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Yue C.,Shanghai Typhoon Institute | Yue C.,Laboratory of Typhoon Forecast Technique China Meteorological Administration | Shou S.,Nanjing University of Information Science and Technology
Atmospheric Research | Year: 2011

The effects of vertical wind shear, radiation, and ice clouds on precipitation associated with the landfall of Typhoon Krosa on 6-8 October 2007 are analyzed based on a series of two-dimensional sensitivity cloud-resolving model simulations. The model is integrated with imposed zonally-uniform vertical velocity, zonal wind, horizontal temperature and vapor advection from National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS) data. The vertical wind shear impacts mean surface rainfall when the shears are large on 6 and 8 October. The mean surface rainfall on 6 October is reduced by either exclusion, or increase, or reversed direction of vertical wind shear through the slowdown in stratiform rainfall. The mean rain rate on 8 October is decreased by the removal of vertical wind shear through the weakened stratiform rainfall and is increased by the doubled vertical wind shear through the strengthened convective rainfall, and is barely affected by the reserved direction of vertical wind shear. The impacts of vertical wind shear on surface rainfall are stronger than those of cloud radiative processes, but they are weaker than those of ice clouds. The elimination of ice clouds reduces the mean surface rainfall by excluding microphysical effects of ice clouds through the severe damage of stratiform cloud and associated rainfall. © 2010 Elsevier B.V.


Wu C.-C.,National Taiwan University | Zhan R.,Shanghai Typhoon Institute | Lu Y.,National Taiwan University | Wang Y.,University of Hawaii at Manoa
Journal of Climate | Year: 2012

As synoptic storms, tropical cyclones (TCs) are highly nonlinear systems resulting from multiscale interactions. In particular, the genesis of TCs involves complex nonlinear processes, exhibiting strong internal variability in climate model simulations. This study attempts to examine such internal variability of dynamically downscaled TCs over the western North Pacific Ocean based on four simulations of 20 typhoon seasons (198222001) initialized on 4 successive days using the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM). The results show that on both seasonal and interannual time scales, the initial conditions significantly affect the downscaled TC activity, with the largest internal variability occurring in August on the seasonal time scale. The spreads between any of the individual simulations and the ensemble mean are comparable to and in some circumstances greater than the interannual variation of the observed TC frequency. The internal variability of the downscaled TC activity is found to be insensitive to the amplitude and the pattern of the initial perturbations. However, day-to-day model solutions are strongly affected by the internal variability. As a result, the development of nonlinear atmospheric instabilities significantly modulates the genesis and development of the TC-like vortices, leading to the large internal variability of the downscaled TC activity. In addition to the traditional initial value problem, criteria (in particular, threshold values) used in the TC detection contribute equally to the internal variability of the downscaled TCs in the simulations. Consistent with earlier studies, the results from this study also show that the ensemble mean provides the better downscaled information on seasonal and interannual frequencies of TC genesis and occurrence. © 2012 American Meteorological Society.


Zhan R.,Shanghai Typhoon Institute | Wang Y.,University of Hawaii at Manoa | Wu C.-C.,National Taiwan University
Journal of Climate | Year: 2011

The impact of the sea surface temperature anomaly (SSTA) in the East Indian Ocean (EIO) on the tropical cyclone (TC) frequency over the western North Pacific (WNP) and the involved physical mechanisms are examined using the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM) driven by the reanalysis and the observed SSTs. The model reproduces generally quite realistic climatic features of the WNP TC activity, including the interannual variability of the WNP TC genesis frequency, the geographical distributions of TC genesis and frequency of occurrence. In particular, the model reproduces the observed statistical (negatively correlated) relationship between the WNP TC frequency and the EIO SSTA, as recently studied by Zhan et al. The experiments with artificially imposed SSTA in the EIO in the year 2004 with normal EIO SST and WNPTC activity confirm that the EIO SSTA does affect the TC genesis frequency in the entire genesis region over the WNP by significantly modulating both the western Pacific summer monsoon and the equatorial Kelvin wave activity over the western Pacific, two major large-scale dynamical controls of TC genesis over the WNP. Additional sensitivity experiments are performed for two extreme years: one (1994) with the highest and one (1998) with the lowest TC annual frequencies in the studied period. The results reveal that after the EIO SSTAs in the two extreme years are removed, the TC frequency in 1998 is close to the climatological mean, while the excessive TCs in 1994 are still simulated. The model results suggest that the warm EIO might be a major factor contributing to the unusually few TCs formed over the WNP in 1998, but the cold EIO seemed to contribute little to the excessive WNP TCs in 1994. © 2011 American Meteorological Society.


Miyoshi T.,University of Maryland University College | Kalnay E.,University of Maryland University College | Li H.,Shanghai Typhoon Institute
Inverse Problems in Science and Engineering | Year: 2013

Usually in data assimilation with geophysical systems, the observation-error covariance matrix R is assumed to be diagonal for simplicity and computational efficiency, although there are studies indicating that several types of satellite observations contain significantly correlated errors. This study brings to light the impact of the off-diagonal terms of R in data assimilation. The adaptive estimation method of Li et al., which allows online estimation of the observation-error variance using innovation statistics, is extended to include off-diagonal terms of R. The extended method performs well with the 40-variable Lorenz model in estimating non-diagonal observation-error covariances. Interestingly, the analysis accuracy is improved when the observation errors are correlated, but only if the observation-error correlations are explicitly considered in data assimilation. Further theoretical considerations relate the impact of observing systems (characterized by both R and an observation operator H) on analysis accuracy. This analysis points out the importance of distinguishing between observation-error correlations (i.e. non-diagonal R) and correlated observations (i.e. non-orthogonal H). In general, observations with a non-diagonal R carry more information, whereas observations with a non-orthogonal H carry less information, but it turns out that the combination of R and H is essential: more information is available from positively (negatively) correlated observations with negatively (positively) correlated errors, resulting in a more accurate analysis. © 2013 Copyright Taylor and Francis Group, LLC.


Zhan R.,Shanghai Typhoon Institute | Wang Y.,University of Hawaii at Manoa
Journal of Geophysical Research: Atmospheres | Year: 2012

Deep convection associated with tropical cyclones (TCs) is considered to play an important role in the tropical troposphere-to-stratosphere transport (TST). In this study, both the AIRS satellite measurements on Aqua and the Interim reanalysis of the European Centre for Medium-Range Weather Forecasts (ERA-Interim) are used to estimate the contribution of TCs to the tropical TST over the western North Pacific (WNP). A case study for Typhoon Nock-ten (2004) demonstrates that both the AIRS and the ERA-Interim data can infer the direct transport of the tropospheric air into the lower stratosphere in the vicinity of the TC eyewall. A contrast analysis for the inactive and active TC seasons of 2003 and 2004 over the WNP based on the same data sets suggests that in the region where TCs occur frequently the TST is significantly enhanced. As a result, the TC-induced TST in the active TC season is about 25% higher than that in the inactive TC season. The results based on the ERA-Interim data for the TC seasons over the WNP from 1989 to 2010 show that TCs can account for more than 10% of the total TST in most of the impact region with the highest contribution up to 30%. Over the entire WNP, TCs account for about 5.6% of the total TST and about 10.9% of the total upward transport across the base of the Tropical Transition Layer (TTL). This suggests that TCs could have a significant contribution to the total upward transport into the TTL over the WNP. © 2012 American Geophysical Union. All Rights Reserved.


Zhan R.,Shanghai Typhoon Institute | Wang Y.,University of Hawaii at Manoa
Journal of Climate | Year: 2016

Ahybrid dynamical-statistical model is developed for predicting the accumulated cyclone energy (ACE)- a measure that can synthesize genesis number, mean life span, and intensity of tropical cyclones (TCs)-in the typhoon season (June-October) over the western North Pacific (WNP) using data from both observations and seasonal forecasts of the National Centers for Environmental Prediction's (NCEP's) Climate Forecast System, version 2 (CFSv2). The model is built on the relationships between the observedACEand the large-scale variables for the period of 1982-2010. Four predictors are selected based on both previous work in the literature and statistical analyses in this study, including vertical zonal wind shear over the equatorial western North Pacific (Ushear), sea surface temperature (SST) gradient (SSTG) between the southwestern Pacific (SWP) and the western Pacific warm pool, Niño-3.4 SST, and SWP SST. Based on the cross validation, the hybrid model is finally constructed with the combination of the summer Niño-3.4 and SWP SST at the 4-to-2-month lead (January-March) and the summer Ushear and the April SSTG at the 1-to-0-month lead (April-May). The hybrid model is shown to be skillful in predicting WNP seasonal ACE starting from January, with the correlation coefficient ranging between 0.58 and 0.81 and the root-mean-square error ranging between 1.26 and 0.91 (scaled by 105m2 s-2) initiated from January to May. The prediction experiments for 2011-13 using the hybrid dynamical-statistical model showed better skill and longer leads than that using the pure statistical models. © 2016 American Meteorological Society.


Li Q.,University of Hawaii at Manoa | Li Q.,Shanghai Typhoon Institute | Wang Y.,University of Hawaii at Manoa
Journal of the Atmospheric Sciences | Year: 2012

The formation and quasi-periodic behavior of outer spiral rainbands in a tropical cyclone simulated in the cloud-resolving tropical cyclone model version 4 (TCM4) are analyzed. The outer spiral rainbands in the simulation are preferably initiated near the 60-km radius, or roughly about 3 times the radius of maximum wind (RMW). After initiation, they generally propagate radially outward with a mean speed of about 5 m s21. They are reinitiated quasi-periodically with a period between 22 and 26 h in the simulation. The inner spiral rainbands, which form within a radius of about 3 times the RMW, are characterized by the convectively coupled vortex Rossby waves (VRWs), but the formation of outer spiral rainbands (i.e., rainbands formed outside a radius of about 3 times the RMW) is much more complicated. It is shown that outer spiral rainbands are triggered by the inner-rainband remnants immediately outside the rapid filamentation zone and inertial instability in the upper troposphere. The preferred radial location of initiation of outer spiral rainbands is understood as a balance between the suppression of deep convection by rapid filamentation and the favorable dynamical and thermodynamic conditions for initiation of deep convection. The quasi-periodic occurrence of outer spiral rainbands is found to be associated with the boundary layer recovery from the effect of convective downdrafts and the consumption of convective available potential energy (CAPE) by convection in the previous outer spiral rainbands. Specifically, once convection is initiated and organized in the form of outer spiral rainbands, it will produce strong downdrafts and consume CAPE. These effects weaken convection near its initiation location. As the rainband propagates outward farther, the boundary layer air near the original location of convection initiation takes about 10 h to recover by extracting energy from the underlying ocean. Convection and thus new outer spiral rainbands will be initiated near a radius of about 3 times the RMW. This will be followed by a similar outward propagation and the subsequent boundary layer recovery, leading to a quasi-periodic occurrence of outer spiral rainbands. In response to the quasi-periodic appearance of outer spiral rainbands, the storm intensity experiences a similar quasi-periodic oscillation with its intensity or intensification rate starting to decrease after about 4 h of the initiation of an outer spiral rainband. The results provide an alternative explanation or one of the mechanisms that are responsible for the quasi-periodic (quasidiurnal) variation in the intensity and in the area of outflow-layer cloud canopy of observed tropical cyclones. © 2012 American Meteorological Society.


Li Q.,University of Science and Technology of China | Li Q.,Shanghai Typhoon Institute | Wang Y.,Shanghai Typhoon Institute
Monthly Weather Review | Year: 2012

The simulated inner and outer spiral rainbands in a tropical cyclone are compared in this study. The inner rainbands are generally active immediately outside the eyewall in the rapid filamentation zone, while the outer rainbands are active in regions outside about 3 times the radius of maximum wind. The inner rainbands are characterized by the convectively coupled vortex Rossby waves. The movement of the outer rainbands follows the low-level vector winds associated with the azimuthally averaged low-level flow and the radially outward cross-band flow caused by the downdraft-induced cold pool in the boundary layer. Convective cells in outer rainbands are typical of convective systems and move cyclonically and radially outward (inward) at large (small) radii. Net upward vertical mass transports (VMTs) appear throughout the depth of the troposphere in the whole inner-rainband region, while net downward VMTs are found below 4-km height in the outer-rainband region. In the whole inner-rainband region, only a very shallow layer with net horizontal convergence appears below 2-km height, while a deep layer with net convergence is found below 7.5-km height with net divergence aloft in the outer-rainband region. The inner rainband shows two tangential wind maxima, respectively, located near the top of the inflow boundary layer and immediately below the upper-tropospheric outflow layer. A secondary horizontal wind maximum occurs at about 4-km height on the inner edge of the outer rainband. Distinct features of the upwind, middle, and downwind sectors of the outer rainband are also discussed. © 2012 American Meteorological Society.


Ma L.-M.,Shanghai Typhoon Institute | Bao X.-W.,Shanghai Typhoon Institute
Boundary-Layer Meteorology | Year: 2016

To reduce the discrepancy between simulated and observed tropical cyclones, we consider a new parametrization scheme for planetary boundary-layer (PBL) height based on helicity, intended to provide an improved description of the overall helical structures of the tropical cyclone PBL simulated in a numerical model. This scheme was preliminarily tested in the Yonsei University (YSU) PBL scheme integrated within the National Center for Atmospheric Research Weather Research and Forecasting model. Based on verification of track simulations for seven tropical cyclones that made landfall over China, tropical cyclone Morakot (2009) was selected for further evaluation of the new scheme. Compared with the original scheme based on the Richardson number (Ri), the new scheme elevated the PBL height associated with intense convection, which is consistent with observation. Importantly, the new scheme improved the numerical simulation of intense rainfall by modulating the PBL environment for convection evolution. Furthermore, the PBL height and 2-m temperature over land at night, which are frequently overestimated by the original YSU scheme, were improved using the new scheme. Because of its effects on PBL structures and convection evolution, the simulation of tropical cyclone Morakot intensity was improved by the new scheme. © 2016 Springer Science+Business Media Dordrecht


Qi L.,Shanghai Meteorological Center | Yu H.,Shanghai Typhoon Institute | Chen P.,Shanghai Typhoon Institute
Quarterly Journal of the Royal Meteorological Society | Year: 2014

This article proposes a selective ensemble-mean technique for tropical cyclone (TC) track forecast based on the errors of ensemble prediction system (EPS) members at short lead times (SLTs, 12 h in this study). The means (SEAV) and weighted means (SEWE) of selected EPS members are applied to EPS products from the European Centre for Medium-range Weather Forecasts (ECMWF), Japan Meteorological Agency (JMA), National Centers for Environmental Prediction (NCEP), and China Meteorological Administration for 35 TCs in the western North Pacific in 2010 and 2011. Verification results show that SEAV behaves better than SEWE, with a skill of 5% to 30% over relevant ensemble means of EPS within 72 h. The SEAV method is the most effective for the JMA EPS, with a skill of 10% even at 96 h. SEAV predictions are compared with the high-resolution deterministic model predictions of ECMWF and several official forecasts, with special consideration given to the time delay associated with numerical model products in operation. The SEAV for the ECMWF EPS can overcome the high-resolution ECMWF deterministic model at 24 h. Case analyses and sensitivity tests on the error thresholds of member selection and SLTs are also presented in this article. © 2013 Royal Meteorological Society.

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