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Marvin A. Geller, Tiehan Zhou, and Peter T. Love

Abstract

Recent satellite determinations of global distributions of absolute gravity wave (GW) momentum fluxes in the lower stratosphere show maxima over the summer subtropical continents and little evidence of GW momentum fluxes associated with the intertropical convergence zone (ITCZ). This seems to be at odds with parameterizations for GW momentum fluxes, where the source is a function of latent heating rates, which are largest in the region of the ITCZ in terms of monthly averages. The authors have examined global distributions of atmospheric latent heating, cloud-top-pressure altitudes, and lower-stratosphere absolute GW momentum fluxes and have found that monthly averages of the lower-stratosphere GW momentum fluxes more closely resemble the monthly mean cloud-top altitudes rather than the monthly mean rates of latent heating. These regions of highest cloud-top altitudes occur when rates of latent heating are largest on the time scale of cloud growth. This, plus previously published studies, suggests that convective sources for stratospheric GW momentum fluxes, being a function of the rate of latent heating, will require either a climate model to correctly model this rate of latent heating or some ad hoc adjustments to account for shortcomings in a climate model’s land–sea differences in convective latent heating.

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Marvin A. Geller, Peter T. Love, and Ling Wang

Abstract

The 1-s-resolution U.S. radiosonde data are analyzed for unstable layers, where the potential temperature decreases with increasing altitude, in the troposphere and lower stratosphere (LS). Care is taken to exclude spurious unstable layers arising from noise in the soundings and also to allow for the destabilizing influence of water vapor in saturated layers. Riverton, Wyoming, and Greensboro, North Carolina, in the extratropics, are analyzed in detail, where it is found that the annual and diurnal variations are largest, and the interannual variations are smallest in the LS. More unstable layer occurrences in the LS at Riverton are found at 0000 UTC, while at Greensboro, more unstable layer occurrences in the LS are at 1200 UTC, consistent with a geographical pattern where greater unstable layer occurrences in the LS are at 0000 UTC in the western United States, while greater unstable layer occurrences are at 1200 UTC in the eastern United States. The picture at Koror, Palau, in the tropics is different in that the diurnal and interannual variations in unstable layer occurrences in the LS are largest, with much smaller annual variations. At Koror, more frequent unstable layer occurrences in the LS occur at 0000 UTC. Also, a “notch” in the frequencies of occurrence of thin unstable layers at about 12 km is observed at Koror, with large frequencies of occurrence of thick layers at that altitude. Histograms are produced for the two midlatitude stations and one tropical station analyzed. The log–log slopes for troposphere histograms are in reasonable agreement with earlier results, but the LS histograms show a steeper log–log slope, consistent with more thin unstable layers and fewer thick unstable layers there. Some radiosonde stations are excluded from this analysis since a marked change in unstable layer occurrences was identified when a change in radiosonde instrumentation occurred.

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Marvin A. Geller, M. Joan Alexander, Peter T. Love, Julio Bacmeister, Manfred Ern, Albert Hertzog, Elisa Manzini, Peter Preusse, Kaoru Sato, Adam A. Scaife, and Tiehan Zhou

Abstract

For the first time, a formal comparison is made between gravity wave momentum fluxes in models and those derived from observations. Although gravity waves occur over a wide range of spatial and temporal scales, the focus of this paper is on scales that are being parameterized in present climate models, sub-1000-km scales. Only observational methods that permit derivation of gravity wave momentum fluxes over large geographical areas are discussed, and these are from satellite temperature measurements, constant-density long-duration balloons, and high-vertical-resolution radiosonde data. The models discussed include two high-resolution models in which gravity waves are explicitly modeled, Kanto and the Community Atmosphere Model, version 5 (CAM5), and three climate models containing gravity wave parameterizations, MAECHAM5, Hadley Centre Global Environmental Model 3 (HadGEM3), and the Goddard Institute for Space Studies (GISS) model. Measurements generally show similar flux magnitudes as in models, except that the fluxes derived from satellite measurements fall off more rapidly with height. This is likely due to limitations on the observable range of wavelengths, although other factors may contribute. When one accounts for this more rapid fall off, the geographical distribution of the fluxes from observations and models compare reasonably well, except for certain features that depend on the specification of the nonorographic gravity wave source functions in the climate models. For instance, both the observed fluxes and those in the high-resolution models are very small at summer high latitudes, but this is not the case for some of the climate models. This comparison between gravity wave fluxes from climate models, high-resolution models, and fluxes derived from observations indicates that such efforts offer a promising path toward improving specifications of gravity wave sources in climate models.

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Stephen D. Eckermann, Jun Ma, Karl W. Hoppel, David D. Kuhl, Douglas R. Allen, James A. Doyle, Kevin C. Viner, Benjamin C. Ruston, Nancy L. Baker, Steven D. Swadley, Timothy R. Whitcomb, Carolyn A. Reynolds, Liang Xu, N. Kaifler, B. Kaifler, Iain M. Reid, Damian J. Murphy, and Peter T. Love

Abstract

A data assimilation system (DAS) is described for global atmospheric reanalysis from 0- to 100-km altitude. We apply it to the 2014 austral winter of the Deep Propagating Gravity Wave Experiment (DEEPWAVE), an international field campaign focused on gravity wave dynamics from 0 to 100 km, where an absence of reanalysis above 60 km inhibits research. Four experiments were performed from April to September 2014 and assessed for reanalysis skill above 50 km. A four-dimensional variational (4DVAR) run specified initial background error covariances statically. A hybrid-4DVAR (HYBRID) run formed background error covariances from an 80-member forecast ensemble blended with a static estimate. Each configuration was run at low and high horizontal resolution. In addition to operational observations below 50 km, each experiment assimilated 105 observations of the mesosphere and lower thermosphere (MLT) every 6 h. While all MLT reanalyses show skill relative to independent wind and temperature measurements, HYBRID outperforms 4DVAR. MLT fields at 1-h resolution (6-h analysis and 1–5-h forecasts) outperform 6-h analysis alone due to a migrating semidiurnal (SW2) tide that dominates MLT dynamics and is temporally aliased in 6-h time series. MLT reanalyses reproduce observed SW2 winds and temperatures, including phase structures and 10–15-day amplitude vacillations. The 0–100-km reanalyses reveal quasi-stationary planetary waves splitting the stratopause jet in July over New Zealand, decaying from 50 to 80 km then reintensifying above 80 km, most likely via MLT forcing due to zonal asymmetries in stratospheric gravity wave filtering.

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