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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. Source


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. Source


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. Source


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. Source


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. Source

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