Scientific challenges of convective-scale numerical weather prediction

Numerical weather prediction (NWP) models are increasing in resolution and becoming capable of explicitly representing individual convective storms. Is this increase in resolution leading to better forecasts? Unfortunately, we do not have sufficient theoretical understanding about this weather regime to make full use of these NWPs. After extensive efforts over the course of a decade, convective–scale weather forecasts with horizontal grid spacings of 1–5 km are now operational at national weather services around the world, accompanied by ensemble prediction systems (EPSs). However, though already operational, the capacity of forecasts for this scale is still to be fully exploited by overcoming the fundamental difficulty in prediction: the fully three–dimensional and turbulent nature of the atmosphere. The prediction of this scale is totally different from that of the synoptic scale (103 km) with slowly–evolving semi–geostrophic dynamics and relatively long predictability on the order of a few days. Even theoretically, very little is understood about the convective scale compared to our extensive knowledge of the synoptic-scale weather regime as a partial–differential equation system, as well as in terms of the fluid mechanics, predictability, uncertainties, and stochasticity. Furthermore, there is a requirement for a drastic modification of data assimilation methodologies, physics (e.g., microphysics), parameterizations, as well as the numerics for use at the convective scale. We need to focus on more fundamental theoretical issues: the Liouville principle and Bayesian probability for probabilistic forecasts; and more fundamental turbulence research to provide robust numerics for the full variety of turbulent flows. The present essay reviews those basic theoretical challenges as comprehensibly as possible. The breadth of the problems that we face is a challenge in itself: an attempt to reduce these into a single critical agenda should be avoided

Extreme precipitation events over north-western Europe: getting water from the tropics

Our capability to adapt to extreme precipitation events is linked to our skill in predicting their magnitude and timing. Synoptic features (such as Atmospheric Rivers) developing over the North Atlantic Ocean are known as the source of the majority of water vapour transport into European mid-latitudes, and are associated with episodes of heavy and prolonged rainfall over UK and north western Europe. Thus, a better understanding of the North Atlantic atmospheric conditions prior the occurrence of extreme precipitation events over Europe could help in improving our capability to predict them. We build on atmospheric re-analyses at high spatial resolution (such as JRA-55 and MERRA), on a daily time scale, to highlight the anomalous path of the vertically integrated water content, transferring water from the western tropical North Atlantic to high latitudes and fuelling the storms developing in the North Atlantic sector, bound to affect Europe as responsible of the most intense precipitation events. The systematic link between anomalous north-eastward transport of vertically integrated water (precipitable water) from the western North Atlantic and anomalously high pressure patterns in the central North Atlantic, developing 5 days prior the extreme precipitation occurrence, suggest the central North Atlantic surface pressure as a potential precursor of extreme precipitation events

The role of the export of tropical moisture into midlatitudes for extreme precipitation events in the Mediterranean region

Abstract The aims of the study are twofold: firstly, to investigate the role of the export of humid tropical air in the formation of cool season heavy precipitating events (HPEs) in the Mediterranean region (MR); and secondly, to examine the possible linkage between the export of humid tropical air and the multiyear trend in extreme precipitation in the region. For this purpose, we analyze the spatial distributions of a number of key atmospheric variables with a reanalysis data for more than 50 intense HPEs for the MR. The results of this evaluation for both individual and composite events suggest that the HPEs are associated with atmospheric rivers (ARs). The MR HPEs are being characterized by the poleward export of humid air of tropical origin into the midlatitude MR from the Atlantic Ocean and Arabian Sea. These export events appear to be associated with the effects of hurricanes or intense cyclones in the North Atlantic. It was also found that the linear trend (for 1979-2013) of the frequency of humid days (days with precipitable water greater than 20 kg m −2 ) is consistent with recent changes in the character of precipitation over the MR and southern Europe

Impacts of atmospheric modes of variability on Mediterranean Sea surface heat exchange

The impacts of variations in the state of the first four modes of atmospheric variability in the North Atlantic/Europe region on air-sea heat exchange in the Mediterranean Sea are considered. Observation-based indices of these modes from the NOAA Climate Prediction Centre are used together with two reanalysis (NCEP/NCAR and ARPERA) surface flux data sets for the period 1958–2006 to determine their relative influence on the mean heat budget of the full Mediterranean basin and the eastern and western subbasins. The modes considered are the North Atlantic Oscillation (NAO), East Atlantic pattern (EA), Scandinavian pattern (SCAN), and East Atlantic/West Russian pattern (EA/WR). Similar results are obtained with both NCEP/NCAR and ARPERA. In each case, winter anomalies dominate the annual mean heat budget and the leading mode, the NAO, has a surprisingly small impact on the full basin winter mean heat budget, <5 Wm?2. In contrast, the EA mode has a major effect, of order 25 Wm?2, with similar impacts on both the eastern and western Mediterranean. The SCAN mode has the weakest influence of those considered. The EA/WR mode plays a significant role but, in contrast to the EA mode, it generates a dipole in the heat exchange with an approximately equal and opposite signal of about 15 Wm?2 on the eastern and western subbasins. A particularly strong impact in the Aegean Sea is observed for the EA/WR mode and this is discussed in the context of episodic deep water formation in this region