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.

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 WisconsinNonhydrostatic 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.

This study tests the hypothesis that Arctic amplification (AA) of global warming remotely affects midlatitudes by promoting a weaker, wavier atmospheric circulation conducive to extreme weather. The investigation is based on the late twenty-first century over greater North America (20°–90°N, 50°–160°W) using 40 simulations from the Community Earth System Model Large Ensemble, spanning 1920–2100. AA is found to promote regionally varying ridging aloft (500 hPa) with strong seasonal differences reflecting the location of the strongest surface thermal forcing. During winter, maximum increases in future geopotential heights are centered over the Arctic Ocean, in conjunction with sea ice loss, but minimum height increases (troughing) occur to the south, over the continental United States. During summer the location of maximum height inflation shifts equatorward, forming an annular band across mid-to-high latitudes of the entire Northern Hemisphere. This band spans the continents, whose enhanced surface heating is aided by antecedent snow-cover loss and reduced terrestrial heat capacity. Through the thermal wind relationship, midtropospheric winds weaken on the equatorward flank of both seasonal ridging anomalies—mainly over Canada during winter and even more over the continental United States during summer—but strengthen elsewhere to form a dipole anomaly pattern in each season. Changes in circulation waviness, expressed as sinuosity, are inversely correlated with changes in zonal wind speed at nearly all latitudes, both in the projections and as observed during recent decades. Over the central United States during summer, the weaker and wavier flow promotes drying and enhanced heating, thus favoring more intense summer weather.

The production of a narrow, heavy, occasionally convective snowband that fell within a modest surface cyclone on 19 January 1995 is examined using gridded model output from a successful numerical simulation performed using the University of Wisconsin-Nonhydrostatic Modeling System. It is found that the snowband was produced by a thermally direct vertical circulation forced by significant lower-tropospheric warm frontogenesis in the presence of across-front effective static stability differences as measured in terms of the equivalent potential vorticity (PV e). The sometimes convective nature of the snowband resulted from the development of freely convective motions forced by frontal lifting of the environmental stratification. Model trajectories demonstrate that a stream of warm, moist air ascended through the trowal portion of the warm-occluded structure that developed during the cyclone life cycle. The lifting of air in the trowal was, in this case, forced by lower-tropospheric frontogenesis occurring in the warm-frontal portion of the warm occlusion. This trowal airstream accounts for the production of the so-called wrap-around precipitation often associated with occluded cyclones and, in this case, accounted for the northern third of the heavy snowband.

Surface and upper-air analyses from the ECMWF Tropical Ocean Global Atmosphere (TOGA) dataset are used to construct a climatology of 177 Alberta clippers over 15 boreal cold seasons (October-March) from 1986/87 to 2000/01. The Alberta clipper (hereafter simply clipper) occurs most frequently during December and January and substantially less frequently during October and March. These cyclones generally move southeastward from the lee of the Canadian Rockies toward or just north of Lake Superior before progressing eastward into southeastern Canada or the northeastern United States, with less than 10% of the cases in the climatology tracking south of the Great Lakes. Characteristics of the structure and evolution of clippers during a 36-h period leading up to departure of the cyclone from the lee of the Canadian Rockies and a 60-h period after departure as the cyclone traverses central and eastern North America are examined through composite analyses. Over the course of the predeparture period, a cyclone over the Gulf of Alaska approaches the west coast of North America, and through its interaction with the mountainous terrain of western North America spawns a surface lee trough, characterized by a thermal ridge at 850 hPa, to the east of the Canadian Rockies. This thermal ridge dampens considerably as the composite clipper moves into central North America away from the immediate lee of the Canadian Rockies. The composite clipper system evolves from a lee cyclone with its nonclassical thermal structure to a more classically structured midlatitude cyclone as it moves through central and eastern North America largely as a result of rotation of the low-level thermal gradient and the increasing westward tilt with height of the composite clipper over the last 36 h of the postdeparture period. The thermal gradient rotation is dynamically linked to convergence of the along-isentrope component of the Q vector and thus to the ascent that sustains the clipper and creates some of its characteristic sensible weather elements. Such dynamical forcing is a direct consequence of the persistent westward displacement of the 500-hPa vorticity maximum with respect to the composite clipper sea level pressure minimum that characterizes the postdeparture period.

The effect of latent heat release on the development of the occluded thermal structure in a major winter storm is examined through comparison of full physics (FP) and no-latent-heat-release (NLHR) simulations of the event performed using the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5). Though both simulations possess a well-developed occluded thermal ridge near the surface, the 3D structure of their respective occluded quadrants is quite different. In particular, the FP simulation depicts the canonical, troposphere-deep warm occluded thermal structure, whereas the NLHR simulation produces only a shallow, poorly developed one. Consistent with these differences in tropospheric thermal structure, the FP cyclone displays a robust treble clef potential vorticity (PV) distribution in the upper troposphere in its postmature phase, while a considerably less robust version characterizes the NLHR simulation. The PV minimum of the treble clef overlies a poleward sloping column of warm, weakly stratified air that extends through the depth of the troposphere and is a signature of the trowal, the essential structural feature of warm occluded cyclones. Consequently, examination of the role played by latent heat release in production of the occluded thermal structure in this case is made through consideration of its influence on the evolution of the upper-tropospheric PV morphology. It is found that direct dilution of upper-tropospheric PV by midtropospheric latent heat release initiates formation of a local, upper-tropospheric PV minimum, or low PV tongue, to the northwest of the surface cyclone center. The production of this PV minimum initiates a cutting off of the upper-tropospheric PV anomaly associated with the surface development. The upper-tropospheric circulation associated with this cutoff anomaly, in turn, forces the advection of low (1 PVU) values of PV into the developing PV trough. This combination of kinematic and diabatic processes acts to produce both the tropopause PV treble clef as well as the underlying warm occluded thermal structure in the FP simulation. In contrast, though an adiabatic kinematic tendency for production of a treble clef PV morphology operates in the NLHR simulation, the resulting PV and thermal structures are weaker and slower to evolve than those produced in the FP simulation. Thus, it is suggested that latent heat release plays an indispensable role in the production of the characteristic occluded thermal structures observed in nature.

A numerical model-based analysis of the quasigeostrophic forcing for ascent in the occluded quadrant of three cyclones is presented based upon a natural coordinate partitioning of the Q vector into its along-and acrossisentrope components, Q s and Q n , respectively. The Q n component describes the geostrophic contribution to the rate of change of the magnitude of ١ p (traditional frontogenesis), whereas the Q s component describes the geostrophic contribution to the rate of change of direction of ١ p (rotational frontogenesis). It is shown that convergence of Q s simultaneously creates the isobaric thermal ridge characteristic of the thermal structure of occluded cyclones and provides the predominant dynamical support for ascent within the occluded quadrant. The absence of significant Q n convergence there suggests that quasigeostrophic (Q-G) frontogenesis plays a subordinate role both in forcing vertical motions and in affecting three-dimensional structural changes in the occluded sector of post-mature phase midlatitude cyclones. A cyclonically ascending, cloud-and precipitation-producing airstream that originates in the warm-sector boundary layer and flows through the trowal portion of the occluded structure is supported by the upward vertical motions implied by the identified Q-G forcing. This airstream is referred to as the ''trowal airstream'' and it is shown to be responsible for the production of the ''wrap around'' cloud and precipitation commonly associated with occluded systems. The relationship of the trowal airstream to previously identified cloud and precipitation producing airflows in cyclones is discussed.

The intensification of frontal characteristics in the region above the mid-latitude jet core within the lower stratospheric portion of an upper-level jet front system (ULJF) is known as lower stratospheric frontogenesis. Four recent cases of lower stratospheric frontogenesis in southwesterly flow are examined in order to elucidate the interaction between lower stratospheric dynamical processes and tropospheric ascent that characterizes such developments. In all of the cases examined the lower stratospheric front was (1) characterized by lower stratospheric quasi-geostrophic forcing for ascent on its cold side, and (2) parallel to a surface cold front.As latent heating associated with ascent along the surface cold front redistributed the potential temperature field within the upper troposphere, the stability of the near-tropopause upper troposphere decreased, thus intensifying the response to lower stratospheric frontal forcing by enhancing the frontogenetic ascent within the upper troposphere. It is therefore suggested that ascent originating in the lower troposphere is able to influence the development of lower stratospheric fronts and substantially alter the structure of the mid-latitude tropopause and its associated horizontal potential vorticity gradient at and above the jet core. The implications of lower stratospheric frontogenetic processes on facilitating tropical-extratropical interactions is discussed.Key Words: lower stratospheric fronts; convection; stability reduction