Enhanced seasonal forecast skill following stratospheric sudden warmings

Advances in seasonal forecasting have brought widespread socio-economic benefits. However, seasonal forecast skill in the extratropics is relatively modest, prompting the seasonal forecasting community to search for additional sources of predictability. For over a decade it has been suggested that knowledge of the state of the stratosphere can act as a source of enhanced seasonal predictability; long-lived circulation anomalies in the lower stratosphere that follow stratospheric sudden warmings are associated with circulation anomalies in the troposphere that can last up to two months. Here, we show by performing retrospective ensemble model forecasts that such enhanced predictability can be realized in a dynamical seasonal forecast system with a good representation of the stratosphere. When initialized at the onset date of stratospheric sudden warmings, the model forecasts faithfully reproduce the observed mean tropospheric conditions in the months following the stratospheric sudden warmings. Compared with an equivalent set of forecasts that are not initialized during stratospheric sudden warmings, we document enhanced forecast skill for atmospheric circulation patterns, surface temperatures over northern Russia and eastern Canada and North Atlantic precipitation. We suggest that seasonal forecast systems initialized during stratospheric sudden warmings are likely to yield significantly greater forecast skill in some regions

Computation of Solar Radiative Fluxes by 1D and 3D Methods Using Cloudy Atmospheres Inferred from A-train Satellite Data

The main point of this study was to use realistic representations of cloudy atmospheres to assess errors in solar flux estimates associated with 1D radiative transfer models. A scene construction algorithm, developed for the EarthCARE satellite mission, was applied to CloudSat, CALIPSO, and MODIS satellite data thus producing 3D cloudy atmospheres measuring 60 km wide by 13,000 km long at 1 km grid-spacing. Broadband solar fluxes and radiances for each (1 km)2 column where then produced by a Monte Carlo photon transfer model run in both full 3D and independent column approximation mode (i.e., a 1D model)

On shear-generated gravity waves that reach the mesophere. Part I : Wave generation

A long unresolved issue in nonorographic gravity wave generation is whether there is significant emission from Kelvin-Helmholtz (KH) shear instability in the lower stratosphere, for instance, just above tropopause jets. Such emission has often been suggested as significant for the angular momentum budget and hence for the wave-driven circulation of the middle atmosphere, most crucially in the summer mesosphere. An idealized model thought experiment is studied in which it is assumed that the KH shear instability rapidly mixes a thin layer, producing a &quot;pancake&quot; of three-dimensional clear-air turbulence, and emitting low-frequency inertia-gravity waves whose aspect ratio matches that of the turbulent layer and whose horizontal wavelength is large enough to avoid back-reflection and hence reach the summer mesosphere. The wave emission is modeled as a linear initial-value problem in which the rapid mixing of mass and momentum achieved by the turbulence is treated as instantaneous, and hence as determining the initial conditions. Care is taken to cast the problem into a form that permits well-conditioned numerical evaluation of the analytical solution, in both rotating and nonrotating cases, which behave very differently. Comparison with fully nonlinear numerical simulations in two dimensions of the same initial-value problem indicates that the linear theory is much better than might be expected on order-of-magnitude grounds. A companion paper (Part II) investigates the transmission of the emitted waves to the mesosphere subject to refraction and radiative damping.</p