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Chi-Sann Liou and Russel L. Elsberry

Abstract

Diagnosis of the conditions associated with explosive maritime cyclogenesis is hindered by the lack of observations. A heat budget approach with analyses and forecasts for a rapid cyclogenesis event during the FGGE period is used in this study. An explosively deepening cyclone in the western North Pacific during 12–15 January 1979 is selected from the analyses by the European Centre for Medium-Range Weather Forecasts. A fairly good numerical prediction of the cyclogenesis is available from the UCLA 9-layer general circulation model. Synoptic feature comparisons and quasi-Lagrangian heat budget results show that the model captured the major mechanisms of the explosive maritime cyclogenesis event. Use of the forecast fields in combination with the analyses allows the inference of heating field distributions during rapid cyclogenesis that are not available from budget studies based on analyzed fields. For both analyzed and predicted cyclogenesis, diagnosed diabatic heating rates are much larger than found in earlier studies of less intense cyclogenesis. Diabatic heating (latent heat release) at 600–700 mb is the primary process to maintain strong vertical motion during the most rapid deepening period. The cyclone deepening rate and the inferred diabatic heating rate are found to be highly correlated, with correlation coefficients of 0.83 and 0.65 for the forecast and analyzed cyclogenesis, respectively. An experimental integration of the UCLA model without latent heat release indicates that 75% of the deepening in this cyclogenesis event can be explained by dry dynamics alone. The latent heat release serves to enhance and modulate the rapid cyclogenesis.

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Rolf H. Langland and Chi-Sann Liou

Abstract

An E–ε parameterization of subgrid-scale vertical turbulent mixing has been installed in NORAPS (Navy operational Regional Atmospheric Prediction System). The 1.5-order parameterization uses full prognostic equations for turbulence kinetic energy E and dissipation ε with no mixing length l assumption. A stable numerical method has been developed to integrate the two prognostic equations; this method has time and memory requirements that are similar to first-order K-theory turbulence parameterization and avoids numerical instabilities reported with El (Mellor–Yamada level 2.5) schemes. The E–ε parameterization produces a more active mixed layer, compared to a first-order K-theory scheme. Improvements are noted in forecasts of mixed-layer depth and near-surface wind speed, with reduction or elimination of spurious noise in the predicted fields of temperature and wind that were related to deficiencies of the first-order K-theory parameterization. In a numerical simulation of the ERICA (Experiment on Rapidly Intensifying Storms over the Atlantic) IOP 5A storm, the E–ε parameterization provides an improved forecast of cyclone central pressure. The better cyclone forecast results primarily from more accurate prediction of wind speed near the surface and in the upper troposphere where first-order K theory may produce unrealistic vertical mixing of momentum and temperature.

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James A. Ridout, Yi Jin, and Chi-Sann Liou

Abstract

A quasi balance with respect to parcel buoyancy at cloud base between destabilizing processes and convection is imposed as a constraint on convective cloud-base mass flux in a modified version of the Kain–Fritsch cumulus parameterization. Supporting evidence is presented for this treatment, showing a cloud-base quasi balance (CBQ) on a time scale of approximately 1–3 h in explicit simulations of deep convection over the U.S. Great Plains and over the tropical Pacific Ocean with the Naval Research Laboratory’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). With the exception of the smaller of two convective events in the Great Plains simulation, a CBQ is still observed upon restriction of the data analysis to instances where the available buoyant energy (ABE) exceeds a threshold value of 1000 J kg−1. This observation is consistent with the view that feedbacks between convection and cloud-base parcel buoyancy can control the rate of convection on shorter time scales than those associated with the elimination of buoyant energy and supports the addition of a CBQ constraint to the Kain–Fritsch mass-flux closure.

Tests of the modified Kain–Fritsch scheme in single-column-model simulations based on the explicit three-dimensional simulations show a significant improvement in the representation of the main convective episodes, with a greater amount of convective rainfall. The performance of the scheme in COAMPS precipitation forecast experiments over the continental United States is also investigated. Improvements are obtained with the modified scheme in skill scores for middle to high rainfall rates.

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Yi Jin, William T. Thompson, Shouping Wang, and Chi-Sann Liou

Abstract

The impact of dissipative heating on tropical cyclone (TC) intensity forecasts is investigated using the U.S. Navy’s operational mesoscale model (the Coupled Ocean/Atmosphere Mesoscale Prediction System). A physically consistent method of including dissipative heating is developed based on turbulent kinetic energy dissipation to ensure energy conservation. Mean absolute forecast errors of track and surface maximum winds are calculated for eighteen 48-h simulations of 10 selected TC cases over both the Atlantic basin and the northwest Pacific. Simulation results suggest that the inclusion of dissipative heating improves surface maximum wind forecasts by 10%–20% at 15-km resolution, while it has little impact on the track forecasts. The resultant improvement from the inclusion of the dissipative heating increases to 29% for the surface maximum winds at 5-km resolution for Hurricane Isabel (2003), where dissipative heating produces an unstable layer at low levels and warms a deep layer of the troposphere. While previous studies depicted a 65 m s−1 threshold for the dissipative heating to impact the TC intensity, it is found that dissipative heating has an effect on the TC intensity when the TC is of moderate strength with the surface maximum wind speed at 45 m s−1. Sensitivity tests reveal that there is significant nonlinear interaction between the dissipative heating from the surface friction and that from the turbulent kinetic energy dissipation in the interior atmosphere. A conceptualized description is given for the positive feedback mechanism between the two processes. The results presented here suggest that it is necessary to include both processes in a mesoscale model to better forecast the TC structure and intensity.

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Chi-Sann Liou, Carlyle H. Wash, Stacey M. Heikkinen, and Russell L. Elsberry

Abstract

A research version of the Navy Operational Regional Atmospheric Prediction System (NORAPS) is used to study cyclogenesis events during the second Intensive Observation Period of the Genesis of Atlantic Lows Experiment (GALE). From 1200 UTC 26 January to 0000 UTC 29 January 1986, two cyclogeneses occurred over the East Coast of the United States. NORAPS analyses reveal that the first cyclone develops rapidly due to the superposition of upper-level jet streak forcing over a shallow surface system associated with a well-developed coastal front. Large latent heat release around the cyclone center is considered to be a contributing factor for the rapid deepening of the first cyclone between 0000UTC and 1200 UTC 27 January. Small static stability at low levels coupled with a new upper-level trough-jet system is considered to be an important factor for the formation of the secondary cyclone off the East Coast at 1800 UTC 27 January.

NORAPS predicted the two cyclogenesis events fairly well up to 0000 UTC 28 January. A prediction of too early and too weak of a cold surge is believed to be the main reason for poor forecasts during later periods.

Extra data available from GALE sounding and surface data tapes are added to the operationally received dataset to study the impact of those extra data on analyzing and predicting the two cyclogenesis events. The GALE data impact investigated in this study is concentrated in the increase of spatial resolution, but not temporal resolution, by GALE networks and dropwindsondes. Because the two cyclogenesis events were over land of close to the coast, the regular operational data coverage over the East Coast of the United States was sufficient for the NORAPAS Optimum Interpolation (OI)_ analysis to analyze the important features for the cyclone developments. As a result, the enhancement of data spatial resolution from GALE soundings and surface reports made only limited improvement on NORAPS analyses and forecasts of these two cyclogenesis cases.

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Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss

Abstract

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems.

The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.

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Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss

Abstract

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems.

The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.

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Chi–Sann Liou, Jen–Her Chen, Chuen–Teyr Terng, Feng–Ju Wang, Chin–Tzu Fong, Thomas E. Rosmond, Hong–Chi Kuo, Chih–Hui Shiao, and Ming–Dean Cheng

Abstract

The global forecast system (GFS), which started its operation in 1988 at the Central Weather Bureau in Taiwan, has been upgraded to incorporate better numerical methods and more complete parameterization schemes. The second-generation GFS uses multivariate optimum interpolation analysis and incremental nonlinear normal-mode initialization to initialize the forecast model. The forecast model is a global primitive equation model with a resolution of 18 sigma levels in the vertical and 79 waves of triangular truncation in the horizontal. The forecast model includes a 1.5-order eddy mixing parameterization, a gravity wave drag parameterization, a shallow convection parameterization, a relaxed version of Arakawa–Schubert cumulus parameterization, grid-scale condensation calculation, and longwave and shortwave radiative transfer calculations with consideration of fractional clouds. The performance of the second-generation GFS is significantly better than the first-generation GFS. For two 3-month periods in winter 1995/96 and summer 1996, the second-generation GFS provided forecasters with 5-day forecasts where the averaged 500-mb height anomaly correlation coefficients for the Northern Hemisphere were greater than 0.6.

Observational data available to the GFS are much less than those at other numerical weather prediction centers, especially in the Tropics and Southern Hemisphere. The GRID messages of 5° resolution, ECMWF 24-h forecast 500-mb height and 850- and 200-mb wind fields available once a day on the Global Telecommunications System are used as supplemental observations to increase the data coverage for the GFS data assimilation. The supplemental data improve the GFS performance both in the analysis and forecast.

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Ling-Feng Hsiao, Chi-Sann Liou, Tien-Chiang Yeh, Yong-Run Guo, Der-Song Chen, Kang-Ning Huang, Chuen-Teyr Terng, and Jen-Her Chen

Abstract

This paper introduces a relocation scheme for tropical cyclone (TC) initialization in the Advanced Research Weather Research and Forecasting (ARW-WRF) model and demonstrates its application to 70 forecasts of Typhoons Sinlaku (2008), Jangmi (2008), and Linfa (2009) for which Taiwan’s Central Weather Bureau (CWB) issued typhoon warnings. An efficient and dynamically consistent TC vortex relocation scheme for the WRF terrain-following mass coordinate has been developed to improve the first guess of the TC analysis, and hence improves the tropical cyclone initialization. The vortex relocation scheme separates the first-guess atmospheric flow into a TC circulation and environmental flow, relocates the TC circulation to its observed location, and adds the relocated TC circulation back to the environmental flow to obtain the updated first guess with a correct TC position. Analysis of these typhoon cases indicates that the relocation procedure moves the typhoon circulation to the observed typhoon position without generating discontinuities or sharp gradients in the first guess.

Numerical experiments with and without the vortex relocation procedure for Typhoons Sinlaku, Jangmi, and Linfa forecasts show that about 67% of the first-guess fields need a vortex relocation to correct typhoon position errors while eliminates the topographical effect. As the vortex relocation effectively removes the typhoon position errors in the analysis, the simulated typhoon tracks are considerably improved for all forecast times, especially in the early periods as large adjustments appeared without the vortex relocation. Comparison of the horizontal and vertical vortex structures shows that large errors in the first-guess fields due to an incorrect typhoon position are eliminated by the vortex relocation scheme and that the analyzed typhoon circulation is stronger and more symmetric without distortions, and better agrees with observations. The result suggests that the main difficulty of objective analysis methods [e.g., three-dimensional variational data assimilation (3DVAR)], in TC analysis comes from poor first-guess fields with incorrect TC positions rather than not enough model resolution or observations. In addition, by computing the eccentricity and correlation of the axes of the initial typhoon circulation, the distorted typhoon circulation caused by the position error without the vortex relocation scheme is demonstrated to be responsible for larger track errors. Therefore, by eliminating the typhoon position error in the first guess that avoids a distorted initial typhoon circulation, the vortex relocation scheme is able to improve the ARW-WRF typhoon initialization and forecasts particularly when using data assimilation update cycling.

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