The Brewer-Dobson circulation : interannual variability and climate change

A quasi-geostrophic theoretical model for the frequency-dependent response of the zonalmean flow to planetary-wave forcing at Northern Hemisphere (NH) midlatitudes was applied to 4D-Var ECMWF data for six extended winter seasons. Linear regression analyses yielded height-dependent estimates for the thermal damping time, and for a scaling parameter which includes the aspect ratio of the meridional to the vertical length scale of the response. The estimated thermal damping time is ~2 days in the troposphere, 7-10 days in the stratosphere, and 2-4 days in the lower mesosphere. The results indicated that the theoretical model is applicable to midlatitude wintertime conditions. In the low-frequency limit, the response to the wave driving is given by the Brewer-Dobson circulation (BDC), which has been the focus of our further research. ERA-40 reanalysis data for the period 1979-2002 were used to examine several factors that significantly affect the interannual variability of the wave driving. The individual zonal wave-1 and wave-2 contributions to the wave driving at 100 hPa exhibit a significant coupling with the troposphere, predominantly their stationary components. The stationary wave-1 contribution to the total wave driving significantly depends on the latitude of the stationary wave-1 source in the troposphere. The results suggest that this dependence is associated with the varying ability of stationary wave-1 activity to enter the tropospheric waveguide at mid-latitudes. The wave driving anomalies were separated into three parts: one part due to anomalies in the zonal correlation coefficient between the eddy temperature an eddy meridional wind, another part due to anomalies in the zonal eddy temperature amplitude, and a third part due to anomalies in the zonal eddy meridional wind amplitude. It was found that year-to-year variability in the zonal correlation coefficient between the eddy temperature and the eddy meridional wind is the dominant factor in explaining the year-to-year variability of the poleward eddy heat flux. Using the ECHAM middle-atmosphere climate model, it was found that the midwinter NH wave driving exhibits a highly significant increase (12%) if CO2 concentrations are doubled. The magnitude and large statistical significance of the increase due to stationary waves only was found to be comparable to that of the total increase. However, dividing the response into the different wavenumber components yielded a more subtle picture, with a decrease in transient wave-1 to less than 50% of the 1×CO2 value and an increase in transient wave-5 to almost the double value. Although transient wave-5 is usually thought not to contribute substantially to the wave driving of the stratosphere, its increase constitutes about 1/6 of the total increase of the wave driving in a 2×CO2 climate

Interannual variability of the stratospheric wave driving during northern winter

The strength of the stratospheric wave driving during northern winter is often quantified by the January–February mean poleward eddy heat flux at 100 hPa, averaged over 40°–80° N (or a similar area and period). Despite the dynamical and chemical relevance of the wave driving, the causes for its variability are still not well understood. In this study, ERA-40 reanalysis data for the period 1979–2002 are used to examine several factors that significantly affect the interannual variability of the wave driving. The total poleward heat flux at 100 hPa is poorly correlated with that in the troposphere, suggesting a decoupling between 100 hPa and the troposphere. However, the individual zonal wave-1 and wave-2 contributions to the wave driving at 100 hPa do exhibit a significant coupling with the troposphere, predominantly their stationary components. The stationary wave-1 contribution to the total wave driving significantly depends on the latitude of the stationary wave-1 source in the troposphere. The results suggest that this dependence is associated with the varying ability of stationary wave-1 activity to enter the tropospheric waveguide at mid-latitudes. The wave driving anomalies are separated into three parts: one part due to anomalies in the zonal correlation coefficient between the eddy temperature and eddy meridional wind, another part due to anomalies in the zonal eddy temperature amplitude, and a third part due to anomalies in the zonal eddy meridional wind amplitude. It is found that year-to-year variability in the zonal correlation coefficient between the eddy temperature and the eddy meridional wind is the most dominant factor in explaining the year-to-year variability of the poleward eddy heat flux

Analysis of the frequency-dependent response to wave forcing in the extratropics

A quasigeostrophic model for the frequency-dependent response of the zonal-mean flow to planetary-wave forcing at Northern Hemisphere (NH) midlatitudes is applied to 4-D-Var ECMWF analysis data for six extended winter seasons. The theoretical response is a non-linear function of the frequency of the forcing, the thermal damping time α−1, and a scaling parameter µ which includes the aspect ratio of the meridional to the vertical length scale of the response. Regression of the calculated response from the analyses onto the theoretical response yields height-dependent estimates for both α−1 and µ. The thermal damping time estimated from this dynamical model is about 2 days in the troposphere, 7–10 days in the stratosphere, and 2–4 days in the lower mesosphere. For the stratosphere and lower mesosphere, the estimates lie within the range of existing radiative damping time estimates, but for the troposphere they are significantly smaller

Coastal Tropical Convection in a Stochastic Modeling Framework

Recent research has suggested that the overall dependence of convection near coasts on large-scale atmospheric conditions is weaker than over the open ocean or inland areas. This is due to the fact that in coastal regions convection is often supported by meso-scale land-sea interactions and the topography of coastal areas. As these effects are not resolved and not included in standard cumulus parametrization schemes, coastal convection is among the most poorly simulated phenomena in global models. To outline a possible parametrization framework for coastal convection we develop an idealized modeling approach and test its ability to capture the main characteristics of coastal convection. The new approach first develops a decision algorithm, or trigger function, for the existence of coastal convection. The function is then applied in a stochastic cloud model to increase the occurrence probability of deep convection when land-sea interactions are diagnosed to be important. The results suggest that the combination of the trigger function with a stochastic model is able to capture the occurrence of deep convection in atmospheric conditions often found for coastal convection. When coastal effects are deemed to be present the spatial and temporal organization of clouds that has been documented form observations is well captured by the model. The presented modeling approach has therefore potential to improve the representation of clouds and convection in global numerical weather forecasting and climate models.Comment: Manuscript submitted for publication in Journal of Advances in Modeling Earth System

Mixed Climatology, Non-synoptic Phenomena and Downburst Wind Loading of Structures

Modern wind engineering was born in 1961, when Davenport published a paper in which meteorology, micrometeorology, climatology, bluff-body aerodynamics and structural dynamics were embedded within a homogeneous framework of the wind loading of structures called today \u201cDavenport chain\u201d. Idealizing the wind with a synoptic extra-tropical cyclone, this model was so simple and elegant as to become a sort of axiom. Between 1976 and 1977 Gomes and Vickery separated thunderstorm from non-thunderstorm winds, determined their disjoint extreme distributions and derived a mixed model later extended to other Aeolian phenomena; this study, which represents a milestone in mixed climatology, proved the impossibility of labelling a heterogeneous range of events by the generic term \u201cwind\u201d. This paper provides an overview of this matter, with particular regard to the studies conducted at the University of Genova on thunderstorm downbursts

On the link between cold fronts and hail in Switzerland

Hail is the costliest atmospheric hazard in Switzerland, causing substantial damage to agriculture, cars and buildings every year. In this study, a 12-year statistic of objectively identified cold fronts and a radar-based hail statistic are combined to investigate the co-occurrence of cold fronts and hail in Switzerland. In a first step, an automated front identification scheme, which has previously been designed for and applied to global reanalysis data, is modified for a high-resolution regional analysis data set. This front detection method is then adapted, tested and applied to the Consortium for Small Scale Modelling (COSMO) analysis data for the extended hail season (May to September) in the years 2002–2013. The resulting cold front statistic is presented and discussed. In a second step, the frequency of cold fronts is linked to a high-resolution radar-based hail statistic to determine the relative fraction of hail initiation events in pre-frontal environments. Up to 45% of all detected hail events in north-eastern and southern Switzerland form in pre-frontal zones. Similar fractions are identified upstream of the Jura and the Black Forest mountains. The percentage of front-related hail formation is highest in regions where hail is statistically less frequent, with the exception of southern Switzerland. Furthermore, it is shown that fronts create wind-sheared environments, which are favourable for hail cells

Analysis of the frequency-dependent response to wave forcing in the extratropics

International audienceA quasigeostrophic model for the frequency-dependent response of the zonal-mean flow to planetary-wave forcing at Northern Hemisphere (NH) midlatitudes is applied to 4-D-Var ECMWF analysis data for six extended winter seasons. The theoretical response is a non-linear function of the frequency of the forcing, the thermal damping time ??1, and a scaling parameter ? which includes the aspect ratio of the meridional to the vertical length scale of the response. Non-linear regression of the calculated response from the analyses onto the theoretical response yields height-dependent estimates for both ??1 and ?. The thermal damping time estimated from this dynamical model is about 2 days in the troposphere, 7?10 days in the stratosphere, and 2?4 days in the lower mesosphere. These estimates generally lie within the range of existing estimates, although the values we find for the troposphere are significantly smaller than those calculated in several radiative transfer modeling studies. At most levels, the estimates for ? are significantly lower than can be derived from scaling arguments that apply outside the forcing region. We illustrate with an example how the response of the meridional circulation inside the forcing area can have a higher aspect ratio than the effective response outside the forcing area

How does the northern-winter wave driving of the Brewer-Dobson circulation increase in an enhanced-CO₂ climate simulation?

Recent climate studies show that the northern-winter wave driving of the Brewer-Dobson circulation is enhanced if greenhouse gas concentrations increase. An explanation for this enhancement does not yet exist. In this study, the enhanced wave driving, as simulated in a doubled-CO2 experiment with the MA-ECHAM4 climate model, is analyzed in detail. The extratropical poleward eddy heat flux increases (decreases) in the stratosphere (troposphere) mainly due to the stationary (transient) heat-flux component. The heat flux at 100 hPa is a measure of the stratospheric wave driving, and is found to increase by 12% in the doubled-CO2 climate. This increase is dominated by the stationary-wave 1 heat flux, which is also enhanced in the midlatitude troposphere. The heat flux increase at 100 hPa is almost entirely due to an increase in the longitudinal temperature variability. The latter increase is mainly due to the well-understood sharpening of the lower-stratospheric meridional temperature gradient

How does the northern-winter wave driving of the Brewer-Dobson circulation increase in an enhanced-CO₂ climate simulation?

Recent climate studies show that the northern-winter wave driving of the Brewer-Dobson circulation is enhanced if greenhouse gas concentrations increase. An explanation for this enhancement does not yet exist. In this study, the enhanced wave driving, as simulated in a doubled-CO2 experiment with the MA-ECHAM4 climate model, is analyzed in detail. The extratropical poleward eddy heat flux increases (decreases) in the stratosphere (troposphere) mainly due to the stationary (transient) heat-flux component. The heat flux at 100 hPa is a measure of the stratospheric wave driving, and is found to increase by 12% in the doubled-CO2 climate. This increase is dominated by the stationary-wave 1 heat flux, which is also enhanced in the midlatitude troposphere. The heat flux increase at 100 hPa is almost entirely due to an increase in the longitudinal temperature variability. The latter increase is mainly due to the well-understood sharpening of the lower-stratospheric meridional temperature gradient