The Clear-Air Coastal Vespertine Radar Bands

Henri Sauvageot and Gilbert Despaux Universite Paul Sabatier, Observatoire Midi-Pyrenees, Laboratoire d'Aerologie, Toulouse, France

ABSTRACT

It is known that the insects observed in the lower layers of the atmosphere produce radar echos and that their spa- tiotemporal distribution is strongly influenced by the meteorological conditions and the thermodynamic structure of the atmosphere. However, the available information on the subject essentially deals with the continental zones. Obser- vations carried out from the Atlantic coast with a polarimetric A^-band Doppler radar are used to show the existence of large band-shaped areas of insects above the coastal Atlantic Ocean, which has a reflectivity 3 x 102 times higher than the values usually observed inland. The insects collectively fly with a common heading. These bands form at dusk and are connected with the coastal atmospheric circulation, which carries along the local insect population.

1. Introduction cous dissipation range of the atmospheric turbulence. At ground level, A, is larger than 2 cm and increases It is well known that so-called clear-air radar ech- at a higher level (Strauch et al. 1984). Thus, AT bands oes are associated with microwave scattering by tur- are not concerned with Bragg scattering. bulent inhomogeneities of the air refractive index on With the narrow beamwidth of a meteorological a scale close to half the radar wavelength (A/2), com- radar, birds, because of their low numerical concen- monly reffered to as Bragg scattering, and are also tration, are a marginal and uncommon source of clear- associated with biota—that is, birds and insects (see air echo (Vaughn 1985). Bird echoes are active fly- the review in Gossard and Strauch 1983; Vaughn ers and cannot be used as a tracer of air motions. 1985; Sauvageot 1992, among others). The term For insects two cases can be distinguished: large "clear air" is a misnomer because these diverse causes insects and small ones. Large insects have a velocity, can be present together in the same volume of clear relative to the air, large enough to control their mo- air but also in the same volume of cloudy air (Knight tions with respect to the ground. Most of them remain and Miller 1993). Thus, interpretation of the clear-air close to the ground and are not seen with radars. Only echoes and of the nonprecipitating, low-reflectivity colonies of long-distance migrating insects flying aloft cloud echoes is sometimes ambiguous. well above the ground can be seen with the radars. It Inhomogeneities of the air refractive index are seems that large insect migrations occur preferentially ubiquitous in the atmosphere, as much in convective at night, with a common heading, usually downwind as in stratified regions. However, Bragg scattering is (Schaefer 1976; Riley and Reynolds 1986; Drake and only detectable by the radars working in frequency Farrow 1988). Radar observations of such migrating bands with A/2 larger than A , the cutoff wavelength insects are not common. Large insects are not valid at the limit between the inertial domain and the vis- tracers of air motions (Achtemeier 1991). Due to a common heading, the radar returns of migrating in- Corresponding author address: Dr. Henri Sauvageot, Universite sects are strongly dependent on polarization orienta- Paul Sabatier, Observatoire Midi-Pyrenees, CRA, 65300 tion. Thus, for a permanent horizontal polarization, the Lannemezan, France. In final form 23 October 1995. radar returns are dependent on the azimuth of the an- ©1996 American Meteorological Society tenna beam.

Bulletin of the American Meteorological Society 673

Unauthenticated | Downloaded 10/09/21 12:43 AM UTC Small insects are those having a near-zero veloc- front attributed to Bragg scattering at S band has also ity relative to the air (insects without wings or small been described by Meyer (1971). spiders on threads) or one slow enough so that it can Above the onshore sea-breeze layer, an offshore be assumed that they are passively carried in the air return-wind layer is often observed. The circulation or flying randomly and the bulk velocity relative to is closed by a subsiding region above the sea. The the air in a volume is negligible. In this case, insects location, structure, and horizontal extent of this region can be used routinely as accurate tracers of air mo- are poorly known. No data or results about the bio- tion (Wilson et al. 1994). Small insects are almost logical field associated with the offshore part of the everywhere in the atmosphere over land at levels coastal cell in sea-breeze conditions have been pub- where the air temperature is above freezing. Their lished. The aim of the present paper is to present some numerical concentration is strongly influenced by at- observations and discussions about this aspect of the mospheric structure, so that sometimes they are also coastal circulation from an experiment performed with structure tracers, notably for convective cells in the a polarimetric Doppler K -band radar. boundary layer and for stratified layers (Johnson 1969; Campistron 1975; Drake and Farrow 1988; Sauvageot 1992). The main axis of small insects is preferentially 2. The radar data horizontal rather than vertical, but orientation may be in any horizontal direction. So echoes from small in- In July and August 1987, the RABELAIS radar was sects are polarization dependent. The reflectivity usu- set on the French Atlantic coast, south of Bordeaux, ally associated with the population of small insects is at the Centre d'Essais des Landes (CEL) (a French low (see section 3). rocket-testing center) (1°16'W, 44°17'N) (Fig. 1). In Above the oceans, far from coasts, it is reasonable to think that there are no or few insects, with the no- table exception of colonies of migrating insects flying aloft. However, African grasshoppers have been reported in the Carribean, and it has been demonstrated that some species are able to migrate over very long dis- tances, sometimes more than a thousand kilometers without touching ground (Drake and Farrow 1988). In the coastal areas, when no large-scale forcing is present, a sea/land-breeze circulation can develop (see Atkinson 1981; Simpson 1994). In the daytime this mesoscale circulation is associated with an onshore wind, the well-known sea breeze, about which many observational studies have been published (Simpson etal. 1977; Carroll 1989; Bantaet al. 1993, among others). The less-studied corresponding night- time offshore wind is the land breeze (Ohara et al. 1989; Zhong and Takle 1992, among others). The coastal mesoscale circulation works as a closed dy- namic cell. In sea-breeze conditions, the inland end of the onshore low-level wind layer is a low-level convergence line, the sea-breeze front, sometimes topped with a line of cumulus clouds. Because it is a region of higher concentration of insects, the sea- breeze front has been frequently detected by meteorological radars, either moving inland during the growing phase of the sea breeze or in a stationary position located typically at a distance of about 20 km from the coast (Drake and Farrow 1988; Wilson et FIG. 1. Map of the observation site. The star shows the location al. 1994). A case of a radar-observed land-breeze of RABELAIS radar. CEL is inside the 25-km circle.

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Unauthenticated | Downloaded 10/09/21 12:43 AM UTC this area, the coast is straight, approximately in a flective material (the proportion of water is 50% to north-south direction. The country east of the radar 70% of the total weight). With insects, a length (ma- is a wide, flat pine forest with an altitude lower than jor axis) to diameter (minor axis) ratio of the liquid- 100 m with three small lakes and a tidal basin, the containing parts of the body of 1:1 to 10:1 can be en- Bassin d'Arcachon. The radar was on the first dune countered. The values of polarimetric radar param- ridge, at an altitude of 10 m, and could thus perform eters that can be expected are smaller in theD2 region low-elevation scans above the sea. RABELAIS is a than in the D6 region of a variations. meteorological polarimetric Doppler radar that oper- The physical quantity appearing in the next sec- ates at 35 GHz (wavelength 8.6 mm) with a very thin tions, in the figures showing the reflectivity distribu- beam (aperture 0.4° at 3 dB). The peak power is 70 tion of the insect population, is the equivalent radar kW with a pulselength of 0.3 /is. The pulse repetition reflectivity factor Ze expressed in dBZ. In reflectivity _1 frequency is 3125/2688 s . The range of unambigu- r], Ze can be converted from the usual relation ous Doppler measurement is ±40 m s-1. The mean (Sauvageot 1992, 112): Doppler velocities are estimated through a pulse-pair processor. The antenna polarization can be linear hori- 77 =2.8 x 10"10 A"4Z ' e zontal (H), linear vertical (V), circular right, or cir- -1 6 3 cular left with commutation at the pulse repetition fre- with r\ in cm , X in cm, and Ze in mm nr . Here, Z quency. For Doppler and conventional reflectivity (in dBZ) = 10 log [Z (in mm6 nr3)]. measurements the polarization used is linear H. Among other programs, RABELAIS was used to observe the dynamics of the coastal atmospheric cell 3. The insects in the coastal cell from the velocity measurements of the insects or from chaff sent at diverse levels with a mortar. At 35 GHz On the Atlantic coast of France, the general atmo- in clear air there is no contribution to the reflectivity spheric circulation is from west-southwest. During the of Bragg scattering, as pointed out in section 1. Ob- summer 1987 campaign when the ambient flow was servations with the RABELAIS radar on strongly tur- prevailing at all levels—that is, when maritime air was bulent air, in and above a very powerful heat source, advected onshore and the coastal cell could not de- and at short distance confirm that even in such extreme velop—no echo (i.e., no insect) was observed above conditions, not the slightest Bragg scattering echo is the sea west of the radar, even at a short distance. This detected (Sauvageot et al. 1982). Thus there is no confirms that there are no small insects in maritime air. ambiguity about the nature of clear-air returns. When no significant large-scale zonal wind was For the interpretation of the reflectivity measure- blowing, in clear-air conditions, the coastal circula- ments from insects, things are not so simple. This tion developed. Figure 2 shows the typical wind vec- point is discussed by Riley (1985) and by Vaughn (1985), who showed the plots of the measured radar backscattering cross section eras a function of the target weight for most of the insect data published up to 1985. In Vaughn's figure, the curve of a for a water sphere at 35 GHz is shown. In the K band, the first mode of Mie's backscattering for a water sphere is at a diameter D = 0.3 A (where A is the radar wavelength) = 2.74 mm, corresponding to an insect weight of about 1.5 x 10~2 g. For the K band, the a of insects ranges from the Rayleigh scattering region up to the beginning of the optical region—that is, D = 3 X = 26 mm. For the Rayleigh region, a is proportional to D6, while for a size larger than 0.3 A—that is, in Mie scattering region—a oscillates around a mean proportional to FIG. 2. Typical wind vector hodograph (with the winds 2 pointing inward) observed on the Atlantic coast at CEL in the D . From the point of view of radar detection, insects presence of a coastal circulation (19 August 1987). The lakes of can be approximated to elongated (prolate) ellipsoids Cazaux and Parentis have an influence on the hodograph at CEL. with an horizontal major axis, made up of highly re- The points are labeled in hours UTC.

Bulletin of the American Meteorological Society 675

Unauthenticated | Downloaded 10/09/21 12:43 AM UTC sides of the coast is due to the fact that the land is the source area of the insects, and their numerical concentration is thus lower above the ocean. The bound- aries of the continuum horizontal extent are determined by the radar sensitivity and the radar proper- ties of the scatterer (size and concentration) and, of course, do not imply that there are no insects beyond. In Fig. 3, the shape of the echo continuum is elongated in the north-south direction (i.e., the echoes seem dependent on the azimuth of the radar beam), which suggests that the insect population shows some tendency for a preferential orientation along the east- west direction. The sea-breeze front was occasionally observed about 10-15 km inland. It was identified as a line of higher echoes in the reflectivity field associated with a line of convergence in the velocity field. In these conditions it is possible to visualize the velocity fields corresponding to the horizontal move- ments on the coastal circulation. In the present paper, Doppler velocities are positive when the motion is toward the radar. Figure 4 shows the mean Doppler FIG. 3. Typical plan position indicator (PPI) distribution of the velocities observed from insects on 12 August 1987 radar reflectivity factor in dBZ at 1351 UTC on 12 August 1987 at about 1340 UTC in the vertical scan (RHI) toward in sea-breeze conditions. The angle of elevation is 2.6°. Range the east and west. At that time the sea breeze was well markers are 5 km apart. The color step is 5 dBZ. established, and its intensity was maximum. In Fig. 4 the sea-breeze layer is just above the surface up to a height of about 500 m, with a maximum velocity of 8 tor hodograph observed at CEL near the surface for a m s_1. Above the sea breeze, between 500 and 1000 24-h cycle, with the usual turning associated with the m of altitude, the return current with a maximum ve- Coriolis effect (Haurwitz 1947). During the night and locity of about the same value as the sea breeze can the early morning, a weak land breeze was observed, be observed. This return current conveys insects off- with a maximum velocity of 1-3 m s-1. The numeri- shore. In Fig. 4a, the sea-breeze layer provides a bet- cal concentration of insects carried in this offshore ter signal than the return flow (the velocity is continu- current was low. At dawn this current decreased with ously determined in the sea-breeze layer, not in the time, and at midmorning, the circulation reversed from return flow). A possible reason could be an insect land breeze to sea breeze. Figure 3 shows the typical concentration higher in the sea-breeze layer than in reflectivity distribution at midday in sea-breeze con- the return flow if the lower layer is thinner than the ditions on 12 August 1987; the radar was surrounded upper one. (The depth of each layer cannot be mea- by an echo continuum. The radar reflectivity was un- sured with great accuracy using a radar in quasi-hori- der 5 x 10~n cm-1. The echoes occupied a layer that, zontal scan working near the limit of detection). although variable from one day to another, had an Above the return current, between 1000 and 1800 m average thickness of 1500 m above the surface. The of altitude, an eastward flow is observed. This flow horizontal extent of the layer, measured from the ra- is the general zonal air circulation. This observation dar, was about 5 km over the ocean west of the coast shows that insects are not confined in the coastal cell and 10 km over the ground in the east. This is small but are carried to higher levels, maybe through dy- for clear-air echoes in comparison to the heights and namic instability mixing processes at the upper limit lengths that can be observed with a A^-band radar in- of the return current where wind shear is present. The land in summer at midlatitudes (typical values are available data do not enable satisfactory clarification 3000-4000 m for altitude and 20-40 km for horizon- on this point. tal length). The asymmetrical distribution on both It can be concluded from above that the insects

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Unauthenticated | Downloaded 10/09/21 12:43 AM UTC appearing in the sea breeze are the insects that have about 5-10 dB between the west-northwest directions been carried offshore by the return current. However, and the south-southwest directions. In the vertical the subsiding area closing the sea-breeze cell circu- plane (not represented), the thickness of the echo zone lation above the sea in the west was never observed is very low, under 450 m above the water. from insect echoes during the day (up to mid- The Doppler velocity of the echo band, shown in afternoon). Fig. 7 at 1741:08 UTC, indicates that the mean horizontal motion of the band is southward and is thus almost parallel to the coast, with a speed of 4-5 m s_1. 4. The vespertine radar bands The horizontal wind measured with an oceanographic buoy, situated 5 km west of the radar, was southward As indicated in Fig. 2, during the afternoon, the at 1800 UTC and had a velocity of 5 m s"1, in agree- wind vector veers (clockwise) in such a way that from ment with the radar measurement. These values sug- midafternoon (1700 UTC) up to 0100 UTC the wind gest that the insects did not have a significant veloc- vector at low levels is from northwest to north. Fig- ity relative to the air. ure 5 shows as an example the radial Doppler velocities observed on 12 August 1987 at 1518 UTC just above the surface (the antenna beam elevation is only 0.5°). The maximum velocity (~8 m s_1) toward the radar is from an azimuth of about 340°. Because the beam is very low on the horizon, it is intercepted by the dunes and other ground obstacles in the east half plane. A curiously shaped colored distribution, look- ing like a butterfly, results from it. During this period, echo bands with an approximately north-south direction appear above the sea, between the coast and a distance longer than 50 km away from the coast. The location, width, reflectivity, and duration of these bands change from one case to another. Figures 3, 4, and 6-8 concern the case of a large band observed on 12 August 1987. Figure 6 represents the reflectivity field at 1740:32 UTC when the phenomenon had reached its maximum width and intensity. The scan is almost a horizontal distribution, as it was obtained from a grazing-angle azimuthal scan above the sea. (The elevation angle is 0.2°. That is, it corresponds to the angular half aperture of the beam). In Fig. 6, the reflectivity is fairly homogeneous. The mean radar reflectivity is 77 = 1.6 x 10~8 cm-1 with 7 -1 maximum values of 77•ma x =5x10 cm . Maximum values have to be considered cautiously because the presence of birds in the bands, as insect predators, is probable in some cases and certain, as the radar detected it,' in another cases. For the K a band,' birds are in the optical scattering domain. They have large backscattering cross sections and no polarization depen- dency. So rj is about 3 x 102 times higher than the 77 observed inland in the coastal area. Following the FIG. 4. Radial mean Doppler velocities in meters per second east-west direction, the highest values are on the east- observed from insects on 12 August 1987 at 1340 UTC in RHI ern side, which corresponds to an abrupt limit. Fol- (top) eastward (azimuth 93°) and (bottom) westward (azimuth _1 lowing the north-south direction, the distribution is 271°). Range markers are 5 km apart. The velocity step is 1 m s . qualitatively the same, with, however, a decrease of The arrows show the direction of air motions.

Bulletin of the American Meteorological Society 677

Unauthenticated | Downloaded 10/09/21 12:43 AM UTC In Fig. 8, the values of ZDR expressed in dB are positive and higher (4-5 dB) for the scanning directions of the west-

northwest sector. Here, ZDR regularly decreases for the scanning direction turning south-southwest. The minimum value of 0 dB is for the 200° azimuth.

Such aZnL)K_ distributio n can be associated with a scatterer population made up (at least partially) of prolate ellipsoids, whose major axis is horizontal, and which are preferentially oriented along the mean direction of the band—that is, north-south and thus in the direction of the mean wind. The axial ratio of the el-

lipsoid corresponding to the ZDR maximum value is about 0.35 if Rayleigh scattering is assumed (this point is justi- fied further). The hypothesis used for the calculation is that the ellipsoid was a mixture of air and water, containing 60% water. These ellipsoids are seen broadside for the west-northwest beam directions and by tail or by head for the south- southwest beam directions. All these observations can be inter- preted without ambiguity as being due to an insect population orientated, on average, in the 200° azimuthal direction, which is more or less than that of the mean wind direction (within about 20°). FIG. 5. Radial mean Doppler velocities in meters per second observed on 12 Such an interpretation is in agreement August 1987 at 1518 UTC above the surface (PPI). The antenna beam elevation with the conclusions of previous inves- is 0.5°. Range markers are 10 km apart. The velocity step is 1 m s_1. tigators, concerning the reflectivity dif- ferences as a function of the sighting direction of insects flying at night at a low altitude and with a common orientation Figure 8 is important in order to confirm the na- (Riley 1985; Mueller and Larkin 1985). However, we ture of the scatterers. It represents the differential do not know if the coincidence between the common reflectivity ZDR on 12 August 1987 at 1741:52 UTC, heading of the insects and the wind direction is fortu- in exactly the same plane as Figs. 6 and 7 and at the itous or intentional. No in situ entomological identi- same time. The quantity ZDR is defined (Seliga and fication of the insect species was done either in the Bringi 1976) by band or inland. However, the proximity of a wide pine forest suggests that species associated with this kind

zDR = 10 log (vu of biotope could be present. An evaluation of the numerical concentration of the where r/H and r\y are the reflectivities measured in a insects is possible, if it is assumed that the insects in horizontal or vertical linear polarization, respectively. this band are the same as those forming the continuum This quantity depends on the form of the scatter- observed close to the radar (Fig. 3). This hypothesis ers and on their common degree of orientation. It is is logical since the bands are observed only when the independent from their numerical concentration. coastal circulation develops. The analysis of P, the

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Unauthenticated | Downloaded 10/09/21 12:43 AM UTC FIG. 6. Radar reflectivity field (PPI) in dBZ just above the sea FIG. 7. Radial mean Doppler velocities (PPI) in meters per (the angle of elevation is 0.2°) observed on 12 August 1987 at second observed on 12 August 1987 at 1741:08 UTC in the same 1740:32 UTC. Range markers are 10 km apart. The vespertine conditions as Fig. 6. The step of velocity is 0.5 m s~l. band is beyond 20 km west of the radar.

2 2 power received by the radar as a function of the radar an and cj is 5.3 x 10~ cm . Such a value corresponds target distance (/?), shows that in the continuum ob- to small insects that can be approximately considered served around the radar, discreet targets can be found as belonging to the Rayleigh scattering domain at 35 (i.e., only one insect at a time in the pulse volume, GHz. In these conditions, the corresponding a value 4 and hence, Pr is proportional to R~ ) up to a distance for the wavelength X - 3.2 cm can be calculated since of about 3 km, then distributed targets beyond (i.e., 2 Pr proportional to R~ ). It is thus found that the back- 2 2 J 0.86 V scattering cross section is about 2.5 x 10~ cm for an °"3.2 ~\~y2~ J °"0-86, insect seen by tail or by head and 8 x 10-2 cm"2 for an insect seen broadside. The numerical concentration of insects around the radar (i.e., at a distance from the where Australia. This value is also quoted as a high density 6 for a depth of 450 m under the assumption of the by Dickison et al. (1986). uniformity of the insects size, it is found that the mass The mass of insects in the band of Fig. 6 can be of insect in the band is about 96 x 103 kg. estimated using the same hypothesis as above. The In August 1987, 12 cases of bands were observed. mean value of CJ (i.e., the arithmetic mean between The intensity of the phenomenon (width, reflectivity,

Bulletin of the American Meteorological Society 679

Unauthenticated | Downloaded 10/09/21 12:43 AM UTC day to compensate for the sea breeze is probable: the bands seem composed of the small insects (not large migrating ones) that the coastal circulation has carried away toward the open sea but does not bring back to land because of the shift in the wind direction at low levels at the end of the day. Thus, the insects stratify above the ocean and possibly disappear into the sea.

5. Conclusions

The coastal atmospheric circulation has been observed from the Atlantic coast of the Bay of Biscay with a polarimetric Z^-band Doppler radar. In this band, Bragg scattering is not possible, so clear-air returns are only due to insects. In the presence of a strong eastward onshore wind flow inhibiting the development of the coastal circulation, no insects are observed above the coastal ocean. When the ambient flow is weak in such a way that the coastal atmospheric circulation develops, radar returns

FIG. 8. Differential reflectivity (ZDR) in dB observed on 12 from insects can be observed in the sea-breeze flow, August 1987 at 1741:52 UTC in the same plane (PPI) as Figs. 6 even at the beginning of the circulation in the morn- and 7. The step for ZDR is 0.6 dB. ing. This provides strong evidence that the sea-/land- breeze circulation carries along the local insect population. duration) and the distance from the coast increase with When the sea-breeze circulation has formed dur- the intensity of the coastal circulation. In all cases, the ing the day, large and shallow band-shaped areas of orientation of the insects, observed through ZDR mea- strong, clear-air insect echoes are observed above the surements as indicated above, is approximately north- sea. These bands appear when the coastal wind vec- south, which was always that of the wind. After the tor in the boundary layer has veered to a southward first detection, which generally occurs between 1700 direction, roughly parallel to the shore line. The bands and 2000 UTC, the band reflectivity intensifies while are polarization dependent. They are made up of rela- the layer depth above the sea decreases by a lower- tively small insects carried along by the coastal atmo- ing of the layer top. Thus, the reflectivity reaches a spheric circulation. The air motion, because of its di- maximum, then the phenomenon enters a receding urnal veering of the low-level wind, does not bring phase, which can continue part of the night. The band back the insects to the coast. The insects have, at least disappears slowly, with a decrease of 7] and of the partly, a common heading approximately along the depth. At the end of the observation, the band become north-south direction. undiscernible from the sea surface echoes. The total The observations suggest that the insects of the duration between the first and last detection ranges vespertine bands disappear at sea. If so, this process from a few tens of minutes to a few hours. is able to massively eliminate the coastal insects and The bands have never been observed when the di- may have some ecological significance. urnal coastal circulation (sea breeze) could not form If this interpretation is correct, the phenomenon is due to strong large-scale flow coming from the west probably not limited to the Bay of Biscay. It also de- and (or) to weak warming of the ground (due to an pends, most certainly, on the local entomological ac- overcast sky). A relationship between the bands and tivity and is probably more intense near lagoon areas, the land-ocean circulation developing aloft during the coastal ponds, and forests.

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Unauthenticated | Downloaded 10/09/21 12:43 AM UTC References Ohara, T., T. Uno, and S. Wakamatsu, 1989: Observed structure of a land breeze head in the Tokyo metropolitan area. J. Appl. Achtemeier, G. L., 1991: The use of insects as tracers for "clear- Meteor., 28, 693-704. air" boundary-layer studies by Doppler radar. J. Atmos. Oce- Riley, J. R., 1985: Collective orientation in night-flying insects. anic Technol., 8, 746-765. Nature, 253, 113-114. Atkinson, B. W., 1981: Mesoscale Atmospheric Circulations. , and D. R. Reynolds, 1986: Orientation at night by high- Academic Press, 495 pp. flying insects. Insect Flight: Dispersal and Migration, W. Banta, R. M., L. D. Olivier, and D. H. Levinson, 1993: Evolu- Danthanarayana, Ed., Springer-Verlag, 71-87. tion of the Monterey Bay sea breeze layer as observed by Sauvageot, H., 1992: Radar Meteorology. Artech House, 366 pp. pulsed Doppler lidar. J. Atmos. Sci., 50, 3959-3982. , G. Lafon, and M. Oruba, 1982: Air motion measurements Campistron, B., 1975: Characteristic distributions of angel ech- in and around thermal plumes using radar and chaff. J. Appl. oes in the lower atmosphere and their meteorological impli- Meteor., 21, 656-665. cations. Bound.-Layer Meteor., 9, 411-426. Schaefer, G. W., 1976: Radar observations of insect flight. Symp. Carroll, J. J., 1989: Analysis of airborne Doppler lidar measure- of the Royal Entomol. Society of London, No. 7, Insect Flight, ments of the extended California sea breeze. J. Atmos. Oce- R. C. Rainey, Ed., 157-197. anic Technol., 6, 820-831. Seliga, T. A., and V. N. Bringi, 1976: Potential use of radar dif- Dickison, R. B. B., M. G. Haggis, R. C. Rainey, and L. M. D. ferential reflectivity measurements at orthogonal polarizations Burns, 1986: Spruce budworm moth flight and storms: Fur- for measuring precipitation. J. Appl Meteor., 15, 59-76. ther studies using aircraft and radar. J. Climate Appl. Meteor., Simpson, J. E., 1994: Sea Breeze and Local Wind. Cambridge 25, 1600-1608. University Press, 234 pp. Drake, V. A., and R. A. Farrow, 1988: The influence of atmo- , D. A. Mansfield, and J. R. Milford, 1977: Inland penetra- spheric structure and motions on insect migration. Annu. Rev. tion of sea breeze fronts. Quart. J. Roy. Meteor. Soc., 103,47- Entomol, 33, 183-210. 76. Gossard, E. E., and R. G. Strauch, 1983: Radar Observation of Strauch, R. G., D. A. Merritt, K. P. Moran, K. B. Earnshaw, and Clear Air and Clouds. Elsevier, 280 pp. D. Van de Kamp, 1984: The Colorado wind-profiling network. Haurwitz, B., 1947: Comments on the sea breeze circulation. J. J. Atmos. Oceanic Technol 1, 37-49. Meteor., 4, 1-8. Vaughn, C. R., 1985: Birds and insects as radar targets: A review. Johnson, C. G., 1969: Migration and Dispersal of Insects by Proc. IEEE, 73, 205-227. Flight. Methuen, 763 pp. Wilson, J. W., T. M. Weckwerth, J. Vivekanandan, R. M. Knight, C. A., and L. J. Miller, 1993: First radar echoes from Wakimoto, and R. W. Russell, 1994: Boundary layer clear- cumulus clouds. Bull. Amer. Meteor. Soc., 74, 179-188. air radar echoes: origin of echoes and accuracy of derived Meyer, J. H., 1971: Radar observations of land breeze fronts. J. winds. J. Atmos. Oceanic Technol, 11, 1184-1206. Appl. Meteor., 10, 1224-1232. Zhong, S., and E. S. Takle, 1992: An observational study of sea Mueller, E. A., and R. P. Larkin, 1985: Insects observed using and land breeze circulation in an area of complex coastal heat- dual polarization radar. J. Atmos. Oceanic Technol, 2,49-54. ing. J. Appl Meteor., 31, 1426-1438.

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Bulletin of the American Meteorological Society 681

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Edited by Kerry A. Emanuel and David J. Raymond

Cumulus convection is perhaps the most complex and perplexing subgrid-scale process that must be represented in numerical models of the atmosphere. It has been recognized that the water vapor content of large parts of the atmosphere is strongly controlled by cloud microphysical processes, yet scant attention has been paid to this problem in formulating most existing convection schemes. This monograph is the fruit of the labors of many of the leading specialists in convection and convective parameterization to discuss this and other issues. Its topics include: an overview of the problem; a review of "classical" convection schemes in widespread use; the special problems associated with the representation of convection in mesoscale and climate models; the parameterization of slantwise convection; and some recent efforts to use explicit numerical simulations of ensembles of convective clouds to test cumulus representations.

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