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J. I. MacPherson and D. Baumgardner

Abstract

Distortion of airflow around aircraft bodies can affect measurements made from these airborne platforms. In particular, particle sizes and concentrations can be in error, and ice crystals incorrectly classified as a result of flow field distortion. The magnitude of distortion has been evaluated for airflow around wingtip-mounted particle measuring probes on a Beechcraft King Air. The flow field in front of the probes was measured with a five-hole pressure probe during wind tunnel and airborne tests. Results from these tests show significant flow distortion and partially explain the preferred orientation of ice crystals which has been observed in previous measurements.

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A. M. Drummond and J. I. MacPherson

Abstract

A theoretical model for the local flow velocity ahead of the wing quarter-chord point on the National Aeronautical Establishment Twin Otter has been developed and experimentally verified by flight measurements using a pitot-static equipped Particle Measuring Systems (PMS) canister. This flow velocity model was used to calculate drop trajectories and the resulting airflow effects on drop images and concentrations. Results indicate that drop images and concentrations measured by the Twin Otter are distorted in the worst case by no more than 25% for an unusually high aircraft lift coefficient (CL) of 0.79, and by only a few percent for lower values of CL typical of normal flight. A method to correct water drop images and concentrations is described and results for a PMS probe mounted at one underwing station are presented.

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P. F. Lester and J. I. MacPherson

Abstract

Instrumented aircraft flights were made near a chinook arch cloud in the lee of the Rocky Mountains in Alberta, Canada. The aircraft data combined with satellite imagery have shown that the arch cloud, based near 5500 m MSL, extended about 50 km in the alongwind direction and more than 900 km in the crosswind direction and was embedded in the crest of a gravity wave 95 km in length. The wave displacement amplitude in the temperature field was about 800 m with estimated vertical motions of 1.6 m s−1. The wave persisted more than 10 h, moving eastward at a, mean speed of about 6 m s−1, somewhat slower than the wind speed at the same height. Light turbulence was found in the wave crests and troughs. The possibility that the wave was partially trapped is discussed with respect to the simultaneous observation of more than one long lee wave cycle in some of the temperature data.

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B. W. Leach and J. I. MacPherson

Abstract

Airborne wind measurement techniques currently being used onboard the National Aeronautical Establishment (NAE) Twin Otter Atmospheric Research Aircraft are described and their fundamental limitations are discussed. In particular, a recently acquired LTN-90-100 strapdown Inertial Reference System (IRS) exhibits significant low frequency errors in its velocity components (primarily Schuler oscillation errors that can attain peak values of 2 to 3 m s−1), actually degrading wind computation accuracy compared with older techniques. A new wind measurement technique, based on a Kalman filter integrated navigation approach, is shown to mitigate this problem and provide wind computation accuracy superior to previous methods. Preliminary results, based on applying the Kalman filter to Twin Otter flight test data, indicate that inertial velocity accuracies of 0.3 m s−1 rms (per axis) are attainable under ideal conditions, with a corresponding improvement in the accuracy of earth-referenced wind components.

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J. I. MacPherson and G. A. Isaac

Abstract

The turbulent characteristics of 17 Canadian cumulus clouds have been documented using the measurements from a specially instrumented T-33 aircraft. Most of the 33 cloud penetrations were made through the tops of cumuli 1–4.5 km in depth. Turbulent energy spectra over a range of wavelengths from 15 to 2500 m have been obtained for the two horizontal and the vertical gust velocities. Mean flow characteristics, especially any expected updrafts, tended to be obscured by turbulent fluctuations. The modal root-mean-square gust velocity was 1.7 m s−1 and the calculated modal turbulent energy dissipation rate was 160 cm2 s−3. Based on measured accelerations, estimates were made of expected vertical forces on several aircraft with a wide range of wing loadings. Cumulus clouds similar to those studied do not pose a safety hazard to these aircraft, and crew and passengers can easily tolerate the turbulence levels.

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G. A. Isaac, J. W. Strapp, R. S. Schemenauer, and J. I. Macpherson

Abstract

A summer (June and July) cumulus cloud seeding experiment was conducted in Canada near Yellowknife in 1975 and 1976, and Thunder Bay in 1977 and 1978. Microphysical and dynamical measurements were made with three instrumented aircraft, flying in the vicinity of and within 58 clouds near Yellowknife and 66 clouds near Thunder Bay. Using wing-mounted AgI pyrotechnic flares, a total of 25 of these clouds were seeded as an aircraft penetrated each cloud at the −5 to −10°C temperature level ∼ 300 m below cloud top. The microphysical properties of each cloud before and after seeding were compared with a statistical summary of the microphysical characteristics of natural or non-seeded cumuli at the same temperature level. Higher concentrations of ice particles were observed after seeding in ∼50% of the seeded clouds, with the magnitude of the increase and the post-seeding concentration being abnormally high in most cases. Approximately 40% of the Yellowknife cumuli produced rain after seeding and examples are given of how this rain could have been produced by an artificially stimulated Bergeron-Findeisen process. Near Thunder Bay, no rain was generated by the AgI. Even though Thunder Bay cumuli contained more cloud water and fewer large particles than Yellowknife cumuli, seeding was less successful because cloud lifetimes were too short for artificial precipitation to form.

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L. Mahrt, Jielun Sun, Dean Vickers, J. I. Macpherson, J. R. Pederson, and R. L. Desjardins

Abstract

Repeated aircraft runs at about 33 m over heterogeneous terrain are analyzed to study the spatial variability of the mesoscale flow and turbulent fluxes. An irrigated area, about 12 km across, generates a relatively cool moist inland breeze. As this air flows out over the warmer, drier surrounding land surface, an internal boundary layer develops within the inland breeze, which then terminates at a well-defined inland breeze front located about 1½ km downstream from the change of surface conditions. This front is defined by horizontal convergence, rising motion, and sharp spatial change of moisture, carbon dioxide, and ozone.

Both a scale analysis and the observations suggest that the overall vertical motion associated with the inland breeze is weak. However, the observations indicate that this vertical motion and attendant vertical transport are important in the immediate vicinity of the front, and the inland breeze does lead to significant modification of the turbulent flux. In the inland breeze downstream from the surface wetness discontinuity, strong horizontal advection of moisture is associated with a rapid increase of the turbulent moisture flux with height. This large moisture flux appears to be partly due to mixing between the thin moist inland breeze and overlying drier air.

As a consequence of the strong vertical divergence of the flux in the transition regions, the fluxes measured even as low as a few tens of meters are not representative of the surface fluxes. The spatial variability of the fluxes is also interpreted within the footprint format. Attempts are made to reconcile predictions by footprint and internal boundary-layer approaches.

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J. Mailhot, J. W. Strapp, J. I. MacPherson, R. Benoit, S. Bélair, N. R. Donaldson, F. Froude, M. Benjamin, I. Zawadzki, and R. R. Rogers

The MERMOZ (Montreal-96 Experiment on Regional Mixing and Ozone) field experiment was conducted in the greater Montreal area in June 1996. The measurement program was designed to examine several aspects of boundary layer dynamics and chemical transport. The project featured high-resolution real-time simulations with a mesoscale meteorological model driving several air quality models; the deployment of a research aircraft, fully instrumented for turbulent flux measurements; and a number of other supporting meteorological measurements such as two boundary layer wind profilers, a Doppler weather radar, and a special network of surface stations, upper-air soundings, tethersondes, and ozonesondes. An overview of the MERMOZ field program is presented with some preliminary results on various aspects of the experiment.

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J. H. Prueger, J. L. Hatfield, T. B. Parkin, W. P. Kustas, L. E. Hipps, C. M. U. Neale, J. I. MacPherson, W. E. Eichinger, and D. I. Cooper

Abstract

A network of eddy covariance (EC) and micrometeorological flux (METFLUX) stations over corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] canopies was established as part of the Soil Moisture–Atmosphere Coupling Experiment (SMACEX) in central Iowa during the summer of 2002 to measure fluxes of heat, water vapor, and carbon dioxide (CO2) during the growing season. Additionally, EC measurements of water vapor and CO2 fluxes from an aircraft platform complemented the tower-based measurements. Sensible heat, water vapor, and CO2 fluxes showed the greatest spatial and temporal variability during the early crop growth stage. Differences in all of the energy balance components were detectable between corn and soybean as well as within similar crops throughout the study period. Tower network–averaged fluxes of sensible heat, water vapor, and CO2 were observed to be in good agreement with area-averaged aircraft flux measurements.

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G. A. Isaac, R. S. Schemenauer, C. L. Crozier, A. J. Chisholm, J. I. MacPherson, N. R. Bobbitt, and L. B. MacHattie

Abstract

A cloud seeding technique is proposed which has the objective of stimulating rainfall from cumulus clouds drifting over forest fires. Preliminary tests of the ice crystal production capability of the cloud seeding technique were conducted on five cumulus clouds near Yellowknife, N.W.T., Canada, during July 1975. These clouds were over forest but not near forest fires. A T-33 turbulence research aircraft performed the seeding by burning wing-mounted TB1 AgI flares while flying through the clouds at the −5 to −10°C level. The T-33 turbulence measurements enabled estimates to be made of the rate of dispersion of the AgI. Microphysical measurements were made before and after seeding by an instrumented DHC-6 Twin Otter aircraft flying at the seeding level, and these were compared with measurements in six untreated cumulus clouds. High concentrations of ice crystals appeared after seeding in four of the five seeded cumulus clouds, and on two occasions precipitation-sized particles appeared at the seeding level. The evidence indicates that the AgI aerosol produced large quantities of ice crystals.

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