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A technique for separating stratiform and convective rain types using the characteristics of two of the main drop size distribution (DSD) parameters is presented. The method was originally developed based on observations from dual-frequency profiler and dual-polarization radar observations in Darwin, Australia. In this paper, we will present the testing of the method using data from 2D video disdrometers (2DVD) from two very different locations, namely, Ontario, Canada, and Huntsville, Alabama, USA. One-minute DSDs from 2DVD are used as input to a gamma-fitting procedure and our separation technique uses the fitted values of log 10 ( N W ) and D 0 (where N W is the scaling parameter and D 0 is the median volume diameter) and an “index” to quantify where the points lie in the log 10 ( N W ) versus D 0 domain. For the Ontario location, the output of the classification is compared with simultaneous observations from a collocated, vertically pointing, X-band Doppler radar. A “bright-band” detection algorithm is used to classify each height profile as either stratiform or convective, depending on whether or not a clearly defined melting layer is present at an expected height. If present, the maximum reflectivity within the melting layer and the corresponding height are determined. Similar testing is carried out for two events in Huntsville and compared with observations from a collocated UHF profiler (with Doppler capability). Additional case studies are required, but these results indicate our separation technique seems to be applicable to many different locations and climatologies based on previously published data.

As part of a long-term observation campaign, over 5000 pair samples of temporally matched 1-minute averaged drop size distribution (DSD) measurements have been recorded by two side-by-side (frequently calibrated) 2D-video disdrometers. The measurement campaign was conducted in Huntsville, Alabama, over a ten-month period, and includes a variety of rain types and regimes. The datasets have been used to examine, (i) the relationship between the mass-weighted mean diameter (D m ) and the standard deviation of the mass spectrum (σ M ) without any assumption on the DSD form, and (ii) the relationship between the shape parameter (μ) and slope parameter (Λ) of the gamma form of the DSD. A number of methods were used to estimate μ and Λ, including: a normalizing procedure, the method of moments, the maximum likelihood method and the L-moment method. The physical validity of the σ M –D m relationship is examined by, (i) relating the estimated D m versus the estimated σ M from the same disdrometer datasets and, (ii) by ‘cross-relating’ D m from one disdrometer with the corresponding σ M from the second disdrometer dataset. The same procedures were repeated to examine the physical validity of the μ–Λ relation. It is shown that the transformed variable σ M ′ = σ M D m − 1.65 , which is uncorrelated with D m , has a narrow histogram and that σ M ′ ≈ constant can form a constraint which may well be applicable to other rain climatologies. For the μ–Λ relationship, the variation between μ from one unit and Λ by the second unit showed, as expected, larger scatter than using estimates from the same unit but not excessively so given that the μ estimates from the two units themselves show some instrument-to-instrument variability. While we cannot ascertain that the removal of any statistical correlations necessarily implies that the μ–Λ relation is physical, we have gone on to show that instrument limitations of accurately measuring the number concentration at the small drop end (D

The raindrop spectra observed in a precipitation system is a complex phenomenon that can help to explain the underlying physical processes of rainfall. This paper explores the characteristics of raindrop spectra in terms of drop size distributions (DSD) using seven years of Joss–Waldvogel disdrometer data within the mid-latitude UK region climatology. A total of 162,415 one-minute “filtered” raindrop spectra obtained from the disdrometer are fitted into a normalized gamma DSD model describing DSDs by the concentration parameter (Nw), the drop diameter (Dm and D0), and the shape parameter (μ). The results show that the rain rates retrieved from the normalized gamma DSD model are in good agreement with the disdrometer measured rain rates, implying the appropriateness of the raindrop spectra as normalized gamma distributions. The DSD characteristics are studied in different seasonal (“cold” and “warm”), atmospheric (“dry” and “wet”) as well as rain type (“stratiform” and “convective”) contexts in a long-term perspective. It has been revealed that the normalized gamma DSD parameters are very sensitive to the rain intensities. The mass weighted mean drop diameter Dm clearly increases exponentially with respect to the rain intensities. Variation of the DSDs in different contexts is also exposed reflecting seasonal, atmospheric and rain type consequence on raindrop spectra. Particularly, the scatterplot between the concentration parameter log10Nw and the median drop diameter D0 exhibits clear separation index between stratiform and convective DSDs. There is a large difference in averaged mass weighted mean drop diameters among stratiform and convective segments (stratiform = 0.988 mm versus convective = 1.99 mm). The DSD inconsistency in different contexts have been further examined in terms of the Z–R relationships (Z = aRb) variability. Additionally, 10 selected events taken place during the study period are also investigated, in which each of the events has shown unique DSD characteristics.

In this paper raindrop shapes from laboratory, field and model investigations are examined in order to distill a consistent picture of raindrop axis ratios as a function of size. The first realistic measurements of raindrop shape were made in the UCLA vertical wind tunnel that was built under the supervision of Hans R. Pruppacher at UCLA in 1968. Subsequent research has investigated raindrop shape in more detail by laboratory studies, field measurements and modeling. Measurements of rain at night in backscattered light have revealed that raindrops larger than 1 mm diameter oscillate in conditions where winds and collisions are too weak to excite oscillations. One apparent source of these oscillations is a resonance with eddy shedding. Subsequent laboratory studies have demonstrated that such oscillations increase the average axis ratio of small raindrops. Details of past theoretical research show that raindrop axis ratios decrease with size in the manner of a sessile drop because of its increasing weight, but also because of an increasing differential in the fore-aft aerodynamic pressure. Modern analytical and numerical models predicted similar axis ratios for water drops falling at terminal velocity and these results are generally consistent with laboratory and field measurements. Although the axis ratios first measured in the UCLA wind tunnel and the more recent laboratory measurements have led to a broader understanding of raindrop axis ratios, new information about raindrop shape is available from recent field measurements using improved techniques. Raindrop axis ratios, orientation and canting angles using 2-D video disdrometer (2DVD) are also reported. The results include measurements from an artificial rain experiment conducted under calm wind conditions where the drops (over 115,000) were allowed to fall a distance of 80 m. Axis ratios and drop shapes show good agreement with the Mainz wind-tunnel data and images, as well as canting angle distributions symmetric about zero degrees with a standard deviations of 7–8° for all drops > 2 mm. Measurements of 2DVD raindrop shape has been collected from Okinawa, Sumatra, Toronto, Huntsville and Brisbane. An unusually wide axis ratio distribution was observed in the D = 1.6–1.8 mm region, which may be evidence for drop oscillations caused by eddy-shedding. Some subtle differences can also be seen for larger drops (e.g. D = 4 mm), the vertical chord being slightly larger than the horizontal chord leading to slightly larger axis ratios compared with the 80 m fall data. Additionally, an example using individual drop information from 2DVD measurements is considered, which enhances agreement with C-band polarimetric radar measurements. In this particular case, a considerable shift in the axis ratio distribution for 3.5 mm drops is observed during the initial strong convective phase of the event. Further ongoing work will combine the 2DVD measurements with polarimetric radar observations in a synergistic manner in order to identify the meteorological or microphysical conditions under which significant changes in drop shapes may occur in natural rain. Improved polarimetric radar estimates of canting angles are reported from an S-band system with high cross-polar performance antenna. The histogram of the standard deviation derived from the radar data obtained in light stratiform rain event with embedded convection shows a canting angle mode of about 7°, with a significant positive skewness. An increase in stability with drop diameter is also inferred from the radar measurements, which is in agreement with direct estimates from the 2DVD. Ongoing experiments in the Mainz wind tunnel are evaluating how the raindrop shapes are affected by collisions with drizzle drops within rain shafts.