April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 133

Relationships Between -Detected Maritime Precipitation and Satellite Viewed Cloud Cover'

By Sidney M. Serebreny and Roy H. Blackmer, Jr. Aerophysics Laboratory, Stanford Research Institute, Menlo Park, California (Manuscript received 9 October1967)

Abstract

Radar data collected by a network of ships, shore stations and aircraft over the Eastern Pacific from mid-February to the end of June 1965, have been studied. Analyses of these radar data and concurrent TIROS IX photographs were made. The data sample included deep cyclones with extensive radar detected precipitation, weaker cyclones with localized rainfall, cold anticyclones with extensive air mass showers, and blocking anticyclones with no precipitation. In the latter case it has been found that the appearance of the cloud cover is a good indicator of areas of anomalous radio propagation. Models have been prepared that illustrate the varying patterned association of cloud and rainfall characteristic of such synoptic situations. Such associations range from comparable cloud-precipitation areas through scattered showers where only a portion of the clouds contain precipitation, to complete absence of precipitation within large areas of low stratiform clouds or fog as in a blocking anticyclone.

determination. However, on a gross scale, 1. Introduction the brightest clouds are found to have been There have been a number of studies of associated with areas of precipitation." concurrent radar and satellite data conducted Other studies such as those by Nagle and by various groups. At Stanford Research Serebreny (1962), Whitney (1963), Blackmer Institute, analyses were made of radar data (1961), Fujita and Izawa (1967), and Hiser and collected around the clock by a widespread Senn (1965), have compared satellite viewed network of radar stations during the period cloud cover and concurrent radar data for TIROS I was taking useful photographs. various types of situations. The data used Some of the problems of working with data were from TIROS satellites similar to TIROS from this satellite were the varying angle I, hence the investigators were still hampered of view at which photographs were taken, by the problems cited above. the lack of daily coverage over a given loca- Recent satellites of the "cartwheel" type tion, and difficulties in determining the geogra- provide continuous coverage in overlapping phical location of the photographed cloud strips, and at the present time a number of cover. In spite of these difficulties, a number such satellites are in operation so that there of cases were studied. The results of these is complete coverage more frequently than studies as reported by Nagle and Blackmer once a day. In addition to improvements in (1962) were that "no characteristics are photographic coverage and frequency of such found that will allow a specific determination coverage, there have also been improvements to be made of clouds that are of a precipitat- (such as APT systems) in making the photo- ing nature, although the limitations of the graphs available to various users. TIROS I data may have precluded such a The purpose of this study was to determine the relationships between cloud photographs 1 These studies were supported by the U.S. obtained from a polar orbiting meteorological Navy Project FAMOS and the National Weather satellite and precipitation echoes observed by Satellite Center (ESSA) Applications Group. radar. Further, to establish whether techni- 134 Journal of the Meteorological Society of Vol. 46, No. 2

P Ocean Station `Papa' (Canadian)

1 U.S. Navy Radar Picket Station 1

3 U.S. Navy Radar Picket Station 3

5 U.S. Navy Radar Picket Station 5

1 U.S. Navy Radar Picket Station

9 U.S. Navy Radar Picket Station 9

N Naselle Air Force Station

K Klamath Air Force Station

SAC Sacramento (USWB)

SXC Santa Catal ina Island (USWB)

Fig. 1. Radar network. ques could be developed for specifying areas This paper emphasizes results of the studies of precipitation and severe weather within a of data from and in the vicinity of the Radar given satellite-observed cloud situation. Picket Ships. The radar used f pr data collec- The present study was conducted using tion aboard these ships was an SPS-8 (8 cm) cloud photographs from TIROS IX (supple- set. The vast majority of echoes detected by mented by data from TIROS VII and VIII) this radar were from precipitating regions between mid-February and mid-June 1965 (122 within the cloud cover. There were a few days). TIROS IX was a polar orbiting satel- occasions, however, when the echo appeared lite-the first of the cartwheel type. Due to to be from dense cloud with no precipitation the orbital characteristics, the same area of visible below the cloud base. the earth's surface was photographed each day at approximately the same time. The area 3. Classification of radar echoes in which we were interested was the eastern Radar films from all stations were examined Pacific Ocean and the west coast of the United for each day to determine whether or not there States. Fig. 1 shows the location and ap- were precipitation echoes at or within two proximate area of coverage of the radar sites hours of the time of the satellite photographs. from which radar data were collected. When echoes were present, the appearance of the echoes at the station with the most 2. Data widespread precipitation was compared with The basic data used in this study were the the cloud configuration shown by the satellite radarscope photographs from the shipborne photographs and the locations of frontal sys- and shore based radar sites shown in Fig. 1 tems as given by the 0000 GMT surface maps and the daily photographs over the area taken prepared at the U.S. Weather Bureau at San by TIROS IX. To aid in interpreting these Francisco International Airport. From this basic data we also examined copies of surface evidence the radar echoes were classified as and upper air observations taken by the ships to whether they were produced by precipita- and shore stations and consulted the surface tion associated with frontal systems (either and upper air charts routinely prepared at warm fronts or cold fronts), or by air-mass the U.S. Weather Bureau at San Francisco shower-type precipitation (see Table 1). In International Airport. this examination it was found that on occasion April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 135

Table 1. Number of days with echoes from frontal Table 2. Satellite photographs data. or air-mass precipitation.

* One apparent squall line and four days with rotating echoes at a low pressure center. the analyzed positions of fronts did not coin- cide with the frontal cloud bands. Such lack of coincidence could be attributed to one or more of the following ; first, the map analysts might not have had access to the satellite photographs and, lacking other data in the vast Pacific Ocean, could have difficulty in determining the precise locations of fronts. Secondly, the time difference between the satellite photographs and the 0000 GMT sur- face map, depending on the speed of move- meridian through the radar station on ment of the frontal systems, could make a the satellite photograph. difference between the positions of photo- c. Superimposed radar detected precipita- graphed cloud bands and analyzed frontal tion (in schematic form) over the por- systems. Thirdly, there is the possibility tion of the associated concurrent satel- that the pre-computed grids for some of the lite photograph within the 150 n. mi. satellite photographs may not be correct, so range of the radar shown by the dashed that the indicated geographical location of a ellipse. frontal band on the gridded satellite photo- d. Selected radiosonde data. graph is not the true geographical position. Finally, it is not unlikely that some differen- Warm fronts. In our sample of 122 days ces could exist between the location of analyz- there were only 13 days with warm fronts ed fronts (based on pressure, wind and tem- with radar detectable precipitation within perature) and the location of frontal cloud range of one or more of the radar network bands and associated radar echoes. For ex- stations. ample, Ligda, et al, (1957) found differences The satellite viewed cloud cover on these in frontal location as determined by map 13 occasions generally showed extensive over- analysis in contrast to radar positioning in cast areas of stratif orm cloud cover within the central where abundant which there were localized areas of much data permit accurate frontal location. brighter cloud. Fig. 2 shows warm frontal cloud cover and 4. Precipitation distribution within satellite radar echoes on 24 April 1965. The frontal viewed cloud cover analysis (on Fig. 2a) shows the warm front Significant features of the cloud cover and extending southeastward across the area of radar echoes for selected situations from our coverage of one of the radar stations (PS 3). data sample are illustrated as follows : In the vicinity of the analyzed surface frontal a. Photographs of satellite viewed cloud position, there is a widespread area of strati- cover (Table 2 lists appropriate data form clouds with embedded regions of bright- for identifying the satellite photographs.) er clouds. The radarscope photograph (Fig. b. Photographs of concurrent radar detect- 2b) shows widespread echo, much of which ed precipitation oriented so that north is stratiform, but there are regions of more on the radarscope is parallel to the convective echo. When the echoes are superim- 136 Journal of the Meteorological Society of Japan Vol. 46, No. 2

Fig. 2. Warm frontal cloud cover and radar echoes on 24 April 1965. posed on the cloud (Fig. 2c), they fall within view of any upper clouds was obscured by an the areas of brighter, more convective-appear- overcast of stratocumulus with a ceiling at ing cloud. Light rain showers were reported 1500 feet. at this station at 1800 GMT and 2100 GMT. Knowledge that an extensive area of cloud Radarscope photographs from PS 5 (five de- cover was produced by warm frontal over- grees further south) showed precipitation running (in contradistinction to other similar echoes northwest of the ship to ranges of 140 appearing widespread areas of cloud well re- miles and rain was reported at PS 5 during moved from any frontal activity), should lead the previous six hours at both 1800 GMT and one to suspect substantial widespread preci- 2100 GMT. pitation within the clouds. There was no upper air sounding from PS 3 at 0000 GMT on the 25th, however, a sound- Warm sectors. Cloud photographs and radar ing at PS 5 (Fig. 2d) shows a shallow, moist, echoes of warm sector precipitation (relatively surface layer, then a dry layer from the 950 infrequent in our data sample) are illustrated mb level to the 780 mb level, then a deep, in Figs. 3 and 4. Both TIROS VIII (Fig. 3a) moist layer extending to the 300 mb level. and TIROS IX (Fig. 4a) photographed cloud As a result, there are probably clouds extend- cover over the same area approximately 35 ing to high altitudes although the observers minutes apart. Differences in appearance of

Fig. 3. Warm sector cloud cover (TIROS VIII) and radar echoes on 11 April 1965. April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 137

Fig. 4. Warm sector cloud cover (TIROS IX) and radar echoes on 11 April 1965.

the clouds are most likely due to differences bands and their relatively small width serve in the camera equipment aboard the satellites as an indication that these clouds do not con- rather than short period changes in the clouds tain much precipitation. themselves. The cloud photographs show a cold frontal band, a warm frontal band and Cold Fronts. Figures 5a and 6a show a por- rifts in the clouds in the warm sector. The tion of a cold frontal cloud band oriented radar echoes (Figs. 3b and 4b) in this case northeast-southwest and extending across two show that the most extensive echo is with the of the radar stations. It is a considerable dis- almost isolated segment of cloud cover in the tance from the low pressure center and is warm sector that is very close to the radar composed of the number of individual narrow station. The lack of extensive echoes in the lines of brighter cloud. The echoes (Figs. 5b frontal cloud bands could be attributed to the and 6b) within this frontal band appear as fact that the height of the radar beam in the narrow lines. No precipitation was reported frontal zones could be so great that only the by observers at PS 3 ; at PS 1 drizzle ended tops of the highest echoes were being detected. before 1800 GMT, so much of this echo must While the soundings (Figs. 3d and 4d) show be only from dense clouds. The soundings, almost saturated air to at least the 400 mb (Fig. 5d and 6d) show conditions close enough level, the streaked appearance of the cloud to saturation to support the presence of cloud

Fig. 5. Cold frontal cloud cover and radar echoes (PS 1) on 12 April 1965. 138 Journal of the Meteorological Society of Japan Vol. 46, No. 2

Fig. 6. Cold frontal cloud cover and radar echoes (PS 3) on 12 April 1965.

Fig. 7. Cold frontal cloud cover and radar echoes on 3 March 1965. cover through a deep layer. The striated rear of the band correspond very closely to appearance of the satellite-viewed cloud cover the size and shape of the bright cloud elements is very similar to that of the echo pattern (Fig. 7c). The sounding (Fig. 7d) was taken (see Fig. 5c and 6c) if one discounts the limita- in the cold air behind the cold front and tions of the radar in detecting distant echoes. shows conditions suitable for development of Figure 7a shows another example of a cold showers up to approximately the 600 mb level. frontal cloud band oriented north-south and extending across PS 9 at the southern end of Showers. A common feature of maritime the radar network. The radar echoes (Fig. cyclones is the presence of widespread shower 7b) are from showery-type cold frontal pre- activity in the cold air to the rear of the cipitation aligned in bands. The line of echoes cyclone. The clouds and associated precipita- within the cloud band east of the ship did not tion echoes in this region vary in size with vary in width as it moved across the radar- distance behind the cold front. The largest scope, showing that there was only a narrow cells usually occur where the cold air has the band of precipitation concentrated at the trail- greatest vertical extent (usually under the ing edge of the cloud. The cloud band over center of the cold dome) and, depending on and west of the ship has showers throughout latitude and season, may contain hail or snow its entire width. The elongated echoes at the pellets. The largest cells usually extend April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 139

Fig. 8. Air mass cellular cloud cover and radar echoes (PS 1) on 9 April 1965.

Fig. 9. Air mass cellular cloud cover and radar echoes (P5 3) on 9 April 1965. almost to the tropopause above the cold dome. south of PS 3 (see Fig. 9a). The radar echoes Previous studies by Kreuger and Fritz (1961) are not quite as extensive as the clouds, be- and Serebreny, Wiegman and Hadfield (1962), cause the latter may be in different stages of have shown that these cells develop in an development, some actively precipitating, some environment in which the wind speed is usual- probably not sufficiently developed for pre- ly less than 30 knots and variations in speed cipitation to have begun, and some possibly and direction along the vertical are small. dissipating. Further, the cloud elements are The size, extent, and brightness of the clouds not all the same height, so that at long ranges contribute to very unique polygonal patterns, the radar beam is above the tops of the clouds easily distinguishable from the smaller rows of smaller vertical extent. The sounding (Fig. and other cellular patterns of cumulus clouds 8d) shows a nearly adiabatic lapse rate and nearer to the cold front. low tropopause characteristic of the center of Figures 8a and 9a show cellular clouds over the cold dome. a vast area of the eastern Pacific on 9 April. Figure l0a shows convective cloud cover as On this day echoes from shower type precipita- photographed in a given area (near 45°N, tion (Figs. 8b and 9b) were also evident on 135°W) by three TIROS satellites over a 31 the radarscope of PS 5, which is 300 miles minute period on 13 April 1965; TIROS VIII 140 Journal of the Meteorological Society of Japan Vol. 46, No. 2

at 2012 GMT, TIROS IX at 2024 GMT and trayed by the different satellites are un- TIROS VII at 2043 GMT. doubtedly due to the relative angles of view, A comparison of the cloud cover (near 45•‹N, differences in resolution, and time changes in 135•‹W) from these three satellite photographs the cloud elements themselves. The distinct shows that they clearly portray the same S-shaped band of cellular cloud cover is

general appearance. Variations in the ap- coincident with the cold low at the 500-mb pearance of individual cloud elements por- level. This cloud cover portrays the dissipat-

Fig. 10. Comparisons of cellular clouds and shower type radar echoes. April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 141

ing stages of an earlier widespread vortical showed up rather brightly on the TIROS IX cloud cover associated with a well developed photograph. The TIROS VII photograph at low pressure area. 2043 GMT still show the cloud element very Five radarscope photographs (Fig. 10b) distinctly and at 2052 GMT the radar showed taken at PS 3 (45•‹N, 135•‹W) between 1950 a well-defined echo from precipitation within GMT and 2052 GMT show a relatively uniform the cloud element. echo pattern during the period. They offer In contradistinction, the cloud element an opportunity to evaluate changes in the marked '2' on the the TIROS VIII and TIROS echoes from precipitation within this cloud IX photographs contained large amounts of cover, shedding some light on short period detectable precipitation prior to the time of changes in cloud elements. Blackmer (1961) the TIROS IX photograph. The echo overlay found 15-minute radarscope photographs use- on the TIROS VII photograph shows that ful in determining continuity in cloud elements radar detectable precipitation no longer exists between successive hourly cloud photographs. within this cloud element. In Figure lOc pairs of echoes shown in Fig. One could conclude that in an area of such lOb are combined in order to assess variations satellite viewed cloud cover, the showery in motion, changes in size, and formation and precipitation will be extensive. However, dissipation o radar-detectable precipitation. individual showers may e short-lived. There- Earlier echoes are shown in black and the fore, from inspection of a satellite photograph later ones in white. at a given instant it is highly unlikely that This series of comparisons shows that one could state specifically which cloud is within this relatively uniform-appearing pat- precipitating. tern, new echoes develop to replace those that However, for short periods (one to two dissipate. The overall number of echoes ap- hours) there is a high degree of probability

pear relatively constant. of precipitation from the majority of the cloud In general, the echoes are moving from the elements in such patterns of cloud cover. northwest at speeds approximating the geo- strophic wind speeds obtained from the contour Cellular clouds with no precipitation. spacing at the 700 mb level. Figure 11 illustrates a situation in which In Figure lOd radar echoes close in time to little or no precipitation occurred. At the the respective satellite photographs are surface there was a well-defined high pres- superimposed over the cloud elements to il- sure area centered just west of PS 5. At lustrate how much of the cloud cover contains the 500 mb level, there was a ridge of mari- radar detectable precipitation at a given time tropical just to the west of the network time, also to demonstrate short term changes with northwesterly flow prevailling over most in precipitation patterns in relation to the of the network. The associated subsidence short term differences in the appearance of inhibits the growth of large cellular clouds the cloud cover. over the network. Thus, in contradistinction From the above it becomes apparent that to the previous photographs of cumulus cloud with this type of cloud cover the vast majori- cover, this cloud cover is composed of very ty of cloud elements contain radar detectable small, closely spaced cumulus containing either

precipitation. The short period changes are no radar detectable precipitation or very few graphically illustrated by a comparison of small showers. changes in radar detectable precipitation Figure lla shows a clear area west of PS within the cloud elements marked '1' and 5 that is associated with the subsidence close '2' in all three photographs . In the first to the center of the high at the surface and

photograph (TIROS VIII) the cloud element aloft, (Figs. llb and llc, respectively). The marked '1' contained only a slight amount sounding at PS 5 (Fig. lld) shows the strong of radar detectable rainfall seven minutes inversion and drying out of the air associated

prior to the satellite photographs. Sixteen with the strong subsidence above the 900 mb minutes later the radar detectable precipita- level. East of the clear area and over most tion had decreased although the cloud element of the Picket Ship network, there are numer- 142 Journal of the Meteorological Society of Japan Vol. 46, No. 2

Fig. 11. Cloud cover with little or no precipitation. ous small, closely spaced cumulus cloud com- and precipitation near the center of a mature plexes none of which contained precipitation cyclone. The surface and 500 mb charts detectable by the aboard the ships. (Figs. 12a and 12b) locate the cyclone center very close to PS 3. The cloud photograph Precipitation patterns at the center of cyclones. (Fig. 12c) shows the vortical portion of the In or near cyclone centers, the radar echo characteristic cloud cover over the station. patterns vary according to the stage of de- The echoes (Fig. 12d) near the center of velopment of the systems. Showers, or an this well-developed cyclone are from wide- absence of echoes, may be the case when spread areas of precipitation that exhibit a the vortex cloud pattern is no longer distinct banded rappearance and these bands show a or possibly even absent. Where clouds are marked curvature. During the period between present, they may appear as an ill-defined 2022 GMT and2314 GMT, the echoes show a vortical system of small cellular clouds (see counter-clockwise rotation around the storm Fig. lOa). center. When this series of radarscope photo- A well-developed, deepening cyclone is usual- graphs was viewed as a movie, the rotation ly characterized by the so-called "comma of the echoes was dramatically apparent and shaped" cloud cover. Near the cyclone cen- the center of rotation could be precisely ter, the precipitation echoes will clearly re- located. This center of rotation was coincident flect the rotational motion within the system. with the low pressure center in the lower Figure 12 shows the distribution of clouds levels of the atmosphere. April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 143

Fig. 12. Cloud cover and radar echoes near a cyclone center.

The sounding at PS 3 (Fig. 12e) shortly The satellite photograph in Fig. 13 shows after the time of the last radar echo exhibit- a typical cloud vortex system containing a ed a fairly uniform lapse rate up to the cold frontal band, and the convective clouds tropopause (approximately the 240 mb level). in the cold air to the rear of the cyclone. Saturation conditions (and probably extensive Radarscope photographs that we have found clouds) existed up to about the 500 mb level. from a large number of situations to be characteristic of the precipitation associated Models. Extensive studies of concurrent with different parts of the cloud cover are cloud photographs and radar echoes make it indicated with lines connecting cloud cover to possible to construct a first approximation to radar echoes. None of these are for the same an operationally useful model. Figure 13 day as the satellite photograph since our illustrates the probable distribution of types radar network was so distributed that it was of precipitation associated with an extra- impossible to sample the complete spectrum tropical maritime cyclone system. The model of echoes around a given cyclonic system at is centered on the low pressure center, in- any one time. cludes the rotating echoes characterizing this The three radarscope photographs labeled part of the low, and the distribution of echoes (A) show typical echoes close to the storm both upstream and downstream from the low. center. These echoes are from a widespread 144 Journal of the Meteorological Society of Japan Vol. 46, No. 2 April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 145 area of precipitation that exhibits a banded appearance. These bands show a marked curvature and over a period of time exhibit couter-clockwise rotations. The echo (B) within the cloud band, a short distance from the center, is from an elongated band of pre- cipitation whose width may be nearly as great as that of the cloud band. Farther along the front, there may be only scattered shower type precipitation associated with the frontal band. Ligda, Serebreny, and Nagle (1961) found similar echo distribution around spiral echo patterns less than 100 miles in extent, i, e., sharp-edged echoes close to the center of the spiral and diffuse echoes along the portion of the spiral arm farthest from the center. In a vortex of the type illustrated, any re- maining warm front is quite vestigial. Indeed, it is questionable if there is any warm front Fig. 14. Schematic sections of cloud and radar with the majority of mature cyclones in the echo distributions with a cold front. eastern Pacific. For purposes of completing the cyclone model, the echo labeled (C) (which is of the type usually found with warm fronts) Blocking Anticyclones. Large maritime is shown at the most likely point of any re- quasi-permanent anticyclones occasionally de- maining warm front. These echoes are quite velop into blocking anticyclones. Figure 15a widespread and indicate intermingled strati- shows the 500 mb chart (000 GMT 11 March f orm and convective precipitation. Such 1965) accompanying such a blocking anti- precipitation, it should be emphasized, is most cyclone in the eastern Pacific. Figure 15b common with developing low pressure systems shows the cloud cover associated with the in which the frontal structure tends more system. In the vicinity of the high pressure toward an open wave rather than occluded center, an extensive clear area is evident due system. As the system occludes, the warm to the intense subsidence extending to the frontal cloud area usually decreases in size, surface. Alder and Serebreny (1964) found leaving only a single frontal band of cloud numerous clear areas in excess of 500,000 extending from the vortex center. square miles associated with such blocking The radar echoes (D) show small showers patterns. Along the periphery of the clear just behind the cold front and close to the area there is dull, indistinct stratiform cloud low pressare center where cold air is in close cover resulting from intense subsidence ex- proximity to the vortical portion of the cloud tending almost to the surface. Actually this cover. Echoes from showers in the cold air cloud cover is very low stratus or stratocumu- behind the cold front (E) are quite large and lus or fog. observations show that they may contain The sounding (Fig. 15d) at PS 1 (0000 GMT copious rainfall, hail, or snow pellets depend- 11 March 1965) illustrates the very strong ing on the latitude and season. inversion as well as the intense humidity Figure 14a illustrates schematically the gradient existant in the lowest layers of the slope of the frontal boundary and the ver- atmosphere. tical extent of post cold frontal cumulus. No radar precipitation echoes were noted at Figure 14b is a plan view of the distribution any of the network stations when they were of these clouds ; the locations of radar de- located in or near the center of a block. tected precipitation echoes identified as D and However, on occasion, radar echoes from E in Fig. 13 are also shown. distant land masses and from the ocean sur- 146 Journal of the Meteorological Society of Japan Vol. 46, No. 2 April 1968 Sidney M, Serebreny and Roy H. Blackmer, Jr. 147 face at extended ranges are observed. Such with distance behind the cold front and have echoes are caused by the trapping of the lifetimes up to two hours. The largest (which radar beam within the low moist layer below may exceed 16 miles in diameter) usually the intense temperature and humidity gra- occur where the cold air has the greatest dients resulting from the extreme subsidence. vertical extent (usually under the center of Radio propagation associated with these at- the cold dome). These clouds generally ex- mospheric conditions is known as anomalous tend almost to the tropopause (bounding the propagation. An example of anomalous echoes cold dome), and grow in an evironment in as observed at PS 1 is shown in Fig. 15c. which the wind speed is usually less than 30 knots with small variations in both speed and 5. Summary and Conclusios direction along the vertical. The size, extentt The situations illustrated in this report in- and brightness of these clouds result in unique clude those with extensive precipitation and patterns of cloud cover easily distiguishable those with little or no precipitation. In such from that of the smaller rows of cumulus situations, very localized areas of cloud cover clouds in closer proximity to the cold front. often may have very similar appearances. The distribution of precipitation echoes is Therefore, the examination merely of a small nearly comparable to the distribution of the area of cloud cover is not sufficient to de- largest, brightest cloud elements, especially termine whether there may be precipitation when they are arranged in polygonal cells ; within a given cloud cover. It is necessary agreement in cases with smaller, less bright to examine the surrounding cloud cover over cloud elements is not as good. In some in- a broad area to determine whether the cloud stances, no precipitation echoes are found cover in a given localized area is part of front- within these less well-developed, (smaller) al bands or is far removed from any active convective cells located either in closer pro- storm systems. The significant broadscale ximity to the cold front or sometimes inter- cloud features are vortical clouds denoting spersed among a well-defined field of large, cyclone centers, bands extending outward bright cloud elements. from the centers denoting the locations of Thus, in the vicinity of mature maritime fronts, and extensive convective clouds to the extratropical cyclones, there will be exten- rear of the cyclone center. If a given local sive banded precipitation within the cloud area of cloud cover is part of one of these band spiraling outward from the storm cen- broadscale cloud features, the following pre- ter and showers in the cold air to the rear cipitations distributions may prevail. of the major band. With such mature sys- In warm frontal cloud cover, the brighter tems, the rainfall generally is as extensive as cloud areas are likely to contain precipitation the cloud cover. which is probably widespread throughout the Cloud cover that is neither frontal nor cold warm frontal cloud shield. Close to the cen- air mass cells may show variations in bright- ters of low pressure, those portions of cold or ness of stratif orm decks, or convective ap- occluded fronts that exhibit extensive brighter pearing cells of various sizes. However, with patches of convective cloud interspersed among this type of cloud, there is much less likeli- the more stratiform cloud, generally contain hood of precipitation. An example of such areas of widespread precipitation, the width cloud cover is that associated with quasi- of which may be nearly as great as the cloud permanent anticyclones of large meridional band. Farther along the front, where the extent and persistence (blocking anticyclones). clouds seem to be arranged more in rows, In this case, large clear areas are observed some clouds look bright enough to contain or the cloud cover pattern is one of dull, in- echo, but the striated appearance serves as a distinct stratiform clouds and fog. This lat- clue to the likelihood of only local precipita- ter type of cloud cover usually indicates that tion or scattered showers, or on occasion, the extensive subsidence associated with clouds in which precipitation does not reach these anticyclones has nearly reached the the ground. surface. Under such circumstances, precipita- Showers within the cold air vary in size tion does not occur.

, S. M. Serebreny, and R. E. Nagle, 1961: Weather radar systems research Final Report Contract AF 19(604)-3067, Stanford Research Institute, Menlo Park, Calif., 82 pp. [DDC AD 267 539 or OTS $8.60] Nagle, R. E., and R. H. Blackmer, Jr., 1962: The use of synoptic-scale weather radar observations in the interpretation of satellite cloud observa- tions. Final Report, Contract AF 19(628) -284, Stanford Research Institute, Menlo Park, Calif. 255 pp. [DDC AD 298 113 or OTS $17.00] and S. M. Serebreny, 1962: Radar precipi- tation echo and satellite cloud observations of a maritime cyclone. J. Appl. Meteor., 1, 279-296. Serebreny, S. M., E. J. Wiegman, and R. G. Hadfield, 1962: Investigation of the operational use of cloud photographs from weather satellites in the North Pacific. Final Report Contract Cwb- 10238, Stanford Research Institute, Menlo Park, Calif., 83 pp. Whitney, L. F., Jr., 1963: Severe storm clouds as seen from TIROS, J. Appl. Meteor., 2, 501-507.

148 Journal of the Meteorological Society of Japan Vol. 46, No. 2

Riser, H. W., and H. V. Senn, 1965: Meso-scale Acknowledgments synoptic analysis of radar and satellite meteoro- The research reported here was made pos- logical data. Final Report, Contract Cwb-10622, sible by the efforts of the , University of Miami, Miami, Fla., 51 pp. Air Defense Command, Environmental Science Krueger, A. F., and S. Fritz, 1961: Cellular cloud Services Administration, and the Canadian patterns revealed by TIROS I, T ellus, 13, 1-7. Ligda, M. G. H., et al., 1957: Middle-latitude pre- Department of Transport. We wish to thank cipitation patterns as observed by radar (a the officers and men of Radar Picket Squad- collection of composite radar observations), ron One, personnel manning the Canadian Scientific Report No. 1, Contract AF 19(604)- ships at Ocean Station "Papa," and person- 1564, Texas A&M College, College Station, nel at ADC and ESSA radar stations, for their 142 pp, efforts and cooperation in taking radarscope photographs. We also wish to thank the late Dr. M.G.H. Ligda, Manager, Aerophysics Laboratory, for his constant interest, suggestions and guidance in the collection and interpretation of the con- siderable quantity of radar data.

References Alder, J, E., and S. M. Serebreny, 1964: The synop- tic climatology of cloud-free areas: Scientific Report 1, Contract AF 19(628) -2363, Stanford Research Institute, Menlo Park, Calif., 78 pp. [DDC AD-433 504, OTS $8.60] Blackmer, R.H., Jr., 1961: Satellite observations of squall line thunderstorms. Proc. Ninth Weather Radar Conf., Boston, Amer. Meteor. Soc., 76- 82. Fujita, T., and T. Izawa, 1967: A model of ty- phoons accompanied by inner and outer rain- bands. J. Appl. Meteor., 6, 3-19.

レーダ で探 知 され た降水 域 と衛 星に よる雲 域 の関係

1965年 の2月 中 旬 よ り6月 末 の期 間,太 平 洋 東 部 に お け る航 空 機,沿 岸 観 測 所,船 舶 レ.一一.ダの 観 測 資 料 を 集 め, これ らとTirosIXの 写 真 を併 せ て解 析 した｡資 料 の範 囲 は広 範 囲 の降 水 エ コ ー を も った 低 気 圧,局 所 的 に しか エ

コー の な い 弱 い 低 気 圧,気 団性 し ゅ う雨 を 伴 った 寒 冷 高 気 圧 ,降 水 を伴 わ な いBlocking高 気 圧 な どを 含 ん でい る｡ 最 後 の場 合 で は 雲 の掩 って い る所 で 電 波 の 異 状 伝播 の あ る こ とが わ か った. ま た上 述 の各 状 態 に あ う雲 と降 水 特 性 に 関 す る関 係 を モ デ ル 化 した. 結論 の概 要 を 次 に のべ る. 局部 的 な 領 域 の 雲 を み る と,全 く同様 に 見 え る とき で も,降 水 領域 は 広 い場 合 か らゼ ロま で 一定 で は な い.雲 は 従 っ'てそ の周 囲 の 状 況 も含 め て 広 範 囲 の 特 徴 を 観 察 す る必 要 が ～ちる.例 え ば,そ れ が前 線 性 バ ソ ドの 一部 で あ る か, April 1968 Sidney M. Serebreny and Roy H. Blackmer, Jr. 149

又 は他 の活 発 な擾 乱 の 一 部 で は な い か な ど を調 べ る必 要 が あ る｡重 要 な パ ター ソ(広 域 ス ケー ル)は 低 気 圧(中 心 の わ か る よ うな)性 渦 状,フ ロソ ト(そ の位 置 がわ か る)性 パ ソ ド状,低 気 圧 中 心 の後 側 に あ らわ れ る対 流 性 雲 の拡 が りで あ る｡若 し広 域 ス ケ ール の一 部 分 の写 真 し か な い場 合 は: 温 暖 前 線 性 の雲 で は,明 る い部 分 は降 雨 を伴 っ て お り,低 気 圧 中 心近 くで は,寒 冷 又 は 閉 塞 前 線 の位 置 に 一様 な 雲 の 中 に よ り輝 い た班 点 状 の 雲 が ひ ろ が って い る と きは 降 水 が 伴 って い る｡更 に,フ ロソ トに伴 う雲 列 で は 一見 降 水 が あ る よ うに 明 る く見 え て も溝(や 縞)の あ る よ うな形 はむ し ろ降 水 の 少 い,例 え ば ご く局 所 的 か,散 在,時 折 り の しゅ う雨,又 は地 上 に達 し な い 降 水 な どの 兆候 とみ て よい｡

寒 気 内 の し ゅ う雨(1ife timeは2時 間 ま で で あ る｡)は 前 線 か らの距 離 に よ っ てそ の大 き さ が かわ っ て い る｡最 大 の 雲 塊(直 径16哩 を こす よ うな)は 普 通cold domeの 中心 部 の よ うな寒 気 の厚 み の最 大 の と ころ で現 わ れ る｡こ の よ うな 雲 は,一 般 に殆 ん ど成 層 圏 ま で達 し,(風 速30ノ ッ ト以 下 の と き)そ の周 囲 に ま で 広 が る｡

雲 が 特 に 多角 形 細 胞状 の 配 列 を な す と きは,明 るい 部 分 と降 水 分 布 は 一 致 す る｡(た だ し,弱 い 小 さい 雲 で は一 致 しな いO)｡明 るい 大 きい 雲 域 や 前 線 の近 傍 で は降 水 が な か ったcaseが あ る｡

海 洋 性(熱 帯 外)の 低 気 圧 では,そ の中 心 か ら出 る スパ イ ラル 状 の降 水 帯 や この 主 バ ソ ドの後 側 の 寒 気 内 で 発達 す る し ゅ う雨 は,雲 の 領 域 とほ ぼ 等 しい 雨 域 を も っ て い る｡