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Peter G. Hess

Abstract

The dynamics responsible for the mixing and dissolution of the polar vortex during final stratospheric warmings is investigated. A GCM and an associated offline N2O transport model are used to simulate the dynamics and transport during a Northern Hemisphere final stratospheric warming. The results are compared with those obtained from LIMS data for the final warming of 1979. In particular we examine the potential vorticity evolution in the two datasets, the modeled N2O evolution and the observed O3 evolution.

With the cessation of Rossby wave activity following the final warming the large horizontal wind shears and vigorous horizontal mixing associated with periods of enhanced wave activity during the winter months are not observed. In the simulated warming the flow field rapidly becomes zonally symmetric and tracer anomalies are trapped in the associated easterly flow regime. In the LIMS dataset potential vorticity anomalies are observed over two months following the breakup of the polar vortex. Ozone anomalies associated with those long-lived vortices are protected from mixing and are also long lived.

Following each warming the remnants of the originally intact vortex (defined in terms of the potential vorticity, N2O, O3 fields) gradually homogenize with the atmosphere at large. Two processes leading to this homogenization have been identified following the final warmings: 1) the potential vorticity field becomes decorrelated from that of the chemical tracer; 2) the vortex remnants begin to tilt dramatically in the vertical direction. As the vortex remnants tilt in the vertical, their vertical depth scale is greatly reduced facilitating vertical mixing; horizontal mixing is enhanced as the decorrelation between the potential vorticity and chemical anomalies leads to a rapid stirring of the chemical anomaly on the horizontal plane.

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Peter G. Hess and James R. Holton

Abstract

A two-dimensional stratospheric tracer transport parameterization is developed using the theory of small amplitude generalized lagrangian means. The parameterization is tested with a severely truncated three-dimensional model for a tracer with a constant tropospheric source and a height dependent stratospheric sink. Results from an explicit three-dimensional calculation of the tracer transport are compared to the results from a two-dimensional version of the model in which eddy fluxes are parameterized using data from the full three-dimensional model. The relative tracer transport by the eulerian, residual and lagrangian velocities is also examined.

Two experiments are run: the first experiment has a steady planetary wave superimposed on a climatological zonal mean wind, while the second simulates the large wave-mean flow interaction processes associated with a sudden stratospheric warming. The semilagrangian tracer transport parameterization is valid for steady state conditions and some transient conditions. The parameterization breaks down during a simulated sudden stratospheric warming. Once the parameterization breaks down, the parameterized transport remains unrelated to the actual transport at all subsequent times.

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Peter G. Hess and James R. Holton

Abstract

Temporal variances in the concentrations of N2O, CF2Cl2, CFCl3 and CH4 in the summer stratosphere at a midlatitude location have been measured by Ehhalt and others. A simple dynamical model is used to argue that these variances are created by irreversible mixing associated with the springtime final stratospheric. warming. Tracer perturbations generated during the warming are advected passively in the zonal mean easterlies so that the tracer variance is effectively frozen into the summertime stratosphere. Temperature perturbations, on the other hand, are subject to radiative dissipation; the temperature variance created during the final warming relaxes quickly to an ambient value.

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Peter G. Hess and Donal O'sullivan

Abstract

The two phases of the quasi-biennial oscillation in ozone are simulated using winds generated by a three-dimensional mechanistic stratospheric model input into an off-line ozone transport model. Ozone chemistry is parameterized in the off-line model. The mechanistic model is run with either easterly or westerly zonal winds in the lower equatorial stratosphere, so as to model the equatorial wind structure during the two phases of the equatorial quasi-biennial oscillation (QBO). When forcing is applied at the lower model boundary in the winter hemisphere, the mechanistic model simulates differences in the global circulation between the easterly and westerly phases of the QBO. The resulting modeled total ozone is larger in the polar regions during the easterly phase of the QBO than during the westerly phase, in agreement with observations. Using the residual-mean formalism the authors find that the difference in the modeled budget of ozone between the two phases of the QBO is due to a modulation of the extratropical planetary wave structure, and consequently the ozone transport, by the equatorial zonal-mean winds. Differences in the residual-mean velocities between the two phases of the QBO explain most of the differences in the ozone transport.

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Jean-Francois Lamarque and Peter G. Hess

Abstract

To study stratosphere–troposphere exchange, an approach based on the nonconservation of potential vorticity (PV) is developed; this approach arises naturally if one defines the tropopause in terms of PV. The evolution of a tropopause fold simulated by a mesoscale model is studied, as well as the evolution of PV at the tropopause level. The PV framework also permits the identification of the physical processes responsible for the cross-tropopause exchange as either diffusive or diabatic. In this model simulation, the diabatic processes are found to be the most important in the exchange. In particular, it is found that the negative heating gradient in the region of the warm sector of the surface cyclone is responsible for most of the diabatic exchange across the tropopause.

The mass exchange during the tropopause folding event is estimated to be around 4.9 × 1014 kg in four days over the domain considered (1600 × 2000 km). This number is shown to correspond to the net difference between exchange from stratosphere to troposphere (23.5 × 1014 kg) and exchange from troposphere to stratosphere (18.6 × 1014 kg). Using the results from the exchange of a passive tracer, the exchange of ozone is estimated to be of the order of 1.1 × 108 kg. Finally, the origin of the air exchanged is found to be from the lower stratosphere and the upper troposphere, for the period of four days studied.

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Peter G. Hess, David S. Battisti, and Philip J. Rasch

Abstract

How the surface boundary heating (sea surface temperature) and cumulus adjustment process affect the location, structure, energetics, and dynamics of the intertropical convergence zones (ITCZs) is investigated. A series of experiments is performed with a general circulation model where the lower boundary is specified to be water at a fixed sea surface temperature (SST), an aqua planet. All experiments are run using equinoctal insolation with no longitudinal variation in SST. Two different convective parameterization schemes (Kuo and moist convective adjustment) and several different zonally symmetric SST distributions are used in these experiments.

The location of the ITCZ is found to be sensitive to the convective parameterization scheme and the SST distribution. The model with the moist convective adjustment scheme produces an ITCZ over the tropical SST maximum, even under conditions where the SST gradient is weak. By contrast, the model with the Kuo convective parameterization is not as sensitive to SST distribution: the model with the Kuo scheme yields two ITCZs straddling the equator at approximately 7° latitude for a wide variety of SST distributions, including when the warmest water is located on the equator. The location of the ITCZ affects the structure and strength of both the time-mean Hadley cells and the subtropical jets. Furthermore, the equatorial wave spectrum is strongly influenced by the type of cumulus parameterization scheme used. The “convective characteristics” for each parameterization scheme are presented in detail to elucidate the influence of the large-scale environment on convection.

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G. Louis Smith, Kory J. Priestley, Phillip C. Hess, Chris Currey, and Peter Spence

Abstract

The Clouds and the Earth’s Radiant Energy System (CERES) instrument is a scanning radiometer for measuring Earth-emitted and -reflected solar radiation to understand Earth’s energy balance. One CERES instrument was placed into orbit aboard the Tropical Rainfall Measuring Mission (TRMM) in 1997; two were aboard the Terra spacecraft, launched in 1999; and two were aboard the Aqua spacecraft, launched in 2002. These measurements are used together with data from higher-resolution instruments to generate a number of data products. The nominal footprint size of the pixel at Earth’s surface is 16 km in the cross-scan direction and 23 km in the scan direction for the TRMM platform and 36 km in the cross-scan direction and 46 km in the scan direction for the Terra and Aqua platforms. It is required that the location on Earth of each pixel be known to 1–2 km to use the CERES data with the higher-resolution instruments on a pixel basis. A technique has been developed to validate the computed geolocation of the measurements by use of coastlines. Scenes are chosen in which the reflected solar radiation changes abruptly from the land surface to the darker ocean surface and the Earth-emitted radiation changes from the warm land to the cool ocean, or vice versa, so that scenes can be detected both day and night. The computed coastline location is then compared with the World Bank II map. The method has been applied to data from the three spacecraft and shows that the pixel geolocations are accurate to within 10% of the pixel size and that the geolocation is adequate for current scientific investigations.

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Kory J. Priestley, G. Louis Smith, Susan Thomas, Denise Cooper, Robert B. Lee III, Dale Walikainen, Phillip Hess, Z. Peter Szewczyk, and Robert Wilson

Abstract

The Clouds and the Earth’s Radiant Energy System (CERES) flight models 1 through 4 instruments were launched aboard NASA’s Earth Observing System (EOS) Terra and Aqua spacecraft into 705-km sun-synchronous orbits with 10:30 p.m. and 1:30 a.m. local time equatorial crossing times. With these instruments CERES provides state-of-the-art observations and products related to the earth’s radiation budget at the top of the atmosphere (TOA). The archived CERES science data products consist of geolocated and calibrated instantaneous filtered and unfiltered radiances through temporally and spatially averaged TOA, surface, and atmospheric fluxes. CERES-filtered radiance measurements cover three spectral bands: shortwave (0.3–5 μm), total (0.3>100 μm), and an atmospheric window channel (8–12 μm).

CERES climate data products realize a factor of 2–4 improvement in calibration accuracy and stability over the previotus Earth Radiation Budget Experiment (ERBE) products. To achieve this improvement there are three editions of data products. Edition 1 generates data products using gain coefficients derived from ground calibrations. After a minimum of four months, the calibration data are examined to remove drifts in the calibration. The data are then reprocessed to produce the edition 2 data products. These products are available for science investigations for which an accuracy of 2% is sufficient. Also, a validation protocol is applied to these products to find problems and develop solutions, after which edition 3 data products will be computed, for which the objectives are calibration stability of better than 0.2% and calibration traceability from ground to flight of 0.25%. This paper reports the status of the radiometric accuracy and stability of the CERES edition 2 instrument data products through April 2007.

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