Within the atmospheric modelling community, a large focus in recent years has been on the concept of Convective Self-Aggregation (CSA): In an environment of radiative convective equilibrium, with homogeneous initial conditions and a constant-temperature tropical sea surface, convection can spontaneously aggregate into domain-wide patterns of persistent dry areas and constrained rainy areas over a temporal timescale of weeks to months. CSA, albeit still a modeling paradigm, could reveal the mechanisms behind some of the convective organization observed in the tropics.

This process of forming domain-wide structure can be accelerated to the order of days by imposing oscillating surface temperatures with a large enough amplitude [1]. The ‘diurnally aggregated’ cloud field is similar to CSA as it also constrains the surface rain field to certain parts of the domain. Further, pattern formation was found to initiate first as persistent dry patches in the uppermost layers of the simulated atmosphere. The dry patches subsequently penetrate through to the subcloud layer [2].

In this work we investigate how diurnal surface temperature amplitudes, typical of tropical land, affect the formation of persistent dry patches and the spatio-temporal extent of the emergent mesoscale convective systems. We run a set of cloud resolving simulations initialized with typical profiles of temperature and humidity. We impose a large-amplitude diurnally oscillating surface temperature, which we then set to constant at different times, to see the effect on the diurnally aggregated cloud field. We present the results of this study, which show a strong dependence on the degree of aggregation over ‘land’, in determining the aggregation over ‘sea’, and a form of hysteresis arises.

1. Haerter, Jan O., Bettina Meyer, and Silas Boye Nissen. ‘Diurnal Self-Aggregation’. Npj Climate and Atmospheric Science 3, no. 1 (30 July 2020): 1–11. https://doi.org/10.1038/s41612-020-00132-z.
2. Jensen, Gorm G., Romain Fiévet, and Jan O. Haerter. ‘The Diurnal Path to Persistent Convective Self-Aggregation’. Journal of Advances in Modeling Earth Systems 14, no. 5 (2022): e2021MS002923. https://doi.org/10.1029/2021MS002923.

How to cite: Kruse, I. L. and Haerter, J. O.: Investigating Convective Self-Aggregation in the Transition from Land to Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6933, https://doi.org/10.5194/egusphere-egu23-6933, 2023.

Squall lines are the consequence of the interaction of low-level shear with cold pools associated with convective downdrafts. Beyond a critical 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 & Fiolleau 2020 show that long lived mesoscale convective systems, including squall lines, are disproportionately involved in rainfall extremes in the tropics. One may then question whether the orientation of squall lines has an impact on rainfall extremes, and if so, why.

Using a cloud-resolving model, we perform idealized simulations of tropical squall lines by imposing a vertical wind shear in radiative-convective equilibrium. Our results show that precipitation extremes in squall lines are 40% more intense in the critical case and remain 30% superior in the supercritical regime. With a theoretical scaling of precipitation extremes (Muller & Takayabu 2019), we show that the condensation rates control the amplification of precipitation extremes in tropical squall lines, mainly due to its dynamic component. The critical case is not only optimal for squall line orientation, but also for the cloud base velocity intensity of new convective cells.

How to cite: Abramian, S., Muller, C., and Risi, C.: Extreme Precipitation in Tropical Squall Lines, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15870, https://doi.org/10.5194/egusphere-egu23-15870, 2023.

Radiative-convective equilibrium is the simplest possible way to phrase many questions about a deep-convecting atmosphere and is accessible by a wide range of model types. The radiative-convective equilibrium project (RCEMIP) provides a common configuration, but reveals a large spread in the simulated climate across models, including profiles of temperature and relative humidity. Here we use simple models and theory to understand the intermodel spread in CAPE, relative humidity, and their responses to warming.

Across the RCEMIP ensemble, temperature profiles are systematically cooler than a moist adiabat, consistent with theory that they are set by dilute ascent. As horizontal grid spacing is reduced in models with explicit convection from 1 km to 200 m, CAPE and relative humidity increase. Across all models, CAPE increases with warming at a rate (14-19%/K) greater than that expected from the Clausius-Clapeyron relation. We find that there is higher CAPE (greater instability) in models that are on average moister in the mid-troposphere, which is consistent with the simple plume model of Romps (2016) in which both instability and relative humidity depend on entrainment and precipitation efficiency. The sign of the relationship suggests that differences in entrainment drive the intermodel spread. This relationship is true across both models with explicit and parameterized convection.

To more explicitly evaluate the drivers of the intermodel spread, we use the Romps (2016) model to diagnose theory-implied values of entrainment and precipitation efficiency given the simulated values of CAPE and relative humidity. We then decompose the the variability across models in CAPE and relative humidity (and their responses to warming) into contributions from entrainment, precipitation efficiency, and the depth of the convecting layer. Targeted microphysics parameter perturbation experiments with an individual cloud-resolving model in which precipitation efficiency is varied and explicitly diagnosed provide proof of concept for this decomposition technique. 

How to cite: Wing, A. and Singh, M.: Control of Tropical Stability and Relative Humidity in Radiative-Convective Equilibrium Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3532, https://doi.org/10.5194/egusphere-egu23-3532, 2023.

Change of the intertropical convergence zone (ITCZ) with global warming has important consequences for the regulation of the tropical climate and for future precipitation projections. Most of the volume of the ITCZ is filled with ice associated with convective anvils, which opens the perspective of using the response of ice clouds to study changes of the tropical convergence zones with global warming. Past studies have shown a decrease of tropical high-cloud fraction with surface warming, whereby the response of anvil clouds is used to interpret the response of ice clouds as a whole. However, tropical clouds organize over a very wide range of scales, meaning that the response of ice clouds is more complex. In particular, precipitating deep convection may represent a small volume of total cloudiness, but it concentrates most of the ascending motion in the tropics and is therefore of crucial importance for the dynamics. Here we show how the high resolution in next generation convection-resolving climate models and in observations can be leveraged to directly measure the response of precipitating deep convection with surface warming in the ice signal. For this purpose, we use the first year-long simulations of global warming ever performed with a Global Storm Resolving Model (GSRM) at 3 km resolution. These simulations use the eXperimental System for High-resolution prediction on Earth-to-Local Domains (X-SHiELD), developed at the Geophysical Fluid Dynamics Laboratory (GFDL). By tracking the response of tropical clouds to surface warming from the response of ice water path (IWP), the vertical integral of ice mixing ratio, we show that the response of precipitating deep convection can be identified at high resolution and that this response, marked by an increase in frequency of very deep convective cores and a decrease in frequency of more moderate convection, is robust in model and active sensor observations. We discuss this result and show how it promotes a simple view of the changes of tropical convergence zones in ice-based coordinates.

How to cite: Bolot, M., Harris, L., Cheng, K.-Y., Blossey, P., Bretherton, C., Clark, S., Kaltenbaugh, A., Merlis, T., Zhou, L., and Fueglistaler, S.: New GSRM global warming simulations and active sensors reveal robust changes of tropical convergence zones in cloud ice space, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4150, https://doi.org/10.5194/egusphere-egu23-4150, 2023.

The convective mass flux within tropical convection influences the large-scale circulation, drives cloud radiative forcing, has integral links to the production of fresh water, and impacts extreme weather. CMF forms the focus of the recently selected Investigation of Convective Updrafts (INCUS) mission to be launched in 2026. This NASA mission is comprised of 3 spacecraft, all of which will carry a Ka-band cloud radar. One spacecraft will also carry a passive microwave radiometer. The 3 smallsats are to be separated by time intervals of 30, 90 and 120 seconds, thus allowing for the rapid and systematic sampling of the same storm with all three spacecraft. These time intervals (delta-ts) also facilitate the investigation of the magnitude and evolution of CMF, which will be examined as a function of storm type, storm lifecycle and environmental properties. INCUS will therefore provide the first global systematic investigation into CMF and its evolution within deep tropical convection.

A wide range of research tasks have been conducted in preparation for the INCUS mission and the development of the INCUS algorithms including: (1) running and analyzing extensive suites of large-domain, high-resolution model simulations; (2) examining ground-based Doppler radar observations obtained using adaptive scanning techniques during several recent field campaigns; and (3) evaluating anvil characteristics using passive microwave radiometer and geoIR data. This talk will focus on three specific highlights arising from these modeling and observational analyses. First, we will examine the temporal scales of updraft variability. Second, we will analyze the relationship between ice water path cores and convective updrafts. Finally, we will demonstrate proof of the INCUS delta-t concept linking changes in reflectivity to CMF through the use of ground-based radar analyses.

How to cite: van den Heever, S., Haddad, Z., Dolan, B., Freeman, S., Grant, L., Kollias, P., Leung, G., Luo, J., Marinescu, P., Posselt, D., Rasmussen, K., Sai, P., Schulte, R., Stephens, G., Storer, R., and Takahashi, H.: Tropical Convection through the Lens of the INCUS Mission, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11285, https://doi.org/10.5194/egusphere-egu23-11285, 2023.

The vertical structure of large-scale tropical circulations is determined by their complex coupling with both deep and shallow convection. This is reflected in our theoretical frameworks for understanding the tropical precipitation distribution, which consist of “deep” theories that focus on overturning circulations that extend throughout the depth of the troposphere and “shallow” theories that focus on low-level convergence driven by boundary-layer pressure gradients. While both types of theories suggest links between low-level thermodynamic fields and the precipitation distribution, shallow theories highlight the importance of the distribution of surface temperature, while deep theories additionally highlight the importance of the low-level humidity. 

Here we use idealised cloud-permitting simulations to elucidate the physical factors that control the vertical structure of tropical circulations. We first demonstrate how the influence of convective entrainment on the lapse rate can act to change the vertical structure of deep tropical circulations, with implications for the behaviour of precipitation in the current and future climate. We further investigate the interaction between deep and shallow tropical circulations  by simulating an idealised overturning circulation over varying surface conditions. By independently varying the sea-surface temperature and moisture availability, the low-level temperature and moisture distributions are manipulated such that the predictions of “deep" and “shallow" theories of the circulation may be distinguished. The results provide insight into the relative roles of oceanic SST gradients and land-ocean contrasts in determining the climatological precipitation distribution in the tropics.

How to cite: Singh, M.: Shallow and Deep Circulations in the Tropical Atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3078, https://doi.org/10.5194/egusphere-egu23-3078, 2023.

We investigate microphysical uncertainties in hailstorms using statistical emulation in a single model framework with the objective to disentangle the relative contributions from aerosols, microphysical parameters and environmental conditions to the uncertainty in cloud-, precipitation- and hail-related parameters.

Our selected case study is the Andreas hailstorm on 28 July 2013 in the Neckar Valley and over the Swabian Jura in Southwest Germany. We perform model simulations on cloud-resolving scale with the numerical weather prediction model ICON coupled with the aerosol module ART (ICON-ART). We use a two-moment cloud microphysics scheme with a representation of ice nucleation by dust aerosols.
We generated a perturbed parameter ensemble (PPE) to sample uncertainties in cloud-, precipitation- and hail related parameters. Six parameters from the categories aerosols, microphysics and environmental conditions were jointly perturbed, namely the cloud condensation nuclei (CCN) and ice nuclei (IN) concentrations, the riming efficiency of graupel and hail, the convective available potential energy (CAPE) and vertical wind shear. The defined parameter ranges are based on forecast analysis and literature. We used the maximin Latin hypercube algorithm to distribute the parameters well-spaced in the six-dimensional parameter uncertainty space. For these six parameters, an ensemble of 90 members was generated and in addition a smaller independent ensemble of 45 members serves for validation.

We used the Gaussian process emulation and developed emulators for hail- and precipitation related output variables. To quantify contributions to the uncertainty in the output variables from the perturbed parameters individually as well as interactions between them, a variance-based sensitivity analysis was performed. We will present first results, which reveal the importance of the CCN concentration for controlling the number concentration of hail particles as well as the CCN concentration and environmental conditions for controlling the amount of hail and precipitation in the model. The geographical distribution of hail and precipitation shows a large variety among the ensemble members, with storm tracks shifted further to the north or south compared to the reference simulation. The path of the storm track is thereby mainly controlled by CAPE and the vertical wind shear, however, aerosol parameters seem to be important for the development of multiple storm tracks. 

How to cite: Frey, L., Hoose, C., Kunz, M., Miltenberger, A., and Kuntze, P.: Using statistical emulation to quantify microphysical uncertainties for the Andreas hailstorm in 2013, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14317, https://doi.org/10.5194/egusphere-egu23-14317, 2023.

This study focuses on the deep-inflow mixing features of the orographically locked diurnal convection, involving interactions between local circulation and the thermodynamic environment of the convection. Under the weak synoptic weather regime, orographically locked diurnal convection is a typical summertime phenomenon in Taiwan, a tropical island in the Asian monsoon region. Numerical simulations are carried out using the vector vorticity equation model with high-resolution Taiwan topography (TaiwanVVM), which can appropriately simulate the characteristics of diurnal convection and the evolution of boundary layer and local circulation. The semi-realistic approach, simplified by observed soundings as the uniform initial condition over the entire domain, emphasizes the decisive environmental factors that modulate the development of convection, representing the variability of the background environment by the ensembles. The analyses by the deep-inflow mixing framework, including the locally-derived convective structures and the upstream moist static energy (MSE) transport, improve the understanding of the interactive physical processes in the boundary layer development and local circulation evolution of orographically locked diurnal convection over complex topography. The convective structures of the deep-inflow mixing, increasing vertical velocity and convective mass flux with height through a deep lower-tropospheric inflow layer, are found in strong convective updraft columns within heavily-precipitating systems over precipitation hotspots. While the topography constrains the location of the convection, enhanced convective development is associated with higher upstream MSE transport through this deep-inflow layer via local circulation, augmenting the rain rate by 35% in precipitation hotspots. The results highlight the importance of non-local dynamical entrainment of the deep-inflow, transporting MSE via local circulation to supply the growth of orographically locked diurnal convection. Thus, the deep-inflow mixing framework can serve as the theoretical basis for describing the orographic locking feature of diurnal convection over complex topography. Guided by the simulations, the Storm Tracker mini-radiosondes are released upstream of the precipitation hotspot, targeting observations within the most common deep-inflow path. Initial field measurements support the presence of high MSE transport within the deep-inflow layer when organized convection occurs at the precipitation hotspot.

How to cite: Chang, Y.-H., Chen, W.-T., Wu, C.-M., Kuo, Y.-H., and Neelin, J. D.: Identifying the Deep-inflow Mixing Features in Orographically Locked Diurnal Convection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4079, https://doi.org/10.5194/egusphere-egu23-4079, 2023.

The initiation of deep convection in a diurnal cycle is still one of the most important uncertainties in a climate numerical model. This is partially due to our poor understanding of the physical mechanisms leading to the transition from shallow to deep convection. In this work, we discuss the role of shallow cumulus clouds in the initiation of deep convection. By using a simple entraining plume model, we show that the interaction between an active and a passive shallow cumulus helps the former to reach higher altitudes and, in the right conditions, may initiate deep convection. It is also shown that the organization of passive and active clouds due to the formation of cold pools may act as a positive feedback. Furthermore, based on the proposed mechanism, a stochastic triggering function is derived, which can be implemented in climate models. As an important feature, the stochastic function is scale-aware, which makes it suitable for simulations at the gray-zone.

How to cite: Vraciu, C.-V.: The role of passive shallow cumuli in the transition from shallow to deep convection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3561, https://doi.org/10.5194/egusphere-egu23-3561, 2023.