Fabrizio Sassi

Toward a Physically Based Gravity Wave Source Parameterization in a General Circulation Model

Abstract Middle atmospheric general circulation models (GCMs) must employ a parameterization for small-scale gravity waves (GWs). Such parameterizations typically make very simple assumptions about gravity wave sources, such as uniform distribution in space and time or an arbitrarily specified GW source function. The authors present a configuration of the Whole Atmosphere Community Climate Model (WACCM) that replaces the arbitrarily specified GW source spectrum with GW source parameterizations. For the nonorographic wave sources, a frontal system and convective GW source parameterization are used. These parameterizations link GW generation to tropospheric quantities calculated by the GCM and provide a model-consistent GW representation. With the new GW source parameterization, a reasonable middle atmospheric circulation can be obtained and the middle atmospheric circulation is better in several respects than that generated by a typical GW source specification. In particular, the interannual NH stratospher...

Distribution and influence of convection in the tropical tropopause region

[1] A global analysis of convective cloud in the tropical tropopause region (12–17 km) is presented. The analysis is based on high-resolution global imagery of cloud brightness temperatures from satellites and from contemporaneous reanalysis temperatures. The coverage by deep convection decreases nearly exponentially with increasing altitude in the tropopause region. Convection is found at temperatures colder than the tropical cold point tropopause over 0.5% (±0.25%) of the tropics. Convection rarely penetrates more than 1.5 km above the tropopause. Large-scale relationships between cold tropopause temperatures and deep convection indicate that where the tropopause is coldest convection penetrates most frequently. Small-scale relationships show that the coldest diurnal tropopause temperatures occur after the diurnal peak in deep convection at tropopause levels over land. The coverage by deep convection is used to estimate the mass exchange or turnover time due to convection in the tropopause region. This turnover time is of the order of weeks at 12 km but increases to longer than a year at 18 km, with significant uncertainties in the tropopause region. INDEX TERMS: 3314 Meteorology and Atmospheric Dynamics: Convective processes; 3362 Meteorology and Atmospheric Dynamics: Stratosphere/ troposphere interactions; 0341 Atmospheric Composition and Structure: Middle atmosphereconstituent transport and chemistry (3334); KEYWORDS: tropical convection, stratospheretroposphere exchange

Modeling the whole atmosphere response to solar cycle changes in radiative and geomagnetic forcing

The NCAR Whole Atmosphere Community Climate Model, version 3 (WACCM3), is used to study the atmospheric response from the surface to the lower thermosphere to changes in solar and geomagnetic forcing over the 11-year solar cycle. WACCM3 is a general circulation model that incorporates interactive chemistry that solves for both neutral and ion species. Energy inputs include solar radiation and energetic particles, which vary significantly over the solar cycle. This paper presents a comparison of simulations for solar cycle maximum and solar cycle minimum conditions. Changes in composition and dynamical variables are clearly seen in the middle and upper atmosphere, and these in turn affect terms in the energy budget. Generally good agreement is found between the model response and that derived from satellite observations, although significant differences remain. A small but statistically significant response is predicted in tropospheric winds and temperatures which is consistent with signals observed in reanalysis data sets.

Climatology of mesopause region temperature, zonal wind, and meridional wind over Fort Collins,Colorado (41°N, 105°W), and comparison with model simulations

[1] Between May 2002 and April 2006, many continuous observations of mesopause region temperature and horizontal wind, each lasting longer than 24 h (termed full-diurnal-cycle observations), were completed at the Colorado State University Na Lidar Facility in Fort Collins, Colorado (41°N, 105°W). The combined data set consists of 120 full-diurnal-cycle observations binned on a monthly basis, with a minimum of 7 cycles in April and a maximum of 18 cycles in August. Each monthly data set was analyzed to deduce mean values and tidal period perturbations. After removal of tidal signals, monthly mean values are used for the study of seasonal variations in mesopause region temperature, zonal and meridional winds. The results are in qualitative agreement with our current understanding of mean temperature and wind structures in the midlatitude mesopause region with an observed summer mesopause of 167 K at 84 km, summer peak eastward zonal wind of 48 m/s at 94 km, winter zonal wind reversal at ∼95 km, and peak summer (pole) to winter (pole) meridional flow of 17 m/s at 86 km. The observed mean state in temperature, zonal and meridional winds are compared with the predictions of three current general circulation models, i.e., the Whole Atmosphere Community Climate Model version 3 (WACCM3) with two different simulations of gravity wavefields, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), and the 2003 simulation of the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM). While general agreement is found between observation and model predictions, there exist discrepancies between model prediction and observation, as well as among predictions from different models. Specifically, the predicted summer mesopause altitude is lower by 3 km, 8 km, 3 km, and 1 km for WACCM3 the two WACCM runs, HAMMONIA, and TIME-GCM, respectively, and the corresponding temperatures are 169 K, 170 K, 158 K, and 161 K. The model predicted summer eastward zonal wind peaks to 71 m/s at 102 km, to 48 m/s at 84 km, to 75 m/s at 93 km, and to 29 m/s at 94 km, in the same order. The altitude of the winter zonal wind reversal and seasonal asymmetry of the pole-to-pole meridional flow are also compared, and the importance of full-diurnal-cycle observations for the determination of mean states is discussed.

Dynamics of the middle atmosphere as simulated by the Whole Atmosphere Community Climate Model, version 3 (WACCM3)

The Whole Atmosphere Community Climate Model, version 3 (WACCM3) is a state-of-the-art climate model extending from the Earths surface to the lower thermosphere. In this paper we present a detailed climatology of the dynamics of the middle atmosphere as represented by WACCM3 at various horizontal resolutions and compare them to observations. In addition to the mean climatological fields, we examine in detail the middle atmospheric momentum budget as well as several lower and upper atmosphere coupling phenomena including stratospheric sudden warmings, the 2-day wave, and the migrating diurnal tide. We find that in large part, differences between WACCM3 and observations and the mean state of the model at various horizontal resolutions are related to gravity wave drag, which is parameterized in WACCM3 (and similar models). All three lower and upper atmosphere coupling processes examined show high sensitivity to the models resolution.

The Role of Equatorial Waves Forced by Convection in the Tropical Semiannual Oscillation

Abstract Recent satellite observations suggest that convection over the tropical continents is capable of exciting wave motions over a wide range of spatial and temporal scales. An equatorial beta-plane model was used to investigate the forcing by convective heating of equatorial waves with zonal wavenumbers from 1 to 15 and a wide range of periods, including diurnal oscillations. Also studied are the propagation of these waves in the equatorial middle atmosphere and their role in driving the tropical semiannual oscillation (SAO). Specification of the heating distribution used to force the model is guided by observations and analyses of tropical convection. It was found that intermediate-scale Kelvin and inertia–gravity waves provide between 25% and 50% of the forcing necessary to drive the westerly phase of the SAO near the stratopause, while the remainder is supplied by planetary-scale Kelvin waves. In the mesosphere, intermediate-scale waves account for an even larger fraction of the force required to ...

The stratopause semiannual oscillation in the NCAR Community Climate Model

Abstract The middle atmospheric version of the NCAR Community Climate Model (CCM2) has been used to study the development of the equatorial semiannual oscillation (SAO) in the stratosphere. The model domain extends from the ground to about 80 km, with a vertical resolution of 1 km. Transport of nitrous oxide (N2O) with simplified photochemistry is included in the calculation to illustrate the influence of tropical circulations on the distribution of trace species. Diagnosis of model output reveals two distinct phases in the evolution of the zonal mean state on the equator. In early December, a strong and broad easterly jet appears near the stratopause in connection with a midlatitude wave event (sudden stratospheric warming) that reverses the winter westerlies of the Northern Hemisphere throughout the upper stratosphere. When the wave forcing dies out, the radiative drive allows the westerlies to recover at midlatitudes, while easterlies persist in the tropics. The resulting strong meridional gradient of ...

Modulation of the mesospheric semiannual oscillation by the quasibiennial oscillation

Recent satellite and radar observations suggest that the semiannual oscillation (SAO) in the mesosphere is modulated by the stratospheric quasibiennial oscillation (QBO). The modulation is only apparent during the SAO easterly phase, which is considerably stronger when QBO winds are westerly than when they are easterly. We use an equatorial beta-plane model to demonstrate how such a modulation could come about through selective damping of the equatorial wave spectrum excited by deep convection. The waves affected most strongly are easterly inertia-gravity waves of phase speeds slower than ∼ −40 m s−1. This is close to the zonal wind speed during the easterly phase of the QBO (−30 to −35 m s−1), so the waves suffer strong thermal damping or even absorption as they propagate through the stratosphere. Because these waves are important for driving the easterly phase of the mesopause SAO in the model, that phase is weaker when the stratospheric QBO winds are easterly. A similar modulation of the westerly phase of the SAO does not occur for two reasons: (1) QBO westerlies are only half as strong as QBO easterlies, and (2) much of the driving of the westerly phase of the SAO is accomplished by Kelvin waves of phase speed ∼40–60 m s−1. As a consequence, the QBO winds have negligible influence on the vertical propagation of waves with westerly phase velocity and hence on the westerly phase of the modeled SAO.

Influence of the Madden-Julian Oscillation on upper tropospheric humidity

[1] Variations of upper tropospheric humidity (UTH) and cold cloud in the tropics reveal coherent changes that propagate eastward from the Indian Ocean into the Pacific. The coherence of UTH is high at periods of 30–90 days along the equator. Near the tropical tropopause, UTH coherence is large over the equator in the Indian Ocean and over the subtropics in the central Pacific. A composite life cycle shows that enhanced UTH in the subtropics coincides with two anticyclonic gyres. They straddle anomalous cold cloud over the equator as it propagates eastward. The enhancement of UTH is consistent with anomalously-cold temperature, which mirrors the anticyclonic gyres. Those temperature anomalies cancel the height anomaly of the gyres overhead. The complex of anomalous UTH, cold cloud, and anticyclonic motion marches eastward across the western Pacific, until reaching the dateline. Anomalous convection over the equator then collapses, along with the subtropical gyres and enhanced UTH accompanying it. INDEX TERMS: 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology; 3399 Meteorology and Atmospheric Dynamics: General or miscellaneous; 3319 Meteorology and Atmospheric Dynamics: General circulation; 3314 Meteorology and Atmospheric Dynamics: Convective processes; KEYWORDS: Madden-Julian Oscillation, Upper Tropospheric Humidity, Tropical Convection

Error growth in a whole atmosphere climate model

It has been well established that the atmosphere is chaotic by nature and thus has a finite limit of predictability. The chaotic divergence of initial conditions and the predictability are explored here in the context of the whole atmosphere (from the ground to the thermosphere) using the NCAR Whole Atmosphere Community Climate Model (WACCM). From ensemble WACCM simulations, it is found that the early growth of differences in initial conditions is associated with gravity waves and it becomes apparent first in the upper atmosphere and progresses downward. The differences later become more profound on increasingly larger scales, and the growth rates of the differences change in various atmospheric regions and with seasons—corresponding closely with the strength of planetary waves. For example, in December– February the growth rates are largest in the northern and southern mesosphere and lower thermosphere and in the northern stratosphere, while smallest in the southern stratosphere. The growth rates, on the other hand, are not sensitive to the altitude where the small differences are introduced in the initial conditions or the physical nature of the differences. Furthermore, the growth rates in the middle and upper atmosphere are significantly reduced if the lower atmosphere is regularly reinitialized, and the reduction depends on the frequency and the altitude range of the reinitialization.