Explicit versus Parameterized Convection in Response to the Atlantic Meridional Mode

Paccini, Laura; Hohenegger, Cathy; Stevens, Björn

doi:10.1175/jcli-d-20-0224.1

Cited by 4 publications

(5 citation statements)

References 43 publications

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“…The stronger large-scale circulation with explicit deep convection is accompanied by stronger trade winds, with an increase of surface wind speed to about 4 m s −1 . Such an increase of surface winds was also shown by a study using the ICON-NWP in a more realistic setup (Paccini et al, 2021). Furthermore, the runs with explicit deep convection exhibit equatorial easterlies in the middle troposphere up to 5-7 km, while P13 exhibits equatorial westerlies there.…”

Section: Dynamical Structuresupporting

confidence: 66%

“…Explicit convection captures the spatial and temporal variability of tropical rainfall more realistically compared to parameterized convection (Stevens et al, 2020). Wind-induced surface exchange of heat and moisture is also improved, as shown for the tropical Atlantic Ocean by Paccini et al (2021). Moreover, explicit deep convection performs better in terms of convectively coupled equatorial waves (Judt and Rios-Berrios, 2021) and gravity wave momentum fluxes, which are often triggered by convection in the the tropics and subtropics (Stephan et al, 2019).…”

Section: Introductionmentioning

confidence: 98%

“…Over oceans, the ITCZ is collocated with low-level convergence and upper-level divergence of the Hadley circulation accompanied by lowlevel easterly trade winds on the flanks (Johnson et al, 1999;Schwendike et al, 2014). Thermodynamic contrasts between the ocean and the air and surface winds modulate surface enthalpy fluxes, of which enhancement increases rainfall by transporting moisture and heat from the ocean into the air (Raymond et al, 2006;Paccini et al, 2021). Cumulonimbus clouds as well as clear-sky, moist columns in the tropics trap outgoing longwave radiation and the moist columns increase the shortwave absorption, while the dry columns and shallow clouds in the subtropics enhance net longwave cooling compared to the tropics (Bony et al, 2015;Lau et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

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Understanding the dependence of mean precipitation on convective treatment in tropical aquachannel experiments

Jung¹,

Knippertz²,

Ruckstuhl³

et al. 2023

Preprint

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Abstract. The intertropical convergence zone (ITCZ) is a key circulation and precipitation feature in the tropics. There has been a large spread in the representation of the ITCZ in global weather and climate models for a long time, the reasons for which remain unclear. This manuscript presents a novel approach with which we disentangle different physical processes responsible for the changeable behavior of the ITCZ in numerical models. The diagnostic tool is based on a conceptual framework developed by Emanuel (2019) and allows for physically consistent estimates of convective mass flux and precipitation efficiency for simulations with explicit and parameterized convection. We apply our diagnostics to a set of tropical aquachannel experiments using the ICOsahedral Nonhydrostatic (ICON) model with horizontal grid resolution of 13 km and with various representations of deep and shallow convection. The channel length corresponds to the Earth's circumference and has rigid walls at 30° N/S. Zonally symmetric sea surface temperatures are prescribed. All four runs share overall similar rainfall patterns and dynamical structures. They simulate an ITCZ at the equator coinciding with the ascending branch of the Hadley circulation, descending branches at 15° N/S with subtropical jets and easterly trade wind belts straddling the ITCZ. Differences are largest between runs with and without parameterized deep convection. With explicit deep convection, rainfall in the ITCZ increases by 35 % and the Hadley circulation as well as surface winds become stronger. Our diagnostic framework reveals that boundary-layer quasi-equilibrium is a key to physically understanding those differences. The stronger surface horizontal winds with explicit deep convection essentially enhance surface enthalpy fluxes and thus perturb quasi-equilibrium in the boundary layer. This is balanced by increasing convective downdraft mass flux that carries low moist static energy from the lower troposphere into the boundary layer. The downdraft strength is proportional to convective updraft mass flux, which is closely linked to rainfall, since – somewhat surprisingly – the convective treatment does not appear to influence precipitation efficiency significantly. Changes in radiative cooling are largely compensated by changes in dry stability, leading to little impact on rainfall. The results highlight the utility of our diagnostics to pinpoint processes important for rainfall differences between models, suggesting applicability for global climate model intercomparison projects.

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Section: Dynamical Structuresupporting

confidence: 66%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Understanding the dependence of mean precipitation on convective treatment in tropical aquachannel experiments

Jung¹,

Knippertz²,

Ruckstuhl³

et al. 2023

Preprint

View full text Add to dashboard Cite

show abstract

“…We conduct a set of simulations using the same approach as Paccini et al. (2021), in which the experimental configuration has been previously tested. Global simulations, at 40 km grid spacing are called the P‐CON simulations and serve as initial and boundary conditions to the one‐way nested domains at finer grid spacing.…”

Section: Methodsmentioning

confidence: 99%

Assessing Precipitation Over the Amazon Basin as Simulated by a Storm‐Resolving Model

Paccini

Stevens

2023

JGR Atmospheres

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The Amazon basin is the largest rainforest in the Earth and of great relevance for the global hydro-climate and biodiversity (Marengo, 2006;Phillips et al., 2008). It is also a region, like many in the tropics, where climate model precipitation biases are both large and systematic. These biases are evident in every aspect of the representation of precipitation, from its spatial and temporal distribution, to its intensity and form. Models systematically have too little precipitation over the northern Amazon (e.g., Fiedler et al., 2020;Yin et al., 2013). The diurnal cycle is characterized by a too early precipitation peak (Betts & Jakob, 2002;Tang et al., 2021) and evidence of convective organization (Mapes & Neale, 2011), which has been estimated to account for up to 50% of the total Amazon rain (Feng et al., 2021), is effectively absent. In this study, we use kilometer-scale "storm-resolving" simulations over large domains to assess the degree to which they reduce these biases and the extent to which this depends on the explicit representation of organized convective systems (OCS). In doing so our premise is that convective features which are not improved, or for which remaining biases show no clear sign of improvement with increases in resolution, are indicative of an important role for non-convective, for example, cloud microphysical, small (sub hectometer) scale mixing or land-surface processes.

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“…Global simulations, at 40 km grid spacing (P-CON simulations), serve as initial and boundary conditions to the one-way nested domains at finer grid spacing. The three inner domains have the convective parameterization switched off (E-CON simulations) and comprise the same regions as described in Paccini et al (2021). The horizontal resolution is successively increased from 20 km to 10 km and to 5 km, with the finest grid spacing covering the tropical Atlantic sector (85°W-25°E; 25°S-25°N).…”

Section: Experimental Set-upmentioning

confidence: 99%

Assessing precipitation over the Amazon basin as simulated by a storm-resolving model

Paccini

Stevens

2022

Preprint

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In this study we investigate whether a better representation of precipitation in the Amazon basin arises through an explicit representation of convection and whether it is related to the representation of organized systems. In addition to satellite data, we use ensemble simulations of the ICON-NWP model at storm-resolving (2.5-5.0 km) scales with explicit convection (E-CON) and coarse resolutions, with parameterized convection (P-CON). The main improvements in the representation of Amazon precipitation by E-CON are in the spatial pattern of precipitation, the distribution of precipitation intensity and the spatial distribution in the diurnal cycle. By isolating precipitation from organized convective systems (OCS), it is shown that many of the well simulated precipitation features in the Amazon arise from the distribution of these systems. The simulated and observed OCS are classified into 6 clusters which distinguish nocturnal and diurnal OCS. While the E-CON ensembles capture the OCS, especially their diurnal cycle, their frequency is reduced compared to observations. Diurnal clusters are influenced by surface processes such as cold pools, which aid to the propagation of OCS. Nocturnal clusters are rather associated with strong low-level easterlies, possibly related to the Amazonian low-level jet. These particular environmental conditions provide insights on the processes that are important for OCS in the Amazon and should be further improved.

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Explicit versus Parameterized Convection in Response to the Atlantic Meridional Mode

Cited by 4 publications

References 43 publications

Understanding the dependence of mean precipitation on convective treatment in tropical aquachannel experiments

Understanding the dependence of mean precipitation on convective treatment in tropical aquachannel experiments

Assessing Precipitation Over the Amazon Basin as Simulated by a Storm‐Resolving Model

Assessing precipitation over the Amazon basin as simulated by a storm-resolving model

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