Clear-air Radar Scatterers
With a few notable exceptions (especially ground clutter), echoes not originating from precipitation are usually overwhelmed by any significant rainfall. For this reason, they are referred to as clear-air echoes. Some radars, including the WSR-88D, allow operations to be switched between a precipitation mode and a clear-air mode. When WSR-88D is used in clear-air mode the volume scan is performed through a smaller range of elevation angles. Thus the antenna can be rotated more slowly and still complete the volume scan in the same time. This slower rotation results in more power per unit volume being transmitted which yields finer resolution (Crum & Alberty, 1993).
Clear echoes have been associated with many different meteorological phenomena, one of which is the sea breeze front. Much of the rest of this paper discusses the nature and origin of clear-air echoes in more detail, and attempts to draw some conclusions about the nature of the echoes observed near sea breeze fronts.
Throughout the development of weather radar technology and science, radar echoes from non-precipitation sources have been referred to by several different names; angel echoes, anomalous propagation and more recently, clear-air echoes.
The origin of clear-air echoes has been attributed to a number of sources. Those most commonly cited are biota (usually birds and airborne insects), dust, gradients in the refractive index of the atmosphere due to temperature and moisture variations and finally ground clutter (returns from obstacles on the ground, usually close to the radar).
Current and recent literature indicates a widely held belief that the primary cause of clear-air echoes is biota and/or strong refractive index inhomogenities. (For example: Campistron, 1975; Wilson, 1994; Rogers et al., 1991; Rabin and Doviak, 1989, and many others)
The literature reviewed indicated that there has been (Atlas, 1964; Serafin et al, 1981; Hardy, 1972), and still is (Wilson et al. 1994) significant debate about the relative contribution of biota and refractive index gradients in the backscattered signal. Insect populations peak during the warmer months. Unfortunately, however, this is also the time when the atmosphere is probably subject to the most dynamic convective effects and hence the strongest gradients in radar refractive index.
Each of these sources listed above is discussed in more detail below. For completeness, short sections are given to describe the effects of ground clutter and dust. However, emphasis is placed on the effects of biota and refractive index gradients which appear to be the main causes of clear-air echoes. They are particularly interesting as they are often manifest in meteorologically interesting situations, for example, near fronts, thunderstorm outflow and strong updrafts.
Ground Clutter
The effects of ground clutter can usually be easily discerned. The returns are usually very large with respect to other echoes, and so can be easily recognized (Accu-Weather NEXRAD Dopplar Radar Information Guide, 1995).
Ground-based obstacles may be immediately in the line of site of the main radar beam, for instance hills or tall buildings. In this case the cause and effect are obvious.
Alternatively, the returns may be from objects close to the radar, which although not directly within the field of view of the main radar beam, are present within one of the sidelobes of the radar beam. In this case, even though the sidelobe power is much lower than that of the main beam, the return may still be large due to the closeness of the obstacle.
Finally, if the radar beam hits a strong enough density gradient, it may be refracted downwards enough for the beam to hit the ground resulting in very large returns. Density gradients which are strong enough to produce this effect may exist at thunderstorm outflow boundaries, near strong cold fronts, or within strong night-time atmospheric inversions.
In most cases, ground clutter produces intense echoes and so is usually easy to detect. Additional verification methods are also simple:
If a Doppler radar is being used, the Base Velocity Data can be checked. Ground clutter usually has no velocity. The exception to this is the last case listed above. Density gradients may move around in time and space and thus ground clutter due to the radar beam being bent towards the earth may not be stationary.
If the return is not present at a higher tilt angle, the backscatter is probably from ground clutter.
In situ observations and satellite pictures can be used to rule out other causes of the echoes.
Scattering by Particulate Matter
The contribution of particulate matter is usually small, but there are instances, which will be discussed, where they may be important.
Particles (including, for example, smoke, dust and large pollen grains) scatter radar energy in much the same way as rain drops. Although there appears to be little literature regarding the contribution of this scattering medium under normal conditions, it may be particularly important in areas with dry climatology and/or near anthropogenic pollution sources. However, the number distribution of airborne particulates large enough to produce significant return from weather radar diminishes rapidly as one moves away from their source. Since backscatter power is proportional to the diameter to the sixth power, the backscatter from a particle of less than 0.1 mm in diameter is very small even if the radar cross section of the particle is relatively large. Particle size distributions (for example the Junge distribution) indicate that particles larger than this generally precipitate out of the atmosphere quite quickly.
Figure 5. Sea breeze front seen advancing as a wall of smoke (after Simpson, 1994).
Scattering by these particles may still be important though. For example, smoke and dust hazes have been observed outlining the boundary of sea breeze fronts (Simpson, 1994). An example of this is shown in Figure 5. The photograph was taken from the air, southwest of Middlesborough, England. It should be noted that Middlesborough is an area of extremely heavy industrial activity.
This pollution that meets the sea breeze front ascends the front and becomes mixed into the sea-air behind the front. This air is the main part of the return flow. The pollutants may then be recirculated, leading to accumulation of high concentrations.
Also, pollution moving from the stable air on the seaward side of a sea breeze front into the more unstable air above land will rapidly mix downward causing fumigation. This is shown in Figure 6.
Figure 6. Fumigation results as smoke moves from cool stable air over water into the mixed layer over land (after Simpson, 1994).
Apart from the obviously undesirable effect such concentrations of pollutants may have on the local community, both fumigation and recirculation could lead to concentrations of particulates great enough to produce a significant radar return.
Scattering by Biota
Clear-air echoes are often attributed to backscatter from biota, particularly insects. Indeed, there is fairly conclusive evidence, which will be discussed in more detail later, that at short wavelengths (e.g. K-Band) insects are the primary contributors to clear-air radar returns.
Since most biota are not spherical, the magnitude of the return is often very dependent on the biota orientation. This idea of orientation is represented using the differential radar reflectivity, ZDR. It is defined as:
(7)
where Zh and Zv are the horizontal and vertical copolarized reflectivity factors (Wilson et al., 1994). The schematic shown in Figure 7 illustrates clearly how orientation can affect the backscattering cross section.
In the first case ZDR is approximately zero. However, in the second case ZDR is much greater than 1. In the third case the insect is oriented somewhat downward and ZDR is consequently smaller than the second case but still much larger than zero.
The presence of a large differential reflectivity factor is a good indicator that the scatterers may be biota. ZDR may also provide information about the type of biota and its behavior; for example, a variation of ZDR over time is indicative of the overall bulk of the biota cloud changing direction and orientation.
Typically, in normal flight, the major axis of an insect will be horizontal (this is especially true of smaller insects), but the insect may be oriented in any direction within this plane (Sauvageot and Despaux, 1996). This results in a significant-to-large ZDR, the magnitude of which depends on the width-to-length ratio of the insect. This observation can also be applied to birds which are generally observed to be in fairly level flight.
The width-to-length ratio of birds is typically between 1:2 and 1:3. For insects it is between 1:3 and 1:10 (Wilson et al., 1994). The result is measurable ZDR which may be up to 10dB for larger insects with elongated bodies. Note that ZDR for rain drops ranges from 0 to 2 dB (Achtemeier, 1991).
It is widely accepted that insects play a greater role in large scale echoes that do birds, which are usually present in relatively small number densities. Insects are comprised largely of water, typically between 50% and 70% (Sauvageot and Despaux, 1996). The backscatter model that should be used when interpreting radar returns from insects varies according to the radar wavelength used and the size distribution of the insects within the scanned volume. For longer-wavelength radar, scattering from insects usually falls within the Rayleigh regime. However, for shorter wavelengths, for example X and K-band, scattering may enter the Mie regime particularly if the insects are large. A distribution of insect sizes may lead to a combination of both.
Many radar studies have been carried out in which insects are used as a tracer to describe the air motion (Wilson et al, 1994; Achtemeier, 1991; Campistron, 1975; Sauvageot, 1996, and many others). However, before any conclusions are drawn about the validity of using insects as tracers of atmospheric motion in radar studies, insect behavior (entomology) must be studied. Much literature is available on this subject including several of the references in this paper. The most important points relevant to this paper are briefly discussed below.
Swarms of insects often fly on a preferred heading. In particular, larger insects often have significant control of their velocity and may migrate en-mass (Mueller and Larkin, 1985). Most of these larger insects tend to remain close to the ground unless migrating and hence are not as frequently observed by radar (Sauvageot and Despaux, 1996). Also, several studies have shown that insect migrations tend to occur at night, with a preferred downwind heading (Schaefer 1976; Riley and Reynolds 1986; Drake and Farrow, 1988)
Insect behavior is dramatically affected by meteorological conditions, particularly temperature. As insects are carried upward by ascending air their ability to fly reduces and at some point they lose the capability of active flight. At this point their orientation is likely to change thus changing their radar reflectivity. This orientation change may also change their coefficient of drag resulting in descent or a reduced rate of ascent. Achtemeier (1991) even suggests that some insects may be able to sense strong lift and either fly downward or point downward and fold their wings, causing them to descend.
The consequences of the insect behavior described above may be manifest in radar observations and impact the validity of using insects as tracers of atmospheric motion. There are two important consequences. Firstly, the Doppler velocity recorded will not be representative of the wind speed or direction. Secondly, the backscatter magnitude will vary according to the polarization of the radar, the orientation of the insect mass and the viewing angle. These effects will be relevant to radar observation of the sea-breeze if insects are contributing to the return.
It has been found that in most cases insects can be used as valid tracers of air motion as long as the possible pitfalls above are considered. As long as the insects are passively carried by the wind or that the general flight of the insect population within the sampling volume is random, the assumption is valid.
Some studies, for example Achtemeier (1991), showed that insects were valid tracers only to a certain point. However, Achtemeier was able to make important connections between the meteorological conditions and insect behavior. (More details of this work are given later). Studies such as this one are providing more knowledge of the general reaction of the insect population to meteorological conditions.
Thus, the backscatter from insects may tell us much about the local dynamics of the atmosphere within the radar sample volume. This is especially true in cases where insects are valid tracers of air motion but also, as a better understanding of entomology is acquired, in cases where they are not.
A study by Sauvageot and Despaux (1996), described in more detail later, demonstrates this by using K-band Doppler radar to observe insects caught up in a sea-breeze circulation.
Scattering by refractive index gradient
Sharp inhomogenities in the refractive index of the atmosphere, such as occur, for example, at air mass boundaries, can result in the backscatter of radar power (Battan, 1973). This phenomenon is often called Bragg scattering and is dealt with in some detail below.
Also, under special circumstances, gradients in refractive index may lead to specular reflections which can contribute significantly to clear-air returns. Partial specular reflection is observed as a result of layered structures in the refractive index field (James, 1979). These layers are common in the stratosphere but may also exist within tropospheric inversions.
Specular reflection of radar power is most important for radars of longer wavelength (particularly above tens of centimeters and into the meter range), especially those which are vertically pointed (generally the lower the elevation angle, the less likely it is that specular reflection will be observed). The reason for this is that the atmosphere is primarily horizontally stratified. Thus stratified layers with very high refractive index gradients at their interfaces reflect some of the radar power directly back to a vertically pointing radar (Gossard, 1984). The effect may be enhanced with altitude when the layers appear concave to the radar due to the earth’s curvature. This focuses the reflected signal (Atlas, 1964). MST (mesosphere-stratosphere-troposphere) radars, are particularly likely to observe some return due to specular reflection. These radars are used to probe the upper reaches of the atmosphere, and are usually operated within a few degrees of the vertical. This type of echo is relatively rarely seen by weather radars that are configured to operate at higher frequencies, which usually employ a steerable dish antenna and are normally operated at elevation angles less then 60°. However, the possibility of partial specular reflection should not be disregarded and an example is given later in which partial specular reflection may be responsible for clear-air returns from thin stable layers within the lower troposphere.
The returns resulting from specular reflection, when present, tend to be coherent, often lasting for several minutes. This suggests that the structures causing the reflection have a relatively prolonged lifetime and are somewhat static. The mechanisms resulting in Bragg scattering, which will be described below, have coherence times of only a few seconds and are transitory.
The fundamental physical principles for Bragg backscatter are the same as those causing refraction of visible light. That is, as the refractive index of the medium changes so does the velocity with which waves travel through the medium. This results in refraction of the wave in accordance with Snell’s law. Where the inhomogenities are sharp enough, for example along inversion boundaries or with pockets of turbulent air, the compounded effect of the refraction can lead to some backscatter of a radar beam back to the radar.
Refractive index of a medium is determined by the physical properties of that medium. For air, the refractive index at radio frequencies (known as the radar or radio refractive index, n) is a function of atmospheric density. Since the density of the atmosphere is a function of pressure, temperature and humidity, the radar refractive index combines three of the most important meteorological parameters.
Within the troposphere, n is close to unity and varies only slightly. For this reason, radar meteorologists use the more convenient term, N = (n - 1) x 106. It can then be shown that:
(8)
where T is temperature (K), P is atmospheric pressure (mb) and e is the vapor pressure (mb)
Spatial gradients of this term are responsible for refraction of a radar beam. The gradient Ñ N is usually denoted with the symbol M. In order to analyze contributions to M, we may break equation 8 into two terms; the dry term 77.6P/T, and the wet term 373256e/T2.
The first term typically contributes at least 60% of N (Bean et al.; 1960). Usually the variation of T and P is not dramatic on the microscale, so this term is not as significant in creating large fluctuations of N as the wet term. However, large gradients of temperature may exist on the boundaries of inversions (and of course close to the ground), and are believed to sometimes be the primary cause of some angel echoes, particularly at higher altitudes where the air is dry. Also, note that because of the hydrostatic nature of the atmosphere the magnitude of the dry term decreases approximately exponentially with height. This has implications in the range vs height relationship which is discussed in the next section.
The contribution of the wet term can vary dramatically, depending on the vapor pressure and temperature. Moisture and temperature gradients typically found at air mass boundaries can produce significant gradients of N. For example, consider the boundary between warm dry continental air and cooler moist maritime air which exists at a sea breeze front. Note also that the moisture content of the atmosphere decreases with height and hence the contribution of the wet term also diminishes with height becoming insignificant in the upper troposphere and above. Thus, generally, in the lower troposphere, refractive index is more sensitive to humidity, while at higher elevations, where water vapor pressure is small, only gradients of temperature make significant contribution to M.
It has already been mentioned that air mass boundaries may produce large enough gradients of refractive index to result in radar backscatter. James (1980) cites several other conditions in which similar gradients may exist: cloud and fog tops, convective boundaries, the tropopause, sheared stable layers, and thunderstorm outflow boundaries. All these conditions have a common consequence: they result in turbulence along the boundary. This turbulence is the responsible for producing the large-scale refractive index gradients which result in return of incident radar power.
Turbulence within the atmosphere can be defined by the use of quantities known as structure parameters. They represent the root mean square difference between an atmospheric variable at two points a unit distance apart. Structure parameters can be defined for practically any atmospheric variable. Of particular interest here is the refractive index structure constant, Cn2 Hardy et al. (1966) showed that the radar reflectivity (h ) is related to the refractive index structure constant, Cn2 (and hence to gradients in refractive index) as follows:
(9)
This relationship assumes that all measured reflectivity is due to the effects of refractive index gradients. Hence, if other scatterers are present, their effect must be filtered out if the relationship is to be valid. Also, the turbulence producing the inhomogenities is assumed to be isotropic and fill the entire radar resolution volume. Finally, one half the radar wavelength must fall within the inertial subrange. The inertial subrange defines the minimum and maximum scales in which turbulence is possible. Below the lower limit of the inertial subrange (termed the limiting microscale), turbulence is heavily damped by viscous dissipation. Above the upper limit, turbulence is no longer isotropic. Within the inertial subrange, turbulent eddies do not lose much energy by viscous processes, but instead transfer of energy from the larger scale to eddies of smaller scale (large eddies make small eddies and so on down to viscosity!).
Tatarski (1961) developed much of the Bragg theory which led to equation 9. He showed that:
(10)
where k is the radar wave number defined as k=4p /l . Fn* is a three-dimensional representation of the refractive index field. Its source originates in a Fourier transform. It can be shown that only the Fourier mode with a wavelength of l /2 produces significant reflectivity. This is why l /2 must be within the inertial subrange. Otherwise no match would exist for any wave number.
The limiting microscale is generally accepted to typically be of the order of 1 to 2 cm at ground level and to increase with height (Sauvageot and Despaux, 1996; Gage and Balsley, 1981). However, James (1979), states that it is of the order of a few millimeters. Even taking this lower Figure, the result is that radars employing K-band (and perhaps even X-band, if the limiting microscale is indeed about 2 cm), will be not detect refractive index inhomogenities. Thus clear-air echoes detected by such radar sets must have some other origin.
The upper limit of the inertial subrange is usually quite large, often several hundred meters in convective conditions. Hence, it is usually not questioned. However, the upper limit may be less than 1 m in thin, statically stable layers, and so conditions stipulated for equation 9 may not be met in these cases (James, 1979).
As well as being of obvious relevance to the subject of this work, Cn2 is an extremely useful quantity as it as it is dependent on the three variables which are most important in the study of the lower atmosphere: pressure, temperature and humidity. In fact Cn2 can be expressed as a linear combination of the structure constants for temperature, absolute humidity and the covariance of temperature and absolute humidity, with each term having a scaling factor which is a function of pressure, temperature and absolute humidity. From this definition numerous equations can be formulated which allow measurements of Cn2 derived from radar reflectivity observations to be used to analyze the structure of the atmosphere.
Many studies have been carried out which attempt to correlate variance of atmospheric parameters, measured using traditional sensors, to the observation of clear-air radar echoes, and many indicate good correlation. For example, Gossard et al. (1984) found that stable elevated layers commonly contained transition zones where very strong gradients of wind, humidity and temperature where found. These zones were found to correspond to strong clear-air echoes. Atlas (1964) hypothesized that air parcels of high refractive index, convected upward to layers of lower index would create especially high refractive index gradients near the base of subsidence inversions when the parcels are suddenly decelerated and diffuse into the surrounding air. Again corresponding echo "layers" where found. Atlas also found point echoes associated with rising convective bubbles. These can again be explained by the strong gradients in humidity and temperature at the interface between the convective parcel and the environmental air through which it is rising. Atlas (1960) attributes observation of clear-air echoes at a sea-breeze front to gradients in refractive index and Meyer (1971) arrives at the same conclusion to explain observation of a land breeze from a radar in Wallops Island.
Although the evidence that Bragg scattering is responsible at least in part for many clear-air echoes, there is much debate concerning the relative contribution of the effect to clear-air echoes and even more controversy about the conditions where the phenomenon is most manifest. Early studies, tended to place much emphasis on this scattering mechanism. For example, Atlas (1960) in a study of echoes associated the sea breezes along the Massachusetts coast concluded that "The most critical condition for the occurrence of sea-breeze (and probably other angel) echoes on a horizontally oriented beam is a sharp vertical lapse of vapor pressure associated with a vertical exchange mechanism to redistribute the moisture so as to present sharp inhomogeneities in the horizontal".
More recent work generally plays down the importance of Bragg scattering within the lower troposphere except for sensitive radars with wavelengths greater than about 10 cm. However, researchers are beginning to determine with more confidence the correlation between atmospheric parameters and the clear-air echoes which result. This work is focused on experiments in which the effect of Rayleigh scatterers is either not relevant or is known. For example, Rabin and Doviak (1989) looked at the effect of a partial solar eclipse (and the resulting changes in meteorological conditions) on radar reflectivity. Crane (1980) conducted a study in which the atmospheric wind profile above the planetary boundary layer (where Rayleigh scatterers are less numerous) computed using the velocity azimuth display (VAD) technique, was compared to observations taken using Rawinsonde and Jimisphere balloon. The results of this experiment are shown in Figure 8.
Gossard et al. (1984), compared in-situ turbulence data, taken by advanced fast-response instruments being conveyed up a 300 m tall tower, with acoustic sounder data and radar data. In one case, the carriage conveying the instruments up the tower was equipped with two booms, 1m apart, each carrying its own set of instruments. This allowed for extremely accurate determination of structure parameters. Gossard noted some discrepancies between his observations and the results expected by Bragg theory in cases where sharp, thin thermally-stable layers existed. In these layers he found extremely large refractive index gradients and suggested that these gradients were large enough for partial specular reflection to occur in cases where the radar signal was incident upon stable layers of a thickness less than half a wavelength. By applying the appropriate wave theory he was then able to find good agreement between in-situ data and the echoes observed by the radar located 550m from the tower.
These experiments found good correlations between the analyzed radar data and the in-situ observations of atmospheric structure parameters. However, compelling evidence that Bragg scattering was not the only (in indeed, perhaps not the primary) agent of scattering was also indicated in some of these, and other works.
Such experiments are more clearly defining the effect of atmospheric structure on radar backscatter. This may ultimately allow routine detailed analysis of the structure of clear air. Also, Doppler techniques provide the capability to study in more detail the dynamics of the scattering phenomena described.
There is little doubt that analysis of Bragg scattering at upper levels, where Rayleigh scatterers are uncommon, is improving our understanding of the atmosphere above the boundary layer. Many studies have indicated fascinating and often beautiful wave structures on radar. An example of one of these structures, found in association with clear-air turbulence (CAT) at an altitude of about 5000 meters in a statically stable layer of strong shear, is shown in Figure 9.
It has been suggested that if it is possible to determine with more certainty the relative effect of Bragg scattering, this may provide the key to detailed analysis of clear-air convection and other boundary layer phenomena, leading to better short-range forecasting (Hardy, 1972; Serafin et al., 1981).
Determining the scattering mechanism
When radar is being used to detect precipitation echoes, the analysis of radar data is a relatively simple task. The origin of the scatterers falls into a small set of possibilities (i.e. the different types of precipitation), for each of which the detailed radar characteristics are known. These characteristics can then be applied to radar theory and the radar data to determine information about the type and intensity of the precipitation. Although there are several unknown variables, enough data exist to allow software algorithms, together with the experience of the radar meteorologist, to interpret with some certainty the radar return. However, in the case of clear-air echoes, the origin of the echoes is much more difficult to determine, as is the scattering mechanism. It may also be the combination of several causes. Without knowledge of the scatterers responsible for the echo and the scattering mechanism it is not possible to make anything more than vague qualitative assessments of the radar return. For this reason a great deal of effort has gone into mapping clear-air returns to in-situ data, into developing ways of determining the nature and characteristics of the scatterers and into developing ways of differentiating between different scatter mechanisms.
To use radar theory and equations to determine the size and number distribution of scatterers, it is necessary to determine the type of scatter. Is it Bragg or particulate scatter? If it is particulate scatter, does it fall within the Rayleigh or Mie ranges? A number of methods have been developed to determine the mechanism. A brief description of some of these is given below.
a) Make simultaneous observations using radars with different wavelengths. It has been shown that the wavelength dependence of Bragg and particulate scattering processes is different (l -1/3 versus l -4). We can use the difference in results between two wavelengths to determine the type of scattering. For Bragg scattering it can be shown that (Wilson et al., 1994):
(11)
Thus, it is possible to predict the change in effective reflectivity that a change of band will produce. If this matches that observed, then the scattering mechanism is likely to be Bragg scatter. If it is determined that the scatter is from particulates, then it is possible to determine whether the more complex Mie range has been entered using a similar approach. Within the Rayleigh range, the effective reflectivity is equal for each wavelength used. However, if the Mie range is entered this will not be the case.
b) Measurements of differential reflectivity, ZDR. Most clear-air particulate scatter is due to biota, particularly insects. Most of these scatterers have length to width ratios of at least 1:3 and hence a random volume of such scatterers will produce a significant differential reflectivity with the magnitude increasing as the scatterers become more elongated.
Bragg scattering assumes that the inhomogenities in refractive index responsible for scattering are due to isotropic turbulence on the scale of half a wavelength. The assumption that turbulence is isotropic is usually valid within the range of wavelengths typically used. Thus, differential reflectivity expected from Bragg scattering should be zero.
Hence, if the measured differential reflectivity is appreciable, particulate scattering is likely cause.
c) Use of the K-Band radar. As mentioned, when millimetric radar is used, it can usually be determined beyond doubt that no Bragg scattering will occur. Therefore, all echoes can attributed to particulate scatter either in the Rayleigh or Mie range.
d) Apply findings of other experiments. The methods above have been used in research to attempt to better understand the contribution of each scattering mechanism and the characteristics of the scatterers. They are often not practical for routine operational radar use. Nonetheless, such research has provided valuable data which can serve as a baseline in routine radar analysis for carefully applying prudent assumptions to radar data.
In general, Bragg scatter is much more prevalent for longer wavelength radar sets (especially wavelengths greater than 20 cm) and typically does not yield reflectivity greater than about -10dBZ. Its intensity is proportional to the magnitude of the refractive index gradients which exist. On the other hand, the magnitude of particulate scatter is dependent both on the scatterer number density and the scatterer size. The reflectivity is usually much higher, typically between -10dBZ to 20dBZ (Wilson et al., 1994), with much higher values possible. Achtemeier (1991) reported reflectivities up to 40dBZ from an insect cloud. This difference will usually enable us to tell if particulate scattering is occurring, but it will not allow us to determine the possible additional contribution of Bragg scatter if any.