Impact of adaptively thinned AIRS cloud-cleared radiances on tropical cyclone representation in a global data assimilation and forecast system.

A simple adaptive thinning methodology for Atmospheric Infrared Sounder (AIRS) radiances is evaluated through a combination of Observing System Experiments (OSEs) and adjoint methodologies. The OSEs are performed with the NASA Goddard Earth Observing System (GEOS, version 5) data assimilation and forecast model. In addition, the adjoint-based forecast sensitivity observation impact technique is applied to assess fractional contributions of sensors in different thinning configurations. The adaptive strategy uses a denser AIRS coverage in a moving domain centered around tropical cyclones (TCs), sparser everywhere else. The OSEs consist of two sets of data assimilation runs that cover the period from September 1st to 10 November 2014, with the first 20 days discarded for spin-up. Both sets assimilate all conventional and satellite observations used operationally. In addition, one ingests clear-sky AIRS radiances, the other cloud-cleared radiances, each comprising multiple thinning strategies. Daily 7-day forecasts are initialized from all these analyses and evaluated with focus on TCs over the Atlantic and the Pacific. Evidence is provided on the effectiveness of this simple TC-centered adaptive radiance thinning strategy, in full agreement with previous theoretical studies. Specifically, global skill increases, and tropical cyclone representation is substantially improved. The improvement is particularly strong when cloud-cleared radiances are assimilated. Finally, the article suggests that cloud-cleared radiances, if thinned more aggressively than the currently used clear-sky radiances, could successfully replace them with large improvements in TC forecasting and no loss of global skill.

Analyzing changes in the complexity of climate in the last four decades using MERRA-2 radiation data.

The energy balance of the Earth is controlled by the shortwave and longwave radiation emitted to space. Changes in the thermodynamic state of the system over time affect climate and are noticeable when viewing the system as a whole. In this paper, we study the changes in the complexity of climate in the last four decades using data from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2). First, we study the complexity of the shortwave and longwave radiation fields independently using Approximate Entropy and Sample Entropy, observing that the rate of complexity change is faster for shortwave radiation. Then, we study the causality of those changes using Transfer Entropy to capture the non-linear dynamics of climate, showing that the changes are mainly driven by the variations in shortwave radiation. The observed behavior of climatic complexity could be explained by the changes in cloud amount, and we research that possibility by investigating its evolution from a complexity perspective using data from the International Satellite Cloud Climatology Project (ISCCP).

An Idealised Test Case For Assessing The Linearization of Tracer Transport Schemes in NWP Models.

The linearized version of a Numerical Weather Prediction (NWP) model, which consists of its tangent linear model (TLM) and adjoint, has a number of important applications in atmospheric modelling. As such it is important that the linearized version of the NWP model can provide an accurate representation of the perturbation growth that occurs in the nonlinear model and does not introduce spurious instability. A suite of test cases, built upon existing frameworks, are developed to assess the accuracy of the linearization of the tracer transport component of the NWP model. Deformation velocities are prescribed that return the tracer back to the initial conditions, thus providing an analytical solution. A selection of smooth and discontinuous tracers and tracer perturbations are used. Example results are shown using second-order and third-order tracer transport schemes, both with and without nonlinear flux limiters. Metrics are offered for assessing the skill of the linearization and predicting when problems will occur. For the example schemes used the results show that linearizations of the nonlinear flux-limited transport schemes behave poorly due to the presence of unstable modes. Some linearized model implementation strategies are offered for situations where the nonlinear scheme should not be linearized.