Large-Scale Controls on Extreme Precipitation

Abstract

Large-eddy simulations with strong lateral forcing representative of precipitation over the Netherlands are performed to investigate the influence of stability, relative humidity (RH), and moisture convergence on precipitation. Furthermore, a simple climate perturbation is applied to analyze the precipitation response to increasing temperatures. Precipitation is decomposed to distinguish between processes affecting the precipitating area and the precipitation intensity. It is shown that amplification of the moisture convergence and destabilization of the atmosphere both lead to an increase in precipitation, but on account of different effects: atmospheric stability mainly influences the precipitation intensity, whereas the moisture convergence mainly controls the precipitation area fraction. Extreme precipitation intensities show qualitatively similar sensitivities to atmospheric stability and moisture convergence. Precipitation increases with RH due to an increase in area fraction, despite a decrease in intensity. The precipitation response to the climate perturbation shows a stronger response for the precipitation intensity than the overall precipitation, with no clear dependency on changes in atmospheric stability, moisture convergence, and relative humidity.

Response of Extreme Precipitating Cell Structures to Atmospheric Warming

“…Up to the end of phase II, at hour 18, convective activity shows a systematic response to the warmer and moister atmospheric conditions. These are imposed by the initial conditions but also by the dependency of the large‐scale moisture convergence which roughly follows the CC relation (Loriaux et al, ). After phase II, the response is less systematic, and because of large temporal variations, it is also more difficult to compare the different runs.…”

“…The experimental setup follows the approach developed in Loriaux et al (), specifically designed to produce a simple but yet realistic forcing for convective situations. Based on a catalog of rainfall events in the Netherlands from the past 20 years (Lenderink et al, ), we create a composite of approximately 300 days with heavy hourly precipitation in the summer season (April to September) in the afternoon hours (1200 to 2000 CET).…”

“…Most extreme convective events in the Netherlands occur with considerable synoptic‐scale convergence, inducing a large‐scale upward vertical velocity (omega) that humidifies and destabilizes the atmosphere (Lenderink et al, ; Loriaux et al, ). This forcing is imposed in DALES through a large‐scale vertical velocity (Loriaux et al, ) and varies in time and with height as shown in Figure S1. The lower right panel in Figure shows the omega forcing ( w ls ) at three example time steps of the simulation.…”

“…More specifically, we conduct simulations of a diurnal cycle with typical conditions for heavy convective precipitation in the Netherlands. This case setup is characterized by substantial time varying forcing from the surface as well as the large‐scale circulation which forces the system out of equilibrium (Loriaux et al, ). Therefore, our simulations are fundamentally different from quasi‐equilibrium simulations performed, for instance, for the tropics, in radiative‐convective equilibrium (e.g., Muller et al, ; Romps, ).…”

Sensitivity of extreme precipitation to temperature: the variability of scaling factors from a regional to local perspective

“…However, further research is needed to test this assumption in our study region. Recent contributions to the literature by Loriaux et al (2016a) and Loriaux et al (2016b) demonstrate that considering relative humidity alone is not sufficient to explain the intensification of extreme precipitation. Through including atmospheric control factors such as large scale moisture convergence and atmospheric stability, they deliver valuable contributions to process understanding.…”