Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

Stevens, Björn; Brogniez, Hélène; Kiemle, Christoph; Lacour, Jean‐Lionel; Crévoisier, Cyril; Kiliani, Johannes

doi:10.1007/s10712-017-9420-8

Cited by 71 publications

(123 citation statements)

References 50 publications

(50 reference statements)

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“…Because radiative cooling from moisture detrained by congestus creates a larger heating contrast with the warm column, subsidence also maximizes near the tops of congestus and in the cloud layer, as suggested before by Pakula and Stephens (2009) and Posselt et al (2011). Even when in real atmospheres the dry layers have a different origin, such as from intrusions of midlatitude air (Yoneyama and Parsons, 1999), the results emphasize the importance of water vapour, as reviewed in recent literature (Mapes et al, 2017;Stevens et al, 2017), and its potential influence in setting convective tops not only through entrainment processes.…”

Section: Discussionsupporting

confidence: 73%

Congestus modes in circulating equilibria of the tropical atmosphere in a two‐column model

Nuijens

Emanuel

2018

Quart J Royal Meteoro Soc

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A two‐column radiative–convective equilibrium (RCE) model is used to study the depth of convection that develops in the subsiding branch of a Walker‐like overturning circulation. The model numerically solves for two‐dimensional non‐rotating hydrostatic flow, which is damped by momentum diffusion in the boundary layer and model interior, and by convective momentum transport. Convection, clouds and radiative transfer are parametrized, and the convection scheme does not include explicit freezing or melting. While integrating the model towards local RCE, the level of neutral buoyancy (LNB) fluctuates between mid‐ and high levels. Evaporation of detrained moisture at the LNB locally cools the environment, so that the final RCE state has a stable layer at mid‐levels (550 hPa ≈ 50–100 hPa below 0 °C), which is unrelated to melting of ice. Preferred detrainment at mid‐ and high levels leaves the middle‐to‐upper troposphere relatively dry. A circulation is introduced by incrementally lowering the sea‐surface temperature in one column, which collapses convection: first to a congestus mode with tops near 550 hPa, below the dry layer created in RCE; then to congestus with tops near 650 hPa; and finally to shallow cumulus with tops near 850 hPa. Critical to stabilizing congestus near 650 hPa is large radiative cooling near moist cumulus tops under a dry upper atmosphere. This congestus mode is very sensitive, and only develops when horizontal temperature gradients created by evaporative and radiative cooling can persist against the work of gravity waves. This only happens in runs with ample momentum diffusion, which are those with convective momentum transport or large domains. Compared to the shallow mode, the congestus mode produces a deep moist layer and more precipitation. This reduces radiative cooling in the cloud layer and enhances stability near the cloud base, which weakens the circulation, and leads to less precipitation over the warm ocean.

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Section: Discussionsupporting

confidence: 73%

Congestus modes in circulating equilibria of the tropical atmosphere in a two‐column model

Nuijens

Emanuel

2018

Quart J Royal Meteoro Soc

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“…Such detrainment can occur into layers of increased stability, and although the altitude of the higher stable layer is not clear in Fig. 2, the 0 C level, which occurs at $ 500 hPa, is enriched with such layers (Zuidema 1998;Stevens et al 2017).…”

Section: The Relationship Of Trade Wind Cold Pools To Cloud Covermentioning

confidence: 98%

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

et al. 2017

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Pools of air cooled by partial rain evaporation span up to several hundreds of kilometers in nature and typically last less than 1 day, ultimately losing their identity to the large-scale flow. These fundamentally differ in character from the radiatively-driven dry pools defining convective aggregation. Advancement in remote sensing and in computer capabilities has promoted exploration of how precipitation-induced cold pool processes modify the convective spectrum and life cycle. This contribution surveys current understanding of such cold pools over the tropical and subtropical oceans. In shallow convection with low rain rates, the cold pools moisten, preserving the near-surface equivalent potential temperature or increasing it if the surface moisture fluxes cannot ventilate beyond the new surface layer; both conditions indicate downdraft origin air from within the boundary layer. When rain rates exceed $ 2 mm h À1 , convective-scale downdrafts can bring down drier air of lower equivalent potential temperature from above the boundary layer. The resulting density currents facilitate the lifting of locally thermodynamically favorable air and can impose an arc-shaped mesoscale cloud organization. This organization allows clouds capable of reaching 4-5 km within otherwise dry environments. These are more commonly observed in the northern hemisphere trade wind regime, where the flow to the intertropical 123Surv Geophys (2017) 38:1283-1305 https://doi.org/10.1007/s10712-017-9447-x convergence zone is unimpeded by the equator. Their near-surface air properties share much with those shown from cold pools sampled in the equatorial Indian Ocean. Cold pools are most effective at influencing the mesoscale organization when the atmosphere is moist in the lower free troposphere and dry above, suggesting an optimal range of water vapor paths. Outstanding questions on the relationship between cold pools, their accompanying moisture distribution and cloud cover are detailed further. Near-surface water vapor rings are documented in one model inside but near the cold pool edge; these are not consistent with observations, but do improve with smaller horizontal grid spacings.

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“…The maximum in Q r increases as the inversion lowers. Larger cloudiness below the inversion can do so, but alone the interaction of long wave radiation with the clear-sky humidity profile would be sufficient (for example, see Stevens et al 2017 in this same book collection). Large increases in DSST further dry the convective layer, leading to less liquid water and cloudiness, and therefore less radiative cooling and a weaker circulation (black dashed lines, Fig.…”

Section: Circulating Equilibria In the Two-column Systemmentioning

confidence: 99%

Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations

et al. 2017

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Space-borne observations reveal that 20-40% of marine convective clouds below the freezing level produce rain. In this paper we speculate what the prevalence of warm rain might imply for convection and large-scale circulations over tropical oceans. We present results using a two-column radiative-convective model of hydrostatic, nonlinear flow on a non-rotating sphere, with parameterized convection and radiation, and review ongoing efforts in high-resolution modeling and observations of warm rain. The model experiments investigate the response of convection and circulation to sea surface temperature (SST) gradients between the columns and to changes in a parameter that controls the conversion of cloud condensate to rain. Convection over the cold ocean collapses to a shallow mode with tops near 850 hPa, but a congestus mode with tops near 600 hPa can develop at small SST differences when warm rain formation is more efficient. Here, interactive radiation and the response of the circulation are crucial: along with congestus a deeper moist layer develops, which leads to less low-level radiative cooling, a smaller buoyancy gradient between the columns, and therefore a weaker circulation and less subsidence over the cold ocean. The congestus mode is accompanied with more surface precipitation in the subsiding column and less surface precipitation in the deep convecting column. For the shallow mode over colder oceans, circulations also weaken with more efficient warm rain formation, but only marginally. Here, more warm rain reduces convective tops and the boundary layer depth-similar to Large-Eddy Simulation (LES) studies-which reduces the integrated buoyancy gradient. Elucidating the impact of warm rain can benefit from large-domain high-resolution simulations and observations. Parameterizations of warm rain may be constrained through collocated cloud and rain profiling from ground, and concurrent changes in convection and rain in subsiding and & Louise Nuijens

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Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

Cited by 71 publications

References 50 publications

Congestus modes in circulating equilibria of the tropical atmosphere in a two‐column model

Congestus modes in circulating equilibria of the tropical atmosphere in a two‐column model

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations

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