Cloud-affected radiances from geostationary satellite sensors provide the first area-wide observable signal of convection with high spatial resolution in the range of kilometers and high temporal resolution in the range of minutes. However, these observations are not yet assimilated in operational convection-resolving weather prediction models as the rapid, non-linear evolution of clouds makes the assimilation of related observations very challenging. To address these challenges, we investigate the assimilation of satellite radiances from visible and infrared channels in idealized observing system simulation experiments (OSSEs) for a day with summer-time deep convection in central Europe. This constitutes the first study assimilating a combination of all-sky observations from infrared and visible satellite channels and the experiments provide the opportunity to test various assimilation settings in an environment, where the observation forward operator and the numerical model exhibit no systematic errors. The experiments provide insights into appropriate settings for the assimilation of cloud-affected satellite radiances in an ensemble data assimilation system and demonstrate the potential of these observations for convective-scale weather prediction. Both infrared and visible radiances individually lead to an overall forecast improvement, but best results are achieved with a combination of both observation types that provide complementary information on atmospheric clouds. This combination strongly improves the forecast of precipitation and other quantities throughout the whole range of 8 h lead time.

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We investigate the turbulent flow through a heterogeneous forest canopy by high-resolution numerical modeling. For this purpose, a novel approach to model individual trees is implemented in our large-eddy simulation (LES). A group of sixteen fractal Pythagoras trees is placed in the computational domain and the tree elements are numerically treated as immersed boundaries. Our objective is to resolve the multiscale flow response starting at the diameter of individual tree elements up to the depth of the atmospheric surface layer. A reference run, conducted for the forest flow under neutral thermal stratification, produces physically meaningful turbulence statistics. Our numerical results agree quantitatively with data obtained from former field-scale LESs and wind tunnel experiments. Furthermore, the numerical simulations resolve vortex shedding behind individual branches and trunks as well as the integral response of the turbulent flow through the heterogeneous forest canopy. A focus is the investigation of the turbulence structure of the flow under stable thermal stratification and in response to the heating of the fractal tree crowns. For the stratified flows, statistical quantities, e.g. turbulent kinetic energy and vorticity, are presented and the turbulent exchange processes of momentum and heat are considered in detail. The onset and formation of coherent structures such as elevated shear layers above the diabatically heated forest canopy are analyzed. For the stably stratified flow, temperature ramps above the forest canopy were simulated in agreement with previous observations. Thermally driven vortices with a typical diameter of the canopy height were simulated when the tree crowns were diabatically heated. The impact of the coherent flow structures on the heat flux is investigated. Keywords Atmospheric boundary layer • Forest canopy • Fractal trees • Heat • Turbulence IntroductionFlow through forest canopies transports momentum, heat, aerosol particles and trace gases as carbon dioxide, oxygen and water vapor. The associated atmospheric exchange processes between the forest and the atmosphere occur on a broad range of scales. Photosynthesis and transpiration on the leaves [32] are examples representing Communicated by R. Klein.

Turbulence and large-scale waves in the tropical region are studied using the spherical shallow-water equations. With mesoscale vorticity forcing, both moist and dry systems show an upscale transfer of kinetic energy which is dominated by rotational modes, scales as a power law with −5∕3 exponent, requires eddy-eddy interactions and ranges from the forcing scale to the respective equatorial deformation radius. At larger planetary scales, the divergent component of the energy increases and we see a footprint of tropical waves. The dry system shows a signature of the entire family of equatorial waves, while the moist simulations show only low-frequency Rossby, Kelvin and mixed Rossby-gravity waves with an equivalent depth that matches rapid condensation estimates. Initially, runs with interactive moisture exhibit a weak inverse transfer of moisture variance as well exponential growth across a range of length-scales. This results in an equilibrium moist energy spectrum obeying a −2 power law and the formation of moisture aggregates. Once formed, aggregates propagate westward in the Tropics with speeds of the order of a few metres per second. In contrast, forcing divergence does not excite an inverse transfer, and injected energy remains trapped at the forcing scale. Height (i.e., temperature or mass) forcing results in a peak at the forcing scale, but also generates large-scale waves and projects onto rotational modes which undergo an inverse energy transfer. Similarly, forcing the moisture field by itself produces an inverse transfer of rotational energy and a well-formed large-scale equatorial wave spectrum. Notably, the heightand moisture-forced inverse transfers are different in nature. Specifically, they require the presence of ambient planetary rotation. In all, these experiments demonstrate that the vortical and divergent wind are inextricably linked with the evolving moisture field, and that large-scale equatorial waves co-exist with synoptic-scale moist turbulence.