the optical properties and generation mechanisms of the cirrus clouds in the vicinity of a jet stream. 1. Introduction Cirrus clouds have a significant influence on the radiative energy balance in the atmosphere because their optical depth is small and they frequently appear in the upper troposphere. Furthermore, these clouds play an important role in the water circulation of the upper troposphere. Many observations have been carried out on the optical and microphysical properties of cirrus cloud to understand these processes. Heymsfield directly sampled the cloud particles in various types of cirriform clouds, and presented *1991, Meteorological Society of Japan crystal habits and size distributions of these clouds particles (Heymsfield and Knollenberg, 1972; Heymsfield, 1975a, 1975b, 1986). Platt studied the optical properties of cirrus clouds in visible and infrared regions, and their temperature dependence (Platt, 1973, 1979; Platt et al., 1974, 1981, 1987, 1989). Recently, Sassen observed supercooled liquid water droplets near the base of convective cells embedded in the cirrostratus using polarization lidar and in situ aircraft measurements (Sassen et al., 1985; Sassen et al., 1989b). On the other hand, many numerical models have been developed to simulate these microphysical processes (Heymsfield and Sabin, 1989; Ramaswamy and Detwiler,

2 402 Journal of the Meteorological Society of Japan Vol. 69, No ; Sassen and Dodd, 1988, 1989a; Starr and Cox, 1985a, 1985b; Wesley and Cox, 1988). Formation of cirrus clouds near the jet stream region has been discussed mainly from a viewpoint of dynamics. It has been suggested that the generation of cirrus clouds relates to ascending motions which occur in the anticyclonic shear portion near the entrance region of the jet stream (Namais and Clapp, 1949; Murray and Daniels, 1953). Some photographs obtained from the satellites showed some features of the horizontal distribution of cirrus clouds in the jet stream regions, and discussions on the dynamics considered the relationship between cirrus clouds and a secondary circulation associated with the jet stream (Doswell III and Schaefer, 1976; Durran and Weber, 1988; Erickson, 1974; Martin and Salomonson, 1970; Ramond et al., 1981; Smith, 1971; Wexler and Skillman, 1979). In recent studies, the zonal mean and geographical distributions of high level clouds have also been presented by some investigators using satellite data (Barton, 1983; Prabhakara et al., 1988; Woodbury and McCormick, 1983, 1986). In most of the previous observations, microphysical and radiative processes of each individual cloud have been discussed. However, there have been only a few discussions on seasonal changes of cloud structure and optical properties, and on their relationships to mesoscale and synoptic organizations and the jet stream. In this paper we present some results of laser radar (lidar) measurements of cirrus clouds, and discuss their optical and structural properties. These results should be useful for calculations of earth radiation field and for the parameterization of cirrus clouds in atmospheric numerical models. We also discuss the differences of the cloud properties between the spring and winter considering the location and the speed of jet stream flow. We have taken observations of cirrus clouds since 1987 using various remote sensors, and the lidar measurements were made as a part of comprehensive measurements on these clouds. The objective of our studies is to derive information on radiative and microphysical properties, internal structure, sustaining mechanisms, and generation mechanisms of cirrus clouds associated with mesoscale and synoptic phenomena. The results obtained from the observations by depolarization techniques, radar and microwave radiometer, which provide information on cloud droplet phase, large ice particle concentration and integrated liquid water content in the clouds, respectively, will be published in following papers. 2. Observations a. The lidar system Lidar measurements of cirrus clouds were made from April 1987 to March 1989 at Nagoya, Japan Laser wavelength : 6943 (A) Laser power (max.) : 1 (J/shot) Transmitter divergence : 0.5 (mrad) Receiver aperture : 1.5 (mrad) Repetition rate : 1 (pps) A/D converter resolution : 12 (bit) Vertical resolution : 15 (m) Signal processing : Analog method b. Data analysis Backscattering light intensity was averaged over 10sec intervals. To derive the backscattering coefficient, we use an analytical method described by Fernald (1984). The boundary condition needed in this method is set near the tropopause height, where it could be considered to be aerosol free. For the scattering parameter S, defined as the ratio of extinction coefficient to backscattering coefficient, we used values varying around 20. Values of S have been selected to satisfy the condition that the backscattering coefficient would be equal to that of atmospheric molecules at a height just below the cloud base. This is based on the assumption that it is also aerosol free at this height. Sometimes S must be a smaller value than 20, such as 10 or 5, to satisfy the condition mentioned above (see Section 4 for a further discussion). In each analysis, S is assumed to be constant with altitude. The scattering ratio, which is used in the following analysis, is defined as the ratio of the total backscattering coefficient to that of atmospheric molecules at each height, and is approximately equal to the mixing ratio of light scattering materials under some assumptions. This quantity is familiar in stratospheric aerosol analysis and is useful for expressing the vertical profiles of the materials when they are not optically very thick. Errors in the backscattering coefficient values are mainly due to the scattering parameter S and the assumption that it is aerosol free at the boundary condition height. When the backscattering signals from the cloud and atmosphere are different by more than two orders of magnitude, then the error will be at least 10 percent because the resolution of the A/D converter is only 12 bits. Further, in daytime observations, errors were relatively large because of the background radiance of the sky. c. Data sets In this paper, we use the data collected during the spring of 1987 and the winter of (Table 1). In these periods, we were able to carry out the observations more continuously compared with

3 June 1991 R. Imasu and Y. Iwasaka 403 Table 1. Date of observations. other periods. When different types of cirrus clouds were observed in a day, we treated them separately and list individual profiles. In the data sets, various types of clouds such as cirrus uncinus, fibrous cirrus, optically thin cirrostratus are included because, in actuality, the definition of cirrus clouds is inaccurate. The meteorological data are based on the radiosonde measurements at Hamamatsu (about 100 km northeast of Nagoya), and in order to discuss the structure and behavior of the jet stream, the meteorological grid data of the Japan Meteorological Agency are used. There are two sets of these data every day (at 9h and 21h JST), and we use whichever is nearer the lidar observation time for each case. We follow the definition of the tropopause found in the *aerological data of Japan*. Observation was made only when there were no optically thick lower-level clouds. So, it may be that our observations do not represent all features of cirrus clouds in each period, but they may represent the gross characteristics of these clouds, at least under these situations, in both of the seasons. 3. Results First of all, we show the temperature at the region where cirrus clouds were observed in Fig. la (spring) and lb (winter). Lines correspond to the cirrus clouds, and closed and open circles depict the first and second tropopauses, respectively. As the lines in each figure are superpositions of vertically varying cirrus clouds in each case, the length of lines does not designate cloud depth directly (the same is true in the following figures). The clouds were sometimes colder than -70*in the spring, on the other hand, most of them were warmer than -50* in the winter. Very cold clouds were observed near -75* in the spring, most of which were observed in advance of the approach of a deep extratropical cyclone or a warm front. Differences were also appreciable in the vertical distance of cloud top from the first tropopause. The distances were frequently very large in the winter compared with those in the spring. Although second tropopauses often existed, the cloud tops were observed below the first tropopause in both of spring and winter except for one case. Fig. 1. Temperature ranges of cirrus clouds in the (a) spring and (b) winter. Closed and open circles indicate the first and second tropopauses, respectively. Cloud depths are shown as a function of cloud top height and temperature in Figs. 2 a and 2b, respectively. Open and closed circles indicate the spring and winter values, respectively. The impressive fea-

4 404 Journal of the Meteorological Society of Japan Vol. 69, No. 3 Fig. 2. Cloud depths versus (a) cloud top height and (b) cloud top temperature. Open and closed circles indicate the spring and winter data, respectively. Lure is that the deepest cloud can be seen at -70*, and the maximum depth for each temperature range linearly decreases with temperature in the spring. The minimum depth for each temperature range increases in proportion to the temperature difference from -40* in both of the seasons. The smallest value, 150m, was observed near -40*. All backscattering coefficient profiles are superposed in Figs. 3a (spring) and 3b (winter). These coefficients are shown as a function of temperature in Figs. 4a and 4b. The values shown in these figures do not include atmospheric molecules. It can be seen that the backscattering coefficient takes the maximum value near -40* in the spring and near -20* in the winter. Then it decreases smoothly according to the decrease of temperature. The maximum value of backscattering coefficient in the winter was smaller than that in the spring in the tern- Fig. 3. Backscattering coefficient profiles as a function of altitude for (a) spring and (b) winter. perature range below -40*. Very dense clouds were sometimes observed in the regions warmer than -40*(spring) or -20* (winter), but the entire profiles of these clouds could not be detected due to strong attenuation of the laser beam. The minimum value of the Backscattering coefficient at each height was not so meaningful because smaller values could be detected at each cloud edge in all cases, except for very stable continuous clouds. Integrated backscattering coefficients (I.B.S.C.) are plotted against cloud top height and cloud top temperature in Figs. 5a and 5b, respectively. The largest value is in the temperature range between -40* and -60*. However, for the reason as described above, the maximum value of I.B.S.C. below this range is not certain. Double-layered cirrus clouds were frequently observed during the observation period. The cloud top heights of each layer are plotted against the first

5 June 1991 R. Imasu and Y. Iwasaka 405 Fig. 4. As in Fig. 3, except for temperature. tropopause height in Fig. 6., where open and closed circles are from the spring and winter data, respectively. The circles of the first and second layers are connected by thin lines for each case. A broken line depicts the condition that the cloud top height is equal to the tropopause height. Most of these clouds were observed in the spring. The upper layers were usually near the tropopause, and the height difference of the two layers was about 2-3 km. In two cases, tripled-layered clouds were observed, and the layers were separated by about 3 km from each other. Sometimes these clouds could be continuously observed for more than one hour. An example is shown in Fig. 7, where the clouds are shown in terms of scattering ratio, all the values being smaller than 30 in this case. The wind speeds were 33 (m/s) and 20 (m/s) at the upper and lower layer height, respectively, and thus the duration of observation corresponds to horizontal spatial scales of 119 km and 72 km under the assumption that the cloud is not generated and/or dissipating within the Fig. 5. Integrated backscattering coefficients versus (a) cloud top height and (b) cloud top temperature. Open and closed circles indicate the spring and winter data, respectively. time period. We studied the relationships between cirrus clouds and jet streams to explain the difference in the temperature range of cirrus clouds observed in the spring and winter, and also to discuss the generation mechanisms of the clouds near the jet stream. Latitude-altitude cross sections of the wind speed and wind flow maps (or isopleth maps) are helpful in deriving the relative position of cirrus clouds with respect to the jet stream core, horizontally and vertically. Here we use meteorological grid data with a horizontal interval of 2.5 degrees. Examples are shown in Figs. 8a and 8b. Figures 9a and 9b show the relative positions of cirrus clouds to the jet stream core for the spring and winter, respectively. The abscissa and ordinate represent the horizontal and vertical distances from the core which is

6 406 Journal of the Meteorological Society of Japan Vol. 69, No. 3 Fig. 6. Cloud top heights as a function of the first tropopause height for double-layered cirrus clouds. The heights of the first and second layers are connected by thin lines for each case. Open and closed circles indicate the spring and winter data, respectively. A broken line depicts the condition that the cloud top height is equal to the tropopause height. Fig. 7. Temporal change of the vertical profiles of the scattering ratio for a typical case of double-layered cirrus clouds on 8 October Fig. 8. Examples of (a) latitude-altitude cross section and (b) isopleth map of the wind speed at 250 mb. This case is on 8 April Vertical lines in (a) depict the range where cirrus clouds were detected. An arrow in (a) and a closed circle in (b) indicates the lidar site. Closed circles in (a) and bold lines in (b) depict the jet stream core. set at the center of each figure. Closed and open circles indicate the first and second tropopauses respectively. In the spring, cirrus clouds are distributed in a very extensive area around the jet stream, from the northern and lower to the far southern and higher area of the jet stream. The maximum horizontal distance from the core was 700 km north and 1700 km south. Whereas, in the winter, as the jet stream was usually located just above Japan, we were. able to observe only the clouds which were located below the jet stream. The clouds were not in the very cold region. This situation strongly affected the temperature range of the winter clouds over Japan. We could not know whether the clouds existed in the north or south area of the jet stream from only one lidar site observation. Another feature can be seen in Fig. 9, namely, cirrus clouds seldom existed in a area close to the jet stream core where the wind speeds were very high. The cloud-free area around the core was larger in the winter than in the spring. The wind speeds of the jet stream were, on the whole, higher in the winter than in the spring during the observation period. When the tropopause was located above the high wind speed area, cirrus clouds were below the jet stream, with a clear area between. Consequently, the cirrus clouds were located far below the tropopause and they were in the warmer region. This situation occurred more frequently in the winter than in the spring. Furthermore, tropopauses were frequently located below the jet stream, and cirrus clouds topped out at that lower height. It can be said that the limitation of cloud temperature in the winter is as much due to the low location of the clouds, caused by the facts described above, as due to the horizontal location of the jet stream.

7 June 1991 R. Imasu and Y. Iwasaka 407 Fig. 9. Relative positions of cirrus clouds to the jet stream core. Abscissa and ordinate are the horizontal and vertical distance from the jet steam core, respectively. The jet stream core is centered. Closed and open circles indicate the first and second tropopauses, respectively for (a) spring (b) winter. The horizontal locations of the jet stream core are shown in Figs. 10a and lob. Thin and thick lines are subtropical and polar jet stream cores, respectively. A closed circle indicates the location of the lidar observation site. About one fourth of winter cases showed the very meandering character of the subtropical jet stream. In the rest of the cases the flow was very straight and wind speeds were very high in the winter. Most of the former cases occurred in December, when the lidar site was covered by an anticyclonically-curved flow area. To examine the moisture fields near the jet stream, we made isopleth maps of relative humidity from meteorological grid data. Absolute values were sometimes less than water or ice saturation values in spite of the existence of clouds. So here we only use the latitude-altitude cross sections of 50* value of relative humidity as an index of large-scale moisture fields. The results are shown in Fig. 11 for (a) spring cases, (b) anticyclonically curved jet cases Fig. 10. Horizontal location of the jet stream core. Thin and thick lines indicate the polar and subtropical jet stream. A closed circle is the lidar site. (a) spring (b) winter. in the winter and (c) strong and straight jet stream cases in the winter. In general, the upper regions beyond the lines were drier than lower regions. In the cases (a) and (b), the wet tongues invaded into the upper troposphere to the south of the lidar site, and cirrus clouds were observed at the poleward edges of the tongues. Whereas, in more than half of the (c) cases, there were no wet tongues, and it can not be said there that the large scale field near the lidar site was moist in the upper troposphere. 4. Discussion Concern over the effect of cirrus clouds on global thermal balance has increased, and various computer simulations have been made including that effect. Cloud top heights of cirrus clouds are frequently assumed to be just below the tropcpause in those models. However such an assumption has not been fully justified on the basis of observation. Present observations showed that the cloud top height was frequently far below the tropopause in winter, and the temperature of the clouds was

8 408 Journal of the Meteorological Society of Japan Vol. 69, No. 3 It is very interesting to compare our results taken in Japan, the midlatitude in the northern hemisphere, with that of Platt's results. They showed the temperature dependence of the cloud properties in terms of mid-cloud temperature instead of cloud top temperature. If the cloud depth distribution of Fig. 2 is plotted against the midcloud temperature, it shows the maximum at -35 to -40*, and this feature is very similar that obtained by Platt et al. (1981, 1987). Platt related the I.B.S.C. to IR emittance in two steps as follows (Platt, 1973; Platt et al., 1987). First, I.B.S.C. is related to the infrared absorption coefficient at irn a through a linear factor cti. where, S is the ratio of extinction to backscattering coefficient at the laser wavelength. Second, * is related to the emittance * through the equation Fig. 11. Latitude-altitude cross sections of the 50 % isopleth of relative humidity. (a) spring, (b) anticyclonically curved jet stream cases in the winter, (c) straight jet stream cases in the winter. Arrows depict the latitude of the lidar site. warmer than that at the tropopause height. Based on the analyses of jet streams shown in Section 3, we pointed out that the vertical location and temperature of cirrus clouds were as much strongly affected by the location and wind speed of the jet stream as the altitude of the tropopause. These points should be considered in the model calculations of terrestrial radiation. Heymsfield and Platt (1984) pointed out that some drastic changes in the crystal habit and the optical properties of cirrus clouds occur in the temperature range from -40 to -50 C. We showed in Fig. l that the temperature range of cirrus clouds was warmer than -50 C in the winter. So. consequently, it might be said that these drastic changes in the cloud properties seldom occur over Japan in winter. To confirm this, direct sampling of cloud particles should be done in future studies. Recently, Platt et al. (1987) presented the mean optical and structural properties of cirrus clouds based on their observations taken at a midlatitude in the southern hemisphere and in a tropical region. The values of the I.B.S.C. were frequently in the range from to 0.1(/strad) in our study, as shown in Fig. 5. If we use the value of 20 for the ratio of extinction/backscattering, the I.B.S.C. corresponds to 98.0 * to 13.5 * of transmittance in the atmosphere. If the I.B.S.C. of Fig. 5 are plotted against the mid-cloud temperature, they take a maximum value of about 0.1 near -40* and then decrease with temperature to a value of near -65*. If we adopt a value of 1.95 for c as used in the studies of Platt and Dilley (1981), the emittance can be evaluated to be 0.64 and 0.05, corresponding to the 0.1 and of I.B.S.C.. If we use 3.5 for a as deduced from observations by Platt et al. (1987), E comes to be 0.44 and Compared with Platt's studies (1987), the emittance in the colder case (-65*) of our study is smaller, whereas, that in the warmer case (-40*) is larger than that of Platt's results. Therefore the slope of the emittance against temperature in our study is larger than that of Platt's. However, the variations are within the observational margins of error. It is necessary to measure the IR optical properties of the clouds simultaneously, and independently, with visible ones to evaluate the emittance of the clouds more directly and deduce the linear factor cr for many types of cirrus clouds. The extinction/backscattering ration, S, used in our analyses corresponds to the reciprocal of k/n, not k/2rn which was used in Platt's studies (Platt, 1973; Platt et al., 1987; Platt and Spinhirne, 1989). In Fernald's method (Fernald, 1984) S is used after being multiplied by the factor of 2. Sometimes we had to use values smaller than 20 (10 or 5) to satisfy the condition described in Section 2b. This is probably due to the *anomalous* backscattering caused by

9 June 1991 R. Imasu and Y. Iwasaka 409 small liquid water particles or horizontally oriented ice plates which produce a high specular reflection of the laser beam. We mentioned that an area more than one hundred km in extent may sometimes be covered by double-layered cirrus clouds, and the optical depth of the clouds was not thick. Such a situation has not been taken into consideration in the previous studies. It is to be expected that the atmospheric radiative process including double-layered clouds will be more complex than that containing only singlelayered ones. But we could not obtain information on the generation mechanism of double-layered clouds in this study. Further investigations are needed on this. With aircraft probing, it was found that the preferred area of cirrus cloud occurrence was from 4 to 6 degrees south of and 1.5 to 3 km below the jet stream core (McLean, 1957; Doswell III and Schaefer, 1976). Compared with their studies, the cirrus clouds observed in our study were located more to the south and upward from the jet stream core in the spring, as shown in Fig. 9a. It is not certain whether the cirrus clouds very distant from the jet stream were influenced by the jet stream. Furthermore, it is impossible to know the actual distribution of cirrus clouds around the jet stream at a point of time from only one-site lidar observations. So, simultaneous observations at many latitudes and over a range of at least 1000 km are needed to make clear the spatial distribution of the clouds around the jet stream and to discuss the generation of the clouds. The theoretical dynamic studies have explained that the generation of cirrus clouds on the equator side of the jet stream are related to the secondary circulation accompanying ascending motion near anticyclonically curved flow areas. Recently, Durran and Weber (1988) offered that wet southerly flow is also necessary to generate the cirrus clouds in the ascending motion area. However, in many of the winter cases of our study, described as case (c), the jet steam flow was very straight and it was not so moist to the south of the jet stream as shown in Fig. 10 and 11. These situations were somewhat different from the basis for the above theories. Therefore, other explanations for the generation of these cirrus clouds must be explored. Possible explanations are that some very strong ascending motion exists and which overcome the dry condition, or that a radiative cooling mechanism is dominant. 5. Conclusion In this paper we show the results of lidar observations of cirrus clouds taken in the spring of 1987 and the winter of The cloud temperatures sometimes reached down to about -75* in the spring, whereas they were limited to near -50* in the winter. We pointed out that this limitation was due to the fact that jet streams usually located just above Japan and only the clouds below the jet stream could be seen in the winter; the clouds were frequently far below the tropopause with a high wind-speed area between; and the tropopause where the clouds were topped out was on several occasions located below the jet stream. The temperature dependence of cloud depth was different in the two seasons, and the minimum depth was observed near -40*. The maximum value of backscattering coefficient of the clouds smoothly decreased with temperature below -40*and -20* in the spring and the winter, respectively, and the value of the winter was smaller than that of the spring in the temperature range below -40*. The emittances of the clouds were tentatively evaluated from I.B.S.C.. The temperature dependences of cloud depth and emittance are compared with those of Platt's observations (1981, 1987). The former is very similar to that of Platt's and the later is somewhat different, but it is within the observational margins of error. Double-layered cirrus clouds were frequently observed during the observation period, mainly in the spring. The upper layers were located near the tropopause, and the height difference of the two layers was about 2-3 km. They sometimes continued for longer than a few hours. We point out that the atmospheric radiative process including those clouds would be more complex than that containing singlelayered ones. The relative positions of cirrus clouds in relation to the jet stream core were studied. In the spring, they were distributed over a very extensive area, from a northern and lower to a far southern and higher area than the jet stream. This area is larger than that reported by McLean (1957) as a preferred area of the clouds. The winter cirrus clouds were observed around the two types of jet stream. One cloud distribution was in the anticyclonically curved area of a meandering jet stream, and the other distribution was in the vicinity of strong and straight jet streams. For the latter cases, some new explanations of the generation of the clouds are needed. We would like to emphasize that simultaneous observations at many stations are needed in order to make clear the actual spatial distribution of cirrus clouds around a jet stream. Further investigations on the generation mechanisms of the double-layered cirrus clouds and the winter cirrus clouds around the strong and straight jet streams would also be interesting. References Barton, I.J., 1983: Upper level cloud climatology from an orbiting satellite. J. Atmos. Sci., 40,

10 410 Journal of the Meteorological Society of Japan Vol. 69, No. 3 Doswell III, C.A. and Schaefer, J.T., 1976: On the relationship of cirrus clouds to Jet stream. Mon. Wea. Rev., 104, Durran, DR. and Weber, D.B., 1988: An investigation of poleward edges of cirrus clouds associated with Midlatitude Jet streams. Mon. Wea. Rev., 116, Erickson, C.O., 1974: A Jet stream cirrus shield. Mon. Wea. Rev., 102, Fernald, F.G., 1984: Analysis of atmospheric lidar observations: some comments. Appl. Opt., 23, Heymsfield, A., 1975a: Cirrus uncinus generating cells and evolution of cirriform clouds. Partl: Aircraft observations of the growth of the ice phase. J. Atmos. Sci., 32, Heymsfield, A., 1975b: Cirrus uncinus generating cells and evolution of cirriform clouds. Part 2: The structure and circulations of the cirrus uncinus generating head. J. Atmos. Sci., 32, Heymsfield, A.J., 1986: Ice particles observation in cirriform cloud at -83 C and implications for polar stratospheric clouds. J. Atmos. Sci., 43, Heymsfield, A.J. and Knollenberg, R.G., 1972: Properties of cirrus generating cells. J. Atmos. Sci., 29, Heymsfield, A.J. and Platt, C.M.R., 1984: A parameterization of the particle size spectrum of ice clouds in tems of ambient temperature and the ice water content. J. Atmos. Sci., 41, Heymsfield, A.J. and Sabin, R. M., 1989: Cirrus crystal nucleation by homogeneous freezing of solution droplets. J. Atmos. Sci., 46, Iwasaka, Y., 1985: Laser radar system for atmospheric studies at Syowa Staton, Antarctica. NEC Res. Dev., 76, Martin, FL. and Salomonson, V.V., 1970: Statistical characteristics of subtropical Jet-stream features in terms of MRIR observations from Nimbus 2. J. Appl. Meteor., 9, McLean, G.S., 1957: Cloud distributions in the vicinity of jet streams. Bull. Amer. Meteor. Soc., 38, Murray, R. and Daniels, SM., 1953: Transverse flow at entrance and exit to jet streams. Quart. J. Roy. Meteor. Soc., 79, Namais, J. and Clapp, P.F., 1949: Confluence theory of the high tropospheric jet stream. J. Meteor., 6, Platt, C.M.R., 1973: Lidar and Radiometric observations of cirrus clouds. J. Atmos. Sci., 30, Platt, C.M.R., 1979: Remote sounding of high clouds: 1. Calculation of visible and infrared optical properties from lidar and radiometer measurements. J. Appl. Meteor., 18, Platt, C.M.R. and Bartusek, B., 1974: Structure and optical properties of some middle-level clouds. J. Atmos. Sci., 31, Platt, C.M.R. and Dilley, AC., 1981: Remote sounding of high clouds. 4: Observed temperature variations in cirrus optical properties. J. Atmos. Sci., 38, Platt, C.M.R., Scoott, J.C. and Dilley, A., 1987: Remote sounding of high clouds. Part 6: Optical properties of midlatitude and tropical cirrus. J. Atmos. Sci., 44, Platt C.M.R. and Spinhirne, J.D., 1989: Optical and microphysical properties of a cold cirrus cloud: Evidence for regions of small ice particles. J. Geophys. Res., 94, Prabhakara, C., Fraser, R.S., Dalu, G., Man-Li, C.Wu and Curran, R.J., 1988: Thin cirrus clouds: Seasonal distribution over oceans deduced from Nimbus- 4 IRIS. J. Appl. Meteor., 27, Ramaswamy, V. and Detwiler, A., 1986: Inredependence of radiation and microphysics in cirrus clouds. J Atmos. Sci., 43, Ramond, D., Corbin, H., Desbios, M., Szejwach, G. and Waldteufel, P., 1981: The dynamics of polar Jet streams as depicted by the METEOSAT WV channel radiance field. Mon. Wea. Rev., 109, Sassen, K. and Dodd, G., 1988: Homogeneous nucleation rate for highly supercooled cirrus cloud droplets. J. Atmos. Sci., 45, Sassen, K. and Dodd, G., 1989a: Haze particle nucleation simulations in cirrus clouds, and applications for numerical and lidar studies. J. Atmos. Sci., 46, Sassen, K., Lion, K.N., Kinne, S. and Griffin, M., 1985: Highly supercooled cirrus cloud water: Confirmation and climatic impactions. Science, 227, Sassen, K., Starr, DO. and Uttal, T., 1989b: Mesoscale and microscale structure of cirrus clouds: Three case studies. J. Atmos. Sci., 46, Smith, A.H., JR., 1971: A cyclonically curved jet stream. Mon. Wea. Rev., 99, Starr, DO. and Cox, S.K., 1985a: Cirrus clouds. Part 1: A cirrus cloud model. J. Atmos. Sci., 42, Starr, DO. and Cox, S.K., 1985b: Cirrus clouds. Part 2: Numerical experiments on the formation and maintenance of cirrus. J. Atmos. Sci., 42, Wesley, D.A. and Cox, S.K., 1988: Radiative processes in upper tropospheric mixed-phase clouds. J. Atmos. Sci., 45, Wexler, R. and Skiliman, W.C., 1979: Satellite detection of a long curving cirrus plume. Mon. Wea. Rev., 107, Woodbury, G.E. and McCormick, M.P., 1983: Global distributions of cirrus clouds determined from SAGE data. Geophys. Res. Lett., 10, Woodbury, G.E. and McCormick, M.P., 1986: Zonal and geographical distributions of cirrus clouds determined from SAGE data. J. Geophys. Res., 91,

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