TY - JOUR AB - 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. AU - Bao, Jiawei AU - Stevens, Bjorn AU - Kluft, Lukas AU - Muller, Caroline J ID - 15047 IS - 8 JF - Science Advances TI - Intensification of daily tropical precipitation extremes from more organized convection VL - 10 ER - TY - JOUR AB - Global storm-resolving models (GSRMs) use strongly refined horizontal grids compared with the climate models typically used in the Coupled Model Intercomparison Project (CMIP) but employ comparable vertical grid spacings. Here, we study how changes in the vertical grid spacing and adjustments to the integration time step affect the basic climate quantities simulated by the ICON-Sapphire atmospheric GSRM. Simulations are performed over a 45 d period for five different vertical grids with between 55 and 540 vertical layers and maximum tropospheric vertical grid spacings of between 800 and 50 m, respectively. The effects of changes in the vertical grid spacing are compared with the effects of reducing the horizontal grid spacing from 5 to 2.5 km. For most of the quantities considered, halving the vertical grid spacing has a smaller effect than halving the horizontal grid spacing, but it is not negligible. Each halving of the vertical grid spacing, along with the necessary reductions in time step length, increases cloud liquid water by about 7 %, compared with an approximate 16 % decrease for halving the horizontal grid spacing. The effect is due to both the vertical grid refinement and the time step reduction. There is no tendency toward convergence in the range of grid spacings tested here. The cloud ice amount also increases with a refinement in the vertical grid, but it is hardly affected by the time step length and does show a tendency to converge. While the effect on shortwave radiation is globally dominated by the altered reflection due to the change in the cloud liquid water content, the effect on longwave radiation is more difficult to interpret because changes in the cloud ice concentration and cloud fraction are anticorrelated in some regions. The simulations show that using a maximum tropospheric vertical grid spacing larger than 400 m would increase the truncation error strongly. Computing time investments in a further vertical grid refinement can affect the truncation errors of GSRMs similarly to comparable investments in horizontal refinement, because halving the vertical grid spacing is generally cheaper than halving the horizontal grid spacing. However, convergence of boundary layer cloud properties cannot be expected, even for the smallest maximum tropospheric grid spacing of 50 m used in this study. AU - Schmidt, Hauke AU - Rast, Sebastian AU - Bao, Jiawei AU - Cassim, Amrit AU - Fang, Shih Wei AU - Jimenez-De La Cuesta, Diego AU - Keil, Paul AU - Kluft, Lukas AU - Kroll, Clarissa AU - Lang, Theresa AU - Niemeier, Ulrike AU - Schneidereit, Andrea AU - Williams, Andrew I.L. AU - Stevens, Bjorn ID - 15097 IS - 4 JF - Geoscientific Model Development SN - 1991-959X TI - Effects of vertical grid spacing on the climate simulated in the ICON-Sapphire global storm-resolving model VL - 17 ER - TY - JOUR AB - The elimination of rain evaporation in the planetary boundary layer (PBL) has been found to lead to convective self‐aggregation (CSA) even without radiative feedback, but the precise mechanisms underlying this phenomenon remain unclear. We conducted cloud‐resolving simulations with two domain sizes and progressively reduced rain evaporation in the PBL. Surprisingly, CSA only occurred when rain evaporation was almost completely removed. The additional convective heating resulting from the reduction of evaporative cooling in the moist patch was found to be the trigger, thereafter a dry subsidence intrusion into the PBL in the dry patch takes over and sets CSA in motion. Temperature and moisture anomalies oppose each other in their buoyancy effects, hence explaining the need for almost total rain evaporation removal. We also found radiative cooling and not cold pools to be the leading cause for the comparative ease of CSA to take place in the larger domain. AU - Hwong, Yi-Ling AU - Muller, Caroline J ID - 15186 IS - 6 JF - Geophysical Research Letters KW - General Earth and Planetary Sciences KW - Geophysics SN - 0094-8276 TI - The unreasonable efficiency of total rain evaporation removal in triggering convective self‐aggregation VL - 51 ER - TY - JOUR AB - Current knowledge suggests a drought Indian monsoon (perhaps a severe one) when the El Nino Southern Oscillation and Pacific Decadal Oscillation each exhibit positive phases (a joint positive phase). For the monsoons, which are exceptions in this regard, we found northeast India often gets excess pre-monsoon rainfall. Further investigation reveals that this excess pre-monsoon rainfall is produced by the interaction of the large-scale circulation associated with the joint phase with the mountains in northeast India. We posit that a warmer troposphere, a consequence of excess rainfall over northeast India, drives a stronger monsoon circulation and enhances monsoon rainfall over central India. Hence, we argue that pre-monsoon rainfall over northeast India can be used for seasonal monsoon rainfall prediction over central India. Most importantly, its predictive value is at its peak when the Pacific Ocean exhibits a joint positive phase and the threat of extreme drought monsoon looms over India. AU - Goswami, Bidyut B ID - 15165 IS - 5 JF - Geophysical Research Letters SN - 0094-8276 TI - A pre-monsoon signal of false alarms of Indian monsoon droughts VL - 51 ER - TY - JOUR AB - The Indian summer monsoon rainfall (ISMR) has been declining since the 1950s. However, since 2002 it is reported to have revived. For these observed changes in the ISMR, several explanations have been reported. Among these explanations, however, the role of the eastern equatorial Indian Ocean (EEIO) is missing despite being one of the warmest regions in the Indian Ocean, and monotonously warming. A recent study reported that EEIO warming impacts the rainfall over northern India. Here we report that warming in the EEIO weakens the low-level Indian summer monsoon circulation and reduces ISMR. A warm EEIO drives easterly winds in the Indo–Pacific sector as a Gill response. The warm EEIO also enhances nocturnal convection offshore the western coast of Sumatra. The latent heating associated with the increased convection augments the Gill response and the resultant circulation opposes the monsoon low-level circulation and weakens the seasonal rainfall. AU - Goswami, Bidyut B ID - 11434 JF - Climate Dynamics SN - 0930-7575 TI - Role of the eastern equatorial Indian Ocean warming in the Indian summer monsoon rainfall trend VL - 60 ER - TY - JOUR AB - We investigate the capabilities of Physics-Informed Neural Networks (PINNs) to reconstruct turbulent Rayleigh–Bénard flows using only temperature information. We perform a quantitative analysis of the quality of the reconstructions at various amounts of low-passed-filtered information and turbulent intensities. We compare our results with those obtained via nudging, a classical equation-informed data assimilation technique. At low Rayleigh numbers, PINNs are able to reconstruct with high precision, comparable to the one achieved with nudging. At high Rayleigh numbers, PINNs outperform nudging and are able to achieve satisfactory reconstruction of the velocity fields only when data for temperature is provided with high spatial and temporal density. When data becomes sparse, the PINNs performance worsens, not only in a point-to-point error sense but also, and contrary to nudging, in a statistical sense, as can be seen in the probability density functions and energy spectra. AU - Clark Di Leoni, Patricio AU - Agasthya, Lokahith N AU - Buzzicotti, Michele AU - Biferale, Luca ID - 12791 IS - 3 JF - The European Physical Journal E SN - 1292-8941 TI - Reconstructing Rayleigh–Bénard flows out of temperature-only measurements using Physics-Informed Neural Networks VL - 46 ER - TY - JOUR AB - The El Niño-Southern Oscillation (ENSO) and the Indian summer monsoon (ISM, or monsoon) are two giants of tropical climate. Here we assess the future evolution of the ENSO-monsoon teleconnection in climate simulations with idealized forcing of CO2 increment at a rate of 1% year-1 starting from a present-day condition (367 p.p.m.) until quadrupling. We find a monotonous weakening of the ENSO-monsoon teleconnection with the increase in CO2. Increased co-occurrences of El Niño and positive Indian Ocean Dipoles (pIODs) in a warmer climate weaken the teleconnection. Co-occurrences of El Niño and pIOD are attributable to mean sea surface temperature (SST) warming that resembles a pIOD-type warming pattern in the Indian Ocean and an El Niño-type warming in the Pacific. Since ENSO is a critical precursor of the strength of the Indian monsoon, a weakening of this relation may mean a less predictable Indian monsoon in a warmer climate. AU - Goswami, Bidyut B AU - An, Soon Il ID - 13256 JF - npj Climate and Atmospheric Science TI - An assessment of the ENSO-monsoon teleconnection in a warming climate VL - 6 ER - TY - JOUR AB - Cumulus parameterization (CP) in state‐of‐the‐art global climate models is based on the quasi‐equilibrium assumption (QEA), which views convection as the action of an ensemble of cumulus clouds, in a state of equilibrium with respect to a slowly varying atmospheric state. This view is not compatible with the organization and dynamical interactions across multiple scales of cloud systems in the tropics and progress in this research area was slow over decades despite the widely recognized major shortcomings. Novel ideas on how to represent key physical processes of moist convection‐large‐scale interaction to overcome the QEA have surged recently. The stochastic multicloud model (SMCM) CP in particular mimics the dynamical interactions of multiple cloud types that characterize organized tropical convection. Here, the SMCM is used to modify the Zhang‐McFarlane (ZM) CP by changing the way in which the bulk mass flux and bulk entrainment and detrainment rates are calculated. This is done by introducing a stochastic ensemble of plumes characterized by randomly varying detrainment level distributions based on the cloud area fraction of the SMCM. The SMCM is here extended to include shallow cumulus clouds resulting in a unified shallow‐deep CP. The new stochastic multicloud plume CP is validated against the control ZM scheme in the context of the single column Community Climate Model of the National Center for Atmospheric Research using data from both tropical ocean and midlatitude land convection. Some key features of the SMCM CP such as it capability to represent the tri‐modal nature of organized convection are emphasized. AU - Khouider, B. AU - GOSWAMI, BIDYUT B AU - Phani, R. AU - Majda, A. J. ID - 14564 IS - 11 JF - Journal of Advances in Modeling Earth Systems KW - General Earth and Planetary Sciences KW - Environmental Chemistry KW - Global and Planetary Change TI - A shallow‐deep unified stochastic mass flux cumulus parameterization in the single column community climate model VL - 15 ER - TY - JOUR AB - 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. AU - Abramian, Sophie AU - Muller, Caroline J AU - Risi, Camille ID - 14453 IS - 10 JF - Journal of Advances in Modeling Earth Systems TI - Extreme precipitation in tropical squall lines VL - 15 ER - TY - JOUR AB - 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. AU - Fildier, B. AU - Muller, Caroline J AU - Pincus, R. AU - Fueglistaler, S. ID - 14752 IS - 3 JF - AGU Advances KW - General Earth and Planetary Sciences TI - How moisture shapes low‐level radiative cooling in subsidence regimes VL - 4 ER - TY - JOUR AB - Through a combination of idealized simulations and real-world data, researchers are uncovering how internal feedbacks and large-scale motions influence cloud dynamics. AU - Muller, Caroline J AU - Abramian, Sophie ID - 14773 IS - 5 JF - Physics Today KW - General Physics and Astronomy SN - 0031-9228 TI - The cloud dynamics of convective storm systems VL - 76 ER - TY - CHAP AB - 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. AU - Haerter, Jan O. AU - Muller, Caroline J ED - Sullivan, Sylvia ED - Hoose, Corinna ID - 14853 SN - 2328-8779 T2 - Clouds and Their Climatic Impacts TI - Mechanisms for the Self‐Organization of Tropical Deep Convection ER - TY - GEN AU - Stöllner, Andrea AU - Lenton, Isaac C AU - Muller, Caroline J AU - Waitukaitis, Scott R ID - 14864 T2 - EGU General Assembly 2023 TI - Measuring spontaneous charging of single aerosol particles ER - TY - GEN AU - Polesello, Andrea AU - Muller, Caroline J AU - Pasquero, Claudia AU - Meroni, Agostino N. ID - 14863 T2 - EGU General Assembly 2023 TI - Intensification mechanisms of tropical cyclones ER - TY - GEN AU - Hwong, Yi-Ling AU - Colin, Maxime AU - Aglas, Philipp AU - Muller, Caroline J AU - Sherwood, Steven ID - 14865 T2 - EGU General Assembly 2023 TI - Evaluating memory properties in convection schemes using idealised tests ER - TY - GEN AU - Abramian, Sophie AU - Muller, Caroline J AU - Risi, Camille ID - 14866 T2 - EGU General Assembly 2023 TI - Extreme precipitation in tropical squall lines ER - TY - JOUR AB - 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. AU - Hwong, Yi-Ling AU - Colin, M. AU - Aglas, Philipp AU - Muller, Caroline J AU - Sherwood, S. C. ID - 14654 IS - 12 JF - Journal of Advances in Modeling Earth Systems TI - Assessing memory in convection schemes using idealized tests VL - 15 ER - TY - GEN AB - 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). AU - Hwong, Yi-Ling AU - Colin, Maxime AU - Aglas, Philipp AU - Muller, Caroline J AU - Sherwood, Steven C. ID - 14991 TI - Data-assessing memory in convection schemes using idealized tests ER - TY - JOUR AB - 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. AU - Abramian, Sophie AU - Muller, Caroline J AU - Risi, Camille ID - 10653 IS - 1 JF - Geophysical Research Letters SN - 0094-8276 TI - Shear-convection interactions and orientation of tropical squall lines VL - 49 ER - TY - JOUR AB - 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. AU - Roca, Rémy AU - De Meyer, Victorien AU - Muller, Caroline J ID - 12107 IS - 24 JF - Geophysical Research Letters SN - 0094-8276 TI - Precipitating fraction, not intensity, explains extreme coarse-grained precipitation Clausius-Clapeyron scaling with sea surface temperature over tropical oceans VL - 49 ER -