Freezing Drizzle Research Articles

Assessing Cloud-Phase Conditions

Abstract In situ microphysics measurements made during the First and Third Canadian Freezing Drizzle Experiments (CFDE I and III, respectively) have been used to assess the relative responses to ice and liquid hydrometeors for several common instruments. These included the Rosemount icing detector, 2D-C monoscale and 2D-C grayscale probes, forward-scattering spectrometer probes (FSSP) on three measurement ranges, Nevzorov liquid water content (LWC) and total water content probes, and King LWC probes. The Nevzorov LWC and King LWC probes responded to between 5% and 30% of the ice water content, with an average response of approximately 20%. The average FSSP measurements of droplet spectra were dominated by ice particles for sizes greater than 35 μm, independent of the measurement range used, when the ice-crystal concentrations exceeded approximately 1 L−1. In contrast, the FSSP measurements of the droplet spectra less than 30 μm appeared free of ice-crystal contamination, independent of the ice-crystal con...

Further Investigation of a Physically Based, Nondimensional Parameter for Discriminating between Locations of Freezing Rain and Ice Pellets

Abstract The general applicability of an isonomogram developed by Czys and coauthors to diagnose the position of the geographic boundary between freezing precipitation (freezing rain or freezing drizzle) and ice pellets (sleet or snow grains) was tested using a 25-yr sounding database consisting of 1051 soundings, 581 where stations were reporting freezing drizzle, 391 reporting freezing rain, and 79 reporting ice pellets. Of the 1051 soundings, only 306 clearly had an environmental temperature and moisture profile corresponding to that assumed for the isonomogram. This profile consisted of a three-layer atmosphere with 1) a cold cloud layer aloft that is a source of ice particles, 2) a midlevel layer where the temperature exceeds 0°C and ice particles melt, and 3) a surface layer where T < 0°C. The remaining soundings did not conform to the profile either because 1) the freezing precipitation was associated with the warm rain process or 2) the ice pellets formed due to riming rather than melting and refr...

Regional and Local Influences on Freezing Drizzle, Freezing Rain,and Ice Pellet Events

Regional and local influences on frequency and type of freezing precipitation (freezing drizzle, freezing rain, and ice pellets) are investigated via in-depth climatologies of six continental United States (CONUS) sounding sites. For each site, wind roses of precipitation type occurrences are compared with those for nearby stations and the aggregate values for the CONUS. Synoptic scenarios and sounding structures are identified for prolonged events of each precipitation type and probable formation mechanisms are discussed. Station location relative to topographic features smaller than 1 km in height, water bodies ranging in size from oceans to small bays, and dominant wintertime storm tracks are shown to play a major role in the determination of the frequency and type of freezing precipitation at each site. Results help to explain the regional maxima and minima of freezing precipitation across the CONUS, as well as the dominance of certain precipitation types and formation mechanisms in different portions thereof. Understanding these differences is necessary for proper development of techniques used to diagnose and forecast surface precipitation type and the occurrence of hazardous aircraft icing conditions associated with freezing precipitation aloft.

Two-Dimensional Airfoil Performance Degradation Because of Simulated Freezing Drizzle

Wind-tunnel tests were conducted to determine the performance degradation of a scaled two-dimensional NACA 23012 airfoil (outboard wing section of the Wyoming King Air 200T ) resulting from 1 ) ice because of various liquid hydrometeor sizes, 2 ) simulated drizzle ice roughness, and 3 ) simulated drizzle ice accretions on a model spar strap. The Wyoming King Air is equipped with a Saunders Fail-Safe Spar Strap that protrudes roughly 6 cm below the wing and extends spanwise from just outside both engine nacelles and may collect ice in large drop regions. The wind-tunnel evaluation facilitated quantifying the effects on aircraft performance degradation because of the King Air spar strap. The airfoil evaluations show that the drizzle drop ice shape and simulated drizzle ice roughness resulted in the highest performance degradation. In general, the ice shapes and simulated freezing drizzle roughness increased proe le drag, reduced angle of attack for maximum lift coefe cient, reduced the maximum lift coefe cient, altered the pitching moment, reduced lift over drag ratio, and marginally changed the lift curve slope. These evaluations also show that the most sensitive surface location on an airfoil is on the suction side between 6 and at least 11% of chord. Ice contaminations in this area are beyond the protective de-icing boots of most aircraft and lead to severe degradations in lift and drag characteristics. In addition, these results suggest that an ice-contaminated spar strap will increase King Air drag by approximately 12% at angles of attack consistent with cruise. Furthermore, any ice that forms on the lower surface of the wing, forward of the spar strap, does not signie cantly increase proe le drag.

Improved measurements of the drop size distribution of a freezing drizzle event

Airborne measurements from a new digital holographic camera are compared with PMS 2D-C measurements during the Canadian Freezing Drizzle Experiment. The digital holographic camera is not affected by drop sizing errors associated with out of focus particles in the same way as the 2D-C probe. Recent theoretical simulations of 2D-C particle sizing errors and approaches for statistically correcting the measurements are discussed. The theoretical simulations predict that in freezing drizzle with drop diameters from about 50–200 μm, uncorrected PMS 2D-C measurements can produce an artificial tail in the drop size distribution that extends out to about 325 μm. The digital holographic camera does not see the artificial tail. 2D-C measurements of drop size distribution in freezing drizzle that have been statistically corrected show an improved agreement with the digital holographic measurements. Some digital images of drizzle drops and ice crystals from a recently improved version of the instrument, called a cloud particle imager (CPI), are also shown.

Dual-Polarization Radar to Identify Drizzle, with Applications to Aircraft Icing Avoidance

Freezing drizzle has been identified as a primary aircraft in-flight icing hazard. New evidence shows that it can be a greater hazard than freezing rain, even though the latter can be significant. Freezing drizzle may form by droplet coalescence without an intermediate melting process, so that it also can be more difficult to detect than freezing rain. Theoretical calculations of microwave scattering and verification by the initial field studies presented here demonstrate that freezing drizzle should be detectable and distinguishable from other hydrometeor types in short-wavelength, dual-polarization radar measurements of elliptical and linear depolarization ratios. A practical procedure is suggested for identifying and monitoring this aviation hazard with the radar and concurrent atmospheric temperature measurements. The WSR-88D radar (NEXRAD) has the potential to add dual-linear polarization to operationally apply this method, within certain constraints

The 1990 Valentine's Day Arctic Outbreak. Part I: Mesoscale and Microscale Structure and Evolution of a Colorado Front Range Shallow Upslope Cloud

Abstract The mesoscale and microscale structure and evolution of a shallow, upslope cloud is described using observations obtained during the Winter Icing and Storms Project (WISP) and model stimulations. The upslope cloud formed within a shallow arctic air mass that moved into the region east of the Rocky Mountains between 12 and 16 February and contained significant amounts of supercooled liquid water for nearly 30 h. Two distinct layers were evident in the cloud. The lower layer was near neutral stability (boundary layer air) and contained easterly upslope flow. The upper layer (frontal transition zone) was thermodynamically stable and contained southerly flow. Overlying the upslope cloud was a dry, southwesterly flow of 20&amp;#x96;25 m s −1, resulting in strong wind shear near cloud top. Within 10 km of the Rocky Mountain barrier, easterly low-level flow was lifted up and over the mountains. The above-described kinematic and thermodynamic structure produced three distinct mechanisms leading to the production of supercooled liquid water: 1) upslope flow over the gently rising terrain leading into the Colorado Front Range, up the slopes of the Rocky Mountains and over local ridges, 2)upglide flow within a frontal transition zone, and 3) turbulent mixing in the boundary layer. Supercooled liquid water was also produced by 1) upward motion at the leading edge of three cold surges and 2) vertical motion produced by low-level convergence in the surface wind field. Large cloud droplets were present near the top of this cloud (approximately 50-µm diameter), which grew by a direct coalescence process into freezing drizzle in regions of the storm where the liquid water content was greater than 0.25 g m −3 and vertical velocity was at 10 cm s −1 Ice crystal concentrations greater than 1 L−1 were observed in the lower cloud layer containing boundary layer air when the top of the boundary layer air when the top of the boundary layer was colder than −12°C. The upper half of the cloud was ice-free despite temperatures as low as −15°C, resulting in long-lived supercooled liquid water in this region of the cloud.

The Form of Cyclonic Precipitation and Its Thermal Impact

A two-dimensional, parameterized kinematic cloud microphysics model is described and used to simulate the form of cyclonic precipitation and its thermal impact in three idealized situations. The first situation represents a strong, warm, or stationary front with a zone of freezing rain and ice pellets separating regions of rain and snow. The second represents a storm with initial surface temperatures slightly above 0°C, and in which precipitation at the ground changes from rain to snow as a result of cooling by melting and evaporation of hydrometeors. The third represents a shallow cloud in a sounding with all temperatures below0°C that initially produces freezing drizzle before a change to snow or ice pellets.