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Abstract A simple analytical model, the zero‐buoyancy plume (ZBP) model, has been proposed to understand how small‐scale processes such as plume‐environment mixing and evaporation affect the steady‐state structure of the atmosphere. In this study, we refine the ZBP model to achieve self‐consistent analytical solutions for convective mass flux, addressing the inconsistencies in previous solutions. Our refined ZBP model reveals that increasing plume‐environment mixing can increase upper‐troposphere mass flux through two pathways: increased cloud evaporation or reduced atmospheric stability. To validate these findings, we conducted small‐domain convection‐permitting Radiative‐Convective Equilibrium simulations with horizontal resolutions ranging from 4 km to 125 m. As a proxy for plume‐environment mixing strength, the diagnosed entrainment rate increases with finer resolution. Consistent with a previous study, we observed that both anvil cloud fraction and upper‐troposphere mass flux increase with higher resolution. Analysis of the clear‐sky energy balance in the simulations with two different microphysics schemes identified both pathways proposed by the ZBP model. The dominant pathway depends on the relative strengths of evaporation cooling and radiative cooling in the environment. Our work provides a refined simple framework for understanding the interaction between small‐scale convective processes and large‐scale atmospheric structure. 

Abstract Studying convection, which is one of the least understood physical mechanisms in the tropical atmosphere, is very important for weather and climate predictions of extreme events such as storms, hurricanes, monsoons, floods and hail. Collecting more observations to do so is critical. It is also a challenge. The OTREC (Organization of Tropical East Pacific Convection) field project took place in the summer of 2019. More than thirty scientists and twenty students from the US, Costa Rica, Colombia, México and UK were involved in collecting observations over the ocean (East Pacific and Caribbean) and land (Costa Rica, Colombia). We used the NSF NCAR Gulfstream V airplane to fly at 13 kilometers altitude sampling the tropical atmosphere under diverse weather conditions. The plane was flown in a ‘lawnmower’ pattern and every 10 minutes deployed dropsondes that measured temperature, wind, humidity and pressure from flight level to the ocean. Similarly, over the land we launched radiosondes, leveraged existing radars and surface meteorological networks across the region, some with co-located Global Positioning System (GPS) receivers and rain sensors, and installed a new surface GPS meteorological network across Costa Rica, culminating in an impressive systematic data set that when assimilated into weather models immediately gave better forecasts. We are now closer than ever in understanding the environmental conditions necessary for convection as well as how convection influences extreme events. The OTREC data set continues to be studied by researchers all over the globe. This article aims to describe the lengthy process that precedes science breakthroughs. 

Key Points Significant sensitivity of mid‐latitude deep convective storm and the associated anvil cirrus cloud to choice of model microphysics schemes Hydrometeor size‐dependent microphysical process are linked with large variability in storm dynamics Six bulk microphysics schemes produced an order of magnitude spread in above‐tropopause water vapor concentrations 

Abstract. Cirrus clouds that form in the tropical tropopause layer(TTL) can play a key role in vertical transport through the uppertroposphere and lower stratosphere, which can significantly impact theradiative energy budget and stratospheric chemistry. However, the lack ofrealistic representation of natural ice cloud habits in microphysicalparameterizations can lead to uncertainties in cloud-related processes andcloud–climate feedbacks. The main goal of this study is to investigate therole of different cloud regimes and the associated ice habits in regulatingthe properties of the TTL. We compare aircraft measurements from theStratoClim field campaign to a set of numerical experiments at the scale of large-eddy simulations (LESs) for the same case study that employ differentmicrophysics schemes. Aircraft measurements over the southern slopes of theHimalayas captured high ice water content (HIWC) up to 2400 ppmv and iceparticle aggregates exceeding 700 µm in size with unusually longresidence times. The observed ice particles were mainly of liquid origin,with a small amount formed in situ. The corresponding profile of ice water content (IWC) fromthe ERA5 reanalysis corroborates the presence of HIWC detrained from deep-convective plumes in the TTL but underestimates HIWC by an order ofmagnitude. In the TTL, only the scheme that predicts ice habits canreproduce the observed HIWC, ice number concentration, and bimodal iceparticle size distribution. The lower range of particle sizes is mostlyrepresented by planar and columnar habits, while the upper range isdominated by aggregates. Large aggregates with sizes between 600 and 800 µm have fall speeds of less than 20 cm s−1, which explains thelong residence time of the aggregates in the TTL. Planar ice particles ofliquid origin contribute substantially to HIWC. The columnar and aggregatehabits are in the in situ range with lower IWC and number concentrations. Forall habits, the ice number concentration increases with decreasingtemperature. For the planar ice habit, relative humidity is inverselycorrelated with fall speed. This correlation is less evident for the othertwo ice habits. In the lower range of supersaturation with respect to ice,the columnar habit has the highest fall speed. The difference in ice numberconcentration across habits can be up to 4 orders of magnitude, withaggregates occurring in much smaller numbers. We demonstrate and quantifythe linear relationship between the differential sedimentation of pristineice crystals and the size of the aggregates that form when pristine crystalscollide. The slope of this relationship depends on which pristine ice habitsediments faster. Each simulated ice habit is associated with distinctradiative and latent heating rates. This study suggests that a modelconfiguration nested down to LES scales with a microphysicalparameterization that predicts ice shape evolution is crucial to provide anaccurate representation of the microphysical properties of TTL cirrus andthus the associated (de)hydration process. 

Abstract It is still debated whether radiative heating observed in the tropical tropopause layer (TTL) is balanced primarily by cooling from convective overshoots, as in an entrainment layer, or by adiabatic cooling from large-scale eddy-driven upwelling. In this study, three-dimensional cloud-resolving model simulations of radiative-convective equilibrium were carried out with three different cloud microphysics schemes and 1-km horizontal resolution. We demonstrate that overshooting cooling in the TTL can be strongly modulated by upper-troposphere stratification. Two of the schemes produce a hard-landing scenario in which convective overshoots reach the TTL with frequent large vertical velocity leading to strong overshooting cooling (~ −0.2 K day -1 ). The third scheme produces a soft-landing scenario in which convective overshoots rarely reach the TTL with large vertical velocity and produce little overshooting cooling (~ −0.03 K day -1 ). The difference between the two scenarios is attributed to changes in the upper-troposphere stratification related to different atmospheric cloud radiative effects (ACRE). The microphysics scheme that produces the soft-landing scenario has much stronger ACRE in the upper troposphere leading to a ~3K warmer and more stable layer which acts as a buffer zone to slow down the convective updrafts. The stratification mechanism suggests the possibility for the ozone variation or eddy-driven upwelling in the TTL to modulate convective overshoots. We further test the sensitivity of overshooting cooling to changes in model resolution by increasing the horizontal resolution to 100 m. The corresponding change of overshooting cooling is much smaller compared with the difference between the hard-landing and soft-landing scenarios. 

Abstract The response of mid-latitude equilibrated eddy length scale to static stability has long been questioned but not investigated in well-controlled experiments with unchanged mean zonal wind and meridional temperature gradient. With iterative use of the linear response function of an idealized dry atmosphere, we obtain a time-invariant and zonally-uniform forcing to decrease the near-surface temperature by over 2 K while keeping the change in zonal wind negligible (within 0.2m s −1 ). In such experiments of increased static stability, energy-containing zonal scale decreases by 3–4%, which matches with Rhines scale decrease near the jet core. Changes in Rossby radius (+2%), maximum baroclinic growth scale (-1%) and Kuo scale (0%) fail to match this change in zonal scale. These findings and well-controlled experiments help with better understanding of eddy–mean flow interactions and hence the mid-latitude circulation and its response to climate change. 

Abstract Soil moisture heterogeneity can induce mesoscale circulations due to differential heating between dry and wet surfaces, which can, in turn, trigger precipitation. In this work, we conduct cloud-permitting simulations over a 100 km × 25 km idealized land surface, with the domain split equally between a wet region and a dry region, each with homogeneous soil moisture. In contrast to previous studies that prescribed initial atmospheric profiles, each simulation is run with fixed soil moisture for 100 days to allow the atmosphere to equilibrate to the given land surface rather than prescribing the initial atmospheric profile. It is then run for one additional day, allowing the soil moisture to freely vary. Soil moisture controls the resulting precipitation over the dry region through three different mechanisms: as the dry domain gets drier, (i) the mesoscale circulation strengthens, increasing water vapor convergence over the dry domain, (ii) surface evaporation declines over the dry domain, decreasing water vapor convergence over the dry domain, and (iii) precipitation efficiency declines due to increased reevaporation, meaning proportionally less water vapor over the dry domain becomes surface precipitation. We find that the third mechanism dominates when soil moisture is small in the dry domain: drier soils ultimately lead to less precipitation in the dry domain due to its impact on precipitation efficiency. This work highlights an important new mechanism by which soil moisture controls precipitation, through its impact on precipitation reevaporation and efficiency. 

Abstract Azimuthally asymmetric perturbations are important to hurricanes because they can influence the track, structure, and intensity of a hurricane. In this work, we applied space‐time spectral analysis on both dynamic and thermodynamic fields of these perturbations and found two distinct power peaks in most of the fields. We obtained the structure of each mode by first filtering the fields through a frequency‐wavenumber spectral window selected for each mode and then regressing these fields on an index based on the filtered radar reflectivity. We found that the fast‐propagating wave is dominated by perturbations near the eyewall, and its structure is similar to that of the unstable mixed vortex Rossby inertia gravity wave. The other peak corresponds to a slow‐propagating wave that has comparable perturbations in and beyond the eyewall. The slow wave has a retrograde intrinsic propagating speed and has a vertical structure that resembles that of convectively coupled waves.