Tropical tropopause layer

Observations of temperature, winds, and atmospheric trace gases suggest that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp ‘‘tropopause.’’ In the tropics, this layer is often called the ‘‘tropical tropopause layer’’ (TTL). We present an overview of observations in the TTL and discuss the radiative, dynamical, and chemical processes that lead to its timevarying, three-dimensional structure. We present a synthesis definition with a bottom at 150 hPa, 355 K, 14 km (pressure, potential temperature, and altitude) and a top at 70 hPa, 425 K, 18.5 km. Laterally, the TTL is bounded by the position of the subtropical jets. We highlight recent progress in understanding of the TTL but emphasize that a number of processes, notably deep, possibly overshooting convection, remain not well understood. The TTL acts in many ways as a ‘‘gate’’ to the stratosphere, and understanding all relevant processes is of great importance for reliable predictions of future stratospheric ozone and climate

Genesis of Pre-hurricane Felix (2007). Part II

J. Atmos. Sci., 67, 1730-1744The article of record as published may be located at http://dx.doi.org/10.1175/2010JAS3435.1The article of record as published may be located at http://dx.doi.org/10.1175/2010JAS3435.

Genesis of Pre-hurricane Felix (2007). Part I

J. Atmos. Sci., 67, 1711-1729The article of record as published may be located at http://dx.doi.org/10.1175/2009JAS3420.1The article of record as published may be located at http://dx.doi.org/10.1175/2009JAS3420.

Dynamical properties of developing tropical cyclones using Lagrangian flow topology

The article of record as published may be found at http://dx.doi.org/10.1002/qj.3196The goal of this study is to characterize the dynamics and structure of tropical cyclone formation from global model analyses to determine thresholds marking the various stages of development which can be computed from the analysis data. We introduce here a new methodology for identifying disturbances that show high likelihood of becoming a tropical depression or tropical storm. We use Lagrangian frame-independent quantities to define intrinsic coordinates for candidate disturbances prior to and post tropical depression declaration in the best-track data (HURDAT2). We use these Lagrangian quantities also as metrics for quantifying the strength of these systems within global model analyses as they are declared depressions, storms or hurricanes in HURDAT2. The criteria proposed are more precise than similar Eulerian criteria since the minimum thresholds for development in the best-track dataset are very close to the threshold at which no false alarms are produced in the global model analyses. Since only very loose thermodynamic thresholds are required, these criteria can be considered dynamically based and require no statistical analysis to compensate for uncertainties in moisture or convection. We describe further the structure of the developing systems using a set of objective profiles where level contours of the Lagrangian averaged rotation rate are mapped to an equivalent radius. Composites of these profiles reveal that the transition to tropical storm strength vortices is marked by the existence of a notable shear sheath outside a region of enhanced solid-body rotation

Lagrangian vortices in developing tropical cyclones

The article of record as published may be found at http://dx.doi.org/10.1002/qj.2616Tracking pre-genesis tropical cyclones is important for earlier detection of developing systems as well as targeting potential locations for dropsondes in field experiments. The use of a reference frame moving with the disturbance gives a more accurate depiction of streamlines and closed circulation than the Earth-relative frame. However, identification of recirculating regions does not require a choice of reference frame when marked by the Galilean invariant Eulerian Okubo– Weiss (OW) parameter. While the Eulerian OW parameter is generally effective at identifying vortex cores at a given place and a given time, it has its limitations in weak disturbances and in time-dependent flows. Integrating the eigenvalue of the velocity gradient tensor along particle trajectories provides a time- smoothing of the Eulerian OW parameter, and provides earlier detection with fewer false alarms. We refer to this integration along trajectories as the Lagrangian OW parameter. When mapped to a horizontal grid it becomes a Lagrangian OW field. The Lagrangian OW field has advantages over the Eulerian OW field in the detail of additional flow structures that it identifies. The Lagrangian OW field shows the Lagrangian boundaries that are present as a disturbance develops from an easterly wave, and a shear sheath that forms when a disturbance becomes self-sustaining, typically at tropical storm strength. Since all of these structures are Lagrangian, they are advected with the flow field, and display the continuous evolution of coherent flow features as the fluid evolves. Examples of the use of the Lagrangian OW field are given for ECMWF forecast data from the 2014 Atlantic hurricane season. All of the Lagrangian coherent structures that can be identified by this field are shown for developing disturbances and mature cyclones. The Lagrangian OW field also shows additional details of vortex mergers, and is used to identify a stable crystal lattice configuration in which vorticity does not aggregate.National Science FoundationNASANaval Postgraduate SchoolGrant AGS-1439283 (NSF)Grant AGS-1313948 (NSF)Grant NNG11PK021 (NASA