The Energetics of African Wave Disturbances as observed During Phase III of GATE

Norquist, Donald C.; Recker, Ernest E.; Reed, Richard J.

doi:10.1175/1520-0493(1977)105<0334:teoawd>2.0.co;2

Cited by 166 publications

(134 citation statements)

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“…Because AEWs north of the AEJ derive the majority of their energy from baroclinic processes associated with the interaction of the low-level meridional temperature gradients in the Sahel−Sahara region and the potential vorticity gradients in the AEJ core, the energetics of AEWs should in part reflect the local changes to surface air temperature that occur in response to enhanced radiative forcing. Stronger gradients in potential temperature result in greater zonal available potential energy for AEW growth (17,24,30). AEWs access this source of zonal available potential energy by fluxing temperature down the meridional temperature gradient, creating eddy available potential energy.…”

Section: Resultsmentioning

confidence: 99%

“…AEWs then grow by converting the eddy available potential energy to eddy kinetic energy through baroclinic overturning. To enable sustained AEW growth, the eddy available potential energy must be replenished by the zonal available potential energy (17,30).…”

Section: Resultsmentioning

confidence: 99%

“…AEWs are convectively triggered in East and Central Africa, and grow through a mixed barotropic−baroclinic mechanism associated with the midtroposphere [600 mbar (mb)] African Easterly Jet (AEJ) (14)(15)(16), which largely controls the structure and wavelength of the AEWs (17,18). AEWs typically have periodicities of 3-5 d, wavelengths between 2,000 km and 4,000 km, and two circulation centers-one to the north of the AEJ and one to the south (19).…”

Section: Greenhouse Gas Forcing | Cmip5mentioning

confidence: 99%

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Projected changes in African easterly wave intensity and track in response to greenhouse forcing

Skinner¹,

Diffenbaugh

2014

Proc. Natl. Acad. Sci. U.S.A.

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Significance African easterly waves (AEWs) are ubiquitous weather disturbances over the tropical Atlantic basin during the summer season. However, despite the critical influence of AEWs on Sahel rainfall, transport of Saharan dust, and initiation of Atlantic hurricanes, little is known about how global warming influences AEW activity. We use an ensemble of general circulation models to investigate the response of AEWs to elevated greenhouse gas concentrations. We find that increases in regional temperature gradients and the strength of convergence and uplift along the Intertropical Front of Africa lead to increases in the strength of AEWs. Given the relationships that are observed in the current climate, changes in AEW strength could influence climate over both West Africa and the larger Atlantic basin.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Greenhouse Gas Forcing | Cmip5mentioning

confidence: 99%

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Projected changes in African easterly wave intensity and track in response to greenhouse forcing

Skinner¹,

Diffenbaugh

2014

Proc. Natl. Acad. Sci. U.S.A.

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“…The fluxes double in magnitude in association with the considerably stronger mid-tropospheric meridional PV gradient as a result of moist convection (Figure 11). The spatial distribution of the vertical heat fluxes below the moist AEJ (not shown) exhibits a minimum located towards 17 • N and an additional maximum compared with the dry life cycle which is located around 10 • N. This is associated with a negative baroclinic conversion probably linked to the overturning associated with evaporative cooling in convective downdraughts, as hypothesized by Norquist et al (1977) in their energetics analysis of GATE data, as observed between 800 and 600 mb by Diedhiou et al (2002), and in a regional modelling study by Hseih and Cook (2007). The minimum at 17 • N is in the right sense for baroclinic growth, and the weak low-level amplitudes in the moist model were closely correlated to increases in this minimum.…”

Section: The Moist Life Cyclementioning

confidence: 90%

The impact of moist processes on the African easterly jet–African easterly wave system

Cornforth

Hoskins

Thorncroft

2009

Quart J Royal Meteoro Soc

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ABSTRACT:The African Easterly Jet-Easterly Wave (AEJ-AEW) system was explored in an idealised model. Prescribed zonally symmetric surface temperature and moisture profiles determine the AEJ which becomes established through meridional contrasts in dry and moist convection.As in previous studies, a realistic AEJ developed with only dry convection. Including moist processes, increased its development rate, but reduced its speed and meridional extent. AEWs grew through barotropic-baroclinic conversions. Negative meridional potential vorticity (PV) gradients arose in the zonally symmetric state through the intrusion of the low-PV Saharan boundary layer. Since moist processes strengthened this significantly through diabatically generated PV in the Intertropical Convergence Zone, moist AEWs were three times stronger. Larger barotropic conversions and faster AEJ development increased the moist wave growth-rate. Jet-level and northerly low-level amplitudes grew, but in the moist case the low-level amplitudes weakened as the AEW interacted with convection, consistent with their absence from observations during the peak monsoon. Striking dependencies between the AEJ, AEW and rainfall existed. Two time-scales governed their evolution, depending on the transfer coefficients: (1) the AEJ's replenishment rate influenced by heat fluxes, and (2) the wave growth-rate, by damping, and the slower jet development rate.Moist AEWs were characterized by intermittent growth/decay, with growth preceded by increased mean rainfall and later, weakening AEJs. These dependencies established an internal 8-10-day variability, consistent with intra-seasonal observations of 9-day rainy sequences. This internal variability offers an alternative explanation to the previously proposed external forcing and a new view of the moist AEW life cycle.

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“…These conversion terms represent the diabatic conversion of latent heat energy and the baroclinic conversion of mean state available potential energy (APE), respectively, to eddy APE. The terms are defined by the following formulas (Norquist et al 1977): …”

Section: E Energy Conversion Calculationsmentioning

confidence: 99%

Mesoscale Convective Vortex Formation in a Weakly Sheared Moist Neutral Environment

Conzemius

Moore

Montgomery

et al. 2007

Journal of the Atmospheric Sciences

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Idealized simulations of a diabatic Rossby vortex (DRV) in an initially moist neutral baroclinic environment are performed using the fifth-generation National Center for Atmospheric Research-Pennsylvania State University (NCAR-PSU) Mesoscale Model (MM5). The primary objective is to test the hypothesis that the formation and maintenance of midlatitude warm-season mesoscale convective vortices (MCVs) are largely influenced by balanced flow dynamics associated with a vortex that interacts with weak vertical shear. As a part of this objective, the simulated DRV is placed within the context of the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) field campaign by comparing its tangential velocity, radius of maximum winds, CAPE, and shear with the MCVs observed in BAMEX.The simulations reveal two distinct scales of development. At the larger scale, the most rapidly growing moist baroclinic mode is excited, and exponential growth of this mode occurs during the simulation. Embedded within the large-scale baroclinic wave is a convective system exhibiting the characteristic DRV development, with a positive potential vorticity (PV) anomaly in the lower troposphere and a negative PV anomaly in the upper troposphere, and the positive/negative PV doublet tilted downshear with height. The DRV warm-air advection mechanism is active, and the resulting deep convection helps to reinforce the DRV against the deleterious effects of environmental shear, causing an eastward motion of the convective system as a whole.The initial comparisons between the simulated DRVs and the BAMEX MCVs show that the simulated DRVs grew within background conditions of CAPE and shear similar to those observed for BAMEX MCVs and suggest that the same dynamical mechanisms are active. Because the BAMEX field campaign sampled MCVs in different backgrounds of CAPE and shear, the comparison also demonstrates the need to perform additional simulations to explore these different CAPE and shear regimes and to understand their impacts on the intensity and longevity of MCVs. Such a study has the additional benefit of placing MCV dynamics in an appropriate context for exploring their relevance to tropical cyclone formation.

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The Energetics of African Wave Disturbances as observed During Phase III of GATE

Cited by 166 publications

References 0 publications

Projected changes in African easterly wave intensity and track in response to greenhouse forcing

Projected changes in African easterly wave intensity and track in response to greenhouse forcing

The impact of moist processes on the African easterly jet–African easterly wave system

Mesoscale Convective Vortex Formation in a Weakly Sheared Moist Neutral Environment

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