Tropical cyclone flow asymmetries induced by a uniform flow revisited

The article of record as published may be found at: http://dx.doi.org10.1002/2015MS000477We investigate the hypothesized effects of a uniform flow on the structural evolution of a tropical cyclone using a simple idealized, three-dimensional, convection-permitting, numerical model. The study addresses three outstanding basic questions concerning the effects of moist convection on the azimuthal flow asymmetries and provides a bridge between the problem of tropical cyclone intensification in a quiescent environment and that in the vertical shear over a deep tropospheric layer. At any instant of time, explicit deep convection in the model generates flow asymmetries that ten to mask the induced flow asymmetries predicted by the dry, slab boundary layer of Shapiro, whose results are frequently invoked as a benchmark for characterizing the boundary layer-induced vertical motion for a translating storm. In sets of ensemble experiment in which the initial low-level moisture field is randomly perturbed, time-averaged ensemble mean fields in the mature stage show a coherent asymmetry in the vertical motion rising into the eyewall and in the total (horizontal) wind speed just above the boundary layer. The maximum ascent occurs about 45 degrees to the left of the vortex motion vector, broadly in support of Shapiro's results, in which it occurs ahead of the storm, and consistent with one earlier more complex numerical calculation by Frank and Ritchie. The total wind asymmetry just above the boundary layer has a maximum in the forward right sector, which is in contrast to the structure effectively prescribed by Shapiro based on an inviscid dry symmetric vortex translating in a uniform flow where, in an Earth-relative frame, the maximum in on the right.G.L.T. and R.K.S. were supported in part by grant SM 30/23-1 from the German Research Council (DFG). R.K.S. is supported also by the Office of Naval Research Global under grant N62909-15-1-N021. M.T.M. acknowledges the supports of NSF grants AGS-0733380 and NSF AGS-0851077 and NASA grants NNH09AK561 and NNG09HG031Approved for public release; distribution is unlimited

A vortical hot tower route to tropical cyclogenesis

J. Atmos. Sci., NASA/CAMEX special issue, 63, 355-386The article of record as published may be located at http://dx.doi.org/10.1175/JAS3604.

Age-related Changes in Lateral Entorhinal and CA3 Neuron Allocation Predict Poor Performance on Object Discrimination

Age-related memory deficits correlate with dysfunction in the CA3 subregion of the hippocampus, which includes both hyperactivity and overly rigid activity patterns. While changes in intrinsic membrane currents and interneuron alterations are involved in this process, it is not known whether alterations in afferent input to CA3 also contribute. Neurons in layer II of the lateral entorhinal cortex (LEC) project directly to CA3 through the perforant path, but no data are available regarding the effects of advanced age on LEC activity and whether these activity patterns update in response to environmental change. Furthermore, it is not known the extent to which age-related deficits in sensory discrimination relate to the inability of aged CA3 neurons to update in response to new environments. Young and aged rats were pre-characterized on a LEGO© object discrimination task, comparable to behavioral tests in humans in which CA3 hyperactivity has been linked to impairments. The cellular compartment analysis of temporal activity with fluorescence in situ hybridization for the immediate-early gene Arc was then used to identify the principal cell populations that were active during two distinct epochs of random foraging in different environments. This approach enabled the extent to which rats could discriminate two similar objects to be related to the ability of CA3 neurons to update across different environments. In both young and aged rats, there were animals that performed poorly on the LEGO object discrimination task. In the aged rats only, however, the poor performers had a higher percent of CA3 neurons that were active during random foraging in a novel environment, but this is not related to the ability of CA3 neurons to remap when the environment changed. Afferent neurons to CA3 in LEC, as identified with the retrograde tracer choleratoxin B (CTB), also showed a higher percentage of cells that were positive for Arc mRNA in aged poor performing rats. This suggests that LEC contributes to the hyperactivity seen in CA3 of aged animals with object discrimination deficits and age-related cognitive decline may be the consequence of dysfunction endemic to the larger network

Midlevel Ventilation's Constraint on Tropical Cyclone Intensity

Midlevel ventilation, or the flux of low-entropy air into the inner core of a tropical cyclone (TC), is a hypothesized mechanism by which environmental vertical wind shear can constrain a tropical cyclone’s intensity. An idealized framework based on steadiness, axisymmetry, and slantwise neutrality is developed to assess how ventilation affects tropical cyclone intensity via two possible pathways: the first through downdrafts outside the eyewall and the second through eddy fluxes directly into the eyewall. For both pathways, ventilation has a detrimental effect on tropical cyclone intensity by decreasing the maximum steady-state intensity significantly below the potential intensity, imposing a minimum intensity below which a TC will unconditionally decay, and providing an upper-ventilation bound beyond which no steady tropical cyclone can exist. Ventilation also decreases the thermodynamic efficiency as the eyewall becomes less buoyant relative to the environment, which compounds the effects of ventilation alone. Finally, the formulation presented in this study is shown to be invariant across a range of thermodynamic environments after a suitable normalization and shows little sensitivity to external parameters.National Science Foundation (U.S.) (Grant No. ATM-0850639

A self-weakening mechanism for tropical cyclones

A mechanism leading to the self-weakening of tropical cyclones is proposed using the Weather and Research and Forecasting model. A comparison between an experiment with variable Coriolis parameter f and one on an f-plane shows that after the initial intensification the former is characterized by a smaller intensity. As opposed to the tropical cyclone on the f-plane, the one with variable f weakens significantly after reaching maturity. Analyses of the 3-D circulation show that the main reason for the weakening is dry intrusion in the mid-upper troposphere from the west. Once the dry intrusion reaches the inner vortex, strong downdrafts reduce the high equivalent potential temperature in the boundary layer inflow. The subsequent updrafts in the eyewall, characterized by lower equivalent potential temperature, are considerably reduced and, consequently, the secondary circulation weakens. Back-trajectories are used to determine the origin of the dry intrusion. It is found that the air parcels expelled from the storm deep convection into the outflow layer recirculate anticyclonically back into the vortex, causing a self-weakening of the tropical cyclone. A time span for the recirculation of at least 48 h allows the air parcels to sink substantially before reaching the vortex circulation. Some implications of the intrinsic nature of this process are briefly discussed