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D. H. Lenschow and E. M. Agee

The field phases of AMTEX, a GARP subprogram on air-sea interaction implemented by Japan, were conducted over the East China Sea in the environs of Okinawa, Japan, during the last two weeks of February in 1974 and 1975. Investigators from Australia, Canada, and the United States also participated in this experiment. The weather was generally very favorable for this study of air mass transformation processes in 1975 because of an extensive cold air outbreak during most of the experimental period. A basic synoptic data set was obtained from 6 h soundings from an array of aerological stations centered at Okinawa. In addition, satellite, hourly surface and surface marine, oceanographic, boundary layer, radiation, radar, cloud physics, and aircraft data were obtained and have been or will be available in published data reports or on magnetic tape.

Preliminary results from 1974 and 1975 reported at the Fourth AMTEX Study Conference and joint United States–Japan Cooperative Science Program Seminar, “Air Mass Transformation Processes over the Kuroshio in Winter,” held in Tokyo, 26–30 September 1975, are presented and discussed.

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D. H. Lenschow and E. M. Agee

The first field phase of AMTEX was conducted during 14–28 February 1974 in the vicinity of the Southwest Islands of Japan, with the operational control center at Okinawa. Investigators from Japan, Australia, and the United States participated in the experiment. The measurements and synoptic situation during the field program as well as some preliminary results are presented. The weather was characterized by a warm period from 14 to 23 February, followed by a cold period from 24 to 28 February when extensive modification of the continental air took place.

Plans for the 1975 field program, scheduled for 16 February to 3 March, are discussed. No major changes from the 1974 program are expected in 1975.

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I. R. Paluch and D. H. Lenschow

Abstract

Observations of stratiform clouds in a region several hundred kilometers west of the southern California coast were made from the NCAR Electra research aircraft in the summer of 1987 during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE). Examples are shown of how heating or cooling of air by the sea and the evaporation of precipitation affect the stability of the temperature profile above the surface layer, which in turn affects the vertical moisture transports and the resulting cloud formation. We expect that sea-surface heating leads to the formation of stratus layers, while sea-surface cooling or cooling from evaporation of precipitation may produce fields of cumuli. The observations lead to a conceptual model of the life cycle of a stratus layer, starting as a thin, rather homogeneous layer, which grows and becomes patchy with time, produces precipitation, followed by formation of small cumuli below, and finally disintegrates, leaving a field of cumuli behind.

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E. N. Brown, C. A. Friehe, and D. H. Lenschow

Abstract

An air-motion sensing technique is described for measurement of attack and sideslip angles and dynamicpressure. The sensing probe consists of an array of five pressure holes in the standard radome of a twin-jetresearch aircraft. Comparisons are made with air motion measurements (angle of attack and dynamic pressure) obtained from a conventional differential pressure flow angle sensor at the tip of a nose boom 1.5fuselage diameters ahead of the aircraft body. The results indicate that the radome system works well downto scale sizes slightly larger than the fuselage diameter. (Finer scale measurements were limited by pressuretransducer response.) An insitu calibration technique is described for the determination of the empiricalradome angle-pressure difference sensitivity factor k, as a function of aircraft Mach number. The value ofk, so determined at low Mach numbers, is in approximate agreement with that calculated for potential flowfor a spherical radome. The in-situ technique applied to the conventional nose boom sensor indicates thatthe value of k based on wind tunnel calibrations may not apply for the present installation.

The time response of the conventional pressure system on the NCAR Sabreliner twin-jet aircraft is estimated on the basis of an in-flight comparison between the conventional pressure probe and a fast-responsegust probe flown together on a nose boom. Comparison of the power spectra of the conventional and radomeangles of attack for a traverse in boundary-layer turbulence indicates that the response of the radome systemis superior to the conventional system due to the shorter pressure lines that can be used.

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I. Gultepe, A. J. Heymsfield, and D. H. Lenschow

Abstract

Techniques are presented to obtain vertical velocity in cirrus clouds from in situ aircraft lateral wind measurements and from ground-based remote Doppler lidar measurements. In general, direct measurements of absolute vertical velocity w from aircraft are currently not feasible because of offsets in the air velocity sensors. An alternative to direct measurement is to calculate w from the integral of the divergence of the horizontal velocity around a closed path. We discuss divergence measurements from both aircraft and Doppler lidar. The principal errors in the calculation of w from aircraft lateral wind measurements are bias in the lateral wind, ground speed errors, and error due to vertical shear of the horizontal wind. For Doppler lidar measurements the principal errors are in the estimate of mean terminal velocity and the zeroth order coefficient of the Fourier series that is fitted to the data. The technique is applied to a cirrus cloud investigated during the FIRE (First International Satellite Cloud Climatology Regional Experiment) Cirrus Intensive Field Observation Program. The results indicate that the error in w is about ±14 cm s−1 from the aircraft technique. We show that this can be reduced to about ±2 to 3 cm s−1 with technical improvements in both ground speed and lateral velocity measurements. The error in w from Doppler lidar measurements, which is about ±8 cm s−1, can be reduced to about ±5 cm s−1 by improvements in the Doppler velocity measurements with technology that is currently available.

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D. H. Lenschow, J. C. Wyngaard, and W. T. Pennell

Abstract

Terms in the budgets of turbulence kinetic energy, temperature and humidity variances, and temperature and humidity fluxes have been evaluated for a baroclinic, convective boundary layer using data obtained from the NCAR Electra aircraft during the Air Mass Transformation Experiment (AMTEX). Although the mean temperature and momentum budgets, which were also evaluated, are strongly influenced by the horizontal temperature gradient, the second-moment budgets are little affected. The mean momentum budget is not well balanced, probably due to a combination of neglect of horizontal advection (aircraft advection measurements are shown to be statistically unreliable) and error in the surface geostrophic wind. For the most part, the measured terms in the second-moment budgets agree with previous estimates. Turbulence dissipation, however, was systematically less than that found in previous tower-based experiments. We find that over most of the mixed layer the temperature variance is maintained by turbulent transport and the temperature flux by buoyant production while, in contrast, the humidity variance and flux are maintained primarily by gradient production. Near the top of the mixed layer both temperature and humidity statistics are strongly affected by entrainment processes.

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R. A. Brost, J. C. Wyngaard, and D. H. Lenschow

Abstract

This paper discusses the turbulence profiles and budgets for two days of radiation, dynamical and thermodynamical observations by the NCAR Electra in shallow marine stratocumulus off the California coast in June 1976.

The boundary layer is characterized by relatively high wind speeds (12–20 m s−1) and low liquid water contents (0.1 g kg−1); the clouds are not very convective and seem to have little influence on the turbulence budgets. In cloud, drizzle has a significant impact on the liquid water budget and occasionally even on the total water budget even though no drizzle is observed at the surface. The stresses, velocity variances, and their budgets behave as in a neutral boundary layer, sometimes with an additional peak in the cross-wind variance at the inversion due to shear production.

There is scant evidence of direct production of vertical velocity variance at cloud top due to radiative cooling or latent heat release; it is maintained principally by the pressure-scrambling terms through redistribution of the shear-produced energy. We find, however, that while the Rotta parameterization for pressure scrambling in the stress budgets works well near the surface and sometimes throughout the layer, it is unsatisfactory in the variance budgets.

Fluctuations of temperature and moisture on a scale of several hundred meters in cloud satisfy the Clausius-Clapeyron equation. When the boundary layer is well mixed in equivalent potential temperature and total water substance, the vertical turbulent fluxes of these quantities are usually almost linear. The efficiency of cloud-top radiative cooling in producing mixed-layer convection is also discussed.

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M. C. vanZanten, B. Stevens, G. Vali, and D. H. Lenschow

Abstract

In situ and radar data from the second field study of the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) have been used to study drizzle in stratocumulus. Measurements indicate that drizzle is prevalent. During five of seven analyzed flights precipitation was evident at the surface, and on roughly a third of the flights mean surface rates approached or exceeded 0.5 mm day−1. Additional analysis of the structure and variability of drizzle indicates that the macroscopic (flight averaged) mean drizzle rates at cloud base scale with H 3/N where H is the flight-averaged cloud depth and N the flight-averaged cloud droplet number concentration. To a lesser extent flight-to-flight variability in the mean drizzle rate also scales well with differences in the 11- and 4-μm brightness temperatures, and the cloud-top effective radius. The structure of stratocumulus boundary layers with precipitation reaching the surface is also investigated, and a general picture emerges of large flight-averaged drizzle rates being manifested primarily through the emergence of intense pockets of precipitation. The characteristics of the drizzle spectrum in precipitating versus nonprecipitating regions of a particular cloud layer were mostly distinguished by the number of drizzle drops present, rather than a change in size of the median drizzle drop, or the breadth of the drizzle spectrum.

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D. H. Lenschow, E. R. Miller, and R. B. Friesen

Abstract

We present procedures to evaluate air motion measurements on two or more aircraft by flying them in formation at a known lateral displacement. The analysis is applied to two formation flights involving three aircraft—the NCAR Electra, Sabreliner and King Air—in a clear convective boundary layer to compare two types of air motion sensing probes mounted on different aircraft. The lateral separation between the Electra in the center, and the other two aircraft was ≈30 m. One sensing system utilized constrained vanes and the other differential pressure measurements across ports on a nose radome to obtain the two airflow angles that are used to calculate the transverse air velocity components. Both systems used a Pitot-static pressure difference for obtaining the longitudinal velocity component. We compare differences in means and variances, spectra and cospectra, and spatial coherences between the same velocity components measured on the different aircraft. The differences are, in most cases, comparable to what is predicted on the basis of making identical measurements of the same variable laterally displaced by 30 m in a turbulent velocity field. Measurements from a constrained vane gust probe and a differential pressure gust probe mounted less than 0.2 m apart on the Electra noseboom also compared well with each other. Thus, we have some assurance that both systems are measuring the true air velocity components.

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R. A. Brost, D. H. Lenschow, and J. C. Wyngaard

Abstract

The mean radiational, dynamical and thermodynamical structure of the marine stratocumulus-topped mixed layers of the California coast is described for two days in June 1976 using data from the NCAR Electra aircraft. We suggest that the synoptic conditions found may be typical of about half of the shallow stratocumulus-topped boundary layers that occur in this region during summer. The inversion was low near the coast and increased in height to the west, consistent with the average westward increase in sea-surface temperature. North–south inversion height change was largely due to entrainment and mean mesoscale vertical motions. Below the inversion, strong winds (12–20 m s−1 from the north) and horizontal inhomogeneities resulted in large advection terms in mean field equations. The sloping inversion often produced large vertical shears of the actual and geostrophic wind velocities across the inversion. Because of low liquid-water contents (0.1 g kg−1), temperature and water vapor could be measured in cloud with in situ instrumentation without significant errors due to wetting.

The longwave radiative extinction length was found to be relatively short; 63% of the cloud-top jump in radiation flux occurred within 40 m. Radiative heat loss was largely balanced by shear-driven entrainment. Compositing vertical gradients provided by individual aircraft ascents and descents is shown to overestimate vertical gradients at the inversion.

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