Description: This study investigates how precipitation-driven cold pools aid the formation of wider clouds that are essential for a transition from shallow to deep convection. In connection with a temperature depression and a depletion of moisture inside developing cold pools, an accumulation of moisture in moist patches around the cold pools is observed. Convective clouds are formed on top of these moist patches. Larger moist patches form with time supporting more and larger clouds. Moreover, enhanced vertical lifting along the leading edges of the gravity current triggered by the cold pool is found. The interplay of moisture aggregation and lifting eventually promotes the formation of wider clouds that are less affected by entrainment and become deeper. These mechanisms are corroborated in a series of cloud-resolving model simulations representing different atmospheric environments. A positive feedback is observed in that, in an atmosphere in which cloud and rain formation is facilitated, stronger downdrafts will form. These stronger downdrafts lead to a stronger modification of the moisture field, which in turn favors further cloud development. This effect is not only observed in the transition phase but also active in prolonging the peak time of precipitation in the later stages of the diurnal cycle. These findings are used to propose a simple way for incorporating the effect of cold pools on cloud sizes and thereby entrainment rate into parameterization schemes for convection. Comparison of this parameterization to the cloud-resolving modeling output gives promising results.

Description: Convection-resolving models (CRMs) can explicitly simulate deep convection and resolve interactions between convective updrafts. They are thus increasingly used in numerous weather and climate applications. However, the truncation of the continuous energy cascade at scales of O (1 km) poses a serious challenge, as in kilometer-scale simulations the size and properties of the simulated convective cells are often determined by the horizontal grid spacing ( Δ x ).In this study, idealized simulations of deep moist convection over land are performed to assess the convergence behavior of a CRM at Δ x = 8, 4, 2, 1 km and 500 m. Two types of convergence estimates are investigated: bulk convergence addressing domain-averaged and integrated variables related to the water and energy budgets, and structural convergence addressing the statistics and scales of individual clouds and updrafts. Results show that bulk convergence generally begins at Δ x = 4  km, while structural convergence is not yet fully achieved at the kilometer scale, despite some evidence that the resolution sensitivity of updraft velocities and convective mass fluxes decreases at finer resolution. In particular, at finer grid spacings the maximum updraft velocity generally increases, and the size of the smallest clouds is mostly determined by Δ x . A number of different experiments are conducted, and it is found that the presence of orography and environmental vertical wind shear yields more energetic structures at scales much larger than Δ x , sometimes reducing the resolution sensitivity. Overall the results lend support to the use of kilometer-scale resolutions in CRMs, despite the inability of these models to fully resolve the associated cloud field.

Description: The observed increase of convective extreme precipitation intensities with temperature beyond the Clausius‐Clapeyron rate has recently directed attention to nonequilibrium processes that might cause the increase. While out‐of‐equilibrium simulations with perturbed heating conditions show clear increases in convective precipitation intensities, it has so far remained unclear, to which extent precipitation intensities can increase, when the atmosphere is in “perpetual equilibrium” (PE). We use the term PE to describe periodically forced diurnal cycles that eventually yield an approximately repetitive atmospheric response from day to day. In PE, as defined here, precipitation extremes increase at rates beyond the Clausius‐Clapeyron rate. When analyzing causes for the increase, we find the variance in near‐surface temperature to increase significantly as precipitation builds up throughout the day and that this temperature variance is larger when surface heating is increased. We propose that enhanced rain evaporation may drive a feedback, by which cold pool activity, and the possible collision of cold pool gust fronts, is strengthened—thereby intensifying subsequent convective updrafts and their precipitation. Plain Language Summary In recent years it has become evident that extreme rain events from thunderstorms react strongly to temperature changes, so that higher temperatures come with much more intense rain—increasing the risk of floods. The origin of the strong temperature sensitivity has so far not been clear. We here propose that the dynamics of the atmosphere is intensified by enhanced rain evaporation, where it falls through the subcloud layer. The effect of this is more cooling below precipitating clouds, leading to larger inequality of temperatures near the surface when comparing cloudy and cloud‐free areas. Where near‐surface air is cold, it is also denser, and this is called a cold pool. It is known that cold pools spread more rapidly along the surface when the lateral temperature difference increases. We here show that this is indeed the case in numerical experiments, as the surface is heated more strongly from one experiment to the next. The implication is that the dynamics of the atmosphere becomes more violent and cold pools, where they collide, can trigger new and stronger subsequent precipitation events. We thereby point to a new feedback, potentially relevant for extreme rain events in a changing climate. Key Points In perpetual equilibrium, convective precipitation extremes increase beyond the Clausius‐Clapeyron rate The variance of the boundary layer temperature distribution is enhanced at higher surface temperature forcing, implying stronger cold pools A dynamical feedback leading to the observed super Clausius‐Clapeyron increase is suggested

Description: Ensembles of convection‐resolving simulations with a simplified land surface are conducted to dissect the isolated and combined impacts of soil moisture and orography on deep‐convective precipitation under weak synoptic forcing. In particular, the deep‐convective precipitation response to a uniform and a nonuniform soil moisture perturbation is investigated both in settings with and without orography. In the case of horizontally uniform perturbations, we find a consistently positive soil moisture‐precipitation feedback, irrespective of the presence of low orography. On the other hand, a negative feedback emerges with localized perturbations: a dry soil heterogeneity substantially enhances rain amounts that scale linearly with the dryness of the soil, while a moist heterogeneity suppresses rain amounts. If the heterogeneity is located in a mountainous region, the relative importance of soil moisture heterogeneity decreases with increasing mountain height: A mountain 500 m in height is sufficient to neutralize the local soil moisture‐precipitation feedback. Plain Language Summary Mountains or regions with patchy soil moisture favor precipitation. While the driving mechanisms have been extensively studied in isolation, only a few investigations have focused on their combined impact. We use kilometer‐scale simulations to systematically compare and quantify their relative importance for deep‐convective rain. We find the strength of the soil moisture‐precipitation feedback to strongly depend on the mountain: once the mountain exceeds a critical, relatively moderate height the relative importance of the local soil moisture distribution becomes less important. Our findings suggest that numerical weather prediction of rainfall is less sensitive to soil moisture patchiness in mountainous areas, as opposed to flat regions. Furthermore, the high‐resolution simulations allow to identify potential misrepresentations of the soil moisture‐precipitation feedback in climate models that are incapable of treating deep convection explicitly. Key Points We simulated the soil moisture‐precipitation feedback in the presence of a mountain For shallow mountains the feedback turns negative if soil moisture heterogeneity is present The strength of the negative feedback is strongly reduced for mountains of moderate height

Description: AbstractThe “gray zone” of convection is defined as the range of horizontal grid-space resolutions at which convective processes are partially but not fully resolved explicitly by the model dynamics (typically estimated from a few kilometers to a few hundred meters). The representation of convection at these scales is challenging, as both parameterizing convective processes or relying on the model dynamics to resolve them might cause systematic model biases. Here, a regional climate model over a large European domain is used to study model biases when either using parameterizations of deep and shallow convection or representing convection explicitly. For this purpose, year-long simulations at horizontal resolutions between 50- and 2.2-km grid spacing are performed and evaluated with datasets of precipitation, surface temperature, and top-of-the-atmosphere radiation over Europe. While simulations with parameterized convection seem more favorable than using explicit convection at around 50-km resolution, at higher resolutions (grid spacing ≤ 25 km) models tend to perform similarly or even better for certain model skills when deep convection is turned off. At these finer scales, the representation of deep convection has a larger effect in model performance than changes in resolution when looking at hourly precipitation statistics and the representation of the diurnal cycle, especially over nonorographic regions. The shortwave net radiative balance at the top of the atmosphere is the variable most strongly affected by resolution changes, due to the better representation of cloud dynamical processes at higher resolutions. These results suggest that an explicit representation of convection may be beneficial in representing some aspects of climate over Europe at much coarser resolutions than previously thought, thereby reducing some of the uncertainties derived from parameterizing deep convection.

Description: Thermally driven upslope flows in mountainous areas provide favorable conditions for diurnal deep moist convection especially during episodes of weak synoptic forcing. The present study investigates the response of deep convection to axisymmetric orography as a function of orographic width and height by running ensembles of idealized convection-resolving simulations with a horizontal grid spacing of Delta x = 1 km, full-physics parameterizations, and an interactive land surface. Deep convection is explicitly resolved and not parameterized. To cover a wide range of orographic scales, simulations are conducted with heights between 250 and 4000 m and widths between 5 and 30 km. The mountain slope strongly affects upslope wind speed characteristics, the timing and intensity of local updrafts, and local rain intensity. Although the day-to-day variability is substantial, the statistical-mean rain amount extracted by the mountain scales almost linearly with the mountain volume. Simulations with alternative mountain geometries, multiple peaks, and large-scale flow suggest that the linear scaling is valid for a surprisingly large portion of the parameter space. The scaling breaks down in the limit of relatively strong large-scale flows, sufficiently tall mountains, or elongated mountains. The existence of the simple linear scaling over such a wide range of configurations suggests that the response of thermally driven orographic deep convection over many mountainous areas is strongly affected by mountain volume. As a consequence, the rain amount is disproportionally dominated by the large horizontal scales of orography, as they contribute mostly to the mountain volume.

Description: Abstract Currently, major efforts are under way to refine the horizontal resolution of weather and climate models to kilometer-scale grid spacing (Δx). Besides refining the representation of the atmospheric dynamics and enabling the use of explicit convection, this will also provide higher resolution in the representation of orography. This study investigates the influence of these resolution increments on the simulation of orographic moist convection. Nine days of fair-weather thermally driven flow over the Alps are analyzed. Two sets of simulations with the COSMO model are compared, each consisting of three runs at Δx of 4.4, 2.2, and 1.1 km: one set using a fixed representation of orography at a resolution of 8.8 km, and one with varying representation at the resolution of the computational mesh. The spatial distribution of precipitation during daytime is only marginally affected by the orographic details, but nighttime convection to the south of the Alps—triggered by cold-air outflow from the valleys—is very sensitive to orography and precipitation is enhanced if more detailed orography is provided. During daytime, the onset of precipitation is delayed. The amplitude of the diurnal cycle of precipitation is reduced, even though more moisture converges toward the Alpine region during the afternoon. The hereby accumulated moisture sustains precipitation during the evening and nighttime over the surrounding plains. For these differences, the effects of changes in orographic detail are more important than changes in grid spacing. In addition, the individual convective cells are weaker, but their number increases with higher resolved orography.

Description: Abstract On summertime fair-weather days, thermally driven wind systems play an important role in determining the initiation of convection and the occurrence of localized precipitation episodes over mountainous terrain. This study compares the mechanisms of convection initiation and precipitation development within a thermally driven flow over an idealized double-ridge system in large-eddy (LESs) and convection-resolving (CRM) simulations. First, LES at a horizontal grid spacing of 200 m is employed to analyze the developing circulations and associated clouds and precipitation. Second, CRM simulations at horizontal grid length of 1 km are conducted to evaluate the performance of a kilometer-scale model in reproducing the discussed mechanisms. Mass convergence and a weaker inhibition over the two ridges flanking the valley combine with water vapor advection by upslope winds to initiate deep convection. In the CRM simulations, the spatial distribution of clouds and precipitation is generally well captured. However, if the mountains are high enough to force the thermally driven flow into an elevated mixed layer, the transition to deep convection occurs faster, precipitation is generated earlier, and surface rainfall rates are higher compared to the LES. Vertical turbulent fluxes remain largely unresolved in the CRM simulations and are underestimated by the model, leading to stronger upslope winds and increased horizontal moisture advection toward the mountain summits. The choice of the turbulence scheme and the employment of a shallow convection parameterization in the CRM simulations change the strength of the upslope winds, thereby influencing the simulated timing and intensity of convective precipitation.

Description: Abstract The importance of soil moisture anomalies on airmass convection over semiarid regions has been recognized in several studies. The underlying mechanisms remain partly unclear. An open question is why wetter soils can result in either an increase or a decrease of precipitation (positive or negative soil moisture–precipitation feedback, respectively). Here an idealized cloud-resolving modeling framework is used to explore the local soil moisture–precipitation feedback. The approach is able to replicate both positive and negative feedback loops, depending on the environmental parameters. The mechanism relies on horizontal soil moisture variations, which may develop and intensify spontaneously. The positive expression of the feedback is associated with the initiation of convection over dry soil patches, but the convective cells then propagate over wet patches where they strengthen and preferentially precipitate. The negative feedback may occur when the wind profile is too weak to support the propagation of convective features from dry to wet areas. Precipitation is then generally weaker and falls preferentially over dry patches. The results highlight the role of the midtropospheric flow in determining the sign of the feedback. A key element of the positive feedback is the exploitation of both low convective inhibition (CIN) over dry patches (for the initiation of convection) and high CAPE over wet patches (for the generation of precipitation).

Description: Soil moisture modifies the state of the atmosphere and thus plays a major role in the climate system. Its spatial distribution is strongly modulated by the underlying orography. Yet the vertical transport of soil water and especially the generation of groundwater runoff at the bottom of the soil column are currently treated in a crude way in most atmospheric and climate models. This potentially leads to large biases in near‐surface temperatures during midlatitude summertime conditions, when the soils may dry out. Here we present a new formulation for groundwater and runoff formation. It is based on Richards equation, allows for saturated aquifers, includes a slope‐dependent groundwater discharge, and enables a subgrid‐scale treatment of the underlying orography. The proposed numerical implementation ensures a physically consistent treatment of the water fluxes in the soil column, using ideas from flux‐corrected transport methodologies. An implementation of this formulation into TERRA_ML, the land surface model of the regional climate model of the COnsortium for Small‐scale MOdeling (COSMO) in CLimate Mode (CCLM), is validated both in idealized and real‐case simulations. Idealized simulations demonstrate the important role of the lower boundary condition at the bottom of the soil column and display a physically meaningful recharge and discharge of the saturated zone. Validation against measurements at selected stations shows an improved seasonal evolution of soil water content. Finally, decade‐long climate simulations over Europe exhibit a realistic representation of the groundwater distribution across continental scales and mountainous areas, an improved annual cycle of surface latent heat fluxes, and as a consequence reductions of long‐standing biases in near‐surface temperatures in semiarid regions. Key Points A formulation for soil water transport and runoff is introduced Simulations exhibit a physically meaningful recharge and discharge of the saturated zone Simulations demonstrate improved surface latent heat fluxes and reductions of long‐standing biases in near‐surface temperatures