Secondary initiation of multiple bands of cumulonimbus over southern Britain. I: An observational case‐studyMorcrette, C. J.; Browning, K. A.; Blyth, A. M.; Bozier, K. E.; Clark, P. A.; Ladd, D.; Norton, E. G.; Pavelin, E.
doi: 10.1256/qj.05.151pmid: N/A
Special observing facilities have been assembled in southern England as part of the Convective Storm Initiation Project (CSIP) to study the mesoscale and convective‐scale processes that determine precisely where warm‐season convective showers will break out. This paper reports the results of a case‐study during the pilot field campaign of CSIP in July 2004. One purpose of the pilot project was to demonstrate the value of various observational facilities and to evaluate the usefulness of a variety of analysis and synthesis techniques. Amongst other things, the case‐study demonstrates the utility of high‐resolution imagery from the Meteosat Second Generation satellite for tracking the early stages of the convective clouds, and of a new clear‐air scanning radar at Chilbolton for mapping both the top of the boundary layer and the initial growth of the convective cells that penetrate it. The particular event studied involved the triggering of convection that developed into three parallel arcs of showers and thunderstorms. The first arc was triggered along the leading edge of the outflow (density current) from an earlier cluster of showers, but the convection in the second and third arcs was triggered by a different mechanism. The paper describes in detail the way in which this convection broke through the stable layer, or lid, at the top of a boundary layer of variable depth. The strength of the lid decreased and the depth of the boundary layer increased with time as a result of diurnal heating, but the precise locations where convection finally broke through were determined by the spatial variability in boundary‐layer depth. The analysis suggests that a wave‐like modulation of the boundary‐layer depth of amplitude 150 m, perhaps due to a gravity‐wave disturbance from the earlier cluster of showers, had a greater influence on where the convection was triggered than the modest hills (typically 200 m high) in southern England. © Royal Meteorological Society, 2006. Contributions by P. A. Clark and M. E. B. Gray are Crown Copyright.
Secondary initiation of multiple bands of cumulonimbus over southern Britain. II: Dynamics of secondary initiationMarsham, J. H.; Parker, D. J.
doi: 10.1256/qj.05.152pmid: N/A
The Convective Storm Initiation Project (CSIP) pilot field programme took place in the summer of 2004 in southern England. During this field campaign a case of a cold‐pool outflow initiating an arc of convection downwind of the generating storm (the ‘primary storm’) was observed. Three further arcs were initiated further downwind of this first arc. These arcs all later gave significant rainfall over south‐east England. Results from the modelling studies described in this paper show that gravity waves, generated by the ‘primary storm’, may have been responsible for initiating the further three arcs of convective showers that were observed.
Formation and release of symmetric instability following Delta‐M adjustmentMorcrette, C. J.; Browning, K. A.
doi: 10.1256/qj.04.108pmid: N/A
Conditional Symmetric Instability (CSI) and Delta‐M adjustment have both been suggested as possible mechanisms to explain the slantwise circulations that lead to two‐dimensional precipitation bands near midlatitude cold fronts. Previous work has shown how the initial state required for CSI to occur is similar to the state produced as a result of Delta‐M adjustment. An idealized dry numerical model is used to test the hypothesis that slantwise circulations can occur due to the release of symmetric instability formed by the Delta‐M adjustment process. The numerical simulation shows the coexistence of two pairs of slantwise circulations: one due directly to Delta‐M adjustment, and the other which occurs in a region of symmetric instability which was formed by the Delta‐M adjustment process. The simulated circulation is qualitatively similar to some high‐resolution Doppler radar observations of vertically stacked slantwise circulations. It is argued that a combined Delta‐M/CSI mechanism may also be responsible for the creation of multiple slantwise circulations in the moist atmosphere. Copyright © 2006 Royal Meteorological Society.
Hydrological evaluation of high‐resolution precipitation forecasts of the Gard flash‐flood event (8–9 September 2002)Chancibault, K.; Anquetin, S.; Ducrocq, V.; Saulnier, G.‐M.
doi: 10.1256/qj.04.164pmid: N/A
On 8 and 9 September 2002, an extreme flash‐flood event occurred over south‐eastern France. This event was simulated with the non‐hydrostatic mesoscale model Meso‐NH. Several experiments were performed, differing in their initial conditions. A first experiment used the 4D‐Var large‐scale French operational analysis, as initial state. For the second experiment, the initial state was provided by a mesoscale initialization using surface observations, radar and satellite data. Finally, in the last experiment, only the mesoscale analysis of surface observations was used as initial state. The aim of this study is to assess the validity of the high‐resolution simulations for hydrological purposes. Then simulated and observed mean areal rainfall depth, over nine watersheds with surface areas ranging from 200 to 2200 km2, were compared. This comparison highlighted the contribution of both initialization procedures to a better location of the mesoscale convective system and to larger amounts of rain, during the first ten hours of simulation. Also, a hydrological model was used to compare stream flows: the simulated and observed rainfall fields were used as input data to a single‐event TOPMODEL version. The comparison of the hydrological responses shows that the mesoscale initialization leads to higher peak flows and more realistic flood timing than with the simulation starting from large‐scale analysis. The full initialization allowed a simulation of peak flows higher than the ones obtained with the mesoscale surface‐observation initialization. Copyright © 2006 Royal Meteorological Society.
On the origin of the tropical Atlantic decadal oscillation based on the analysis of the ICOADSLim, Gyu‐ho; Suh, Yong‐cheol; Kim, Baek‐min
doi: 10.1256/qj.05.01pmid: N/A
From an analysis of the International Comprehensive Ocean–Atmosphere Data Set, we were able to confirm salient features of the tropical Atlantic decadal oscillation illustrated in previous works. These features include antisymmetric behaviour of surface air pressure and sea surface temperature with two centres of action residing in the tropical North and South Atlantic Oceans. In addition to the recognized features of the oscillation, we found a new aspect: the principal mechanism regulating the oscillation differs between the north and south centres of action. Decadal components of the relative humidity time series show a maximum value of lag cross‐correlation at a lag of −2 years. Such a temporal evolution was irrespective of the centres of action and the reference time series. The two reference series employed are the sunspot numbers and the southerlies over the western equatorial Atlantic. Associated with this, statistically significant phase shifts between the decadal components of variables exist, especially in the tropical North Atlantic Ocean. For the decadal oscillation, relative humidity change was dominated by specific humidity change over the tropical North Atlantic and by air temperature change over the tropical South Atlantic. For the frequency band of the solar cycle, the variation of relative humidity seemed to amplify the Earth's response to the sun's radiation changes that had previously been considered too small to affect the Earth's climate. By incorporating our new findings with the known features of the oscillation, we can suggest that the variability of solar radiation may be crucial as an originator, or at least as a regulator, of the oscillation in combination with the climate distribution of clouds and water vapour over the tropical Atlantic. Copyright © 2006 Royal Meteorological Society.
Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreakKnippertz, Peter; Fink, Andreas H.
doi: 10.1256/qj.05.109pmid: N/A
In 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.
New version of the TOMCAT/SLIMCAT off‐line chemical transport model: Intercomparison of stratospheric tracer experimentsChipperfield, M. P.
doi: 10.1256/qj.05.51pmid: N/A
We describe the development of a new three‐dimensional off‐line chemical transport model (CTM). The CTM has been produced by combining the existing, and closely related, TOMCAT and SLIMCAT models. The new CTM (TOMCAT/SLIMCAT) has a flexible vertical coordinate which can use both σ‐p and σ‐θ levels. A novel approach is used for the σ‐θ coordinate to ensure a smooth transition between levels. The CTM has different options for calculating vertical transport in the stratosphere, depending on the coordinate chosen. The CTM also has different options for other processes such as advection scheme, radiation scheme and meteorological forcing.
Retrievals of sea surface temperature from infrared imagery: origin and form of systematic errorsMerchant, C. J.; Horrocks, L. A.; Eyre, J. R.; O'carroll, A. G.
doi: 10.1256/qj.05.143pmid: N/A
We show that retrievals of sea surface temperature from satellite infrared imagery are prone to two forms of systematic error: prior error (familiar from the theory of atmospheric sounding) and error arising from nonlinearity. These errors have different complex geographical variations, related to the differing geographical distributions of the main geophysical variables that determine clear‐sky brightness‐temperatures over the oceans. We show that such errors arise as an intrinsic consequence of the form of the retrieval (rather than as a consequence of sub‐optimally specified retrieval coefficients, as is often assumed) and that the pattern of observed errors can be simulated in detail using radiative‐transfer modelling. The prior error has the linear form familiar from atmospheric sounding. A quadratic equation for nonlinearity error is derived, and it is verified that the nonlinearity error exhibits predominantly quadratic behaviour in this case. Copyright © 2006 Royal Meteorological Society.