J. Barkmeijer

Sensitivity of the tropospheric circulation to changes in the strength of the stratospheric polar vortex

Abstract The sensitivity of the wintertime tropospheric circulation to changes in the strength of the Northern Hemisphere stratospheric polar vortex is studied using one of the latest versions of the ECMWF model. Three sets of experiments were carried out: one control integration and two integrations in which the strength of the stratospheric polar vortex has been gradually reduced and increased, respectively, during the course of the integration. The strength of the polar vortex is changed by applying a forcing to the model tendencies in the stratosphere only. The forcing has been obtained using the adjoint technique. It is shown that, in the ECMWF model, changes in the strength of the polar vortex in the middle and lower stratosphere have a significant and slightly delayed (on the order of days) impact on the tropospheric circulation. The tropospheric response shows some resemblance to the North Atlantic Oscillation (NAO), though the centers of action are slightly shifted toward the east compared to tho...

Optimal atmospheric forcing perturbations for the Cold Ocean Warm Land pattern

The atmospheric Integrated Forecast System model from the European Centre for Medium-Range Weather Forecasts is used to calculate forcing perturbations which are optimal in producing atmospheric response patterns resembling the ‘cold-ocean-warm-land’ (COWL) flow regime over 4 d. Similar initial state perturbations are computed for comparison. COWL is relevant for recent climate change. The perturbations are optimal in a tangent-linear sense but are validated by non-linear calculations. Calculations cover 836 cases during 22 winter seasons. The method effectively estimates flow-dependant perturbations which produce response patterns resembling COWL. The optimal forcing is more geographically confined with relatively smaller remote amplitudes and larger spatial scale than the initial state perturbations. The quality of the 4-d response is highly dependent on the actual optimal perturbation. Averaging over just a few cases drastically reduces this optimal property, but the long-term climate response of an average forcing does produce signatures of the COWL regime. The results are discussed in view of Palmer’s non-linear dynamical perspective on climate change, and several key elements are confirmed: the climate sensitivity is flow dependent; efficient forcing structures do not resemble the response (non-normality); sensitivity and predictability are negatively correlated; and flow characteristics for high sensitivity differ from those for low sensitivity.

Local Skill Prediction for the ECMWF Model Using Adjoint Techniques

Abstract Two experiments are performed to predict the regional forecast skill over western Europe using a three-level quasigeostrophic model with truncation T21 (T21QG). The predictor of skill, the maximal forecast error over the area of consideration, is obtained assuming linear error growth, no model error, and use of the tangent and adjoint of the T21QG model. A small maximal error implies small error growth and therefore good actual forecasts. A large maximal error, on the other hand, may or may not be associated with a large actual forecast error depending on whether the analysis error projects strongly on the fast-growing modes. In the first experiment, the forecast trajectory is obtained directly using the T2IQG model. In the second experiment, actual 96-h ECMWF forecasts are interpolated every 12 h using the T21QG model. Both experiments indicate that the maximal growth can provide useful discrimination about very good and very poor forecasts. The improvement upon climatology is of the order 10%–1...

Simulating the extreme 2013/2014 winter in a future climate

How would the spell of extreme weather observed over North America and western Europe during the 2013/2014 winter manifest itself in a warmer climate? Here a forced sensitivity method is used to calculate optimal model tendency perturbations which result in a simulation which has its upper atmospheric circulation shifted in the direction of the January 2014 jet stream pattern. When applied to a simulation run under present-day conditions the main features of the observations of this event are reproduced, such as the more zonal position and increased strength of the midlatitude North Atlantic storm track, the anomalous temperature pattern over North America, and the excessive precipitation in parts of Europe. When this method is applied to a future warmer climate, the North Atlantic storm track changes to a more zonal orientation, but its strength does not see the significant increase that is obtained in the present-day simulations. Despite southward advection of polar air into northeastern America, which has also occurred during the 2013/2014 winter, the associated drop in temperature is less in the future climate compared to the present-day climate because of Arctic amplification. The less steep drop in temperatures over northeastern America leads to a smaller land-sea temperature contrast, less baroclinic instability, and a reduced increase of midlatitude storminess. While one may expect the future increased atmospheric moisture content to augment the net precipitation, for this specific event, this effect is counterbalanced by the dynamics, leading to a similar net increase in precipitation in the future simulations compared to the present day.

Properties of singular vectors using convective available potential energy as final time norm

We study the feasibility of using the singular vector technique to create initial condition perturbations for shortrange ensemble prediction systems (SREPS) focussing on predictability of severe local storms and in particular deep convection. For this a newfinal time semi-norm based on the convective available potential energy (CAPE) is introduced. We compare singular vectors using the CAPE-norm with SVs using the more common total energy (TE) norm for a 2-week summer period in 2007, which includes a case of mesoscale extreme rainfall in the south west of Finland. The CAPE singular vectors perturb the CAPE field by increasing the specific humidity and temperature of the parcel and increase the lapse rate above the parcel in the lower troposphere consistent with physical considerations. The CAPE-SVs are situated in the lower troposphere. This in contrast to TE-SVs with short optimization times which predominantly remain in the high troposphere. By examining the time evolution of the CAPE singular values we observe that the convective event in the south west of Finland is clearly associated with high CAPE singular values.

The 2010 Pakistan floods in a future climate

The summer 2010 floods hitting Pakistan were the severest on record. Coinciding with these events was the 2010 heatwave over eastern Europe and Russia, which also ranks among the severest ever recorded in the region. Both events were related to an anomalously widespread and intense quasi-stationary anticyclonic circulation anomaly over western Russia which provided favourable conditions, in combination with monsoonal forcing factors, for the Pakistan precipitation events. Here, a data assimilation technique is used which results in a climate model simulation which has its mean upper atmospheric circulation shifted in the direction of the anomalous anticyclonic circulation of summer 2010. This primes the climate model to reproduce, much more frequently than in a climate simulation without this technique, to simulate the conditions which led to the Pakistan 2010 floodings. These experiments are conducted under both present-day and future climatic conditions. In the present-day climate, the main features of the 2010 Pakistan precipitation events are modeled realistically, although the amplitude of the extreme precipitation is underestimated. The simulated future equivalent of the observed extreme precipitation events shows a stronger precipitation over the Bay of Bengal to Kashmir in northern India and northern Pakistan, and from the Arabian Sea to northern Pakistan. In the model context, these precipitation increases are substantial with 50–100% increases in rainfall rates. This implies that the future equivalent of the 2010 Pakistan floodings may have even stronger socio-economic impacts.