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In this study, the ability of a turbulent kinetic energy (TKE)-based boundary layer scheme to reproduce the rapid evolution of the planetary boundary layer (PBL) observed during two clear convective days is examined together with the impact of including nonlocal features in the boundary layer scheme. The two cases are chosen from the Montreal-96 Experiment on Regional Mixing and Ozone (MERMOZ): one is characterized by strong buoyancy, a strong capping inversion, and weak vertical wind shear; the other displays moderate buoyancy, a weaker subsidence inversion, and significant wind shear near the PBL top. With the original local version of the turbulence scheme, the model reproduces the vertical structures and turbulent quantities observed in the well-developed boundary layer for the first case. For the second case, the model fails to reproduce the rapid evolution of the boundary layer even though the TKE and sensible heat fluxes are greatly overpredicted. Some nonlocal aspects of the turbulence scheme are tested for these two cases. Inclusion of nonlocal (countergradient) terms in the vertical diffusivity equation has little impact on the simulated PBL. In contrast, alternative formulations of the turbulent length scales that follow the strategy proposed by Bougeault and Lacarrere have a greater influence. With the new turbulent lengths, entrainment at the top of the boundary layer is enhanced so that the depth of the well-mixed layer is much larger compared to that of the local simulations even though the turbulent sensible heat fluxes are smaller. Comparison with observations reveals, however, that the inclusion of these modifications does not improve all aspects of the simulation. To improve the performance and reduce somewhat the arbitrariness in the Bougeault-Lacarrere technique, a relationship between the two turbulent length scales (mixing and dissipation) used in the turbulence scheme is proposed. It is shown that, in addition to reducing the sensitivity of the results to the particular formulations, the simulated boundary layer agrees better with observations.

The microphysics associated with observations of supercooled drizzle drops, which formed through a condensation and collision-coalescence process, are reported and discussed. The growth environment was an 1100-m-thick stratiform cloud with cloud-base and cloud-top temperatures of -7.5 [degrees] and -12 [degrees] C, respectively. The cloud was characterized by a low droplet concentration of 21 [cm.sup.-3] and a large droplet median volume diameter of 29 [[micro]meter], with a concentration of interstitial aerosol particles of less than 15 [cm.sup.-3] (larger than 0.13 [[micro]meter] in diameter). The evolution of drizzle drops was traced downward from cloud top, with a maximum diameter of 500 [[micro]meter] observed at cloud base. The air mass was sufficiently clean to ensure only a small number of active cloud condensation nuclei. Consequently, small concentrations of cloud droplets led to concentrations of over 300 [L.sup.-1] for droplets larger than 40 [[micro]meter], which set up strong conditions for continued growth by collision-coalescence. Ice crystals in concentrations of 0.08 [L.sup.-1] were measured simultaneously with the drizzle drops and were not effective in glaciating the cloud, even though the drizzle drops were estimated to have taken at least 1-2 h to form. While the growth of precipitation-sized drops through collision-coalescence has been well documented, there are few measurements of this phenomena at temperatures less than 0 [degrees] C. This study provides a well-documented example of such an event at subfreezing temperatures. The applicability of this measurement in terms of hazardous aircraft icing is discussed.

Measurements of aircraft icing environments that include supercooled large drops (SLD) greater than 50 [micro]m in diameter have been made during 38 research flights. These flights were conducted during the First and Third Canadian Freezing Drizzle Experiments. A primary objective of each project was the collection of in situ microphysics data in order to characterize aircraft icing environments associated with SLD. In total there were 2793 30-s averages obtained in clouds with temperatures less than or equal to 0 [degrees] C, maximum droplet sizes greater than or equal to 50 [micro]m, and ice crystal concentrations less than 1 [L.sup.-1]. The data include measurements from 12 distinct environments in which SLD were formed through melting of ice crystals followed by supercooling in a lower cold layer and from 27 distinct environments in which SLD were formed through a condensation and collision-coalescence process. The majority of the data were collected at temperatures between 0 [degrees] and -14 [degrees] C, in stratiform winter clouds associated with warm-frontal or low pressure regions. For in-cloud measurements with temperatures less than or equal to 0 [degrees] C, the relative fraction of liquid-, mixed-, and glaciated-phase conditions were 0.4, 0.4, and 0.2, respectively. For each 30-s (3 km) measurement, integrated drop spectra that spanned 1-3000 [micro]m were determined using measurements from forward-scattering spectrometer probes and 2D-C and 2D-P probes. The integrated liquid water content (LWC) for each drop spectrum was compared with the LWC measured with a Nevzorov total water content probe and a Rosemount icing detector. The agreement was within the errors expected for such comparisons. This provides confidence in the droplet spectra measurements, particularly in the assessment of extreme conditions. The 99.9th-percentile LWC value was 0.7 g [m.sup.-3], and the 99th-percentile LWC for drops greater than 50 [micro]m in diameter was 0.2 g [m.sup.-3]. The 99.5th-percentile values of LWC and droplet concentrations are determined for different horizontal length scales and droplet diameter intervals, and are used to characterize the extreme icing conditions observed. The largest median volume diameters (MVD) observed were approximately 1000 [micro]m and represent cases in which the aircraft was flown below cloud base in freezing-rain conditions. In one case, SLD was observed to form at -21 [degrees] C, and the associated icing was rated as severe. Approximately 3% of the data for which SLD were observed had LWC greater than 0.2 g [m.sup.-3] and MVD greater than 30 [micro]m. Such conditions are believed to represent conditions that have the largest potential effects on aircraft performance. The analysis is presented in a format that is suitable for several applications within the aviation community, and comparisons are made to four common icing-envelope formulations. The data should be beneficial to regulatory authorities who are currently attempting to assess certification requirements for aircraft that are expected to encounter freezing-precipitation conditions.

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 [micro]m, independent of the measurement range used, when the ice-crystal concentrations exceeded approximately 1 [L.sup.-1]. In contrast, the FSSP measurements of the droplet spectra less than 30 [micro]m appeared free of ice-crystal contamination, independent of the ice-crystal concentrations observed. Glaciated cloud conditions always had FSSP-measured median volume diameters greater than 30 [micro]m and particle concentrations less than 15 [cm.sup.-3], whereas similar measurements in entirely liquid-phase clouds were observed less than 4% of the time. Images of drops greater than or equal to 125 [micro]m in diameter, which were collected in warm clouds greater than 0 [degrees] C, were used to calibrate geometric criteria, which were, in turn, used to segregate 2D images into circular and noncircular categories. It is shown that, on average, between 5% and 40% of ice crystals greater than or equal to 125 [micro]m in diameter will be classified as circular, depending on the particle size, with the percentage decreasing with increasing particle size. In liquid-phase clouds, between 85% and 95% of the 2D images will be correctly classified as circular for all particle sizes. At temperatures less than -4 [degrees] C, a Rosemount icing-detector threshold of 2 m V [s.sup.-1], corresponding to a maximum LWC of 0.002 g [m.sup.-3], was used to help to identify glaciated and nonglaciated conditions. A methodology for segregating liquid, mixed, and glaciated cloud regions, based on instrument responses to liquid and ice hydrometeors, was developed and applied to the CFDE dataset. The results were used to determine the relative frequency of liquid, mixed, and glaciated cloud conditions for the data collected during the two field projects. Approximately 40% of the in-cloud observations at temperatures less than 0 [degrees] C were assessed as liquid phase. The fractions of mixed-phase and glaciated-phase conditions were 26% and 34% for CFDE I and 46% and 14% for CFDE III, respectively. Because the ice-crystal responses for each instrument depend on the aircraft sampling speed and the ice-crystal sizes and concentrations, the results may be limited to conditions similar to those in clouds in midlatitude winter storms. Regardless, the results may have application to several fields, including development of parameterizations for numerical modeling, precipitation formation, remote sensing, ice multiplication, radiative transfer, and aircraft icing investigations. Important implications for aircraft icing investigations are discussed.