Inertial Available Kinetic Energy and the Dynamics of Tropical Plume Formation

Mecikalski, John R.; Tripoli, Gregory J.

doi:10.1175/1520-0493(1998)126<2200:iakeat>2.0.co;2

Cited by 33 publications

(21 citation statements)

References 23 publications

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“…While the straight cloud band over Mali caused rather little precipitation, some rainfall was associated with the widespread deep convection over Nigeria. It is conceivable that low inertial stability at the anticyclonic flank of the subtropical jet streak accompanying the upper trough has provided good outflow conditions for convection (Mecikalski and Tripoli 1998;Knippertz 2005). The situation resembles the dry-season precipitation case discussed in KF08, who suggested a relation between the pressure fall over West Africa and the combined affects of warm advection and an enhanced greenhouse effect under the TP.…”

Section: An Example Case Studymentioning

confidence: 69%

Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

Knippertz

Fink

2009

Weather and Forecasting

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Precipitation during the boreal winter dry season in tropical West Africa is rare but occasionally results in significant impacts on the local population. The dynamics and predictability of this phenomenon have been studied very little. Here, a statistical evaluation of the climatology, dynamics, and predictions of dry-season wet events is presented for the region 7.58-158N, 108W-108E. The analysis is based upon Global Precipitation Climatology Project (GPCP) merged satellite-gauge pentad rainfall estimates and 5-day 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) precipitation forecasts, and covers the 23 dry seasons (November-February) during 1979/80-2001/02. Wet events are defined as pentads with an area-averaged precipitation anomaly of more than 1200% with respect to the mean seasonal cycle. Composites of the 43 identified events indicate an association with a trough over northwestern Africa, a tropical plume on its eastern side, unusual precipitation at the northern and western fringes of the Sahara, and reduced surface pressure over the Sahara, which allows an inflow of moist southerlies from the Gulf of Guinea to feed the unusual dry-season rainfalls. The results give evidence for a preconditioning by another disturbance about 1 week prior to the precipitation event. The ERA-40 forecasts show a high temporal correlation with observations, a general wet bias, but a somewhat too low number of wet events. With 53% of all identified events correctly forecasted and only 32% of forecasted events not verified, the model shows moderate skill in contrast to the prediction of many other tropical precipitation systems. A separate consideration of hits, misses, and false alarms corroborates the previously proposed hypothesis that a strong extratropical influence enhances the quality of predictions in this region. The results should encourage weather services in West Africa to take advantage of available dry-season precipitation forecasts in terms of the dissemination of early warnings.

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Section: An Example Case Studymentioning

confidence: 69%

Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

Knippertz

Fink

2009

Weather and Forecasting

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“…12 of TF97. Inertial instability and extreme subsidence have been described in various other TP studies (Mecikalski and Tripoli 1998;Knippertz and Martin 2005), but not the related imbalance in the low-level height field.…”

Section: (C) Further Unbalanced Processesmentioning

confidence: 84%

Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak

Knippertz

Fink

2006

Quart J Royal Meteoro Soc

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SUMMARYIn March 2004, a large-scale, strong and persistent outbreak of Saharan dust on to the adjacent tropical and subtropical Atlantic Ocean was observed in satellite imagery. Surface observations show that the event was accompanied by unusual weather conditions across large parts of North Africa including low temperatures and strong winds over the Sahara, extreme precipitation in Libya and a heat wave on the Guinea coast. The dust outbreak was associated with an unseasonally southerly position of the Intertropical Convergence Zone and a delayed northward progression of the African monsoon.The dust front was initially related to a density current caused by strong evaporational cooling along a precipitating cloud-band that penetrated into the northern Sahara ahead of an upper-level trough. At later stages, massive upper-level convergence, sinking, low-level divergence and an explosive anticyclogenesis over northwest Africa caused strong northerly flow and a quick spreading of the dust front to the south and west. The strong pressure gradients over North Africa were further enhanced by the formation of a cyclone ahead of the uppertrough. A dynamical analysis based on potential-vorticity (PV) theory provides new insights into the generation of North African harmattan surges. Using a PV inversion, it is shown that the explosive anticyclogenesis was caused largely by unbalanced dynamics related to the density current, divergent outflow from the precipitation over Libya and inertial instability in the entrance region of the strong subtropical jet and tropical plume along the eastern side of the upper trough. These processes enhanced the effects of the balanced advection of negative vorticity and cold air by the large-scale wave. Moreover, we show the importance of an explosive cyclogenesis event over the central North Atlantic for the amplification of the downstream wave associated with the penetration of the cold front into North Africa.

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“…Attempts were made to investigate the role of inertial (and symmetric) instability in aiding development of the TTTs (Mecikalski and Tripoli 1998;Knippertz and Martin 2005); however, calculating these parameters from the 2.58 3 2.58 NCEP2 reanalysis produce no regions of instability, which is not unexpected, given the coarse resolution of the data and the mesoscale nature of instability regions.…”

Section: Discussionmentioning

confidence: 99%

Tropical–Extratropical Interactions over Southern Africa: Three Cases of Heavy Summer Season Rainfall

Hart

Reason

Fauchereau

2010

Monthly Weather Review

127

121

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The synoptic evolution of three tropical-extratropical (TE) interactions, each responsible for extreme rainfall events over southern Africa, is discussed in detail. Along with the consideration of previously studied events, common features of these heavy rainfall producing tropical temperate troughs (TTTs) over southern Africa are discussed. It is found that 2 days prior to an event, northeasterly moisture transports across Botswana, set up by the Angola low, are diverted farther south into the semiarid region of subtropical southern Africa. The TTTs reach full maturity as a TE cloud band, rooted in the central subcontinent, which is triggered by upper-level divergence along the leading edge of an upper-tropospheric westerly wave trough. Convection and rainfall within the cloud band is supported by poleward moisture transports with subtropical air rising as it leaves the continent and joins the midlatitude westerly flow. It is shown that these systems fit within a theoretical framework describing similar TE interactions found globally.Uplift forcing for the extreme rainfall of each event is investigated. Unsurprisingly, quasigeostrophic uplift is found to dominate in the midlatitudes with convective processes strongest in the subtropics. Rainfall in the semiarid interior of South Africa appears to be a result of quasigeostrophically triggered convection.Investigation of TTT formation in the context of planetary waves shows that early development is sometimes associated with previous anticyclonic wave breaking south of the subcontinent, with full maturity of TTTs occurring as a potential vorticity trough approaches the continent from the west. Sensitivity to upstream wave perturbations and effects on anticyclonic wave breaking in the South Indian Ocean are also observed.

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Inertial Available Kinetic Energy and the Dynamics of Tropical Plume Formation

Cited by 33 publications

References 23 publications

Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak

Tropical–Extratropical Interactions over Southern Africa: Three Cases of Heavy Summer Season Rainfall

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