340 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

Hurricane Squall Lines

MYRON G. H. LIGDA 1

Texas A. & M. College, College Station, Texas

ABSTRACT

Are the outer precipitation bands of hurricanes and typhoons important phenomena? In suggesting that the answer to this question is "yes>" several features of these bands are de- scribed or postulated. Methods of estimating the severity of attendant weather conditions by radar observation are offered.

N the course of a detailed analysis 2 of the a model of the surface pressure pattern which radar film made by the University of Florida must have existed. This model, indicating one I [1] of Hurricane II of 1949, it was found band, is shown in FIGURE 3. When this pattern that, as nearly as could be determined, the pas- was constructed it was recognized that if the sage of two outer precipitation bands of the storm kinks were conserved in the moving pressure sys- coincided with two dips in the barograph trace tem there should have been a momentary veering at West Palm Beach, Florida. The precipitation of the wind from the northeast to the east and bands, as detected by the radar at Gainesville, and south, then rapid backing into the northeast again pressure dips are indicated by the arrows in Fig- as a band passed over a station. Unfortunately, ures 1 and 2, respectively. no detailed wind records could be located for this particular storm, but in another storm of very similar path and characteristics analyzed by Wex- ler [2] such veering was observed, also coincident

FIG. 1. The PPI scope at Gainesville, Florida, at 2300 EST, 26 August 1949. (From Latour & Bunting.)

Assuming that similar dips in the barograph trace would have been observed at other stations over which the bands passed,3 one can construct 1 Department of Oceanography, Texas A. & M. College. 2 The analysis was carried out by the author at Massa- chusetts Institute of Technology under Signal Corps Engineering Laboratories Contract No. DA-36-039 FIG. 2. Microbarograph trace at West Palm Beach, SC-42625. Florida, during passage of hurricane of August 23-29, 3 Such fluctuations in the barograph trace are very 1949. frequently observed.

Unauthenticated | Downloaded 09/23/21 09:41 PM UTC VOL. 36, No. 7, SEPTEMBER, 1955 341 with precipitation band passage and entirely simi- lar pressure fluctuations. FIGURE 4, reproduced from Wexler's article, shows the phenomenon oc- curring at about 1700 EWT September 15, 1945, at Avon Park, Florida. The same phenomenon together with a thunderstorm was also observed at Melbourne, Florida, during the same storm, as may be seen by inspection of FIGURE 5 in the same article. Outer precipitation bands or squall lines are apparently a quasi-permanent feature in hurri- canes which may persist in the same quadrant of the storm for many hours. They may be dif- fuse or sharply defined when viewed on radar- scopes. In some hurricanes and typhoons they have not been observed at all. While their origin and cause are unknown, in some respects these outer precipitation bands have FIG. 4. Weather elements and radar bands at Avon characteristics which are similar to instability Park, Florida, associated with hurricane of 15-16 Sep- lines associated with some extratropical cyclones. tember 1945. (From Wexler.) As indicated by radar and visual observation the bands are usually convective in nature ; thunder- and severity of the hurricane. In conversation storms and tornadoes have been observed coinci- with a radar operator stationed on Okinawa dur- dent with their passage over a station. Also, they ing passage of Typhoon Nina of 1953, the writer are nonfrontal. According to the local surface learned that passage of a precipitation band over wind variations which have been observed coinci- the radar station was accompanied by wind gusts dent with their passage, there must be localized in excess of 100 mph, a velocity about double areas of strong divergence and convergence which that which was observed when the eye of the probably extend to at least moderate heights. storm passed about 60 miles southwest of the Major L. G. Starrett, in a paper delivered before station, its point of closest approach. the American Meteorological Society at Miami At the present stage of research it is nearly Beach, November 1954, reported observing a pres- sure fall and turbulence coincident with passage impossible to give quantitative rules for predic- of his aircraft through an outer precipitation band. tion of the wind velocities which may accompany these squall lines. When more detailed observa- It is suggested that passage of these bands over tions have been obtained this problem should be a station may be accompanied on occasion by brought under attack. There are, however, some weather conditions which are sufficiently severe to merit the attention of weather forecasters con- indications (obtainable from radar observation) cerned with issuing advisories on the progress which may help the forecaster evaluate the sever- ity of the squall line: 1. If the radar echo from precipitation band is narrow, continuous, intense, and has well-defined edges, wind velocities will probably be much higher and gusts will be stronger than if the echo is broad, broken, and diffuse. In the former case the band is probably young and developing, in the latter, old and dissipating. 2. Bands containing well defined weather echoes which can be detected to appreciable heights are more likely to be accompanied by high, gusty winds than those whose echoes only extend to moderate heights. 3. Bands whose echoes can be detected to great FIG. 3. Surface pressure pattern in hurricane with distances (over 100 miles from the radar) are single outer precipitation band. likely to be quite severe.

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4. Bands whose echoes are exceptionally in- quadrant of the storm. Since the most severe tense in comparison with precipitation detected bands are probably rather narrow, the time the elsewhere in the periphery of the storm are likely public needs to seek safety will normally be rather to be accompanied by severe weather conditions. short—perhaps an hour or so. When more is In the single storm studied, the outer bands learned concerning the nature and behavior of maintained their orientation, bearing and distance these bands the public should be informed in from the eye, and extent during the time they order that warnings will be correctly interpreted could be detected by radar. This, of course, may and proper protective measures can be taken. not hold true in all hurricanes and typhoons and is a subject for further study. At any rate, it ACKNOWLEDGMENT should not be a difficult matter to determine band The writer would like to acknowledge the kind- position and velocity by radar and to issue suit- ness of the University of Florida, Department of able warnings a few hours in advance of the time Engineering, in lending him their classic radar they will pass over populated areas lying in their hurricane film, without which this article could path. Again, it is pointed out that such areas not have been written. may lie many miles to one side or another of the predicted path of high winds associated with the REFERENCES eye, and people living there may have a false sense [1] Latour, M. H., and Bunting, D. C., "Radar Observa- of security as to their safety. Also, as in the case tion of Florida Hurricane, August 26-27, 1949," University of Florida, College of Engineering; analyzed, the bands may precede eye passage by Bull. Ser., Vol. 3, No. 8 (1949). many hours and catch people totally unprepared. [2] Wexler, H., "Structure of Hurricanes as Deter- mined by Radar," Ann. N. Y. Acad. Sci., Vol. 48, Bands seem to be best defined in the right front pp. 821-844 (1947).

service, student activities, etc.) of about $75 per semester. ANNOUNCEMENT Out-of-State tuition is waived. Cost of room and food is estimated to be $870 per calendar year for single students. Most Assistants will be expected to participate in The Department of Meteorology, Florida State Uni- research projects being pursued under sponsorship of the versity, announces the availability of Graduate Assistant- ships for the academic year 1956-1957. Any candidate Air Force Cambridge Research Center, the Office of for the M.S. or Ph.D. degree in Meteorology and Clima- Naval Research, and the U. S. Weather Bureau. Pri- tology is eligible to apply. No previous meteorological mary research fields are synoptic meteorology, theoreti- education is necessary, but the applicant's undergraduate cal meteorology, tropical meteorology and theoretical education must have included at least one year of calculus climatology. and one year of university-level physics. Applications for assistantships should be filed before A Graduate Assistantship carries a stipend of $1680 April 15, 1956. Later applications will be considered if per calendar year for students holding a bachelor's degree, funds are available. For further information and appli- $1980 for students holding a master's degree in meteorol- cation blanks, write Dr. Werner A. Baum, Head, De- ogy. The student is permitted to carry ten hours of partment of Meteorology, Florida State University, Talla- course work. Assistants pay resident fees (for health hassee, Florida.

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