The role of latent heating in atmospheric blocking: climatology and numerical experiments

Abstract

The formation of persistent anticyclonic circulation anomalies, denoted as atmospheric blocking, represents an important aspect of mid-latitude weather variability. These circulation anomalies disrupt or block the prevailing westerly flow and the eastward progression of synoptic eddies, often causing anomalous, sometimes extreme weather. In spite of the importance of atmospheric blocking for weather and climate, the blocking mechanism is not yet fully understood, and accurately simulating the formation and duration of blocks is a challenge for numerical weather forecasting and climate models. Theories about blocking are based on dry dynamics, describing the dry-adiabatic interactions between transient baroclinic waves and the large-scale diffluent flow, during which air masses with low potential vorticity (PV) are advected poleward, setting up an intense anticyclonic PV anomaly. While moist diabatic processes, and in particular latent heating (LH) due to cloud formation in ascending airstreams, are known to play a significant role in modifying the upper-level large-scale flow, they have been far less considered in theories about blocking. The aim of this thesis is to better understand the role of LH in atmospheric blocking dynamics. This goal is addressed with three different approaches: (i) climatological investigations of diabatic contributions to atmospheric blocking using reanalysis, (ii) detailed model-based sensitivity experiments to study the causal relationship and sensitivity of blocking to changes in LH, and (iii) evaluation of climate simulations and the assessment of projected future changes. To this end, trajectory-based diagnostics along with a combination of statistical analyses are applied to different data sets (reanalysis, weather and climate model output), making extensive use of the PV perspective. The first part of the thesis describes a comprehensive global climatology of blocking and LH along kinematic 7-day backward trajectories from the upper-level low-PV regions, using the ERA-Interim reanalysis dataset between 1979 and 2016. The feature-based climatological analysis, based on 4270 blocking events and ~30 million trajectories, reveals that the flow associated with blocking is actually never perfectly adiabatic. Between 31 - 46% of all blocking air masses have been heated by more than 2K, with a median heating of 8.5K, during the final 3 days before entering the blocking anticyclone, and this number increases to 58 - 67% (12.5K in the median) when considering a 7-day period. The other blocking air masses, which enter the block with the upper-level jet stream, have been mostly cooled by a median -4K in 3 days. While the importance of LH varies considerably between individual blocking events and different regions, in particular between ocean and continents, LH is generally most important during blocking onset and (re-)intensification and in more intense and larger blocks. LH is often connected to upstream baroclinic developments in the mid-latitude storm tracks and provides the required flow amplification in addition to dry-dynamical interaction between synoptic eddies and blocking, thereby contributing to both the high- (fast onset and fluctuation in intensity and size) and low-frequency (maintenance and quasi-stationarity during maturation phase) properties of blocking anticyclones. This amplification of the upper-level circulation by LH is due to a combination of two effects: the direct injection of air masses with low PV into the upper troposphere in strongly ascending `warm conveyor belt' (WCB) airstreams, and the indirect effect owing to the interaction of the associated divergent outflow with the upper-level PV structure. Both effects act to slow down eastward propagation and amplify the intensity of the block. The second part of the thesis explicitly explores the causal effect of LH on the development of five different blocking cases. Numerical sensitivity experiments are performed with the global weather prediction model IFS in which LH is artificially altered in clouds upstream of the blocks. The elimination of LH has substantial effects on the vertical motion and upper-tropospheric circulation in all case studies, but there is also significant case-to-case variability: some blocking systems do not develop at all without upstream LH, while for others the amplitude of the blocking anticyclone is merely reduced. This sensitivity is related to the state of the background flow. The presence of a large-scale diffluent flow supports the meridional amplification of synoptic-scale Rossby waves also in the absence of LH. These sensitivity experiments demonstrate the complex two-way interaction between moist diabatic and dry baroclinic processes, and the vital role of LH in the formation and maintenance of blocking. The last part of the thesis presents a systematic and quantitative investigation of the influence of LH on blocking in a warmer and moister climate as projected for the end of the 21th century in CESM-LE large ensemble climate simulations. Blocking and diabatic processes are reasonably well represented in the present-day reference simulations (1990-2000), although blocking frequencies are underestimated over both the Pacific and Euro-Atlantic sectors when compared to ERA-Interim. In future climate (2091-2100) under the RCP8.5 emissions scenario, the responses of blocking to global warming are projected to be weak with a complex regional and seasonal variation. There is substantial uncertainty in many regions concerning the sign of the change. This is related to the fact that changes in blocking frequencies are influenced by the combined effect of future changes in the jet stream, cyclone activity and diabatic heating. Nevertheless, a robust conclusion is that the increase in temperature and hence moisture in the atmosphere leads to an increase in the importance of LH. This can mainly be attributed to a 50% increase in WCB airmasses with strong diabatic heating, which can initiate Rossby wave amplification and breaking along a stronger future jet stream. However, it is still unclear how enhanced LH will affect blocking intensity and duration. In conclusion, moist diabatic processes, in particular the release of latent heat in ascending airstreams, are of first-order importance for the formation of prolonged circulation anomalies related to atmospheric blocking. Such moist diabatic processes thus have to be taken into account to obtain a more comprehensive understanding of blocking dynamics.