Physical processes determining the dynamic and thermodynamic structure of a tropical cyclone boundary layer (TCBL) are quite different from anywhere else in the atmospheric boundary layer due to the substantial contribution of latent heating and frictional convergence. These processes regulate the radial and vertical distributions of momentum and enthalpy fluxes that are closely related to storm development and intensification. Our current understanding of TCBL is limited by the number of observations in this region, and a majority of the observational studies assume an axisymmetric structure. Three-dimensional observations and numerical studies show that substantial asymmetric structure exists in the TCBL. This study investigates the link between the asymmetric structure and small-scale processes using a Moist Potential Vorticity (MPV) framework. The simulated TCBL is uniquely characterized as a region of negative MPV with a robust and coherent layer of high-magnitude negative MPV embedded within, referred to as the Potential Vorticity Minimum Layer (PVML). The PVML can interact with the local flow anomalies such as those associated with roll vortices provided they are vertically collocated. The small-scale dynamical processes set the thermodynamic structure inside the TCBL and this interplay modulates the height of the PVML. Since the height of the PVML combines information about the local wind and thermal structures using a materially conserved variable, it is a valuable proxy to study the evolving 'topography' of a simulated TCBL.
ABSTRACT:Recently available satellite observations from the water vapor channel (6.5-7.1 µm) of the Imager on-board India's geostationary satellite, INSAT-3D have been used to estimate Upper Tropospheric Humidity (UTH). In this study, operationally retrieved UTH product has been compared and validated for the period of Jan-Jun, 2014, using in-situ and satellite measurements. In-situ measurements of UTH have been indirectly derived using humidity profiles obtained from a network of radiosonde stations from NOAA/ESRL database. Meteosat-7 UTH products have been used as satellite measurements. The validation of INSAT-3D UTH against UTH derived from radiosonde profiles shows reasonable agreement, with linear correlation coefficients ranging from 0.78 to 0.87 and the slope of the regression line ranging from 0.52 to 0.77. The UTH tends to overestimate observed humidity by ~4% with RMS difference of ~12%. Comparison of INSAT-3D UTH product with Meteosat-7 UTH product suggests a good match with RMS difference of 7.61% and a mean bias of -0.43%, linear correlation coefficients varying from 0.88 to 0.93 and slope of the regression line varying from 0.64 to 1.08. The UTH products from INSAT-3D and Meteosat-7 have also been inter-compared by validating the two against the UTH derived from a set of collocated radiosonde observations. INSAT-3D UTH shows a RMSD of 10.65% and bias of 0.78% which matches very well with Meteosat-7 UTH with a RMSD of 10.31% and bias of -0.53%.
The Tropical Cyclone Boundary Layer (TCBL) is uniquely favorable for the formation of highly asymmetric coherent structures called roll-vortices. They are kilometer-scale eddies that are aligned in the mean tangential wind direction and are generally associated with a combination of shear and convective instability. Roll-vortices can cause periodic enhancements of winds as well as surface fluxes and may assist the intensification process via vertical transport of tangential momentum. Furthermore, the up-gradient transfer of surface heat and momentum fluxes associated with roll-vortices can energize the large-scale motions, possibly resulting in an increased convective activity. It is therefore important to understand and quantify the effects of roll-vortices in setting the overall TCBL structure as well as near-surface winds. Synthetic Aperture Radars onboard low-earth orbit (LEO) satellites such as Sentinel-1A/B and RadarSat-2 have often been used to identify TCBL roll-vortices. Based on the positive correlation between microwave backscattering and sea surface roughness, sea surface wind speed can be retrieved at high spatial resolution (1–3 km), without any saturation for high wind speed. However, the spatial distribution of roll-vortices in the TCBL and their subsequent impact on the vertical momentum and enthalpy fluxes are not well understood.  In this study, we use standard two-dimensional spectral analysis to detect roll-vortices in SAR-observed winds over multiple tropical cyclones (TCs) and use TC-relative composites to analyze their spatial distributions. We will also study the vertical wind and thermodynamic structures in the vicinity of SAR-observed roll-vortices using collocated aircraft observations whenever available. This may also allow us to extract empirical relationships between the roll-averaged vertical profiles of near-surface fluxes and maximum near-surface winds.
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