Lagrangian characterization of heat waves: The perspective matters

Atmospheric heat waves pose a threat to natural ecosystems and society, which is projected to become more severe due to anthropogenic global warming. However, the mechanisms that determine the formation of heat waves are not yet sufficiently understood. In particular, there is still quite some debate about the relative contribution of three key processes: horizontal temperature transport, adiabatic heating due to subsidence, and diabatic heating. Here, we quantify these processes from a Lagrangian persepctive using a method that provides essential Lagrangian information about the atmospheric flow on an Eulerian grid. The method is based on the advection of passive tracer fields and includes a relaxation term. For each grid point at any time, the method allows us to decompose a temperature anomaly into the effect of horizontal transport across climatological temperature gradients, the combined effect of vertical transport across climatological temperature gradients and adiabatic heating, and the parcel-based diabatic heating. The tracer method thus provides a field-based view on the three processes under discussion. We analyse several recent heat wave episodes and quantify the contributions from horizontal transport, vertical transport, and diabatic heating. We then continue to analyse whether and to what extent these absolute fields are anomalous with respect to their corresponding climatologies. It turns out that the anomaly-based perspective leads to significant differences regarding the relative importance of the various processes compared to the perspective in terms of absolute fields. Our work complements previous studies based on trajectories, which generally considered significantly fewer air masses and did not take into account a climatological background. The results further our knowledge on important mechanisms and drivers of heat waves, which in turn may help to improve their forecasts.