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Peter Knippertz

Abstract

A simple algorithm for the identification of upper-level trough axes is introduced. In contrast to the assumption of a basically circular geometry in many comparable routines found in the literature, the presented identification scheme is based on the east–west gradient in the 500-hPa geopotential height field, accounting for the meridionally elongated troughs often found in the subtropics. On the basis of the NCEP–NCAR reanalysis data from 1958 to 1998, climatological aspects are discussed for the area 22.5°–47.5°N, 38.5°W–18.5°E. For the period from 1977 to 1998, 12-hourly precipitation reports from 37 stations in northwestern Africa are used to investigate the influence of the identified troughs on winter precipitation. It is demonstrated that more than 90% of the precipitation in this region can be assigned to nearby trough axes. A comparison of rainy periods with the climatology reveals a significant enhancement of the occurrence of trough axes in a range of 10°–15° longitude close to the region considered. While in northwestern Morocco most precipitation falls in connection with the convective instability and dynamical lifting close to and ahead of the trough axes, the orography modifies this relation in the areas along the Mediterranean coast and south of the Atlas Mountains. In the former region, a considerable part of the precipitation falls to the west of the trough axis, where the associated northerly flow is directed against the Tell Atlas. A similar relation is derived for the latter region, where precipitation appears to occur primarily east of the trough axis in the flow that is directed against the southern side of the Atlas range. Here, a particularly strong relation is found between the rainfall and the relatively rare occurrence of upper-level troughs. It is suggested that the consideration of upper-level features like the ones identified by the presented algorithm should be an integrative part of investigations of precipitation variability in the equatorward part of the midlatitudes and the subtropics.

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Peter Knippertz

Abstract

In several case studies, tropical–extratropical interactions (TEIs) have been shown to contribute to the transition-season precipitation in subtropical northwest Africa. Such TEI situations are characterized by a moisture source in the Tropics, a midlevel moisture transport into the subtropics to the east of an upper-level trough, and precipitation generation over northwest Africa through upper-level divergence and orographic effects in the Atlas Mountains.

In this paper, an automatic algorithm to identify TEI episodes on the basis of a 20-yr (December 1978–November 1998) climatology of 4-day backward trajectories starting at 400 hPa over northwest Africa, calculated from ECMWF (re-) analysis, is introduced. Twelve-hourly precipitation reports from 36 synoptic stations in northwest Africa are used to investigate the climatological relevance of TEI situations for different seasons. Results show that the region with the highest relative importance of TEIs is the semiarid southern foothills of the High Atlas (up to 40% of the annual precipitation amount). Relevance clearly decreases toward the much wetter Atlantic coast. TEI contributions are largest in the transition seasons, when TEI situations are most distinct, and in summer, when TEI situations are most frequent. It is suggested to consider TEIs in future studies on the observed and modeled precipitation variability of the region around the Atlas chain.

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Peter Knippertz

Abstract

Tropical plumes (TPs) are elongated bands of upper- and midlevel clouds stretching from the Tropics poleward and eastward into the subtropics, typically accompanied by a subtropical jet (STJ) streak and a trough on their poleward side. This study uses ECMWF analyses and high-resolution University of Wisconsin–Nonhydrostatic Modeling System trajectories to analyze the multiscale complex tropical–extratropical interactions involved in the genesis of a pronounced TP and STJ over the NH Atlantic Ocean in late March 2002 that was associated with extreme precipitation in arid northwest Africa. Previous concepts for TP genesis from the literature are discussed in the light of this case study.

Analysis of the upper-level flow prior to the TP formation shows a northeastward propagation and a continuous acceleration of the STJ over the Atlantic Ocean equatorward of a positively tilted upper-level trough to the west of northwest Africa. Both dynamic and advective processes contribute to the generation of the accompanying cloud band. The northern portion of the TP consists of parcels that exit a strong STJ streak over North America, enter the deep Tropics over South America, and then accelerate into the Atlantic STJ, accompanied by strong cross-jet ageostrophic motions, rising, and cloud formation. The southern portion is formed by parcels originating in the divergent outflow from strong near-equatorial convection accompanying the TP genesis. A local increase in the Hadley overturning is found over the tropical Atlantic and east Pacific/South America and appears to be related to low inertial stability at the outflow level and to low-level trade surges associated with the cold advection, sinking, and lower-level divergence underneath two strong upper-level convergence centers in the eastern portions of both a subtropical ridge over North America and an extratropical ridge over the North Atlantic Ocean. Evidence is presented that the convective response lags the trade surge by several days.

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Florian Meier and Peter Knippertz

Abstract

In January 2002 the Cape Verde region in tropical West Africa was hit by an exceptionally heavy precipitation event. Rain rates of up to 116 mm (48 h)−1 caused harmful impacts on the local population. The rainfall was triggered by a series of two upper-level disturbances penetrating from the extratropics to the West African coast. This study investigates the dynamics and predictability of this event on the basis of simulations with the global model Global Model Europe (GME) of the German Weather Service [i.e., Deutscher Wetterdienst (DWD)] initialized by the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data. Free forecasts satisfactorily reproduce the upper-level disturbances and the precipitation up to a lead time of 7 days. Several sensitivity experiments are conducted to unveil the reasons for this comparably high predictability and to identify dynamical precursors. The relevance of the upper-level wave structure in the extratropics is examined by modifications of the initial conditions using a quasigeostrophic potential vorticity (PV) inversion technique. While a reservoir of high PV over the North Atlantic and a PV ridge over Europe are found to be crucial for the upper-level wave amplification and the rainfall over West Africa, latent heating over the North Atlantic affects the event rather little in contrast to previous case studies. Surface properties like orography and sea surface temperature anomalies modify the precipitation quantity, but appear not to be essential for the occurrence of the extreme event on the simulated time scale.

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Peter Knippertz and Heini Wernli

Abstract

Case studies have shown that heavy precipitation events and rapid cyclogenesis in the extratropics can be fueled by moist and warm tropical air masses. Often the tropical moisture export (TME) occurs through a longitudinally confined region in the subtropics. Here a comprehensive climatological analysis of TME is constructed on the basis of seven-day forward trajectories started daily from the tropical lower troposphere using 6-hourly 40-yr ECMWF Re-Analysis (ERA-40) data from the 23-year period 1979–2001. The objective TME identification procedure retains only those trajectories that reach a water vapor flux of at least 100 g kg−1 m s−1 somewhere north of 35°N. The results show four distinct activity maxima with different seasonal behavior: (i) The “pineapple express,” which connects tropical moisture sources near Hawaii with precipitation near the North American west coast, has a marked activity maximum in boreal winter. (ii) TME over the west Pacific is largest in summer, partly related to the East Asian monsoon and the mei-yu–baiu front. This region alone is responsible for a large portion of TME across 35°N. (iii) The narrow activity maximum over the Great Plains of North America is rooted over the Gulf of Mexico and the Caribbean Sea and has a clear maximum in summer and spring. (iv) TME over the western North Atlantic shows the smallest annual cycle with a maximum in winter and autumn. The interannual variability of (i) and (iv) is significantly modulated by El Niño. Over the African–European–Asian region, high orographic barriers impede TME. A typical TME trajectory evolution is poleward and quasi-horizontal in the subtropics and then more eastward and upward in the southern midlatitudes, where TME contributes up to 60% to climatological precipitation. The TME dataset presented here can serve as a basis for future studies on extreme events.

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Peter Knippertz and Andreas H. Fink

Abstract

Precipitation is a major socioeconomic factor in the Guineo-Soudanian zone of tropical West Africa with its distinct summer rainy season from May to October. Albeit rare, precipitation during the dry season can have substantial impacts on the local hydrology and human activities reaching from the rotting of harvests to improved grazing conditions. This study provides an observationally based synoptic and dynamical analysis of an abundant rainfall event during the dry season of 2003/04 that affected the countries of Nigeria, Benin, Togo, and Ghana. The results point to a forcing of the rainfalls from the extratropics in the following ways: 1) Upper-level clouds and moisture to the east of a weak, quasi-stationary extratropical disturbance enhance the greenhouse effect over the Sahel and the adjacent Sahara, and thereby cause a net-column warm anomaly and falling surface pressure. 2) One day before the precipitation event, negative pressure tendencies are further enhanced through warm advection and subsidence associated with the penetration of a more intense upper-trough into Algeria. 3) The resulting northward shift and intensification of the weak wintertime heat low allows low-level moist southerlies from the Gulf of Guinea to penetrate into the Soudanian zone. 4) Finally, daytime heating of the land surface and convective dynamics initiate heavy rainfalls. Operational forecasts of this event were promising, pointing to a strong control by the comparatively well-predicted extratropical upper-level circulation.

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Peter Knippertz and Jonathan E. Martin

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In this study the term moisture conveyor belt (MCB) is defined as an elongated band of enhanced poleward water vapor fluxes (WVFs) above the PBL that is rooted in the Tropics. This new terminology is illustrated through an exemplary detailed case study of an MCB over the northeastern Pacific during 9–13 November 2003 that provides the moisture for a significant precipitation event in the dry southwestern United States. The analysis of the involved moisture transports and dynamics comprises both Eulerian and Lagrangian approaches, and is based upon output from a simulation with the University of Wisconsin-Nonhydrostatic Modeling System, as well as analysis data, surface observations, and satellite images.

The formation of the MCB is related to a quasi-stationary upper-level cutoff low (COL) resulting from a wave-breaking event over the North Pacific. A pronounced upper-tropospheric baroclinic zone and a strong, inertially unstable subtropical jet (STJ) are found to the east of the COL, where at later stages an elongated tropical cloud plume developed in association with a marked flare-up of ITCZ convection. Part of the extratropical air that subsides to the west of the COL becomes involved in this convection; another part feeds the so-called dry slot at the base of the COL. The actual MCB consists of midlevel trajectories that curve anticyclonically away from the moist tropical easterlies and cause a northeastward-directed WVF maximum at around 700 hPa over the subtropical northeast Pacific and a marked humidity gradient toward the subsided extratropical air. At late stages, frontogenetic circulations lead to WVF convergence involving air from the midlevel subtropical troposphere. At the surface, cyclogenesis and thermal contrasts are weak, and northeasterly trade winds prevail, which clearly distinguishes this MCB from a classical extratropical warm conveyor belt. Other important differences are the high elevation of the WVF maximum, as well as the quasi-horizontal track and origin above the PBL of most moist trajectories. Three precipitation regions with different influence factors can be distinguished. 1) Close to the COL center, moist tropical air is overrun by the dry slot, resulting in convective instability and extreme hail in the Los Angeles, California, area. 2) To the north and east, quasigeostrophic forcing and midlevel warm frontogenesis generate ascent, where the northern branch of the MCB circulates around the COL. 3) Along the anticyclonic shear side of the STJ, convection forms within potentially unstable MCB air benefiting from the inertial instability at the outflow level. It is suggested that this set of circumstances is quite similar to those that conspire to produce heavy precipitation events in subtropical West Africa.

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Peter Knippertz and Andreas H. Fink

Abstract

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 7.5°–15°N, 10°W–10°E. 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 +200% 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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Gregor Gläser, Peter Knippertz, and Bernd Heinold

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On 2 March 2004 a marked upper-level trough and an associated surface cold front penetrated into the Sahara. High winds along and behind this frontal system led to an extraordinary, large-scale, and long-lived dust outbreak, accompanied by significant precipitation over parts of Algeria, Tunisia, and Libya.

This paper uses sensitivity simulations with the limited-area model developed by the Consortium for Small-Scale Modeling (COSMO) together with analysis data and surface observations to test several hypotheses on the dynamics of this case proposed in previous work. It is demonstrated that air over central Algeria is cooled by evaporation of frontal precipitation, substantially enhancing winds at the leading edge of the cold front. This process is supported by very dry low-level air in the lee of the Atlas Mountains associated with a foehn situation. Flattening the mountain chain in a sensitivity experiment, however, has complex effects on the wind. While reduced evaporative cooling weakens the front, the elimination of the orographic blocking accelerates its penetration into the Sahara. The simulations also indicate high winds associated with a hydraulic jump at the southern slopes of the Tell Atlas.

Feeding the simulated winds into a dust emission parameterization reveals reduced emissions on the order of 20%–30% for suppressed latent heating and even more when effects of the increased precipitation on soil moisture are considered. In the experiment with the Atlas removed, effects of the overall increase in high winds are compensated by an increase in precipitation. The results suggest that a realistic representation of frontal precipitation is an important requisite to accurately model dust emission in such situations.

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Peter Knippertz, Andreas H. Fink, and Susan Pohle

Abstract

Hydrostatic tendency equations of pressure and geopotential have been used in various forms since the beginning of the twentieth century. In contrast to classical mass flux divergence formulations, all forms that involve vertical integrals of temperature tendencies contain an additional term related to geopotential tendency at the upper limit of the integral. In many previous studies, including a recent work by two of the authors on a case of unusual wintertime precipitation in tropical West Africa, it has been assumed that there exists a pressure level in the stratosphere (usually 100 or 50 hPa) where these tendencies become negligible. This assumption implies a direct relation between net column heating and surface pressure fall. Prompted by a critique of Spengler and Egger, the validity of the concept of a stratospheric level of insignificant dynamics is tested here for the previously studied case on the basis of operational analyses from the ECMWF. At least for low latitudes, significant tendencies with some spatial and temporal variations are found up to 10 hPa, which renders a general neglect of this term on a fixed pressure level problematic. These results call for a more detailed investigation of the dynamical causes of the analyzed tendencies, a reevaluation of some previous work, and a more careful design of future studies on this subject.

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