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Douglas J. Parker
and
Ralph R. Burton

Abstract

This paper uses simple one- and two-dimensional models to investigate the influence of a propagating line of convective forcing on a tropical jet, representative of the African easterly jet. The results are used to infer changes in the environment of the forcing region, which would in reality tend to influence the evolution of the storm through convective mechanisms, which are not resolved here.

From linear analytical solutions with a rigid lid it is found that the influence of the propagation of the forcing region is to intensify the response on the upstream side of the forcing: this result is confirmed in two-dimensional nonlinear simulations. When linear wave modes are computed in a basic state that includes the jet structure, small sensitivities to the basic-state jet are found.

The two-dimensional nonlinear model has been used further to compute the change in the environmental structure as a result of the forcing. Principally, it is found that the modes of response to the forcing may be associated with characteristic changes in the basic-state shear, convective available potential energy (CAPE), and convective inhibition (CIN), which would be expected to have significant influence on the convective system itself.

Full access
Ioana Colfescu
,
Joseph B. Klemp
,
Massimo A. Bollasina
,
Stephen D. Mobbs
, and
Ralph R. Burton

Abstract

On 20 October 2016, aircraft observations documented a significant train of lee waves above and downstream of the Snæfellsnes Peninsula on the west coast of Iceland. Simulations of this event with the Weather Research and Forecasting (WRF) Model provide an excellent representation of the observed structure of these mountain waves. The orographic features producing these waves are characterized by the isolated Snæfellsjökull volcano near the tip of the peninsula and a fairly uniform ridge along its spine. Sensitivity simulations with the WRF Model document that the observed wave train consists of a superposition of the waves produced individually by these two dominant orographic features. This behavior is consistent with idealized simulations of a flow over an isolated 3D mountain and over a 2D ridge, which reproduce the essential behavior of the observed waves and those captured in the WRF simulations. Linear analytic analysis confirms the importance of a strong inversion at the top on the boundary layer in promoting significant wave activity extending far downstream of the terrain. However, analysis of the forced and resonant modes for a two-layer atmosphere with a capping inversion suggest that this wave train may not be produced by resonant modes whose energy is trapped beneath the inversion. Rather, these appear to be vertically propagating modes with very small vertical group velocity that can persist far downstream of the mountain. These vertically propagating waves potentially provide a mechanism for producing near-resonant waves farther aloft due to interactions with a stable layer in the midtroposphere.

Open access
R. R. Burton
,
A. M. Blyth
,
Z. Cui
,
J. Groves
,
B. L. Lamptey
,
J. K. Fletcher
,
J. H. Marsham
,
D. J. Parker
, and
A. Roberts

Abstract

The ability to predict heavy rain and floods in Africa is urgently needed to reduce the socioeconomic costs of these events and increase resilience as climate changes. Numerical weather prediction in this region is challenging, and attention is being drawn to observationally based methods of providing short-term nowcasts (up to ∼6-h lead time). In this paper a freely available nowcasting package, pySTEPS, is used to assess the potential to provide nowcasts of satellite-derived convective rain rate for West Africa. By analyzing a large number of nowcasts, we demonstrate that a simple approach of “optical flow” can have useful skill at 2-h lead time on a 10-km scale and 4-h lead time at larger scales (200 km). A diurnal variation in nowcast skill is observed, with the worst-performing nowcasts being those that are initialized at 1500 UTC. Comparison with existing nowcasts is presented. Such nowcasts, if implemented operationally, would be expected to have significant benefits.

Significance Statement

A freely available, easy-to-use nowcasting package has been applied to satellite-retrieved rainfall rates for West Africa, and extrapolations have useful skill at up to 4 h of lead time.

Open access

The JET2000 Project: Aircraft Observations of the African Easterly Jet and African Easterly Waves

Aircraft Observations of the African Easterly Jet and African Easterly Waves

C. D. Thorncroft
,
D. J. Parker
,
R. R. Burton
,
M. Diop
,
J. H. Ayers
,
H. Barjat
,
S. Devereau
,
A. Diongue
,
R. Dumelow
,
D. R. Kindred
,
N. M. Price
,
M. Saloum
,
C. M. Tayor
, and
A. M. Tompkins

Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 are presented. The Met Research Flight CI30 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde, and Niamey, Niger, and two “box” flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5°–10° intervals. The two box flights also included low-level flights that sampled north–south variations in boundary layer properties in the baroclinic zone beneath the AEJ.

Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross sections of the AEJ on 28 and 29 August (after the passage of a mesoscale system) are presented and discussed. Analysis of dropsonde data on 28 August indicates contrasting convective characteristics north and south of the AEJ with dry convection more dominant to the north and moist convection more dominant to the south. The consequences of this for the AEJ and the relationship with the boundary layer observations are briefly discussed.

Preliminary NWP results indicate little sensitivity to the inclusion of the dropsonde data on the AEJ winds in European Centre for Medium-Range Weather Forecasts (ECMWF) and Met Office analyses. It is proposed that this may be due to a good surface analysis and a realistic model response to this. Both models poorly predict the AEJ in the 5-day forecast indicating the need for more process studies in the region.

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