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  • Author or Editor: David R. Ryglicki x
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David R. Ryglicki and Daniel Hodyss

Abstract

A deeper analysis of possible errors and inconsistencies in the analysis of vortex asymmetries owing to the placement of centers of tropical cyclones (TCs) in mesoscale models is presented. Previous works have established that components of the 2D and 3D structure of these TCs—primarily radial wind and vertical tilt—can vary greatly depending on how the center of a model TC is defined. This work will seek to expand the previous research on this topic, but only for the 2D structure. To be specific, this work will present how low-wavenumber azimuthal Fourier analyses can vary with center displacement using idealized, parametric TC-like vortices. It is shown that the errors associated with aliasing the mean are sensitive primarily to the difference between the peak of vorticity inside the radius of maximum winds and the average vorticity inside the core. Tangential wind and vorticity aliasing occur primarily in the core; radial wind aliasing spans the whole of the vortex. It is also shown that, when adding low-wavenumber asymmetries, the aliasing is dependent on the placement of the center relative to the location of the asymmetries on the vortex. It is also shown that the primary concern for 2D analysis when calculating the center of a TC is correctly resolving azimuthal wavenumber 0 tangential wind, because errors here will alias onto all higher wavenumbers, the specific structures of which are dependent on the structure of the mean vortex itself.

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David R. Ryglicki and Robert E. Hart

Abstract

A variety of tropical-cyclone (TC) center-finding methods aggregated from previous works of mesoscale modeling and operational analysis are compared. The previous methods used can be divided into three classes: local extreme, weighted grid point, and minimization of azimuthal variance. To analyze these methods, four representative separate TC forecasts from three operational models—the Coupled Ocean–Atmosphere Mesoscale Prediction System Tropical Cyclone version, a Geophysical Fluid Dynamics Laboratory model, and the Hurricane Weather Research and Forecasting Model—are examined. It is found that for this dataset the spread of the derived TC centers is fairly small between 1000 and 600 hPa but begins to increase rapidly at higher levels. All models exhibit increased center spread at upper levels when the TCs’ strengths fall below approximately hurricane strength. On a given pressure level, tangential wind differences calculated from different centers are generally small and localized, whereas radial wind differences are often much larger in both space and relative magnitude. Center-finding techniques that use mass fields to calculate centers exhibit the smallest vertical tilts for hurricane-strength TCs. Conversely, potential vorticity centroids with large weighting areas produce the largest tilts. Given the potential sensitivity of center determination and implied tilt for various other measures of TC structure (radius of maximum winds), these results may have large repercussions on both past and future analyses.

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James R. Campbell, Erica K. Dolinar, Simone Lolli, Gilberto J. Fochesatto, Yu Gu, Jasper R. Lewis, Jared W. Marquis, Theodore M. McHardy, David R. Ryglicki, and Ellsworth J. Welton

Abstract

Cirrus cloud daytime top-of-the-atmosphere radiative forcing (TOA CRF) is estimated for a 2-yr NASA Micro-Pulse Lidar Network (532 nm; MPLNET) dataset collected at Fairbanks, Alaska. Two-year-averaged daytime TOA CRF is estimated to be between −1.08 and 0.78 W·m−2 (from −0.49 to 1.10 W·m−2 in 2017, and from −1.67 to 0.47 W·m−2 in 2018). This subarctic study completes a now trilogy of MPLNET ground-based cloud forcing investigations, following midlatitude and tropical studies by Campbell et al. at Greenbelt, Maryland, and Lolli et al. at Singapore. Campbell et al. hypothesize a global meridional daytime TOA CRF gradient that begins as positive at the equator (2.20–2.59 W·m−2 over land and from −0.46 to 0.42 W·m−2 over ocean at Singapore), becomes neutral in the midlatitudes (0.03–0.27 W·m−2 over land in Maryland), and turns negative moving poleward. This study does not completely confirm Campbell et al., as values are not found as exclusively negative. Evidence in historical reanalysis data suggests that daytime cirrus forcing in and around the subarctic likely once was exclusively negative. Increasing tropopause heights, inducing higher and colder cirrus, have likely increased regional forcing over the last 40 years. We hypothesize that subarctic interannual cloud variability is likely a considerable influence on global cirrus cloud forcing sensitivity, given the irregularity of polar versus midlatitude synoptic weather intrusions. This study and hypothesis lay the basis for an extrapolation of these MPLNET experiments to satellite-based lidar cirrus cloud datasets.

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