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Peter H. Stone and John H. Carlson

Abstract

Lapse rates, moist adiabatic lapse rates and the critical lapse rate for baroclinic adjustment are calculated and compared for the mean annual, January and July states in the Northern Hemisphere. In the troposphere above the planetary boundary layer zonal mean lapse rates are within 20% of the moist adiabatic lapse rate from the equator up to about 30°N in January and 50°N in July, but are appreciably more stable in higher latitudes. The latitudinal distribution of tropospheric mean lapse rates clearly delineates two regimes in the atmosphere—a low-latitude regime where the lapse rates are essentially moist adiabatic, and a high-latitude regime where the lapse rates are essentially the critical lapse rate for baroclinic adjustment. The dividing point between the two regimes shifts from 28°N in January to 47°N in July, and the transition is less sharp in July than in January. The absence of appreciable seasonal changes in lapse rates in midlatitudes can be attributed to counterbalancing seasonal changes in the strength of moist convection and baroclinic eddies. Hemispheric mean lapse rates in the mid and lower troposphere are within 0.4 K km−1 of the moist adiabatic lapse rate in July, but are as much as 1.9 K km−1 less in January. Implications for simple climate models are discussed. A principal conclusion is that the vertical temperature structure could be well approximated by a radiative-convective equilibrium model with two critical lapse rates—the moist adiabatic lapse rate and the critical lapse rate for baroclinic adjustment.

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Kenneth H. Bergman and Toby N. Carlson

Abstract

A method for objective analysis of aircraft observations in tropical cyclones has been developed. Quasi-horizontal fields of motion, temperatures, mixing ratios, and D-values are analyzed using a modified version of the method of successive corrections. The weighting functions are specified so that the high degree of circular symmetry characteristic of tropical cyclones is incorporated in the analyses. The analyses are performed on a 25 by 25 Cartesian grid of 5 n mi spacing which is centered on the storm. A special feature is the analysis of vertical motions as determined from aircraft flight characteristics. Three Atlantic storms are analyzed in detail: Hurricanes Inez (1966), Debbie (1969), and Ginger (1971). The analyses show the significant larger-scale features and major asymmetries of these storms. Both Inez and Debbie, which were well organized hurricanes, display characteristic vertical motion patterns in which a ring of strong ascent is found immediately surrounding the eye, with marked descent just outside of the annulus of strong ascent. Maximum ascent and descent rates were each indicated to be a few meters per second in these storms. Ginger was a marginal hurricane with poorly organized eye structure and relatively weak and disorganized patterns of vertical motion.

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V. Cardone, H. Carlson, J. A. Ewing, K. Hasselmann, S. Lazanoff, W. McLeish, and D. Ross

Abstract

The surface wave environment in the GATE B/C scale is described from wave measurements made from buoys and aircraft during Phase III (September 1974). Particular emphasis is given to the wave measurements made from the pitch-roll buoy deployed in the B-scale array from the ship Gilliss and a similar buoy deployed in the C-scale array from Quadra. Reduction of the pitch-roll buoy measurements provided estimates of the one-dimensional wave spectrum as well as of the mean direction and spread of wave energy as a function of frequency. The data clearly revealed the importance of external forcing on the wave climate in GATE. Most of the wave energy present in the GATE areas was found to be swell imported from the trade wind circulations of both hemispheres and from an intense extratropical cyclone which crossed the North Atlantic at high latitudes early in Phase III. Locally generated waves were clearly evident in the wave spectra, but their energy level way have been modulated significantly by the low-frequency swell. The GATE wave data set can provide a powerful test of contemporary numerical wave-prediction models. The present study defines the, attributes which are required of such models for meaningful application to the GATE needs.

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Thomas B. Sanford, James A. Carlson, John H. Dunlap, Mark D. Prater, and Ren-Chieh Lien

Abstract

An instrument has been developed that measures finescale velocity and vorticity in seawater based on the principles of motional induction. This instrument, the electromagnetic vorticity meter (EMVM), measures components of the gradient and Laplacian of the electrostatic potential field induced by the motion of seawater through an applied magnetic field. The principal innovation described here is the development of a sensor for measuring small-scale vorticity. The sensor head consists of a strong NdFeB magnet, a five-electrode array, low-noise preamplifiers, and 20-Hz digitizers. The main electronics includes attitude sensors, batteries, a microprocessor, and a hard disk. The vorticity sensors are usually carried on a heavy towed vehicle capable of vertically profiling to 200 m and at tow speeds of several knots.

The theoretical response functions of the EMVM are evaluated for velocity and vorticity. Extensive measurements were obtained in Pickering Passage, Washington, as the sensor vertically profiled in an unstratified tidal channel. During periods of strong flow, the vertical structure of all properties confirmed expectations for a fully developed turbulent bottom boundary layer. EMVM observations of velocity and vorticity are shown to be in agreement with the theoretical response function for isotropic turbulence. A principal result is that the vertical flux of spanwise vorticity (i.e., wωy) is positive (i.e., flux is away from seabed) and vertically uniform. The vertical eddy diffusivity for vorticity is about 5 × 10−2 m2 s−1, which is about the same value as for momentum.

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Barry H. Lynn, Toby N. Carlson, Cynthia Rosenzweig, Richard Goldberg, Leonard Druyan, Jennifer Cox, Stuart Gaffin, Lily Parshall, and Kevin Civerolo

Abstract

A new approach to simulating the urban environment with a mesocale model has been developed to identify efficient strategies for mitigating increases in surface air temperatures associated with the urban heat island (UHI). A key step in this process is to define a “global” roughness for the cityscape and to use this roughness to diagnose 10-m temperature, moisture, and winds within an atmospheric model. This information is used to calculate local exchange coefficients for different city surface types (each with their own “local roughness” lengths); each surface’s energy balances, including surface air temperatures, humidity, and wind, are then readily obtained. The model was run for several summer days in 2001 for the New York City five-county area. The most effective strategy to reduce the surface radiometric and 2-m surface air temperatures was to increase the albedo of the city (impervious) surfaces. However, this caused increased thermal stress at street level, especially noontime thermal stress. As an alternative, the planting of trees reduced the UHI’s adverse effects of high temperatures and also reduced noontime thermal stress on city residents (and would also have reduced cooling energy requirements of small structures). Taking these results together, the analysis suggests that the best mitigation strategy is planting trees at street level and increasing the reflectivity of roofs.

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