@article{15047, abstract = {Tropical precipitation extremes and their changes with surface warming are investigated using global storm resolving simulations and high-resolution observations. The simulations demonstrate that the mesoscale organization of convection, a process that cannot be physically represented by conventional global climate models, is important for the variations of tropical daily accumulated precipitation extremes. In both the simulations and observations, daily precipitation extremes increase in a more organized state, in association with larger, but less frequent, storms. Repeating the simulations for a warmer climate results in a robust increase in monthly-mean daily precipitation extremes. Higher precipitation percentiles have a greater sensitivity to convective organization, which is predicted to increase with warming. Without changes in organization, the strongest daily precipitation extremes over the tropical oceans increase at a rate close to Clausius-Clapeyron (CC) scaling. Thus, in a future warmer state with increased organization, the strongest daily precipitation extremes over oceans increase at a faster rate than CC scaling.}, author = {Bao, Jiawei and Stevens, Bjorn and Kluft, Lukas and Muller, Caroline J}, issn = {2375-2548}, journal = {Science Advances}, number = {8}, publisher = {American Association for the Advancement of Science}, title = {{Intensification of daily tropical precipitation extremes from more organized convection}}, doi = {10.1126/sciadv.adj6801}, volume = {10}, year = {2024}, } @article{14453, abstract = {Squall lines are substantially influenced by the interaction of low-level shear with cold pools associated with convective downdrafts. Beyond an optimal shear amplitude, squall lines tend to orient themselves at an angle with respect to the low-level shear. While the mechanisms behind squall line orientation seem to be increasingly well understood, uncertainties remain on the implications of this orientation. Roca and Fiolleau (2020, https://doi.org/10.1038/s43247-020-00015-4) show that long lived mesoscale convective systems, including squall lines, are disproportionately involved in rainfall extremes in the tropics. This article investigates the influence of the interaction between low-level shear and squall line outflow on squall line generated precipitation extrema in the tropics. Using a cloud resolving model, simulated squall lines in radiative convective equilibrium amid a shear-dominated regime (super optimal), a balanced regime (optimal), and an outflow dominated regime (suboptimal). Our results show that precipitation extremes in squall lines are 40% more intense in the case of optimal shear and remain 30% superior in the superoptimal regime relative to a disorganized case. With a theoretical scaling of precipitation extremes (C. Muller & Takayabu, 2020, https://doi.org/10.1088/1748-9326/ab7130), we show that the condensation rates control the amplification of precipitation extremes in tropical squall lines, mainly due to its change in vertical mass flux (dynamic component). The reduction of dilution by entrainment explains half of this change, consistent with Mulholland et al. (2021, https://doi.org/10.1175/jas-d-20-0299.1). The other half is explained by increased cloud-base velocity intensity in optimal and superoptimal squall lines.}, author = {Abramian, Sophie and Muller, Caroline J and Risi, Camille}, issn = {1942-2466}, journal = {Journal of Advances in Modeling Earth Systems}, number = {10}, publisher = {Wiley}, title = {{Extreme precipitation in tropical squall lines}}, doi = {10.1029/2022MS003477}, volume = {15}, year = {2023}, } @article{14654, abstract = {Two assumptions commonly applied in convection schemes—the diagnostic and quasi-equilibrium assumptions—imply that convective activity (e.g., convective precipitation) is controlled only by the large-scale (macrostate) environment at the time. In contrast, numerical experiments indicate a “memory” or dependence of convection also on its own previous activity whereby subgrid-scale (microstate) structures boost but are also boosted by convection. In this study we investigated this memory by comparing single-column model behavior in two idealized tests previously executed by a cloud-resolving model (CRM). Conventional convection schemes that employ the diagnostic assumption fail to reproduce the CRM behavior. The memory-capable org and Laboratoire de Météorologie Dynamique Zoom cold pool schemes partially capture the behavior, but fail to fully exhibit the strong reinforcing feedbacks implied by the CRM. Analysis of this failure suggests that it is because the CRM supports a linear (or superlinear) dependence of the subgrid structure growth rate on the precipitation rate, while the org scheme assumes a sublinear dependence. Among varying versions of the org scheme, the growth rate of the org variable representing subgrid structure is strongly associated with memory strength. These results demonstrate the importance of parameterizing convective memory, and the ability of idealized tests to reveal shortcomings of convection schemes and constrain model structural assumptions.}, author = {Hwong, Yi-Ling and Colin, M. and Aglas, Philipp and Muller, Caroline J and Sherwood, S. C.}, issn = {1942-2466}, journal = {Journal of Advances in Modeling Earth Systems}, number = {12}, publisher = {Wiley}, title = {{Assessing memory in convection schemes using idealized tests}}, doi = {10.1029/2023MS003726}, volume = {15}, year = {2023}, } @article{14752, abstract = {Radiative cooling of the lowest atmospheric levels is of strong importance for modulating atmospheric circulations and organizing convection, but detailed observations and a robust theoretical understanding are lacking. Here we use unprecedented observational constraints from subsidence regimes in the tropical Atlantic to develop a theory for the shape and magnitude of low‐level longwave radiative cooling in clear‐sky, showing peaks larger than 5–10 K/day at the top of the boundary layer. A suite of novel scaling approximations is first developed from simplified spectral theory, in close agreement with the measurements. The radiative cooling peak height is set by the maximum lapse rate in water vapor path, and its magnitude is mainly controlled by the ratio of column relative humidity above and below the peak. We emphasize how elevated intrusions of moist air can reduce low‐level cooling, by sporadically shading the spectral range which effectively cools to space. The efficiency of this spectral shading depends both on water content and altitude of moist intrusions; its height dependence cannot be explained by the temperature difference between the emitting and absorbing layers, but by the decrease of water vapor extinction with altitude. This analytical work can help to narrow the search for low‐level cloud patterns sensitive to radiative‐convective feedbacks: the most organized patterns with largest cloud fractions occur in atmospheres below 10% relative humidity and feel the strongest low‐level cooling. This motivates further assessment of favorable conditions for radiative‐convective feedbacks and a robust quantification of corresponding shallow cloud dynamics in current and warmer climates.}, author = {Fildier, B. and Muller, Caroline J and Pincus, R. and Fueglistaler, S.}, issn = {2576-604X}, journal = {AGU Advances}, keywords = {General Earth and Planetary Sciences}, number = {3}, publisher = {American Geophysical Union}, title = {{How moisture shapes low‐level radiative cooling in subsidence regimes}}, doi = {10.1029/2023av000880}, volume = {4}, year = {2023}, } @article{14773, abstract = {Through a combination of idealized simulations and real-world data, researchers are uncovering how internal feedbacks and large-scale motions influence cloud dynamics.}, author = {Muller, Caroline J and Abramian, Sophie}, issn = {1945-0699}, journal = {Physics Today}, keywords = {General Physics and Astronomy}, number = {5}, publisher = {AIP Publishing}, title = {{The cloud dynamics of convective storm systems}}, doi = {10.1063/pt.3.5234}, volume = {76}, year = {2023}, } @inbook{14853, abstract = {Organization – or departure from a random pattern – in tropical deep convection is heavily studied due to its immediate relevance to climate sensitivity and extremes. Low-latitude convection has motivated numerical model idealizations, where the Coriolis force is removed and boundary conditions are simplified spatially and temporally. One of the most stunning aspects of such idealized simulated cloud organization is the spontaneous clumping of convection that can occur without any predetermining external perturbation, such as inhomogeneous surface boundary conditions or large-scale waves. Whereas individual convective rain cells measure only few kilometers in horizontal diameter, the clusters they form can often span hundreds or even thousands of kilometers. Hence, organization may emerge from the very small scales but can show effects at the synoptic scale. We refer to such emergent organization as convective self-organization. Convective self-organization thus features characteristics of emergence, such as non-trivial system-scale pattern formation or hysteresis. We summarize observational evidence for large-scale organization and briefly recap classical idealized modeling studies that yield convective self-aggregation – emergent organization under strongly idealized boundary conditions. We then focus on developing research, where temporal variation, such as the diurnal cycle, or two-way interactive surface properties yield distinct organizational modes. Convectively generated cold pools and mesoscale convective systems, both ubiquitous in nature, are thereby found to potentially play key roles in promoting – rather than suppressing – sustained system-scale organization.}, author = {Haerter, Jan O. and Muller, Caroline J}, booktitle = {Clouds and Their Climatic Impacts}, editor = {Sullivan, Sylvia and Hoose, Corinna}, isbn = {9781119700319}, issn = {2328-8779}, pages = {179--193}, publisher = {Wiley}, title = {{Mechanisms for the Self‐Organization of Tropical Deep Convection}}, doi = {10.1002/9781119700357.ch8}, year = {2023}, } @inproceedings{14863, author = {Polesello, Andrea and Muller, Caroline J and Pasquero, Claudia and Meroni, Agostino N.}, booktitle = {EGU General Assembly 2023}, location = {Vienna, Austria & Virtual}, publisher = {European Geosciences Union}, title = {{Intensification mechanisms of tropical cyclones}}, doi = {10.5194/egusphere-egu23-6157}, year = {2023}, } @inproceedings{14864, author = {Stöllner, Andrea and Lenton, Isaac C and Muller, Caroline J and Waitukaitis, Scott R}, booktitle = {EGU General Assembly 2023}, location = {Vienna, Austria & Virtual}, publisher = {European Geosciences Union}, title = {{Measuring spontaneous charging of single aerosol particles}}, doi = {10.5194/egusphere-egu23-6166}, year = {2023}, } @inproceedings{14865, author = {Hwong, Yi-Ling and Colin, Maxime and Aglas, Philipp and Muller, Caroline J and Sherwood, Steven}, booktitle = {EGU General Assembly 2023}, location = {Vienna, Austria & Virtual}, publisher = {European Geosciences Union}, title = {{Evaluating memory properties in convection schemes using idealised tests}}, doi = {10.5194/egusphere-egu23-4968}, year = {2023}, } @inproceedings{14866, author = {Abramian, Sophie and Muller, Caroline J and Risi, Camille}, booktitle = {EGU General Assembly 2023}, location = {Vienna, Austria & Virtual}, publisher = {European Geosciences Union}, title = {{Extreme precipitation in tropical squall lines}}, doi = {10.5194/egusphere-egu23-15870}, year = {2023}, } @misc{14991, abstract = {This repository contains the data, scripts, WRF codes and files required to reproduce the results of the manuscript "Assessing Memory in Convection Schemes Using Idealized Tests" submitted to the Journal of Advances in Modeling Earth Systems (JAMES).}, author = {Hwong, Yi-Ling and Colin, Maxime and Aglas, Philipp and Muller, Caroline J and Sherwood, Steven C.}, publisher = {Zenodo}, title = {{Data-assessing memory in convection schemes using idealized tests}}, doi = {10.5281/ZENODO.7757041}, year = {2023}, } @article{10656, abstract = {Idealized simulations of the tropical atmosphere have predicted that clouds can spontaneously clump together in space, despite perfectly homogeneous settings. This phenomenon has been called self-aggregation, and it results in a state where a moist cloudy region with intense deep convective storms is surrounded by extremely dry subsiding air devoid of deep clouds. We review here the main findings from theoretical work and idealized models of this phenomenon, highlighting the physical processes believed to play a key role in convective self-aggregation. We also review the growing literature on the importance and implications of this phenomenon for the tropical atmosphere, notably, for the hydrological cycle and for precipitation extremes, in our current and in a warming climate.}, author = {Muller, Caroline J and Yang, Da and Craig, George and Cronin, Timothy and Fildier, Benjamin and Haerter, Jan O. and Hohenegger, Cathy and Mapes, Brian and Randall, David and Shamekh, Sara and Sherwood, Steven C.}, issn = {1545-4479}, journal = {Annual Review of Fluid Mechanics}, pages = {133--157}, publisher = {Annual Reviews}, title = {{Spontaneous aggregation of convective storms}}, doi = {10.1146/annurev-fluid-022421-011319}, volume = {54}, year = {2022}, } @article{10653, abstract = {Squall lines are known to be the consequence of the interaction of low-level shear with cold pools associated with convective downdrafts. Also, as the magnitude of the shear increases beyond a critical shear, squall lines tend to orient themselves. The existing literature suggests that this orientation reduces incoming wind shear to the squall line, and maintains equilibrium between wind shear and cold pool spreading. Although this theory is widely accepted, very few quantitative studies have been conducted on supercritical regime especially. Here, we test this hypothesis with tropical squall lines obtained by imposing a vertical wind shear in cloud resolving simulations in radiative convective equilibrium. In the sub-critical regime, squall lines are perpendicular to the shear. In the super-critical regime, their orientation maintain the equilibrium, supporting existing theories. We also find that as shear increases, cold pools become more intense. However, this intensification has little impact on squall line orientation.}, author = {Abramian, Sophie and Muller, Caroline J and Risi, Camille}, issn = {1944-8007}, journal = {Geophysical Research Letters}, number = {1}, publisher = {Wiley}, title = {{Shear-convection interactions and orientation of tropical squall lines}}, doi = {10.1029/2021GL095184}, volume = {49}, year = {2022}, } @article{12107, abstract = {The sensitivity of coarse-grained daily extreme precipitation to sea surface temperature is analyzed using satellite precipitation estimates over the 300–302.5 K range. A theoretical scaling is proposed, linking changes in coarse-grained precipitation to changes in fine-scale hourly precipitation area fraction and changes in conditional fine-scale precipitation rates. The analysis reveals that the extreme coarse-grained precipitation scaling with temperature (∼7%/K) is dominated by the fine-scale precipitating fraction scaling (∼6.5%/K) when using a 3 mm/h fine-scale threshold to delineate the precipitating fraction. These results are shown to be robust to the selection of the precipitation product and to the percentile used to characterize the extreme. This new coarse-grained scaling is further related to the well-known scaling for fine-scale precipitation extremes, and suggests a compensation between thermodynamic and dynamic contributions or that both contributions are small with respect to that of fractional coverage. These results suggest that processes responsible for the changes in fractional coverage are to be accounted for to assess the sensitivity of coarse-grained extreme daily precipitation to surface temperature.}, author = {Roca, Rémy and De Meyer, Victorien and Muller, Caroline J}, issn = {1944-8007}, journal = {Geophysical Research Letters}, number = {24}, publisher = {Wiley}, title = {{Precipitating fraction, not intensity, explains extreme coarse-grained precipitation Clausius-Clapeyron scaling with sea surface temperature over tropical oceans}}, doi = {10.1029/2022GL100624}, volume = {49}, year = {2022}, } @article{9151, abstract = {We investigate how mesoscale circulations associated with convective aggregation can modulate the sensitivity of the hydrologic cycle to warming. We quantify changes in the full distribution of rain across radiative‐convective equilibrium states in a cloud‐resolving model. For a given SST, the shift in mean rainfall between disorganized and organized states is associated with a shift in atmospheric radiative cooling, and is roughly analogous to the effect of a 4K SST increase. With rising temperatures, the increase in mean rain rate is insensitive to the presence of organization, while extremes can intensify faster in the aggregated state, leading to a faster amplification in the sporadic nature of rain. When convection aggregates, heavy rain is enhanced by 20‐30% and nonlinear behaviors are observed as a function of SST and strength of aggregation feedbacks. First, radiative‐ and surface‐flux aggregation feedbacks have multiplicative effects on extremes, illustrating a non‐trivial sensitivity to the degree of organization. Second, alternating Clausius‐Clapeyron and super‐Clausius‐Clapeyron regimes in extreme rainfall are found as a function of SST, corresponding to varying thermodynamic and dynamic contributions, and a large sensitivity to precipitation efficiency variations in some SST ranges. The potential for mesoscale circulations in amplifying the hydrologic cycle is established. However these nonlinear distortions question the quantitative relevance of idealized self‐aggregation. This calls for a deeper investigation of relationships which capture the coupling between global energetics, aggregation feedbacks and local convection, and for systematic tests of their sensitivity to domain configurations, surface boundary conditions, microphysics and turbulence schemes.}, author = {Fildier, Benjamin and Collins, William D. and Muller, Caroline J}, issn = {1942-2466}, journal = {Journal of Advances in Modeling Earth Systems}, keywords = {Global and Planetary Change, General Earth and Planetary Sciences, Environmental Chemistry}, number = {2}, publisher = {American Geophysical Union}, title = {{Distortions of the rain distribution with warming, with and without self‐aggregation}}, doi = {10.1029/2020ms002256}, volume = {13}, year = {2021}, } @unpublished{9124, abstract = {The couplings among clouds, convection, and circulation in trade-wind regimes remain a fundamental puzzle that limits our ability to constrain future climate change. Radiative heating plays an important role in these couplings. Here we calculate the clear-sky radiative profiles from 2001 in-situ soundings (978 dropsondes and 1023 radiosondes) collected during the EUREC4A field campaign, which took place south and east of Barbados in January–February 2020. We describe the method used to calculate these radiative profiles and present preliminary results sampling variability at multiple scales, from the variability across all soundings to groupings by diurnal cycle and mesoscale organization state, as well as individual soundings associated with elevated moisture layers. This clear-sky radiative profiles data set can provide important missing detail to what can be learned from calculations based on passive remote sensing and help in investigating the role of radiation in dynamic and thermodynamic variability in trade-wind regimes. All data are archived and freely available for public access on AERIS (Albright et al. (2020), https://doi.org/10.25326/78).}, author = {Albright, Anna Lea and Fildier, Benjamin and Touzé-Peiffer, Ludovic and Pincus, Robert and Vial, Jessica and Muller, Caroline J}, booktitle = {Earth System Science Data}, publisher = {Copernicus Publications}, title = {{Atmospheric radiative profiles during EUREC4A}}, doi = {10.5194/essd-2020-269}, year = {2020}, } @article{9125, abstract = {This study investigates the feedbacks between an interactive sea surface temperature (SST) and the self‐aggregation of deep convective clouds, using a cloud‐resolving model in nonrotating radiative‐convective equilibrium. The ocean is modeled as one layer slab with a temporally fixed mean but spatially varying temperature. We find that the interactive SST decelerates the aggregation and that the deceleration is larger with a shallower slab, consistent with earlier studies. The surface temperature anomaly in dry regions is positive at first, thus opposing the diverging shallow circulation known to favor self‐aggregation, consistent with the slower aggregation. But surprisingly, the driest columns then have a negative SST anomaly, thus strengthening the diverging shallow circulation and favoring aggregation. This diverging circulation out of dry regions is found to be well correlated with the aggregation speed. It can be linked to a positive surface pressure anomaly (PSFC), itself the consequence of SST anomalies and boundary layer radiative cooling. The latter cools and dries the boundary layer, thus increasing PSFC anomalies through virtual effects and hydrostasy. Sensitivity experiments confirm the key role played by boundary layer radiative cooling in determining PSFC anomalies in dry regions, and thus the shallow diverging circulation and the aggregation speed.}, author = {Shamekh, S. and Muller, Caroline J and Duvel, J.‐P. and D'Andrea, F.}, issn = {1942-2466}, journal = {Journal of Advances in Modeling Earth Systems}, keywords = {Global and Planetary Change, General Earth and Planetary Sciences, Environmental Chemistry}, number = {11}, publisher = {American Geophysical Union}, title = {{Self‐aggregation of convective clouds with interactive sea surface temperature}}, doi = {10.1029/2020ms002164}, volume = {12}, year = {2020}, } @article{9126, abstract = {The goal of this study is to understand the mechanisms controlling the isotopic composition of the water vapor near the surface of tropical oceans, at the scale of about a hundred kilometers and a month. In the tropics, it has long been observed that the isotopic compositions of rain and vapor near the surface are more depleted when the precipitation rate is high. This is called the “amount effect.” Previous studies, based on observations or models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts and rain evaporation. But the relative importance of these processes has never been quantified. We hypothesize that it can be quantified using an analytical model constrained by large‐eddy simulations. Results from large‐eddy simulations confirm that the classical amount effect can be simulated only if precipitation rate changes result from changes in the large‐scale circulation. We find that the main process depleting the water vapor compared to the equilibrium with the ocean is the fact that updrafts stem from areas where the water vapor is more enriched. The main process responsible for the amount effect is the fact that when the large‐scale ascent increases, isotopic vertical gradients are steeper, so that updrafts and downdrafts deplete the subcloud layer more efficiently.}, author = {Risi, Camille and Muller, Caroline J and Blossey, Peter}, issn = {1942-2466}, journal = {Journal of Advances in Modeling Earth Systems}, keywords = {Global and Planetary Change, General Earth and Planetary Sciences, Environmental Chemistry}, number = {8}, publisher = {American Geophysical Union}, title = {{What controls the water vapor isotopic composition near the surface of tropical oceans? Results from an analytical model constrained by large‐eddy simulations}}, doi = {10.1029/2020ms002106}, volume = {12}, year = {2020}, } @article{9127, abstract = {Nearly all regions in the world are projected to become dryer in a warming climate. Here, we investigate the Mediterranean region, often referred to as a climate change “hot spot”. From regional climate simulations, it is shown that although enhanced warming and drying over land is projected, the spatial pattern displays high variability. Indeed, drying is largely caused by enhanced warming over land. However, in Northern Europe, soil moisture alleviates warming induced drying by up to 50% due to humidity uptake from land. In already arid regions, the Mediterranean Sea is generally the only humidity source, and drying is only due to land warming. However, over Sahara and the Iberian Peninsula, enhanced warming over land is insufficient to explain the extreme drying. These regions are also isolated from humidity advection by heat lows, which are cyclonic circulation anomalies associated with surface heating over land. The cyclonic circulation scales with the temperature gradient between land and ocean which increases with climate change, reinforcing the cyclonic circulation over Sahara and the Iberian Peninsula, both diverting the zonal advection of humidity to the south of the Iberian Peninsula. The dynamics are therefore key in the warming and drying of the Mediterranean region, with extreme aridification over the Sahara and Iberian Peninsula. In these regions, the risk for human health due to the thermal load which accounts for air temperature and humidity is therefore projected to increase significantly with climate change at a level of extreme danger.}, author = {Drobinski, Philippe and Da Silva, Nicolas and Bastin, Sophie and Mailler, Sylvain and Muller, Caroline J and Ahrens, Bodo and Christensen, Ole B. and Lionello, Piero}, issn = {1436-3798}, journal = {Regional Environmental Change}, keywords = {Global and Planetary Change}, number = {9}, publisher = {Springer Nature}, title = {{How warmer and drier will the Mediterranean region be at the end of the twenty-first century?}}, doi = {10.1007/s10113-020-01659-w}, volume = {20}, year = {2020}, } @article{9128, abstract = {This paper reviews recent important advances in our understanding of the response of precipitation extremes to warming from theory and from idealized cloud-resolving simulations. A theoretical scaling for precipitation extremes has been proposed and refined in the past decades, allowing to address separately the contributions from the thermodynamics, the dynamics and the microphysics. Theoretical constraints, as well as remaining uncertainties, associated with each of these three contributions to precipitation extremes, are discussed. Notably, although to leading order precipitation extremes seem to follow the thermodynamic theoretical expectation in idealized simulations, considerable uncertainty remains regarding the response of the dynamics and of the microphysics to warming, and considerable departure from this theoretical expectation is found in observations and in more realistic simulations. We also emphasize key outstanding questions, in particular the response of mesoscale convective organization to warming. Observations suggest that extreme rainfall often comes from an organized system in very moist environments. Improved understanding of the physical processes behind convective organization is needed in order to achieve accurate extreme rainfall prediction in our current, and in a warming climate.}, author = {Muller, Caroline J and Takayabu, Yukari}, issn = {1748-9326}, journal = {Environmental Research Letters}, keywords = {Renewable Energy, Sustainability and the Environment, Public Health, Environmental and Occupational Health, General Environmental Science}, number = {3}, publisher = {IOP Publishing}, title = {{Response of precipitation extremes to warming: What have we learned from theory and idealized cloud-resolving simulations, and what remains to be learned?}}, doi = {10.1088/1748-9326/ab7130}, volume = {15}, year = {2020}, }