Dr. M. Moncrieff comments

Moncrieff, M. W.

doi:10.1136/adc.52.10.804-a

Cited by 16 publications

(40 citation statements)

References 12 publications

(8 reference statements)

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“…An important aspect, the impact of environmental vertical shear on convective development, was highlighted in the previous section. While shear can disrupt the vertical development of small convective clouds, it is central to the mesoscale organization of convection (Moncrieff 1992, and references therein). In traditional parametrizations, the latter aspect is seldom considered (an exception being Donner 1993).…”

Section: Discussionmentioning

confidence: 99%

“…vertical shear) on the development of convection. While wind shear tends to have an organizing effect on deep convection (intense thunderstorms and squall lines; see Moncrieff (1992) and references therein), it tends to disrupt shallow cumulus, especially when the shear is unidirectional, as within the westerly wind burst area. A cumulus cloud consists of an aggregate of towers, which rise in succession before dissipating on the down-shear side of the parent cumulus.…”

Section: (A) Summary Of the Emanuel Convection Schemementioning

confidence: 99%

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Moisture–convection feedback in the tropics

Grabowski

Moncrieff

2004

Quart J Royal Meteoro Soc

143

111

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SUMMARYThis paper discusses the large-scale moisture-convection feedback in the tropics, where spatial fluctuations of deep convection cause perturbations of free-tropospheric moisture which, in turn, affect the spatial distribution of deep convection. A simple heuristic argument using the timescale of free-tropospheric humidity change explains why moisture-convection feedback is particularly relevant for tropical intraseasonal oscillations.The large-scale dynamical context for moisture-convection feedback is investigated in idealized rotating constant-sea-surface-temperature ('tropics everywhere') aquaplanet using cloud-resolving convection parametrization (CRCP; super-parametrization) and a traditional convective parametrization (the Emanuel scheme). The large-scale organization of convection within the equatorial waveguide takes the form of MJO-like (Madden-Julian Oscillation) coherent structures. First, CRCP simulations are performed in which development of large-scale free-tropospheric moisture perturbations is artificially suppressed using relaxation with a timescale of one day. As in previous simulations where much shorter relaxation timescale was used, MJO-like coherences do not develop and, if already present, they disintegrate rapidly. Second, CRCP simulations that start from planetaryscale moisture perturbation in the free troposphere are conducted. The ensuing large-scale velocity perturbations have e-folding times of five and seven days, respectively, for interactive and prescribed radiation simulations. This supports the conjecture that interactive radiation enhances moisture-convection feedback; an enhanced largescale circulation results from differences in radiative cooling between areas having enhanced and suppressed convectively-generated moisture and cloudiness. Additional support for the role of moisture-convection feedback in intraseasonal oscillations is seen in simulations that apply the Emanuel scheme. The standard configuration of the Emanuel scheme is insensitive to free-tropospheric humidity and results in weak MJO-like coherences. A simple modification of the Emanuel scheme that enhances its sensitivity to free-tropospheric humidity dramatically improves the simulated MJO-like coherences.

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Section: Discussionmentioning

confidence: 99%

Section: (A) Summary Of the Emanuel Convection Schemementioning

confidence: 99%

Moisture–convection feedback in the tropics

Grabowski

Moncrieff

2004

Quart J Royal Meteoro Soc

143

111

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“…Such systems may have a dynamical impact on the AEJ through the vertical and meridional transport of momentum (e.g. Moncrieff 1992;LeMone and Moncrieff 1994;Caniaux et al 1995), as well as through modification of the thermodynamic profiles and consequent geostrophic adjustment. The extent to which the AEJ is accelerated or decelerated by these processes remains unclear.…”

Section: Introductionmentioning

confidence: 99%

Analysis of the African easterly jet, using aircraft observations from the JET2000 experiment

Parker

Thorncroft

Burton

et al. 2005

Q. J. R. Meteorol. Soc.

140

153

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SUMMARYAnalyses of the African easterly jet (AEJ) are presented which are based on meridional transects of highresolution dropsonde observations made during JET2000, an aircraft campaign conducted in the last week of August 2000. The observations have confirmed that the AEJ is closely defined by geostrophic balance. The baroclinicity between the extreme northern and southern profiles accurately determines the altitude of the jet core, while the location and morphology of the jet core correspond to a locally-defined geostrophic wind measure. The potential-vorticity (PV) structure has also been found to accord with theoretical expectations, with distinctive positive-and negative-PV anomalies equatorward and poleward of the jet core respectively.The thermodynamic structure of the AEJ environment can be categorized into coherent layers. The monsoon layer is a humid zone connected to the land surface, extending northwards into the Sahel and increasing in depth towards the south. This layer is affected by the land surface on diurnal time-scales, through the growing convective mixed layer and through shallow cumulus clouds. Above the monsoon layer is the Saharan air layer (SAL), which can be identified as a layer of low static-stability and low PV. The SAL is deep where it merges with the Saharan boundary-layer in the north, and becomes thinner toward the south. It has been shown that the boundaries of the SAL can be approximated to good accuracy as adiabatic surfaces, meaning that the SAL comprises air which is adiabatically connected to the land surface via the Saharan boundary-layer. The upper region of the SAL is identified as a layer of high relative-humidity where altocumulus and stratocumulus layers are observed. Finally, the troposphere above the SAL is again almost pseudoadiabatic, with small baroclinicity which determines the closure of the AEJ core aloft. Through inspection of thermodynamic tracers, evidence of convective and lateral transport and exchange between these layers is also presented.

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“…For the tropical squall lines, the LCL is low, CAPE, and thus the Richardson number, are large, so neither steering-level-type convection nor the mixed jump and steering-level model can develop favourably because the moist updraught tends to become downshear. This kind of environment may be in favour of the fast-propagating 3-D jumping convection suggested by Moncrieff (1981). For the mid-latitude squall lines, the LCL is high and the Richardson number is not too large, so it is in favour of an upshear mixed jump and steering-level convection.…”

Section: Discussionmentioning

confidence: 94%

“…In those two studies, squall lines were simulated numerically as initial value problems, which were very different from the inviscid steady state model of Moncrieff (1978). By including shallow downdraught cooling and non-constant shear, Thorpe et al (1982) were able to obtain a quasi-steady upshear-sloping updraught in their 2-D model.…”

Section: Introductionmentioning

confidence: 99%

On the Two‐Dimensional Steady Upshear‐Sloping Convection

Chang

1987

Quart J Royal Meteoro Soc

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SUMMARYA 2-D inviscid steering-level model due to M. W. Moncrieff is reexamined both analytically and numerically. It is found that the thermal effects on the distribution of energy and vorticity play opposite roles in determining the interface slope, with the former being dominant. By separating the two effects, a useful insight is obtained into how the interface is controlled by the internal flow regime. Consequently, a mixed jump and steering-level model is proposed, in which the returning updraught and downdraught are separated by adjacent jump updraught and downdraught. By solving for the internal flow, upshear-sloping convection is obtained in the mixed model. Numerical results indicate that the inferface slope increases towards the vertical, or even becomes downshear-sloping, as (i) the jump outflow becomes shallow, (ii) the lifting condensation level (LCL) becomes low, and/or (iii) the Richardson number (proportional to the ratio between convective available potential energy and kinetic energy in the inflow) increases (except for the cases where the jump outflow is very deep and LCL is very high). These features are discussed in connection with the recent results of A. J. Thorpe et al., and the observed differences between tropical propagating squall lines and mid-latitude steering-level-type squall lines.

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Dr. M. Moncrieff comments

Cited by 16 publications

References 12 publications

Moisture–convection feedback in the tropics

Moisture–convection feedback in the tropics

Analysis of the African easterly jet, using aircraft observations from the JET2000 experiment

On the Two‐Dimensional Steady Upshear‐Sloping Convection

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