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Daniel J. Halperin, Andrew B. Penny, and Robert E. Hart

Abstract

Operational forecasting of tropical cyclone (TC) genesis has improved in recent years but still can be a challenge. Output from global numerical models continues to serve as a primary source of forecast guidance. Bulk verification statistics (e.g., critical success index) of TC genesis forecasts indicate that, overall, global models are increasingly able to predict TC genesis. However, as global model configurations are updated, TC genesis verification statistics will change. This study compares operational and retrospective forecasts from three configurations of NCEP’s Global Forecast System (GFS) to quantify the impact of model upgrades on TC genesis forecasts. First, bulk verification statistics from a homogeneous sample of model initialization cycles during the period 2013–14 are compared. Then, composites of select output fields are analyzed in an attempt to identify any key differences between hit and false alarm events. Bulk statistics indicate that TC genesis forecast performance decreased with the implementation of the 2015 version of the GFS, but then modestly recovered with the 2016 version of the model. In addition, the composite analysis suggests that false alarm forecasts in the 2015 version of the GFS may have been the result of inaccurately forecasting the location and/or strength of upper-level troughs poleward of the TC. There is also evidence of convective feedbacks occurring, such as ridging above the low-level circulation and upper-level convective outflow that were too strong, in this same set of false alarm forecasts. Overall, analyzing retrospective forecasts can assist forecasters in determining the strengths and weaknesses associated with a new configuration of a global model with respect to TC genesis.

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Andrew B. Penny, Joshua P. Hacker, and Patrick A. Harr

Abstract

A nondeveloping tropical disturbance, identified as TCS025, was observed during three intensive observing periods during The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment. The low-level circulation of the disturbance was relatively weak, asymmetric, and displaced a considerable distance from the midlevel circulation. An ensemble of high-resolution numerical simulations initialized from global model analyses was used to further examine TCS025. These simulations tended to unrealistically overdevelop the TCS025 disturbance. This study extends that work by examining the impact of assimilating in situ observations of TCS025 and dual-Doppler radial velocities from the airborne Electra Doppler Radar (ELDORA) using the Data Assimilation Research Testbed (DART) ensemble data assimilation system.

The assimilation of observations results in a more accurate vortex structure that is consistent with the observational analysis. In addition, forecasts initialized from the state of the ensemble after data assimilation exhibit less development than both the control simulation and an ensemble of forecasts without prior data assimilation.

A composite analysis of developing and nondeveloping forecasts from the ensemble reveals that convection was more active in developing simulations, especially near the low-level circulation center. This led to larger diabatic heating rates, spinup of the low-level circulation from vorticity stretching, and greater alignment of the low- and midlevel vorticity centers. In contrast, nondeveloping simulations exhibited less convection, and the circulation was more heavily impacted by vertical wind shear.

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Andrew B. Penny, Patrick A. Harr, and Michael M. Bell

Abstract

Large uncertainty still remains in determining whether a tropical cloud cluster will develop into a tropical cyclone. During The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment, over 50 tropical cloud clusters were monitored for development, but only 4 developed into a tropical cyclone. One nondeveloping tropical disturbance (TCS025) was closely observed for potential formation during five aircraft research missions, which provided an unprecedented set of observations pertaining to the large-scale and convective environments of a nondeveloping system.

The TCS025 disturbance was comprised of episodic convection that occurred in relation to the diurnal cycle along the eastern extent of a broad low-level trough. The upper-level environment was dominated by two cyclonic cells in the tropical upper-tropospheric trough (TUTT) north of the low-level trough in which the TCS025 circulation was embedded. An in-depth examination of in situ observations revealed that the nondeveloping circulation was asymmetric and vertically misaligned, which led to larger system-relative flow on the mesoscale. Persistent environmental vertical wind shear and horizontal shearing deformation near the circulation kept the system from becoming better organized and appears to have allowed low equivalent potential temperature () air originating from one of the TUTT cells to the north (upshear) to impact the thermodynamic environment of TCS025. This in turn weakened subsequent convection that might otherwise have improved alignment and contributed to the transition of TCS025 to a tropical cyclone.

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Andrew B. Penny, Patrick A. Harr, and James D. Doyle

Abstract

An analysis of in situ observations from the nondeveloping tropical disturbance named TCS025 revealed that a combination of unfavorable system-scale and environmental factors limited further development. In this study, a multiphysics ensemble of high-resolution simulations of TCS025 are analyzed and compared. A simulation that overdeveloped the TCS025 disturbance is compared with one that correctly simulated nondevelopment and reveals that convection was stronger and diabatic heating rates were larger in the developing simulation. This led to continued spinup of the low-level circulation primarily through vorticity stretching. In contrast, convection was much weaker in the nondeveloping simulation, and after an initial period of deep convection, average vorticity tendencies from stretching became weakly negative, which allowed for the frictional spindown of the low-level circulation.

Convective-scale differences identified early in the simulations appear to have resulted from the explicit representation of graupel in the developing simulation. The net impacts resulting from these differences in convection are manifest in the average diabatic heating profiles that are important for determining the developmental outcome. Additional simulations are conducted whereby the diabatic heating rates are artificially adjusted. Relatively small changes in the diabatic heating rate led to significantly different outcomes with respect to storm development, and the degree of overdevelopment is largely dictated by the diabatic heating rate. These findings suggest the correct representation of convective processes and associated diabatic heating are necessary to adequately forecast tropical cyclogenesis, especially for systems near a threshold of development like TCS025.

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John P. Cangialosi, Eric Blake, Mark DeMaria, Andrew Penny, Andrew Latto, Edward Rappaport, and Vijay Tallapragada

Abstract

It has been well documented that the National Hurricane Center (NHC) has made significant improvements in Atlantic basin tropical cyclone (TC) track forecasting during the past half century. In contrast, NHC’s TC intensity forecast errors changed little from the 1970s to the early 2000s. Recently, however, there has been a notable decrease in TC intensity forecast error and an increase in intensity forecast skill. This study documents these trends and discusses the advancements in TC intensity guidance that have led to the improvements in NHC’s intensity forecasts in the Atlantic basin. We conclude with a brief projection of future capabilities.

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Myung-Sook Park, Andrew B. Penny, Russell L. Elsberry, Brian J. Billings, and James D. Doyle

Abstract

Latent heating and cooling rates have a critical role in predicting tropical cyclone formation and intensification. In a prior study, Park and Elsberry estimated the latent heating and cooling rates from aircraft Doppler radar [Electra Doppler Radar (ELDORA)] observations for two developing and two nondeveloping tropical disturbances during the Tropical Cyclone Structure 2008 (TCS-08) field experiment. In this study, equivalent retrievals of heating rates from two mesoscale models with 1-km resolution are calculated with the same radar thermodynamic retrieval. Contoured frequency altitude diagrams and vertical profiles of the net latent heating rates from the model are compared with the ELDORA-retrieved rates in similar cloud-cluster regions relative to the center of circulation.

In both the developing and nondeveloping cases, the radar-equivalent retrievals from the two models tend to overestimate heating for less frequently occurring, intense convective cells that contribute to positive vorticity generation and spinup in the lower troposphere. The model maximum cooling rates are consistently smaller in magnitude than the heating maxima for the nondeveloping cases as well as the developing cases. Whereas in the model the cooling rates are predominantly associated with melting processes, the effects of evaporative cooling are underestimated in convective downdraft regions and at upper levels. Because of the net warming of the columns, the models tend to overintensify the lower-tropospheric circulations if these intense convective cells are close to the circulation center. Improvements in the model physical process representations are required to realistically represent the evaporative cooling effects.

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Anu Simon, Andrew B. Penny, Mark DeMaria, James L. Franklin, Richard J. Pasch, Edward N. Rappaport, and David A. Zelinsky

Abstract

This study discusses the development of the Hurricane Forecast Improvement Program (HFIP) Corrected Consensus Approach (HCCA) for tropical cyclone track and intensity forecasts. The HCCA technique relies on the forecasts of separate input models for both track and intensity and assigns unequal weighting coefficients based on a set of training forecasts. The HCCA track and intensity forecasts for 2015 were competitive with some of the best-performing operational guidance at the National Hurricane Center (NHC); HCCA was the most skillful model for Atlantic track forecasts through 48 h. Average track input model coefficients for the 2015 forecasts in both the Atlantic and eastern North Pacific basins were largest for the European Centre for Medium-Range Weather Forecasts (ECMWF) deterministic model and the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) ensemble mean, but the relative magnitudes of the intensity coefficients were more varied. Input model sensitivity experiments conducted using retrospective HCCA forecasts from 2011 to 2015 indicate that the ECMWF deterministic model had the largest positive impact on the skill of the HCCA track forecasts in both basins. The most important input models for HCCA intensity forecasts are the Hurricane Weather Research and Forecasting (HWRF) Model and the Coupled Ocean–Atmosphere Mesoscale Prediction System-Tropical Cyclone (COAMPS-TC) model initialized from the GFS. Several updates were incorporated into the HCCA formulation prior to the 2016 season. Verification results indicate HCCA continued to be a skillful model, especially for short-range (12–48 h) track forecasts in both basins.

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Peter Black, Lee Harrison, Mark Beaubien, Robert Bluth, Roy Woods, Andrew Penny, Robert W. Smith, and James D. Doyle

Abstract

The High-Definition Sounding System (HDSS) is an automated system deploying the expendable digital dropsonde (XDD) designed to measure wind and pressure–temperature–humidity (PTH) profiles, and skin sea surface temperature (SST) within and around tropical cyclones (TCs) and other high-impact weather events needing high sampling density. Three experiments were conducted to validate the XDD.

On two successive days off the California coast, 10 XDDs and 14 Vaisala RD-94s were deployed from the navy’s Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft over offshore buoys. The Twin Otter made spiral descents from 4 km to 60 m at the same descent rate as the sondes. Differences between successive XDD and RD-94 profiles due to true meteorological variability were on the same order as the profile differences between the spirals, XDDs, and RD-94s. XDD SST measured via infrared microradiometer, referred to as infrared skin SST (SSTir), and surface wind measurements were within 0.5°C and 1.5 m s−1, respectively, of buoy and Twin Otter values.

A NASA DC-8 flight launched six XDDs from 12 km between ex-TC Cosme and the Baja California coast. Repeatability was shown with good agreement between features in successive profiles. XDD SSTir measurements from 18° to 28°C and surface winds agreed well with drifting buoy- and satellite-derived estimates.

Excellent agreement was found between PTH and wind profiles measured by XDDs deployed from a NASA WB-57 at 18-km altitude offshore from the Texas coast and NWS radiosonde profiles from Brownsville and Corpus Christi, Texas. Successful XDD profiles were obtained in the clear and within precipitation over an offshore squall line.

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Dan Lubin, Damao Zhang, Israel Silber, Ryan C. Scott, Petros Kalogeras, Alessandro Battaglia, David H. Bromwich, Maria Cadeddu, Edwin Eloranta, Ann Fridlind, Amanda Frossard, Keith M. Hines, Stefan Kneifel, W. Richard Leaitch, Wuyin Lin, Julien Nicolas, Heath Powers, Patricia K. Quinn, Penny Rowe, Lynn M. Russell, Sangeeta Sharma, Johannes Verlinde, and Andrew M. Vogelmann
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Dan Lubin, Damao Zhang, Israel Silber, Ryan C. Scott, Petros Kalogeras, Alessandro Battaglia, David H. Bromwich, Maria Cadeddu, Edwin Eloranta, Ann Fridlind, Amanda Frossard, Keith M. Hines, Stefan Kneifel, W. Richard Leaitch, Wuyin Lin, Julien Nicolas, Heath Powers, Patricia K. Quinn, Penny Rowe, Lynn M. Russell, Sangeeta Sharma, Johannes Verlinde, and Andrew M. Vogelmann

Abstract

The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) West Antarctic Radiation Experiment (AWARE) performed comprehensive meteorological and aerosol measurements and ground-based atmospheric remote sensing at two Antarctic stations using the most advanced instrumentation available. A suite of cloud research radars, lidars, spectral and broadband radiometers, aerosol chemical and microphysical sampling equipment, and meteorological instrumentation was deployed at McMurdo Station on Ross Island from December 2015 through December 2016. A smaller suite of radiometers and meteorological equipment, including radiosondes optimized for surface energy budget measurement, was deployed on the West Antarctic Ice Sheet between 4 December 2015 and 17 January 2016. AWARE provided Antarctic atmospheric data comparable to several well-instrumented high Arctic sites that have operated for many years and that reveal numerous contrasts with the Arctic in aerosol and cloud microphysical properties. These include persistent differences in liquid cloud occurrence, cloud height, and cloud thickness. Antarctic aerosol properties are also quite different from the Arctic in both seasonal cycle and composition, due to the continent’s isolation from lower latitudes by Southern Ocean storm tracks. Antarctic aerosol number and mass concentrations are not only non-negligible but perhaps play a more important role than previously recognized because of the higher sensitivities of clouds at the very low concentrations caused by the large-scale dynamical isolation. Antarctic aerosol chemical composition, particularly organic components, has implications for local cloud microphysics. The AWARE dataset, fully available online in the ARM Program data archive, offers numerous case studies for unique and rigorous evaluation of mixed-phase cloud parameterization in climate models.

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