Air Mass Transformation Experiment Research Articles

Cold surge event observed by radiosonde observation from the research vessel \u201cHakuho-maru\u201d over the Philippine Sea in December 2012

The thermal energy transfer from the sea surface to the atmosphere associated with a cold surge event was investigated with observations from radiosondes on the research vessel “Hakuho-maru” over the Philippine Sea in December 2012. These observations were analyzed, and the results were compared with those obtained from observations over the East China Sea in the Air Mass Transformation Experiment in 1974 (AMTEX ‘74). The horizontal advection of cold and dry air associated with the cold surge dominated at heights below 850 hPa. In spite of this strong advection, the local temporal variations in the temperature and moisture were small, because the advection was balanced by the transfer of heat and moisture from the sea surface, which is qualitatively the same behavior as observed during the cold surge event in AMTEX ‘74. The eddy transport of the total heat energy from the sea surface to the atmosphere was estimated at about 410 W/m2, which is about half of the maximum value of 780 W/m2 observed during AMTEX ‘74. This result shows the existence of considerable heat transfer from the sea surface to the atmosphere over the Philippine Sea, which is the downstream region of the cold surge, after it passed through the East China Sea.

A Polar Mesoscale Cyclone Formed over the East China Sea and Developed into a Secondary Cyclone over the Northwestern Pacific — An Observational Case Study on 19-22 February 1975 —

Polar mesoscale cyclones (PMCs) frequently develop over the Japan Sea. Genesis of PMCs over the East China Sea is rare, but can occur under certain synoptic-scale conditions. In this observational case study, the feature of a PMC generated over the eastern East China Sea on 20 February 1975 is studied using observation data including those obtained during Air-mass Transformation Experiment, satellite cloud images, and objective-reanalysis data.

Estimating Overwater Turbulence Intensity from Routine Gust-Factor Measurements

AbstractFor overwater diffusion estimates the Offshore and Coastal Dispersion (OCD) model is preferred by the U.S. Environmental Protection Agency. The U.S. Minerals Management Service has recommended that the OCD model be used for emissions located on the outer continental shelf. During southerly winds over the Gulf of Mexico, for example, the pollutants from hundreds of offshore platforms may affect the gulf coasts. In the OCD model, the overwater plume is described by the Gaussian equation, which requires the computation of σy and σz, which are, in turn, related to the turbulence intensity, overwater trajectory, and atmospheric stability. On the basis of several air–sea interaction experiments [the Barbados Oceanographic and Meteorological Experiment (BOMEX), the Air-Mass Transformation Experiment (AMTEX), and, most recently, the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE)] and the extensive datasets from the National Data Buoy Center (NDBC), it is shown that under neutral and stable conditions the overwater turbulence intensities are linearly proportional to the gust factor (G), which is the ratio of the wind gust and mean wind speed at height z (Uz) as reported hourly by the NDBC buoys. Under unstable conditions, it is first shown that the popular formula relating the horizontal turbulence intensity (σu,υ/u∗, where u∗ is the friction velocity) to the ratio of the mixing height (h) and the buoyancy length (L) (i.e., h/L) suffers from a self-correlation problem and cannot be used in the marine environment. Then, alternative formulas to estimate the horizontal turbulence intensities (σu,υ/Uz) using G are proposed for practical applications. Furthermore, formulas to estimate u∗ and z/L are fundamentally needed in air–sea interaction studies, in addition to dispersion meteorology.

Errors in airborne flux measurements

We present a general approach for estimating systematic and random errors in eddy correlation fluxes and flux gradients measured by aircraft in the convective boundary layer as a function of the length of the flight leg, or of the cutoff wavelength of a highpass filter. The estimates are obtained from empirical expressions for various length scales in the convective boundary layer and they are experimentally verified using data from the First ISLSCP (International Satellite Land Surface Climatology Experiment) Field Experiment (FIFE), the Air Mass Transformation Experiment (AMTEX), and the Electra Radome Experiment (ELDOME). We show that the systematic flux and flux gradient errors can be important if fluxes are calculated from a set of several short flight legs or if the vertical velocity and scalar time series are high‐pass filtered. While the systematic error of the flux is usually negative, that of the flux gradient can change sign. For example, for temperature flux divergence the systematic error changes from negative to positive about a quarter of the way up in the convective boundary layer.

Mean and Turbulence Statistics in a Wintertime Convectively-mixed Boundary Layer Over Lake Michigan

First, second, and third moment statistics for u, v, w, θ ν, and q have been derived from the Lake-Effect Snow Studies (LESS) 20 Hz aircraft data set collected in a northerly flow cold air outbreak over Lake Michigan on 10 January 1984. Vertical profiles of mean and turbulence statistics in a Type I Cloud-Topped Boundary Layer (CTBL) have been presented, discussed, and compared with similar observational case studies during the Air Mass Transformation Experiment (AMTEX) and the Mesoscale Air-Sea Exchange (MASEX), as well as with numerical model simulations of convective boundary layers as presented by Deardorff (1974, 1980) and Moeng (1984). LESS results show the importance of a more dynamically-active cloud layer in the normalized vertical profile of vertical velocity variance, covariance of horizontal and vertical wind components, buoyancy, turbulent kinetic energy, TKE, vertical transport of TKE, and vertical velocity skewness in Type I CTBLs. Turbulence statistics are also examined with respect to the effect of strong heating at the air-lake interface and entrainment effects near the top of the CTBL. Comparison with model simulations continue to show the need for better agreement with observational results. Finally, selected profiles of LESS turbulence statistics are presented and discussed to show the sensitivity (or lack thereof) between results derived from raw as opposed to detrended data.

Modified Eady Waves and Frontogenesis Part I: Linear Stability Analysis

Baroclinic instability in an inviscid fluid with parabolic potential temperature profiles is investigated. Unlike the classical Eady model, there is no short wave cutoff. In addition to the longwave disturbances, which are similar to the Eady waves, shortwave disturbances can also develop in the lower atmosphere, where the stratification is weaker. The growth rate of the short waves increases with increasing stratification aloft. The results show that shortwave disturbances can penetrate into the upper stable layer. The growth rate and disturbances of those waves may be associated with an effective Burger number, which is defined as(The equation is abbreviated)where h(superscript *)is the height of the maximum vertical heat flux(w(superscript ')θ(superscript '))and λ is the horizontal wavelength. Numerical simulations obtained from a nonlinear mesoscale model in Part Ⅱ also confirm that the short waves can develop into a surface front within a few days. Those short waves may correspond to the medium-scale disturbances observed over the AMTEX (Air Mass Transformation EXperiment) region.

Bivariate conditional sampling of buoyancy flux during an intense cold-air outbreak

Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top. The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux. Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.

Nearly steady convection and the boundary-layer budgets of water vapor and sensible heat

The budgets of water vapor and sensible heat in the convective atmospheric boundary (mixed) layer are analyzed by means of a simple slab approach adapted to steady large-scale advective conditions with radiation and cloud activity. The entrainment flux for sensible heat is assumed to be a linear function of the surface flux. The flux of water vapor at the top of the mixed layer is parameterized by extending the first-order Betts-Deardorff approach, i.e., by adopting linear changes for both the specific humidity and the flux across the mixed layer and across the inversion layer of finite thickness. In this way the dissimilarity of sensible heat and water vapor transport in the mixed layer can be taken into account. The experimental data were obtained from the Air Mass Transformation Experiment (AMTEX). The entrainment constant for sensible heat at the top of the mixed layer was found to have values similar to those observed in other weakly convective situations, i.e., around 0.4 to 0.6. This appears to indicate that the effect of mechanical turbulence was not negligible; however, the inclusion of this effect in the formulation did not improve the correlation. In contrast to the first-order approach, the zero-order approach, i. e., the jump equation commonly used for the flux of a scalar at the inversion, (ovw′c′ ) h = we δc (where w e is the entrainment velocity and δc the concentration jump across the inversion), was found to be invalid and incapable of describing the data.

Turbulence in a Convective Marine Atmospheric Boundary Layer

The structure and kinetic energy budget of turbulence in the convective marine atmospheric boundary layer as observed by aircraft during a cold air outbreak have been studied using mixed layer scaling. The results are significantly different from those of previous studies under conditions closer to free convection. The normalized turbulent kinetic energy and turbulent transport are about twice those found during the Air Mass Transformation Experiment (AMTEX). This implies that for a given surface heating the present case is dynamically more active. The difference is mainly due to the greater importance of wind shear in the present case. This case is closer to the roll vortex regime, whereas AMTEX observed mesoscale cellular convection which is closer to free convection. Shear generation is found to provide a significant energy source, in addition to buoyancy production, to maintain a larger normalized turbulent kinetic energy and to balance a larger normalized dissipation. The interaction between turbulent pressure and divergence (i.e., pressure scrambling) is also found to transfer energy from the vertical to the horizontal components, and is expected to be stronger in roll vortices than in m esoscale cells. The sensible heat flux is found to fit well with a linear vertical profile in a clear or subcloud planetary boundary layer (PBL), in good agreement with the results of Lenschow et al., (1980). The heat flux ratio between the PBL top and the surface, derived from the linear fitted curve, is approximately -0.14, in good agreement with that derived from the lidar data for the same case. Near the PBL top, the heat flux profiles are consistent with those of Deardoff (1979) and Deardorff et al. (1980).

Comparisons of Aircraft and Tower Measurements around Tarama Island during the AMTEX ‘75

Abstract During the field observation period of the Air Mass Transformation Experiment in 1975 (AMTEX ‘75), one of the boundary-layer research flights of the National Center for Atmospheric Research (NCAR) Electra aircraft was conducted in the vicinity of Tarama Island. A 50-m onshore observation tower was located on the southwestern coast of Tarama Island, and a 12-m offshore observation tower on the northern reef. We compare and discuss mean profiles of dry- and wet-bulb temperature, wind speed and direction, and turbulent fluxes of latent and sensible heat and momentum observed by these three observing platforms. We found that the 50.m onshore tower, which was about 3.5 km downwind of the shoreline, and the airplane flight legs over the island at 140 and 160 m height were in the island-modified air. This resulted in large differences between ocean and island measured fluxes, smaller differences in the variances and almost no differences in the means. Thus, we found that for the conditions of this exper...

Simple formulation of heat flux in the unstable atmospheric boundary layer

A formulation for the magnitude of the upward heat flux is developed for the common situation where information on temperature fluctuations is available but measurement of vertical motion fluctuations are unavailable or of inadequate quality. A relationship is also developed which allows inference of the vertical structure of the heat flux from the stability. This development uses data from the Air Mass Transformation Experiment and a number of existing studies in the literature.

Simulations of Cold Surges over Oceans with Application to AMTEX'75

Comparative numerical experiments and observational verifications for cold surges over the ocean are conducted using a primitive equation planetary boundary layer (PBL) model. The comparative experiments illustrate the importance of large-scale environmental conditions on air mass transformation. Such factors as baroclinity, static stability, moisture content, upwind inversion strength and height exert strong controls on the downwind evolution of the PBL and clouds. Most results are obtained for clouds in the cumulus mode, but two comparisons are made with simpler stratocumulus simulations. The PBL flow is influenced by the synoptic geostrophic wind distribution and the PBL depth is also sensitive to large-scale vertical velocity. The responses of this advective PBL to stronger wind are found to be different from that of a horizontally homogeneous PBL. A simulation of an intense cold air outbreak observed during the Air Mass Transformation Experiment, 1975 (AMTEX'75) is performed. Downwind variations of dynamic and thermodynamic PBL properties and cloud distribution and type are well reproduced. The steep sea-surface temperature gradient produces strong PBL baroclinity and a strongly divergent PBL flow. The simulated large cross-isobar angle associated with intense cold air advection and vigorous momentum mixing is in favorable agreement with both observation and theory.

Heat Transport in the Atmospheric Boundary Layer

Abstract The structure of turbulence and transport of heat is examined from data obtained from 11 aircraft soundings executed in heated boundary layers during the Air Mass Transformation Experiment. Various influences on the turbulent transport are revealed by analyzing properties of the joint frequency distribution in polar coordinate. Such an analysis allows determination of a correlation coefficient and fluctuation amplitude as a function of properties of different types of motion comprising the total flow. In particular, the contribution of various events to the total flux is revealed without a priori specification of sampling criteria. In the present case, shear generation of turbulence and broken stratocumulus at the boundary layer top modify the profiles of virtual and sensible heat flux. The upward virtual heat flux shows the usual linear decrease with height but vanishes and becomes downward at the relatively low level of 60% of the boundary layer top. The vertical structure of the heat flux is e...

The effects of mesoscale convective cells on the surface wind field over the ocean

Near-surface wind velocities were measured on an array of anchored spar buoys in the East China Sea in February 1975 as a part of the Japanese AMTEX '75 (Air Mass Transformation Experiment), a subprogram of GARP. These data were used to determine the effects of atmospheric convection associated with mesoscale convective cells (MCC) on near-surface winds over the ocean. When MCC were present, a peak occurred in the near-surface wind spectrum in the so-called spectral gap.

Numerical Study of an AMTEX'75 Oceanic Cyclone

Abstract The diabatic effects of latent heat release, boundary layer moisture and heat flux from the ocean surface and large-scale forcing due to upper-level systems are three physical processes affecting oceanic cyclogenesis. Detailed analyses of a major winter storm which occurred during the initial phase of the Air Mass Transformation Experiment in 1975 (AMTEX '75) indicated that the role of these three processes was vital to the Cyclone's development. To gain further insight into their influence, a control and three numerical experiments were performed using a multi-level moist primitive equation model with fine vertical resolution in the boundary layer. The simulation which included complete physics faithfully reproduced the major feature of the observed system. It was found that latent heating had a profound impact on the middle-level baroclinicity, the intensity and phase speed of the storm, and the vertical coupling within the simulated system. Without the surface moisture and heat source, the eff...

Cloud-Top Entrainment Instability Observed in AMTEX

Abstract Aircraft data collected at the top of a cloud-topped mixed layer during the Air Mass Transformation Experiment are analyzed. Entrainment instability due to evaporation of liquid water is found to occur intermittently. This instability and upward heat flux occur frequently at the downstream edge of cloudy mixed-layer motions which have penetrated upward into the faster moving dry air of the inversion layer. Such circulations appear to occur mainly on scales of the order of 100 or 200 m. At the level of the aircraft at the mixed-layer top, the net heat flux is upward in cloudy regions and downward in the driest regions.

Mean vertical velocity and turbulence intensity inside and outside thermals

Aircraft data from the Air Mass Transformation Experiment (AMTEX) are used to estimate mean vertical velocities and turbulence intensities of a field of convective elements (thermals) and its environment. Thermals are defined here by segments of the time series (which has first been high-pass filtered with a 5 km wavelength cutoff filter) where humidity exceeds half a standard deviation above the mean. Mixed layer normalization is used to generalize the results for application to simplified models of pollution dispersion from elevated sources.

Wintertime Current Meter Measurements from the East China Sea

Abstract An array of three current meters were anchored on the continental shelf of the East China Sea during the last half of February 1975 as part of the Japanese Air Mass Transformation Experiment, AMTEX-75. The results indicate that the currents are dominated by the rotational semidiurnal M2 component superimposed on a slow mean drift to the northeast. Differences in direction of several days duration between two of the current meters suggest the presence of transient mesoscale eddies or meanders in the flow regime. Rotary spectra indicated that the tidal currents are characterized by a clockwise ellipse of eccentricity ≈0.6 and that there was some inertial energy present. The current rose for bandpassed velocity fluctuations (0.2–2 eph) was elongated in a direction 25–30° to the right of the wind direction indicative of Ekman veering.

AMTEX中の冬季東シナ海域に見られる逆転層を持つ大気境界層

The observed thermodynamic structure of the winter monsoon ABL during the air-mass transformation experiment is studied with a simple steady-state inversion model, taking into account the effects of large-scale subsidence, turbulent mixing, advection and radiative cooling. The model output provides the heights of cloud base and inversion, the profiles of static energy and mixing ratio and also the vertical profiles of the turbulent fluxes.The results of this steady-state model are in accordance with observations and other computations, and indicate, that the main processes are included into the model. The strong dependence of the results on some of the input parameters is shown by a sensitivity test.

Mean-Field and Second-Moment Budgets in a Baroclinic, Convective Boundary Layer

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 turbul...