Tropical Cyclogenesis in a Tropical Wave Critical Layer: Easterly Waves

The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, is identified herein as the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside

Coarse, Intermediate and High Resolution Numerical Simulations of the Transition of a Tropical Wave Critical Layer to a Tropical Storm

Recent work has hypothesized that tropical cyclones in the deep Atlantic and eastern Pacific basins develop from within the cyclonic Kelvin cat's eye of a tropical easterly wave critical layer located equatorward of the easterly jet axis. The cyclonic critical layer is thought to be important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing flow and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the developing proto-vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. This genesis sequence and the overarching framework for describing how such hybrid wave-vortex structures become tropical depressions/storms is likened to the development of a marsupial infant in its mother's pouch, and for this reason has been dubbed the "marsupial paradigm". Here we conduct the first multi-scale test of the marsupial paradigm in an idealized setting by revisiting the Kurihara and Tuleya problem examining the transformation of an easterly wave-like disturbance into a tropical storm vortex using the WRF model. An analysis of the evolving winds, equivalent potential temperature, and relative vertical vorticity is presented from coarse (28 km), intermediate (9 km) and high resolution (3.1 km) simulations. The results are found to support key elements of the marsupial paradigm by demonstrating the existence of rotationally dominant region with minimal strain/shear deformation near the center of the critical layer pouch that contains strong cyclonic vorticity and high saturation fraction. This localized region within the pouch serves as the "attractor" for an upscale "bottom up" development process while the wave pouch and proto-vortex move together

Utilizing data-driven technology tools for community-led solutions to vacant properties and urban blight

As a city that has lost more than 1/3 of its population over the past 6 decades, some Baltimore neighborhoods suffer from a disproportionate number of vacant and abandoned properties, mired in issues of unclear ownership and “under-water” lien burdens. Cloudy legal and financial restrictions cause properties to cycle through a speculative system that strips them of all equity, and causes them to move out of reach for redevelopment. Evidence suggests that existing processes for addressing these issues, such as tax lien sales and foreclosures, can actually play a role in increasing vacancy rates and amplify neighborhood disinvestment (Dewar, Seymour, and Druță, 2015). Policies aimed at real property tax reform and foreclosure prevention can improve conditions, yet communities, non-profits, and city agencies in Baltimore lacked a unified data system to guide their reform and outreach efforts. One challenge is that property data are housed at various agencies, each using its own system of data storage and dissemination, making it difficult to use different datasets for a single property. The Baltimore City Open Land Data (BOLD) web application arose out of the need to streamline the data gathering process by integrating various datasets for easier use by stakeholders working to stabilize their communities, preserve homeownership, and break the cycle of vacant properties. This presentation will give an overview how BOLD was designed, a short demonstration of the application, and show how it can be used to further research the impact of tax sales and foreclosures in Baltimore City

Sub-seasonal variations in lower stratospheric water vapor

Observations of water vapor with high temporal and spatial resolution and good horizontal coverage just above the tropical tropopause have been scarce, but a preliminary version of such data has been developed using radiance measurements of the Microwave Limb Sounder. These data reveal distinct variations with periods in the ranges 10-25 days and 30-70 days, consistent with (respectively) slow Kelvin waves and the tropical intraseasonal oscillation

Sponge layer feedbacks in middle-atmosphere models

Middle-atmosphere models commonly employ a sponge layer in the upper portion of their domain. It is shown that the relaxational nature of the sponge allows it to couple to the dynamics at lower levels in an artificial manner. In particular, the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating is shown to induce a drag force in the sponge that modifies the response expected from the “downward control” arguments of Haynes et al. [1991]. In the case of an imposed local force the sponge acts to divert a fraction of the mean meridional mass flux upward, which for realistic parameter values is approximately equal to exp(−Δz/H), where Δz is the distance between the forcing region and the sponge layer and H is the density scale height. This sponge-induced upper cell causes temperature changes that, just below the sponge layer, are of comparable magnitude to those just below the forcing region. In the case of an imposed local diabatic heating, the sponge induces a meridional circulation extending through the entire depth of the atmosphere. This circulation causes temperature changes that, just below the sponge layer, are of opposite sign and comparable in magnitude to those at the heating region. In both cases, the sponge-induced temperature changes are essentially independent of the height of the imposed force or diabatic heating, provided the latter is located outside the sponge, but decrease exponentially as one moves down from the sponge. Thus the effect of the sponge can be made arbitrarily small at a given altitude by placing the sponge sufficiently high; e.g., its effect on temperatures two scale heights below is roughly at the 10% level, provided the imposed force or diabatic heating is located outside the sponge. When, however, an imposed force is applied within the sponge layer (a highly plausible situation for parameterized mesospheric gravity-wave drag), its effect is almost entirely nullified by the sponge-layer feedback and its expected impact on temperatures below largely fails to materialize. Simulations using a middle-atmosphere general circulation model are described, which demonstrate that this sponge-layer feedback can be a significant effect in parameter regimes of physical interest. Zonally symmetric (two dimensional) middle-atmosphere models commonly employ a Rayleigh drag throughout the model domain. It is shown that the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating, in this case, is noticeably modified from that expected from downward control, even for a very weak drag coefficien

The equivalent barotropic structure of waves in the tropical atmosphere in the Western Hemisphere

Tropical waves are generally considered to have a baroclinic structure. However, analysis of ERA-Interim and NOAA OLR data for the period 1979-2010 shows that in the equatorial and Northern Hemisphere near-equatorial regions in the tropical western hemisphere (WH), westward and eastward-moving transients, zonal wavenumber 2-10, period 2-30 days, have little tilt in the vertical, and can be said to be equivalent barotropic. The westward-moving transients in the equatorial region have large projection onto the westward mixed Rossby-Gravity (WMRG) wave and those in the near-equatorial region project onto the gravest Rossby wave and also the WMRG. The eastward-moving transients have large projections onto the Doppler shifted eastward-moving versions of these waves. To examine how such equivalent barotropic structures are possible in the tropics, terms in the vorticity equation are analysed. It is deduced that waves must have westward intrinsic phase speed and can exist in the WH with its large westerly vertical shear. Throughout the depth, the advection of vorticity by the zonal flow and the β-term are large and nearly cancel. In the upper troposphere the zonal advection by the strong westerly flow wins and the residual is partially balanced by vortex shrinking associated with divergence above a region of ascent. Below the region of ascent the β-term wins and is partially balanced by vortex stretching associated with the convergence. An equivalent barotropic structure is therefore maintained in a similar manner to higher latitudes. The regions of ascent are usually associated with deep convection and, consistently, WH waves directly connected to tropical convection are also found to be equivalent barotropic

On the existence of the logarithmic surface layer in the inner-core of hurricanes

Q. J. R. Meteorol. Soc., 1394128