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.
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This study investigates the diurnal cycle of tropical organized deep convection and the feedback in large-scale circulation. By considering gravity wave phase speeds, we find that the circulation ...adjustment into weak temperature gradient (WTG) balance occurs rapidly (<6 h) relative to diurnal diabatic forcing on the spatial scales typical of organized convection (≤500 km). Convection-permitting numerical simulations of self-aggregation in diurnal radiative–convective equilibrium (RCE) are conducted to explore this further. These simulations depict a pronounced diurnal cycle of circulation linked to organized convection, which indeed maintains WTG balance to first order. A set of sensitivity experiments is conducted to assess what governs the diurnal cycle of organized convection. We find that the “direct radiation–convection interaction” (or lapse-rate) mechanism is of primary importance for diurnal precipitation range, while the “dynamic cloudy–clear differential radiation” mechanism amplifies the range by approximately 30%, and delays the nocturnal precipitation peak by around 5 h. The differential radiation mechanism therefore explains the tendency for tropical heavy rainfall to peak in the early morning, while the lapse-rate mechanism primarily governs diurnal amplitude. The diurnal evolution of circulation can be understood as follows. While nocturnal deep convection invigorated by cloud-top cooling (i.e., the lapse-rate mechanism) leads to strong bottom-heavy circulation at nighttime, the localized (i.e., differential) top-heavy shortwave warming in the convective region invigorates circulation at upper levels in daytime. A diurnal evolution of the circulation therefore arises, from bottom heavy at nighttime to top heavy in daytime, in a qualitatively consistent manner with the observed diurnal pulsing of the Hadley cell driven by the ITCZ.
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What controls the variability of daily precipitation averaged over the tropics? Are these the most numerous precipitation rates or the most intense ones? And do they relate to a specific cloud type? ...This work addresses these questions using precipitation from the one-year simulation of the global-coupled storm-resolving ICOsahedral Non-hydrostatic model run in its Sapphire configuration (ICON-Sapphire) and observations. Moreover, we develop a framework to analyze the precipitation variability based on the area covered by and the mean intensity of different groups of precipitation rates. Our framework shows that 60 % of the precipitation variability is explained by precipitation rates between 20 and 70 mm d−1, but those precipitation rates only explain 46 % of the mean precipitation in the tropics. The decomposition of the precipitation variability into the area fraction and mean intensity of a set of precipitation rates shows that this variability is explained by changes in the area fraction covered by precipitation rates between 20 and 70 mm d−1, not by changes in the mean intensity. These changes in the area fraction result from changes in the area covered by congestus clouds, not by cumulonimbus or shallow clouds, even though congesti and cumulonimbi contribute equally to the mean tropical precipitation. Overall, ICON-Sapphire reproduces the probability density function of precipitation rates and the control of specific precipitation rates on the tropical mean precipitation and variability compared to observations.
Motivated by the results of idealized studies on the self‐aggregation of convection, we investigate a potential relationship between the degree of organization of the Intertropical Convergence Zone ...(ITCZ) and humidity based on reanalysis data. We focus on the Atlantic ITCZ and use the number of long convergence lines occurring per month to define the degree of organization. The latter shows a weak enhancement during June to August (JJA) and a large interannual variability. On an interannual time scale and particularly during JJA, a relationship exists between organization and humidity: Years with more organized ITCZs are associated with a moister ITCZ region and drier subtropics. Even though we cannot demonstrate any causality and cannot rule out the presence of another agent, we show that these moisture anomalies are not incompatible with an effect of organization. We also note that the annual cycle in sea surface temperature (SST) gradient may contribute to the intra‐annual variability in organization.
Plain Language Summary
The large‐scale organization of individual convective cells into a coherent structure over the tropics, the Intertropical Convergence Zone (ITCZ), is an easily recognizable feature of satellite imagery. At times, the ITCZ appears well organized, forming a long line, whereas at other times, it appears much more scattered and made of disconnected convective clusters. We investigate the hypothesis, derived from previous idealized studies, that a relationship exists between organization and atmospheric humidity. We quantify organization by counting the number of times per month the ITCZ is organized in a long line. The study shows for the first time that, indeed, a relationship exists between the large‐scale degree of convective organization and humidity in the real world: Years with a more organized ITCZ are associated with drier subtropics and a moister ITCZ region. Given the importance of subtropical humidity for the energy budget of the climate system, our results ask for more studies investigating this relationship and its causality.
Key Points
The large‐scale organization of the Atlantic ITCZ is quantified using a 35‐year data set of convergence lines
Relationships derived from reanalysis data between convective organization and humidity are consistent with results from idealized studies
Years with a more organized ITCZ structure are associated with drier conditions in the subtropics
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A determination of the sign and magnitude of the soil moisture–precipitation feedback relies either on observations, where synoptic variability is difficult to isolate, or on model simulations, which ...suffer from biases mainly related to poorly resolved convection. In this study, a large-eddy simulation model with a resolution of 250m is coupled to a land surface model and several idealized experiments mimicking the full diurnal cycle of convection are performed, starting from different spatially homogeneous soil moisture conditions. The goal is to determine under which conditions drier soils may produce more precipitation than wetter ones. The methodology of previous conceptual studies that have quantified the likelihood of convection to be triggered over wet or dry soils is followed but includes the production of precipitation. Although convection can be triggered earlier over dry soils than over wet soils under certain atmospheric conditions, total precipitation is found to always decrease over dry soils. By splitting the total precipitation into its magnitude and duration component, it is found that the magnitude strongly correlates with surface latent heat flux, hence implying a wet soil advantage. Because of this strong scaling, changes in precipitation duration caused by differences in convection triggering are not able to overcompensate for the lack of evaporation over dry soils. These results are further validated using two additional atmospheric soundings and a series of perturbed experiments that consider cloud radiative effects, as well as the effect of large-scale forcing, winds, and plants on the soil moisture–precipitation coupling.
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This study investigates the interplay between atmospheric moisture and deep convective clouds via cold‐pool dynamics in the absence of large‐scale forcing in a series of cloud‐resolving modelling ...studies. More specifically, the contribution of moisture advection, evaporation of rain and surface fluxes to the moisture budget over particular regions of the domain is investigated. This is done both for a continental case and an oceanic case although both cases show very similar behaviour. The accumulation of moisture in confined regions of the sub‐cloud layer that constitute preferred locations for future cloud development mainly results from the advection of moisture. The latter contributes ~86%, minor evaporation of precipitation contributes ~4%, whereas surface moisture fluxes yield ~11% in the continental case. In the oceanic case advection contributes ~125%, surface moisture fluxes ~−32% and evaporation of precipitation ~7%.
To further identify the origin of the advected moisture, additional scalars marking moisture originating from the surface and from the evaporation of rain are introduced into the model. It is seen that the surface moisture and the evaporated rain water released within the last 2 h only make 55% of the moisture accumulated in the moist patches in the land case, the rest stemming from older moisture. In the ocean case, this share increases to 72%. The contribution of recently released moisture drops to 28% at cloud base in the continental case and to 56% in the ocean case. The contribution at cloud base is dominated by surface fluxes; the evaporation of rain is negligible.
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Storm-resolving simulations where deep convection can be explicitly resolved are performed in the idealized radiative-convective equilibrium framework to explore the climatic role of interactive leaf ...phenology. By initializing the system with different initial soil moisture and leaf area index (LAI) conditions, we find three categories of potential equilibrium climatic and vegetation states: a hot desert planet without vegetation, an intermediate sparsely vegetated planet, and a wet fully vegetated planet. The wet fully vegetated equilibrium category occurs over the widest range of initial soil moisture as it occurs as soon as soil saturation is 19% higher than the permanent wilting point (35%). This indicates that a quite harsh environment is needed in our modeling system to force leaves to be shed. The attained equilibrium states are only dependent upon the initial soil moisture, not the initial LAI. However, interactive leaves do allow an earlier transition from the intermediate to the wet vegetated equilibrium category. Hence, interactive leaves make the vegetation-atmosphere system more stable and more resilient to drying. This effect could be well approximated by just prescribing the LAI to its maximum value. Finally, our sensitivity experiments reveal that leaves influence the climate equally through their controls on canopy conductance and vegetation cover, whereas albedo changes play a negligible role.
Abstract
A conceptual bulk model for a dry, convective boundary layer with prescribed horizontally homogeneous and heterogeneous low-level radiative cooling rates is developed. For horizontally ...homogeneous radiative cooling, the response of the system to varying its prescribed parameters is explored and formulated in terms of nondimensional parameters. Large-eddy simulations with prescribed radiative cooling rates match the results of the bulk model well. It is found that, depending on the strength of the surface coupling, the height of the boundary layer (BL) either increases or decreases in response to increasing radiative BL cooling. Another property of the system is that, for increasing surface temperature, the BL temperature decreases if the prescribed radiative BL cooling rates are strong. This counterintuitive behavior is caused by the formulation of the entrainment rate at the inversion. Heterogeneous radiative BL cooling is found to cause a circulation induced by pressure deviations between the area of weak radiative BL cooling and the area of strong radiative BL cooling. Including the feedback of the induced circulation on the BL in a two-column model leads to a modified equilibrium state, in which a weakened horizontal BL flow of about 1 m s−1 is maintained for differences in radiative BL cooling rates larger than 1 K day−1. Such a circulation strength is comparable to a shallow circulation caused by surface temperature differences of a few kelvins. Spatial differences in radiative BL cooling should therefore be considered as a first-order effect for the formation of shallow circulations.
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An activity designed to characterise patterns of mesoscale (20 to 2,000 km) organisation of shallow clouds in the downstream trades is described. Patterns of mesoscale organisation observed from ...space were subjectively defined and learned by 12 trained scientists. The ability of individuals to communicate, learn and replicate the classification was evaluated. Nine‐hundred satellite images spanning the area from 48°W to 58°W, 10°N to 20°N for the boreal winter months (December–February) over 10 years (2007/2008 to 2016/2017) were classified. Each scene was independently labelled by six scientists as being dominated by one of six patterns (one of which was “no‐pattern”). Four patterns of mesoscale organisation could be labelled in a reproducible manner, and were labelled Sugar, Gravel, Fish and Flowers. Sugar consists of small, low clouds of low reflectivity, Gravel clouds form along apparent gust fronts, Fish are skeletal networks (often fishbone‐like) of clouds, while Flowers are circular clumped features defined more by their stratiform cloud elements. Both Fish and Flowers are surrounded by large areas of clear air. These four named patterns were identified 40% of the time, with the most common pattern being Gravel. Sugar was identified the least and suggests that unorganised and very shallow convection is unlikely to dominate large areas of the downstream trade winds. Some of the patterns show signs of seasonal and interannual variability, and some degree of scale selectivity. Comparison of typical patterns with radar imagery suggests that even this subjective and qualitative visual inspection of imagery appears to capture several important physical differences between shallow cloud regimes, such as precipitation and radiative effects.
Sugar: MODIS‐Aqua scenes from Worldview. The images cover the area from 60°W to 48°W and 10°N to 20°N. For these images the scenes have been extended to the west to include Barbados, coloured in artificial green, on the far left. For a sense of scale, Barbados fits in a rectangle of east–west dimension of 25 km and north–south dimension of 30 km. Depending on the quality of the reproduction, some features distinguishing these from other patterns may be difficult to discern from printed (rather than electronic) renditions of this article. From left to right the images correspond to 31 December 2014, 5 December 2015 and 20 January 2016.
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The relationship between mesoscale convective organization, quantified by the spatial arrangement of convection, and oceanic precipitation in the tropical belt is examined using the output of a ...global storm‐resolving simulation. The analysis uses a 2D watershed segmentation algorithm based on local precipitation maxima to isolate individual precipitation cells and derive their properties. 10° by 10° scenes are analyzed using a phase‐space representation made of the number of cells per scene and the mean area of the cells per scene to understand the controls on the spatial arrangement of convection and its precipitation. The presence of few and large cells in a scene indicates the presence of a more clustered distribution of cells, whereas many small cells in a scene tend to be randomly distributed. In general, the degree of clustering of a scene (Iorg) is positively correlated to the mean area of the cells and negatively correlated to the number of cells. Strikingly, the degree of clustering, whether the cells are randomly distributed or closely spaced, to a first order does not matter for the precipitation amounts produced. Scenes of similar precipitation amounts appear as hyperbolae in our phase‐space representation, hyperbolae that follow the contours of the precipitating area fraction. Finally, including the scene‐averaged water vapour path (WVP) in our phase‐space analysis reveals that scenes with larger WVP contain more cells than drier scenes, whereas the mean area of the cells only weakly varies with WVP. Dry scenes can contain both small and large cells, but they can contain only few cells of each category.
Convection often appears organized in satellite imagery which raises the question as to the importance of organization for climate. Here we found that convection does not appear to use organization to rain more. We also investigated the relationship between precipitation, moisture and precipitating area fraction and found that convection rains more in a moister atmosphere, not because the area of its cells increases, but because it triggers more convective cells.
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