Theoretical and Numerical Studies of Deep Convection Over Orography and Soil-Moisture Heterogeneities

Abstract

Summer precipitation in many extratropical land regions largely results from deep convection in rain showers and thunderstorms. Deep convection is the widespread, diurnal response of the atmosphere to its vertical destabilization by surface warming and radiative cooling aloft. As a side effect of deep-convective dynamics strong updrafts, lightning strikes, devastating hail, and gusty winds are produced. The most serious impact however relates to abrupt flooding after heavy rain making deep convection not only one of the most memorable but also the most severe weather phenomena. Despite the extraordinary importance of deep convection for weather and climate, numerical weather prediction and climate simulations continue to suffer from the side effects of the misrepresentation of many underlying physical processes in the numerical models. The challenge to represent deep convection is not surprising, given that deep convection is non-linear, multi-scale, multi-phase, three-dimensional, turbulent flow that is highly sensitive to initial and boundary conditions. In order to better understand the role of deep convection for the hydrological cycle and to improve the representation of deep convection in atmospheric models, a deeper understanding of the basic physical processes is therefore required. This is particularly pressing during episodes of weak synoptic forcing, when the abrupt formation of deep convective thunderstorms is exceedingly difficult to simulate. However it is promising, that numerous observations reveal that thunderstorms initiate at preferential locations, which seem to be strongly tied to heterogeneities on the Earth's surface. This thesis therefore aims to shed light on the physical influence of two major components of the Earth's surface on deep convection. On the one hand the focus is put on orography (mountains), the most dominant and widespread surface characteristic controlling weather and climate far beyond its immediate surrounding. On the other hand a focus is put on soil moisture which exerts a much more uncertain influence on deep convection due to its substantial variability in space and time. In order to gain a better understanding on the influence of these two land surface components on deep convection, a state-of-the-art computer model of the coupled atmosphere-land system is used. With that, ensembles of idealized convection-resolving simulations with a numerical grid spacing of 1 km and full physics parameterizations (except for shallow and deep convection) are systematically run. The first part of the thesis focuses on the feedback between soil moisture and precipitation. In particular the relative importance of soil moisture heterogeneity in flat and mountainous terrain is quantified. The sign of the feedback strongly depends on the patchiness of soil moisture. While the feedback is positive for horizontally uniform soil moisture (more rain over increased soil moisture), the feedback turns strongly negative at localized soil moisture anomalies (more rain over dry soil anomalies). The negative feedback stems from the explicitly resolved shallow circulations in the planetary boundary layer that develop at soil moisture transition zones. A similar mechanism emerges in presence of an isolated mountain. Mountains drier than their surrounding thermally develop stronger upslope flows and therefore receive more rain than moister mountains. However, the strength of the feedback sensitively depends on the dominance of the mountain. A relatively low mountain of 500 m in height is sufficient to neutralize the soil-moisture-precipitation feedback, i.e. the rain amounts over a dry or a moist mountain do not differ. The second part of the thesis is dedicated to the relative importance of orographic scales for deep convection. To that end deep convective activity at an isolated mountain of varying height and width is simulated. Despite a strong impact of the mountain slope on the converging upslope flows, and the intensity of deep convection, an approximately linear scaling between mountain rain and mountain volume emerges in the statistical mean. Tests with alternative mountain profiles, multiple peaks and large-scale flow suggest, that the scaling is present over a surprisingly large portion of parameter space. Only in the limit of tall mountains and relatively strong large-scale flow a systematic breakdown of the scaling is found. A simple conceptual framework relates the linear scaling to the enhanced energy density in the atmosphere above mountains. The breakdown of the scaling is explained in terms of the venting of the elevated mountain heat anomaly by the large-scale flow. Since the large horizontal scales of mountains contribute disproportionally to the mountain volume, the volume scaling suggests that small-scale terrain (such as single peaks in a mountainous area) are, in the statistical mean, of minor importance for deep convective precipitation during episodes of weak synoptic forcing. In summary, this thesis is testimony to the dominance of orography, and its large-scale features in particular, in controlling the statistical behavior of deep convection and ultimately the continental water cycle.