Mr. E. A. Nicholson, President of the Institute for the Biennium 1981–83

Nicholson, Elizabeth

doi:10.1080/00049158.1981.10674318

Cited by 3 publications

(2 citation statements)

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“…This situation is in qualitative agreement with the idea of Soudano-Saharan depressions resulting from an interaction between low-level African easterly waves and troughs in the upper-level westerlies (see Fig. 3 in Nicholson 1981).…”

Section: Discussionsupporting

confidence: 78%

Tropical plumes and extreme precipitation in subtropical and tropical West Africa

Knippertz

Martin

2005

Quart J Royal Meteoro Soc

113

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SUMMARYThree cases of extreme precipitation in October, January and March/April over subtropical north-western and tropical West Africa have been selected in order to study the moisture transports, precipitation generation mechanisms and large-scale dynamics involved. All cases show strong tropical-extratropical interactions and elongated cloud bands extending from the Tropics into the subtropics, usually referred to as tropical plumes (TPs). Investigations are based on observational data and high-resolution output from simulations using the University of Wisconsin-Nonhydrostatic Modeling System. Trajectory analyses show that moisture is transported from the tropical easterly mid-level flow, the monsoonal southerlies and the north-easterly trade winds to provide precipitable water and potential instability at mid-tropospheric levels. The instability is released through ascent related both to weak quasi-geostrophic forcing to the east of an upper-level trough and to the strong, inertially unstable and highly divergent upper-level subtropical jet (STJ) accompanying the TP. In the January and March cases the passage of a precursor upper-level trough over the same location precedes the development and initiates the moisture transport from the deep Tropics. The associated north-westerly cold advection supports the generation of precipitation by triggering convection over the tropical Atlantic Ocean, whose outflow feeds into the STJ over Africa, and by enhancing the vertical mass transport within the intertropical discontinuity over and near West Africa. In October, when the still active African monsoon facilitates the extraction of tropical moisture, a wave in the tropical easterlies and strong trade winds from the southern hemisphere are dominant factors. Low-level cyclogenesis and frontogenesis only play roles in the late stages of the developments. The elongated, mostly positively tilted potential-vorticity (PV) streamers associated with the observed troughs form as a result of an equatorward transport of high-PV air downstream of a large ridge over the North Atlantic. The rapid amplification of the ridge is achieved through a combination of negative horizontal PV advection and diabatic reduction of upper-level PV through latent heating within a cloud band that forms in connection with an explosive baroclinic development near the east coast of North America.

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Section: Discussionsupporting

confidence: 78%

Tropical plumes and extreme precipitation in subtropical and tropical West Africa

Knippertz

Martin

2005

Quart J Royal Meteoro Soc

113

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“…Despite the broad predictability of the monsoon progression, this region exhibits marked inter-and intraseasonal variability (Le Barbé et al 2002) and, indeed, Thorncroft et al (2003) point out that 2000 was unusual, with a dry late season and a stronger sustained jet displaced to the south (two characteristics commonly observed in tandem over this region, e.g. Nicholson 1981;Newell and Kidson 1984;Grist and Nicholson 2001;Nicholson and Grist 2003). This can be seen in Fig.…”

Section: Introduction (A) the African Easterly Jetmentioning

confidence: 79%

The African easterly jet in the ECMWF Integrated Forecast System: 4D‐Var analysis

Tompkins¹,

Diongue-Niang²,

Parker³

et al. 2005

Quart J Royal Meteoro Soc

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SUMMARYIn August 2000, the Met Office C-130 research flight aircraft conducted four sorties over the western Sahel region, making dropsonde measurements at an unprecedented resolution. These were used as an independent validation source to assess the quality of the analysis of the African easterly jet (AEJ) in the ECMWF Integrated Forecast System (IFS). The model was found to produce a reasonably accurate representation of the AEJ location and structure. Peak zonal winds were within 5 m s −1 of the observed values for the two flights that performed transects across the jet. Temperature errors were less than 1 K above the boundary layer, and the gross features of the humidity distribution were reproduced. The analysis was also able to capture the weak African easterly wave activity present during the campaign, but was not capable of reproducing the observed daily variability in boundary-layer height, with a too deep mixed layer on two days resulting in a significant dipole error structure in temperature and relative humidity.Analysis experiments were conducted in which various observations were excluded from the system to determine which were important for the diagnosis of the jet. Satellite data had a minor influence on the jet structure. Conventional platforms such as radiosondes and pilot balloons were more important. The thermodynamical information from radiosondes had a greater impact on the jet winds than the direct assimilation of the wind observations, made possible by the 4D variational assimilation system where thermodynamic information can impact on the dynamics. In contrast, the jet structure improved when surface observations (SYNOPs) were removed from the system, possibly due to the disparity between the radiosonde and SYNOP station humidity information over these regions. The investigation implied that all observational information is incorporated into the analysis in a well-balanced manner, rather than being dominated by a single source. The system was able to assimilate the dropsonde data, which led to an improved dynamical and thermodynamical structure of the jet.

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A profile of the Australian forestry profession

Jennings¹,

Bacon²

1983

Australian Forestry

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Mr. E. A. Nicholson, President of the Institute for the Biennium 1981–83

Cited by 3 publications

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Tropical plumes and extreme precipitation in subtropical and tropical West Africa

Tropical plumes and extreme precipitation in subtropical and tropical West Africa

The African easterly jet in the ECMWF Integrated Forecast System: 4D‐Var analysis

A profile of the Australian forestry profession

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