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Journal ArticleDOI

Drop size distribution measurements in convective storms with a vertically pointing 35‐GHz Doppler radar

01 Jan 1984-Radio Science (John Wiley & Sons, Ltd)-Vol. 19, Iss: 1, pp 177-183
TL;DR: In this paper, the authors used a 35 GHz pseudonoise coded high-resolution radar at Johannesburg, South Africa, to measure vertical incidence in convective storms with a 35GHz pseudonooise coded radar at very short ranges (≈ 19 m).
Abstract: Measurements at vertical incidence in convective storms with a 35-GHz pseudonoise coded high-resolution radar at Johannesburg, South Africa, are discussed. The radar is bistatic and has the capability to measure targets at very short ranges (≈ 19 m). This capability is used to derive an accurate experimental mean velocity-reflectivity ( v―-Z) relationship in convective storm systems. It is shown that at an altitude of 19 m the standard deviation of the data points from the mean v―-Z relationship is about 0.25 m s−1, indicating that the contribution of the vertical air velocity at the low altitude of 19 m is less than about ±0.5 m s−1 (95% confidence limit). The experimental v―-Z relationship is then used to derive drop size distributions from the measured Doppler spectra in convective rainfall with high time resolution (10 s). Some of the measured drop size distributions are discussed and related to specific kinematic features of the storm under study. For example, drop size distributions measured in a severe squall line show a large abundance of smaller drops (< 1.5 mm in diameter) in regions of wind shear near the edges of the main downdraft core. A possible explanation of this large concentration of smaller drops is sorting of the hydrometeors caused by shear in the horizontal wind that is generated by the divergence of the downdraft at the ground. These observations show the usefulness of this radar measurement technique to detect features of drop size distributions in convective systems and relate them to storm structure and kinematics.
Citations
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Journal ArticleDOI
TL;DR: In this paper, a 35GHz scanning Doppler radar was designed, assembled, and tested to observe fog, mounted on a flatbed vehicle for portability, transmits peak powers of 100 kW in a pulse of 0.5µs width and a beamwidth of0.3°.
Abstract: To observe fog, a 35-GHz scanning Doppler radar was designed, assembled, and tested. The radar, mounted on a flatbed vehicle for portability, transmits peak powers of 100 kW in a pulse of 0.5-µs width and a beamwidth of 0.3°. Thus, a reflectivity factor Z of −20 dBZ at a range of 10 km generates a signal-to-noise ratio of 0 dB. Doppler velocity measurements are made by sampling the radio frequency phase within each pulse transmitted by a magnetron oscillator and referencing the phases of the received echoes to the transmitted phase. A Nyquist velocity of approximately 9.7 m s–1 is obtained in real time using the spaced pulse-pair method, and aliases of radial velocities are corrected using software. The three-dimensional structure of sea fog and its advection are depicted with the radar.

45 citations


Cites background or methods from "Drop size distribution measurements..."

  • ...Several 35-GHz research radars have been developed for the purpose of cloud observation in the vertical direction (e.g., Pasqualucci 1984; Pasqualucci et al. 1983; Hobbs et al. 1985)....

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  • ...The attenuation by fog is estimated by using the empirical formula of Benoit (Ulaby et al. 1981, 259–314), and the attenuation by the atmosphere (without fog) is estimated using the Millimeter-wave Propagation Model (MPM) of Liebe (1985). Assume that uniform fog has a depth of 600 m, a constant relative humidity (100%), and water content (W 5 0....

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Journal ArticleDOI
TL;DR: In this article, the correlation between the copolarized and cross-polarized components of the received signal has given new insight into the falling behavior of precipitation particles, and a great deal of new information will evidently be made available by combining coherent signal processing techniques with variable polarization measurements.
Abstract: From the earliest years of radar meteorology, efforts have been devoted to multiparameter observations: the measurement of one or more signal properties, ordinarily in addition to the signal intensity, to give added information about the precipitation. Studies employing variable polarization or Doppler frequency measurement were among the first such techniques and over the years have proved fruitful in terms of the information provided. During the past decade there has been such an increase in the use of Doppler radar in meteorological research that radar-measured wind fields are becoming nearly as routine as reflectivity fields. Research has continued in variable polarization techniques, which appear to provide a means of distinguishing remotely between various precipitation types. Investigations of the correlation between the copolarized and cross-polarized components of the received signal have given new insight into the falling behavior of precipitation particles. A great deal of new information will evidently be made available by combining coherent signal processing techniques with variable polarization measurements.

23 citations

Journal ArticleDOI
TL;DR: A special fog observation campaign was conducted in the Miyoshi basin, Hiroshima prefecture, Japan during the period November 7-15, 2000 as discussed by the authors, where the spatial distributions of fogs and their movements using a millimeter-wave scanning radar were observed.
Abstract: A special fog observation campaign was conducted in the Miyoshi basin, Hiroshima prefecture, Japan during the period November 7–15, 2000. We observed the spatial distributions of fogs and their movements using a millimeterwave scanning radar. This is the first time that the distribution of basin fogs associated with fog development and decay processes has been examined. Echo intensity observed with the radar, which is mainly associated with fog particle size, was almost under −23 dBZ at levels below 200 m in height. The horizontal distribution of echo intensity changed with time. Namely, weak echoes were observed over nearly all observation areas at first, and then the echoes gradually became stronger as the fogs developed, although the echoes were weaker at higher levels. After sunrise, the echoes decayed. During the developing periods, the occurrence ratio of the echo intensity between −38 and −23 dBZ increased from the lower height, while the ratio decreased from the higher levels during the decay periods. This feature in the developing period is consistent with the results of optical measurements but the feature in the decaying period is inconsistent. It is suggested that this inconsistency is due to the difference in sensitivity between the two measurement approaches.

10 citations


Cites methods from "Drop size distribution measurements..."

  • ...Mm-wave radars also have been used for cloud studies (e.g., Kropfli et al., 1984; Pasqualucci, 1984)....

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Book ChapterDOI
01 Jan 2014
TL;DR: In this article, the notion of radar detectability is first shown and basic techniques used to retrieve wind velocity from radar measurements are then discussed, and the methods used for estimating turbulence parameters and other physical parameters obtained from atmospheric radar measurements were discussed.
Abstract: Refractive index perturbations of half the radar wavelength in scale mostly contribute to atmospheric radar backscattering. From the Doppler observations of upper atmosphere, vertical profiles of turbulence parameters such as energy dissipation rate and vertical eddy diffusivity can be deduced on certain conditions. Combined with the radar acoustic sounding system (RASS), atmospheric radar measurements can also provide vertical profiles of background virtual temperature and water vapor.In this chapter, the notion of radar detectability is first shown. Basic techniques used to retrieve wind velocity from radar measurements are then discussed. Next, the methods used for estimating turbulence parameters and other physical parameters obtained from atmospheric radar measurements are discussed. Finally, radar interferometric techniques for improving the spatial resolution are described.

5 citations

References
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Journal ArticleDOI
TL;DR: In this article, a theoretical analysis based on energy concepts predicted that the coalescence efficiency is given by the equation $\epsilon =2.40\left(\frac{\sigma}{U^{2}r\rho}\right)f(R/r)$, where $f(r/f)$ is a function which varies from 1.3 to 3.8 for R/r = 3.0.
Abstract: Experimental studies have been conducted of the interaction of falling water drops of radii R and r (R > r), density $\rho $ and surface tension $\sigma $ colliding in air with a relative velocity U and a perpendicular distance X between the centre of one drop and the undeflected trajectory of the other. R and r were varied from 150 to 750 $\mu m$, R/r from 1.0 to 2.5, U from 0.3 to 3.0 m s$^{-1}$ and X from 0 (head-on collisions) to the maximum value for contact, R+r. Four types of interaction were observed: (1) bouncing; (2) permanent coalescence; (3) coalescence followed by separation; (4) coalescence followed by separation and the formation of satellite drops. The principal effort was devoted to a study of the critical conditions under which drops will separate after coalescence. It was found that there was a critical value of X, denoted by $X\_{\text{c}}$, below which the coalesced drops remained united and above which there was sufficient angular momentum for the drops to separate after coalescence. For a wide range of values of r, R and U the coalescence efficiency $\epsilon =X\_{\text{c}}/R+r)^{2}$ was found to lie between 0.1 and 0.4 for drops of identical size and about 0.2 and 0.6 for drops with R/r = 2.0. A theoretical analysis based on energy concepts predicted that the coalescence efficiency is given by the equation $\epsilon =2.40\left(\frac{\sigma}{U^{2}r\rho}\right)f(R/r)$, where $f(R/f)$ is a function which varies from 1.3 for R/r = 1 and 3.8 for R/r = 3. The predictions of this equation were in excellent agreement with the experimental results over the entire range of conditions studied. Investigations are also described of the critical conditions for the bouncing of colliding drops, the influence of electric charges upon the interactions and the elongation and splitting of a rotating drop.

307 citations

Journal ArticleDOI
TL;DR: In this article, the collision and subsequent breakup of water drops moving essentially vertically and at terminal velocity has been studied for five drop pairs: the diameters Ds of the large drops were 4.8, 3.6 and 3.0 mm; the small drops were 1.8 mm and 1.5 mm; and three distinct types of collision breakup were found with the following occurrence: necks 27%, sheets 55% and disks 18%. Bag breakups were insignificant with < 0.5%.
Abstract: The collision and subsequent breakup of water drops moving essentially vertically and at terminal velocity has been studied for five drop pairs: the diameters Ds of the large drops were 4.8, 3.6 and 3.0 mm; the diameters D3 of the small drops were 1.8 mm and 1.0 mm. 712 collisions were obtained in 25,000 individually recorded attempts. Three distinct types of collision-breakup were found with the following occurrence: necks 27%, sheets 55% and disks 18%. Bag breakups were insignificant with <0.5%. All types are defined and corresponding examples shown. Fragment size and number distributions for the different types and the overall situation give further reasons for the breakdown into the different types. The disk collision has been found to be the major cause for the depletion in number of large drops, hence the cutoff of large drops in rain. The results also form the first data bank for numerically modelling the evolution of raindrop size spectra and the Langmuir chain process.

101 citations

Journal ArticleDOI
TL;DR: In this paper, the intercept n0 and slope λ of the exponential distribution are given by differential equations expressing the conservation of raindrop concentration and rainwater content M. The differential equations are solved numerically using published experimental data on coalescence efficiency and spontaneous breakup.
Abstract: The processes of condensation, coalescence and drop breakup tend to produce exponential raindrop size spectra. The intercept n0 and slope λ of the exponential distribution are given by differential equations expressing the conservation of raindrop concentration and rainwater content M. The differential equations are solved numerically using published experimental data on coalescence efficiency and spontaneous breakup. The number of fragments resulting from a collisional breakup S0 is taken as a variable parameter. Calculations show that 1) the effects of collisional breakup usually predominate over those of spontaneous breakup, 2) for dM/dt=0, a stationary λ results which is a function of S0, and 3) for dM/dt>0, λ tends to a quasi-static value which depends upon S0 and (1/M2) (dM/dt) but is close to the stationary value for the same S0. In each case n0 is determined by the values of λ and M. Binary interactions, i.e., drop coalescence and collisional breakup, tend to produce raindrop size spectra...

68 citations

Journal ArticleDOI
TL;DR: In this article, an artificial rain column was generated by letting a continuous stream of water fall under zero pressure from an open hose, and within a meter of fall the stream broke into large drops, and interaction between the drops was noted during the remaining 60 m of fall.
Abstract: An artificial rain column was generated by letting a continuous stream of water fall under zero pressure from an open hose. Within a meter of fall the stream broke into large drops. These large drops further subdivided, and interaction between the drops was noted during the remaining 60 m of fall. At the bottom of the rain column drop-size distributions were measured at intensities that ranged from 190–1900 mm hr−1. Over this range the largest drops were about 9 mm in diameter, and the number of these drops per cubic meter was constant. For smaller drops, however, the concentration increased with intensity. Drops ≳5 mm are unstable. It is felt, had the rain column fallen several hundred meters more, that these drops would have broken up, causing an increase in the slope of the drop distribution. Calculations suggest that an exponential type distribution, similar to that found in natural rain, would have been produced. This lends credence to the hypothesis that in heavy rain the shape of the drop-...

53 citations