This theme is concerned with increasing our knowledge and understanding of the processes that lead to the selected hazards and hence their potential predictability. It deals both with the slowly evolving large scale processes that create the environment for high impact weather and with the fast, small scale, processes associated with the hazard itself.  Better understanding is needed of the processes governing convective-scale development and their dependence on the initial state. Synoptic scale precursors need to be correctly represented to achieve useful downscaled forecasts and to enable early preparation and issue of warnings.

Hazards in tropical, subtropical and extra-tropical regions will, in general, be associated with different types of weather systems.  However, some science questions are independent of these differences. Interaction between the free atmosphere and the urban canopy is of crucial relevance to high impact weather in the future as an increasing proportion of the world’s population lives in megacities. Processes that enhance or diminish a hazard are also important – e.g. radiation effects of the built environment, concentration and dilution of pollutants, frost and fog hollows.

Research is required on processes that determine onset of and changes in flow regimes. Better knowledge is required of the relationship of forecast error growth to weather regimes on all scales, for use in data assimilation, ensemble predictions and in forecast postprocessing and interpretation. Further questions pertain to the representation of synoptic situations associated with different high impact weather events in medium-range forecasts. Are errors in intensity and structure of precipitation fields due to low resolution or to an inadequate representation of the processes involved? 

Process understanding comes from consistently relating cause and effect and depends critically on the availability of data – both observations and model output. This theme will draw heavily on the “field experiments and demonstration projects” cross-cutting activity for these datasets. The TIGGE archive and YOTC dataset remain as powerful resources for these investigations. 

In mid-latitudes, generation of high impact weather events is typically associated with precursors at upper levels. The prominent tropopause-level jetstream is characterized by an intense meridional potential vorticity (PV) gradient on isentropic surfaces, which acts as a waveguide for synoptic scale Rossby waves. Nonlinear amplification of these waves can result in wave breaking events leading to filamentary PV streamers and cut-offs, and to anomalous meridional moisture fluxes. These structures may ultimately result in urban flooding, disruptive winter weather or localized strong winds. Alternatively, they may lead to the blocking events that may result in heat waves, wildfires and air pollution episodes. It has been shown that wave evolution can be strongly modified by moist diabatic processes. Successful prediction of mid-latitude weather hazards thus presupposes a correct representation of (i) the structure of the waveguide (i.e., the jet location and intensity), (ii) waveguide disturbances (typically in the form of PV anomalies approaching the jet), and (iii) the modulation of the disturbances by intense convective or large-scale cloud systems. Davies and Didone (2013) reported significant PV errors in global medium-range forecasts, probably due to inaccurate representation of moist processes. Research is needed into the interaction of Rossby wave dynamics with moist diabatic processes and specifically the intensity, evolution and interactions of upper-level jetstreams (Martius et al. 2011).

In regions with high topography, mesoscale orographic effects (e.g., flow blocking, channeling, lifting, downslope wind storms) can lead to particular localised hazards, e.g. foehn storms, bora, potential for flash floods or landslides. The advent of very high-resolution numerical models and/or coupling of precipitation and river/sewer flow or landslide models offer forecasts of unprecedented detail, but predictability of the hazards needs to be investigated. Bifurcations between different flow regimes remain a critical issue that can lead to substantial miss-forecasts of orographic precipitation and intense wind events.

Convection-permitting NWP models have shown remarkable realism in their simulation of severe convective storm events. However, research is needed to establish the sensitivity of forecast accuracy to details of the microphysics and turbulence parametrisations in these models, and to characterize this for use in data assimilation and ensemble prediction schemes. Furthermore, a better process understanding is needed to improve nowcasting systems which close the gap between observations and NWP in the timescale of minutes.

Interactions between the boundary layer and surface conditions need to be studied to establish the complexity of coupling that is relevant to nowcasting and very short range forecasting. Particular areas of complexity requiring improved understanding include atmospheric responses to the complex topographic gradients of mountainous areas; the influence of the urban fabric and the role of coastal ocean circulations, including tides, in modulating land-sea contrasts. Local surface coupling may also be important where intense precipitation changes the surface properties, saturating the ground, temporarily covering large areas with water and feeding contaminated fresh water into the coastal ocean.

In the Tropics, a diverse range of synoptic scale disturbances, e.g. Kelvin waves, Easterly waves, Equatorial Rossby waves, Monsoon disturbances and Tropical Cyclones, create the dynamical environment in which high impact weather events occur. Within these systems a variety of convective systems develop that may produce high impact weather. Understanding the inception and interaction of synoptic scale tropical disturbances remains weak. Observational studies typically capture only part of the spectrum of interaction, while modelling studies depend on the ability of the models to react realistically to parametrized convection fluxes.

Many of these areas of research deal with atmospheric processes that are also of importance to scientists modelling and predicting climate change. A characteristic of work is this theme will therefore be collaboration with WCRP initiatives and programmes, including the Global Energy & Water Exchanges project (GEWEX), the Climate and Ocean: Variability, Predictability and Change project (CLIVAR) and several of the WCRP Grand Challenges.