Aarnout van Delden

The synoptic setting of thunderstorms in western Europe

This paper discusses the synoptic factors contributing to the formation of thunderstorms in western Europe and in particular gives reasons for the existence of the preferred areas for thunderstorms. These areas are all found in the vicinity of the Alps. The synoptic features playing a role in promoting thunderstorm formation in particular areas in western Europe are identified. The principle synoptic features or processes promoting the formation of intense thunderstorms are a high level of potential instability, convergence lines associated with frontogenesis and cyclogenesis and upper level potential vorticity advection. Additional features playing a role in thunderstorm formation are land and seabreeze circulations and (thermally) forced upward motion at slopes.

Incorporating circulation statistics in bias correction of GCM ensembles: Hydrological application for the Rhine basin

Abstract An adapted statistical bias correction method is introduced to incorporate circulation-dependence of the model precipitation bias, and its influence on estimated discharges for the Rhine basin is analyzed for a historical period. The bias correction method is tailored to time scales relevant to flooding events in the basin. Large-scale circulation patterns (CPs) are obtained through Maximum Covariance Analysis using reanalysis sea level pressure and high-resolution precipitation observations. A bias correction using these CPs is applied to winter and summer separately, acknowledging the seasonal variability of the circulation regimes in North Europe and their correlation with regional precipitation rates over the Rhine basin. Two different climate model ensemble outputs are explored: ESSENCE and CMIP5. The results of the CP-method are then compared to observations and uncorrected model outputs. Results from a simple bias correction based on a delta factor (NoCP-method) are also used for comparison. For both summer and winter, the CP-method offers a statistically significant improvement of precipitation statistics for subsets of data dominated by particular circulation regimes, demonstrating the circulation-dependence of the precipitation bias. Uncorrected, CP and NoCP corrected model outputs were used as forcing to a hydrological model to simulate river discharges. The CP-method leads to a larger improvement in simulated discharge in the Alpine area in winter than in summer due to a stronger dependence of Rhine precipitation on atmospheric circulation in winter. However, the NoCP-method, in comparison to the CP-method, improves the discharge estimations over the entire Rhine basin.

The dynamics of meso-scale atmospheric circulations

Abstract A review is given of the dynamics of meso-scale atmospheric circulations. An analysis of the instabilities, oscillations and adjustment processes which are possible in a rotating atmosphere in which the rotation rate does not vary in space, yields several characteristic time scales and length scales which can be used to define the meso-scale. Special attention is paid to showing the relation between the circulation associated with convection, the circulation associated with sea breeze, the circulation perpendicular to a front, and the radial circulation in a tropical cyclone. Other important meso-scale weather phenomena which are discussed are gravity-inertia waves, shallow cumulus convection, downslope winds and thunderstorms.

A case study of tropopause cyclogenesis

This paper is concerned with the transformation of an upper level trough into a shear line and a tropopause cyclone or cut-off low. During this event, the isentropic potential vorticity of air columns between fixed isentropes approaching the trough axis at its level of maximum intensity from the west is well conserved over a period of about 12 to 24 hours, but there is a significant repartitioning of the constituent parts, from a relatively large static stability to a relatively large absolute vorticity. This indicates stretching of vertical columns of air. The effect that is inducing the vortex stretching is identified within the context of quasi-geostrophic theory as ‘opposite vector-frontogenesis’. This implies that Q-vectors point in opposite directions, respectively, above and below the level of maximum trough-intensity. Isentropic analysis indicates that the criterion for unstable isentropic downgliding is fulfilled in the upper troposphere in a restricted area, covering several hundred thousand square kilometres, near the base of the trough when and where the cut-off low is formed. This area of unstable isentropic downgliding, to which we assign the term ‘baroclinic downburst’, and which maintains its identity during a period of time in the order of 24 hours, coincides approximately with the dry intrusion as identified on a ‘water vapour’ image. It is argued that unstable isentropic downgliding is in fact necessary in order to realise a cyclone-core characterised by relatively high static stability, relatively high absolute vorticity and relatively high potential vorticity, as is demanded by the invertibility principle for potential vorticity. The events described above are triggered by the interaction of two potential vorticity anomalies.

Linear dynamics of hydrostatic adjustment to horizontally homogeneous heating

The process of hydrostatic adjustment to horizontally homogeneous heating in a stably stratifiedatmosphere of arbitrary thermal structure is investigated in the limit of small perturbations. Alinear differential equation is derived for the vertical pressure distribution in the final balancedstate. Solutions of this equation are compared with the time dependent solution which is foundby numerically integrating the equations in time. During the process of hydrostatic adjustmentacoustic-buoyancy oscillations are generated. The amplitudes of these oscillations become sogreat that static instability is generated at heights above 100 km, depending on where and howabruptly the heat is added. As a crude representation of the unstable breakdown and dampingof these waves, Rayleigh damping is introduced. If the associated damping coefficient in theupper atmosphere is sufficiently large (greater than the Brunt Väisälä frequency), the oscillationsvanish. Below a height of about 50 km the steady state predicted by the above mentioneddifferential equation is reached approximately in 10 min.

PV-θ view of the zonal mean state of the atmosphere

ABSTRACT The relation between zonal mean potential vorticity (PV) in potential temperature (θ) coordinates and the zonal mean zonal wind in January and in July is studied. PV-anomalies are defined with respect to a reference state that is at rest with respect to the rotating earth. Two important PV-anomalies are identified. One PV-anomaly, the ‘Ex-UTLS PV-anomaly’, coincides approximately with the extratropical tropopause (310–360 K). It is a permanent feature of the zonal mean state. The other PV-anomaly is located higher in the stratosphere. It exhibits a strong seasonal cycle, i.e. in winter, it is strongly positive, while in summer, it is weakly negative. In the Northern Hemisphere winter, the Ex-UTLS PV-anomaly and the stratospheric PV-anomaly are separated by a ‘surf-zone’, which is characterised by a negative PV-anomaly pole-wards of a positive PV-anomaly. Piecewise PV-inversion reveals that (1) the Ex-UTLS PV-anomaly induces the westerly winds in the troposphere and the lower stratosphere, including the subtropical jet, that (2) the positive stratospheric PV-anomaly induces the stratospheric polar night jet and that (3) the negative polar cap stratospheric PV-anomaly in summer reduces the westerly wind speeds in the troposphere and induces easterly winds in the stratosphere. The Ex-UTLS PV-anomaly is manifest mainly as an isentropic density- (or mass-) anomaly. Piecewise PV-inversion of these anomalies in isolation should account for this by an appropriate adjustment of the lower boundary condition.

Resolution dependence of extreme precipitation and deep convection over the Gulf Stream

Modeled wintertime precipitation over the Atlantic Gulf Stream region is shown to be sensitive to the horizontal resolution of the driving Global Circulation Model (GCM). By contrasting simulations with the EC-Earth GCM over a range of horizontal resolutions (T159, T319, T799), it is shown that especially the precipitation extremes become more populated if resolution is higher. Higher resolution also appears to strengthen the communication from the sea surface toward the troposphere. With increasing resolution, deep convection over the Gulf Stream region, diagnosed via wind-convergence and vertical motion, occurs more frequently and the former is in better agreement with observations. Likewise the frequency increase of the precipitation extremes over the region for increasing resolution makes them agree better with observations, despite large natural variability and discrepancies between different observational sources.

How Gulf-Stream SST-fronts influence Atlantic winter storms

The strong horizontal gradients in sea surface temperature (SST) of the Atlantic Gulf Stream exert a detectable influence on extratropical cyclones propagating across the region. This is shown in a sensitivity experiment where 24 winter storms taken from ERA-Interim are simulated with HARMONIE at 10-km resolution. Each storm is simulated twice. First, using observed SST (REF). In the second simulation a smoothed SST is offered (SMTH), while lateral and upper-level boundary conditions are unmodified. Each storm pair propagates approximately along the same track, however their intensities (as measured by maximal near-surface wind speed or 850-hPa relative vorticity) differ up to ± 25%. A 30-member ensemble created for one of the storms shows that on a single-storm level the response is systematic rather than random. To explain the broad response in storm strength, we show that the SST-adjustment modifies two environmental parameters: surface latent heat flux (LHF) and low-level baroclinicity (B). LHF influences storms by modifying diabatic heating and boundary-layer processes such as vertical mixing. The position of each storm’s track relative to the SST-front is important. South of the SST-front the smoothing leads to lower SST, reduced LHF and storms with generally weaker maximum near-surface winds. North of the SST-front the increased LHF tend to enhance the winds, but the accompanying changes in baroclinicity are not necessarily favourable. Together these mechanisms explain up to 80% of the variability in the near-surface maximal wind speed change. Because the mechanisms are less effective in explaining more dynamics-oriented indicators like 850 hPa relative vorticity, we hypothesise that part of the wind-speed change is related to adjustment of the boundary-layer processes in response to the LHF and B changes.

Characteristics and development of European cyclones with tropical origin in reanalysis data

Major storm systems over Europe frequently have a tropical origin. This paper analyses the characteristics and dynamics of such cyclones in the observational record, using MERRA reanalysis data for the period 1979-2013. By stratifying the cyclones along three key phases of their development (tropical phase, extratropical transition and final re-intensification), we identify four radically different life cycles: the tropical cyclone and extratropical cyclone life cycles, the classic extratropical transition and the warm seclusion life cycle. More than 50% of the storms reaching Europe from low latitudes follow the warm seclusion life cycle. It also contains the strongest cyclones. They are characterized by a warm core and a frontal T-bone structure, with a northwestward warm conveyor belt and the effects of dry intrusion. Rapid deepening occurs in the latest phase, around their arrival in Europe. Both baroclinic instability and release of latent heat contribute to the strong intensification. The pressure minimum occurs often a day after entering Europe, which enhances the potential threat of warm seclusion storms for Europe. The impact of a future warmer climate on the development of these storms is discussed.

PV- θ view of diabatic-dynamical interaction in the general circulation

This paper studies the question how the zonal mean potential vorticity (PV) distribution in potential temperature (θ) coordinates is established in the atmosphere by the interaction of diabatic processes (cross-isentropic transport of mass) with adiabatic dynamical processes (isentropic transport of mass and potential vorticity substance). As an aid in dissecting this interaction, a simplified model of the general circulation is constructed, which contains parametrisations of radiative transfer, wave drag and water cycle. This model reproduces the following four observed features of the atmosphere below 10 hPa: (1) a permanently present eastward subtropical jet, which in winter is separated from an eastward stratospheric jet by a zone (referred to as the ‘surf zone’), between θ=380 K and θ=550 K, where planetary wave drag reduces PV over the polar cap; (2) a stratospheric zonal wind reversal in spring or beginning of summer; (3) a tropical cold layer at 100 hPa, and (4) a realistic distribution of zonal mean cross-isentropic flow. The strength of the cross-isentropic flow depends on wave drag, latent heat release and the thermal inertia of both the atmosphere and the earths surface. Of special interest is the layer between θ=315 K and θ=370 K (the ‘Middleworld’), which lies in the troposphere in the tropics and in the stratosphere in the extratropics. Mass converges diabatically into this layer in the deep tropics, mainly due to latent heat release, and diverges out of this layer elsewhere due to radiation flux divergence. Meridional isentropic vorticity flux divergence in the tropical Middleworld, associated with the upper branch of the Hadley circulation, creates a region in the subtropics, at θ=350 K and adjacent isentropic levels, with a marked isentropic meridional PV-gradient, forming the isentropic dynamical tropopause.