Recent Progress in Atmospheric Sciences

The following sections are included:

• Preface

• Contents

The Pacific and Indian Oceans are closely linked to each other through atmospheric circulation and oceanic throughflow. Climate variations in one ocean basin often interact with those in the other basin. This includes phenomena such as the El Niño–Southern Oscillation (ENSO), biennial monsoon variability, and the Indian Ocean zonal/dipole mode. Increasing evidence suggests that these interbasin interactions and feedbacks are crucial in determining the period, evolution, and pattern of ENSO and its decadal variability. This article reviews recent efforts in using a series of basin-coupling CGCM (coupled atmosphere–ocean general circulation model) experiments to understand the physical processes through which ENSO interacts with the Indian Ocean and monsoon, the impacts of interbasin interactions on the characteristics of ENSO, the relative roles of the Pacific and Indian Oceans in monsoon variability, and ENSO's role in the Indian Ocean zonal/dipole mode.

In this review article, we summarize mechanisms limiting the poleward extent of the summer monsoon rain zones, particularly for the Asian summer monsoon. They include local processes associated with net heat flux into the atmosphere and soil moisture, ventilation by cross-continental flow, and the interactive Hodwell–Hoskins (IRH) mechanism, defined as the interaction between monsoon convective heating and baroclinic Rossby wave dynamics. The last two mechanisms, ventilation and the IRH mechanism, also induce an east–west asymmetry of the summer monsoon rain zones. Processes that change land–ocean heating contrast, and differences in net heat flux into the atmosphere rather than in surface temperature, are also discussed. Convection associated with the Asian summer monsoon is initiated by net heat flux into the atmosphere and modified by soil moisture via an evaporation process. In Asia, ventilation by moisture advection is particularly important, and the IRH mechanism tends to favor interior arid regions and east coast precipitation. Land surface conditions, such as surface albedo and topography, and ocean heat transport tend to modify land–ocean heating contrast, in terms of a tropospheric temperature gradient, and then change the Asian summer monsoon circulation and its associated rain zone. The stronger meridional gradient of tropospheric temperature tends to enhance the summer monsoon rainfall and extend the rain zone farther northward. Local SST, such as the warm SST anomalies in the western North Pacific during El Niño, is also important in the summer monsoon rainfall and its position.

A strong in-phase relationship between the intraseasonal oscillation (ISO) and the tropical cyclone (TC) was observed in the tropical western North Pacific from June through October 2004. The ISO, which is characterized by the fluctuations in the East Asian monsoon trough and the Pacific subtropical anticyclone, modulated the TC activity and led to the spatial and temporal clustering of TCs during its cyclonic phase. This clustering of strong TC vortices contributed significant positive vorticity during the cyclonic phase of the ISO and therefore enlarged the intraseasonal variance of 850 hPa vorticity. This result indicates that a significant percentage (larger than 50%) of observed intraseasonal variance along the clustered TC tracks in the tropical western North Pacific came from TCs. Numerical simulation confirmed that the presence and enhancement of TCs in the models enlarged the simulated intraseasonal variance. This implies that the contribution of TCs has to be taken into account to correctly estimate and interpret the intraseasonal variability in the tropical western North Pacific.

Understanding convective–radiative-mixing processes is crucial in making better predictions about tropical climate. The cloud-resolving model and the mixed-layer model, combined with observations, are powerful tools for studying these physical processes interacting with climate. In this article, the authors' research work of the past 15 years on tropical climate processes is reviewed. The topics reviewed include climate equilibrium study, tropical convective responses to radiative and microphysical processes, the diurnal cycle, cloud clustering and associated cloud-microphysical processes, precipitation efficiency, air–sea exchanges and ocean-mixing processes at diurnal-to-intraseasonal scales, and coupled boundary layer and forced oceanic responses. Representation of these processes in climate models and future perspectives are also discussed.

The atmosphere, like other parts of nature, is full of phenomena that involve rapid transitions from one (quasi-)equilibrium state to another, i.e. catastrophes. These (quasi-)equilibria are the multiple solutions of the same dynamical system. Unlocking the mystery behind a catastrophe reveals not only the physical mechanism responsible for the transition, but also how the (quasi-) equilibria before and after the transition are maintained. Each catastrophe is different, but they do have some common traits. Understanding these common traits is the first step in studying these catastrophes. In this article, three examples are reviewed to show how atmospheric catastrophes can be studied.

The climatological winter Atlantic storm track is distinctly more intense (by about 10%) than the Pacific storm track even though the Atlantic jet is about 30% weaker than the Pacific jet. It is hypothesized that this counterintuitive feature is partly attributable to having statistically stronger seeding disturbances upstream of the Atlantic jet. The difference in the seeding disturbances may stem from the geographical distribution of continents and oceans in the northern hemisphere, especially when differential friction over land versus water surfaces is taken into consideration. This hypothesis is shown to be valid even in the context of a barotropic model. The proxy forcing in the model is introduced in the form of relaxation of the instantaneous flow toward a reference flow, which has two localized jets broadly resembling the winter Pacific and Atlantic jets. A linear modal instability analysis of these jets is first presented. The nonlinear model Atlantic storm track is found to be more intense than its counterpart over the Pacific. The difference in the relative intensity of the two model storm tracks becomes more pronounced when the drag coefficient over the land sectors is several times larger than that over the ocean sectors. These results may be taken as evidence in support of the hypothesis.

Meiyu is a unique feature of East Asia, and the Meiyu frontal system is the key synoptic feature which causes the maximum seasonal rainfall. Numerous studies have been focused on various aspects of the Meiyu frontal system. The main purpose of this article is to present an overview on the recent research on the Meiyu frontal system, which includes the structure and dynamics of the related phenomena, such as Meiyu frontogenesis, frontal movement, frontal disturbances, and low-level jets (LLJ's) during the Meiyu season. Particularly, the recent studies of the role of convective latent heating in frontogenesis, cyclogenesis, and LLJ formation using the piecewise PV inversion technique will be emphasized.

The advance in the dynamics and targeted observations of hurricane movement is reviewed in this article. In celebration of the 50th anniversary of the Department of Atmospheric Sciences, National Taiwan University, special emphasis is put on the author's major scientific contributions to the following issues: the baroclinic effect on tropical cyclone motion, the potential vorticity diagnosis of the tropical cyclone motion, and the targeted observations from DOTSTAR (Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region) in understanding and improving the tropical cyclone track predictability.

An important issue in the formation of concentric eyewalls in a typhoon is the development of a symmetric structure from asymmetric convection. As an idealization of the interaction of a tropical cyclone core with nearby weaker vorticity of various spatial scales, we consider nondivergent barotropic model integrations to illustrate that concentric vorticity structures result from the interaction between a small and strong inner vortex (the tropical cyclone core) and neighboring weak vortices (the vorticity induced by the moist convection outside the central vortex of a tropical cyclone). In particular, the core vortex induces a differential rotation across the large and weak vortex, to strain out the latter into a vorticity band surrounding the former without a merging of the two. The straining out of a large, weak vortex into a concentric vorticity band can also result in the contraction of the outer tangential wind maximum. The dynamics highlight the essential role of the vorticity strength of the inner core vortex in maintaining itself, and in stretching, symmetrizing and stabilizing the outer vorticity field.

Our binary vortex experiments from the Rankine vortex suggest that the formation of a concentric vorticity structure requires: (1) a very strong core vortex with a vorticity at least six times stronger than the neighboring vortices, (2) a neighboring vorticity area that is larger than the core vortex, and (3) a separation distance between the neighboring vorticity field and the core vortex that is within three to four times the core vortex radius. On the other hand, when the companion vortex is four times larger than the core vortex in radius, a core vortex with a vorticity skirt produces concentric structures when the separation distance is five times greater than the smaller vortex. At this separation distance, the Rankine vortex produces elastic interaction. Thus, a skirted core vortex of sufficient strength can form a concentric vorticity structure at a larger radius than what is allowed by an unskirted core vortex. This may explain the wide range of radii for concentric eyewalls in observations.

The formation of Typhoon Vamei on 27 December 2001 in the southern South China Sea was the first-observed tropical cyclogenesis within 1.5 degrees of the equator. This rare event was first detected by observations of typhoon strength winds from a US navy ship, and the existence of an eye structure was confirmed by satellite and radar imageries. This paper reviews these observations, and discusses the dynamic theory that may explain the process suggested by Chang et al. (2003) in which a strong cold surge event interacting with the Borneo vortex led to the equatorial development. As pointed out by Chang et al., the most intriguing question is not how Vamei could form so close to the equator, but is why such a formation was not observed before then.

The improvement in the forecasting skills of short range numerical weather prediction is attributable to two major advances since the inception of operational numerical weather prediction in the late 1950's: the advent of satellite remote sensing since the 1970's, which has vastly increased the number of observations in the operational database, and the improvement in the treatment of model physics, numerics and spatial resolution of the global forecast models, and their associated atmospheric data analysis and data assimilation systems, which are capable of effectively using the vast amounts of satellite data observations for NWP operations. The evolution of the operational global data assimilation systems at NCEP is briefly described, followed by a review of various satellites during the last two decades that have been designed to provide global coverage of ocean surface winds. These include SEASAT, NSCAT, ERS-1/2, SSM/I and the most current operational satellite, QuikSCAT. On board these satellites, data characteristics of two microwave instruments for measuring the ocean surface winds from both an active scatterometer and a passive radiometer are discussed. The procedures for effectively using these satellite ocean surface wind data in the global data assimilation experiments, and that for assessing the impact of any particular data set, are described. Results of preimplementation impact investigations on the use of these satellite surface winds are discussed, and based on the results of these investigations, these data are implemented in NCEP's NWP operations. It is fair to state that from the gross statistics based on many cases of forecasts, the impact of satellite ocean surface winds on the short range NWP forecasts is mostly positive and significant, albeit small because the data are of a single level nature and are available only over the ocean surface. A case study is presented which shows that the use of satellite scatterometer ocean surface winds has a significantly large positive impact on the storm intensity and circulation over the southwestern Pacific Ocean.

This article summarizes our research related to geofluid dynamics and numerical modeling. In order to have a better understanding of the motion in the atmosphere, we have been working on various forms of the Navier–Stokes equations, including the linearized and nonlinear systems as well as turbulence parametrization, cumulus parametrization, cloud physics, soil–snow parametrization, atmospheric chemistry, etc. We have also been working on numerical methods in order to solve the equations more accurately. The results show that many weather systems in the initial/growing stage can be qualitatively described by the linearized equations; on the other hand, many developed weather phenomena can be quantitatively reproduced by the nonlinear Purdue Regional Climate Model, when the observational data or reanalysis is used as the initial and lateral boundary conditions. The model can also reveal the detailed structure and physics involved, which sometimes can be misinterpreted by meteorologists according to the incomplete observations. However, it is also noted that systematic biases/errors can exist in the simulations and become difficult to correct. Those errors can be caused by the errors in the initial and boundary conditions, model physics and parametrizations, or inadequate equations or poor numerical methods. When the regional model is coupled with a GCM, it is required that both models should be accurate so as to produce meaningful results. In addition to the Purdue Regional Climate Model, we have presented the results obtained from the nonhydrostatic models, the one-dimensional cloud model, the turbulence-pollution model, the characteristic system of the shallow water equations, etc. Although the numerical model is the most important tool for studying weather and climate, more research should be done on data assimilation, the physics, the numerical method and the mathematic formulation in order to improve the accuracy of the models and have a better understanding of the weather and climate.

This article reviews our current understanding of the interactive processes between aerosols and clouds pertaining to the issues of climate change and the hydrological cycle. We first introduce the aerosol effects on clouds by the classification of hygroscopic aerosols, carbonaceous aerosols and mineral dust according to the aerosol chemical contents. Following that are discussions on how clouds influence aerosols via microphysical and chemical mechanisms, scavenging by precipitation, and vertical transport of cloud venting, as well as some indirect effects. The main topics covered here include cloud microphysics, cloud chemistry, photochemistry and gas-to-particle conversion, radiation and climate impact, and interactions with the biosphere. The examples given focus more on the work conducted by the Cloud and Aerosol Research Group in the Department of Atmospheric Sciences, National Taiwan University.

In tropical oceanic regions, surface heat fluxes influence the climate at all spatial and temporal scales, ranging from diurnal variations of the sea surface temperature (SST) and clouds to intraseasonal variations of atmospheric disturbances, as well as interannual variations of the El Niño/Southern Oscillation. In the past, research on the impact of sea surface heat fluxes on climate was limited by the lack of quality global data sets with high temporal and spatial resolutions. More recently, good quality sea surface heat flux data sets with a temporal resolution of 1 day and a spatial resolution of 1° × 1° latitude–longitude have been produced, based on satellite radiation measurements. We have applied these data sets to study climate in the tropical Pacific Ocean and eastern Indian Ocean. These studies include the climatology of surface heat budgets (SHB's), the role of SHB's in the regulation of Pacific warm pool temperature, the correlation between local SHB's and the rate of change of the SST, and the correlation between the interannual variations of the tropical Pacific basin-wide mean SHB and SST. We review the results of those studies in this article.

A number of unsolved problems in atmospheric radiative transfer are presented, including the light scattering and absorption by aerosols, the effect of mountains on radiation fields, and radiative transfer in the atmosphere–ocean system, with a specific application to the Asia–Pacific region. We discuss the issues of two nonspherical and inhomogeneous aerosol types, dust and black carbon, regarding climate radiative forcings. Reduction in uncertainties of their radiative forcings must begin with improvement of the knowledge and understanding of the fundamental scattering and absorption properties associated with their composition and size/shape. The effects of intensive topography, such as that in the Tibetan Plateau, on surface radiation and the radiation field above, are significant and the solution requires a three-dimensional radiative transfer program. We show that in a clear atmosphere, surface net solar flux averaged over a mesoscale domain can differ by 5–20 W/m² between a slope and a flat surface. Accurate calculations and parametrizations of both solar and thermal infrared radiative transfer in mountains must be developed for effective incorporation in regional and global models. The wind-driven air–sea interface is complex and affects the transfer of solar flux from the atmosphere into the ocean and the heating in the ocean mixed layer. We point out the requirement of reliable and efficient parametrizations of the ocean surface roughness associated with surface winds. The scattering and absorption properties of irregular phytoplankton and other species in the ocean must also be determined on the basis of the rigorous theoretical and experimental approaches for application to remote sensing and climate research.

Doppler radars have played a critical role in observing atmospheric vortices including tornados, mesocyclones, and tropical cyclones. The detection of the dipole signature of a mesocyclone by pulsed Doppler weather radars in the 1960s led to an era of intense research on atmospheric vortices. Our understanding of the internal structures of atmospheric vortices was primarily derived from a limited number of airborne and ground-based dual-Doppler datasets. The advancement of single Doppler wind retrieval (SDWR) algorithms since 1990 [e.g. the velocity track display (VTD) technique] has provided an alternate avenue for deducing realistic and physically plausible two- and three-dimensional structures of atmospheric vortices from the wealth of data collected by operational and mobile Doppler radars.

This article reviews the advancement in single Doppler radar observations of atmospheric vortices in the following areas: (1) single Doppler radar signature of atmospheric vortices, (2) SDWR algorithms, in particular the VTD family of algorithms, (3) objective vortex center-finding algorithms, and (4) vortex structures and dynamics derived from the VTD algorithms. A new paradigm that improves the VTD algorithm, displaying and representing the atmospheric vortices in V_dD/R_T space, is presented. The VTD algorithm cannot retrieve the full components of the divergent wind which may be improved by either implementing physical constraints on the VTD closure assumptions or combining high temporal resolution data with a mesoscale vorticity method. For all practical perspectives, SDWR algorithms remain the primary tool for analyzing atmospheric vortices in both operational forecasts and research purposes in the foreseeable future.

The western North Pacific Ocean and the surrounding seas are among the world's oceans where tropical cyclones, highest both in number and in intensity, are found. There has long been interest in studying the typhoon–ocean interaction processes in this vast oceanic region. However, observations are rare and it has been difficult to study these complex, dynamic, and interdisciplinary processes. With the advancement of satellite remote sensing, especially microwave remote sensing with cloud-penetrating capabilities, it has finally become possible to catch a glimpse of some of these processes in the western North Pacific. In this article, we review a number of recent papers using these new satellite observations to study (1) the interaction between typhoons and warm ocean eddies, (2) enhancement of ocean primary production induced by typhoons, and (3) posttyphoon air–sea interaction.

Taiwan, located downwind of dust storm outbreaks from China, in the sink region of biomass-burning aerosols from Southeast Asia, and at the outflow of urban-industrial pollutants from the Pearl and Yangtze River Delta, is exposed to a seasonal milieu of natural and anthropogenic aerosols in the atmosphere. In the springtime, outbreaks of Asian dust storms occur frequently in the arid and semiarid areas of northwestern China — about 1.6 × 10⁶ square kilometers, including the Gobi and Taklimakan deserts — with continuous expansion of spatial coverage. These airborne dust particles, originating in desert areas far from polluted regions, interact with anthropogenic sulfate and soot aerosols emitted from Chinese megacities during their transport over the mainland. Adding the intricate effects of clouds and marine aerosols, dust particles reaching the marine environment can have drastically different properties than those from their sources.

Together with anthropogenic pollutants, airborne dust particles may alter regional hydrological cycles by aerosol direct/indirect radiative forcing, influence fisheries by causing nutrient deposition anomalies, and increase adverse health effects on humans by trace metal enrichment. In addition to their local-to-regional impact, these dust aerosols can be transported swiftly across the Pacific Ocean to reach North America in less than a week, resulting in an even larger scale effect. Asian dust aerosols can be distinctly detected by their colored appearance on modern Earth-observing satellites [e.g. MODerate-resolution Imaging Spectroradiometer, (MODIS), Total Ozone Mapping Spectrometer (TOMS), Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Geostationary Meteorological Satellites (GMSs)], and their evolution monitored by satellites and surface networks [e.g. AErosol RObotic NETwork (AERONET), Micro-Pulse Lidar Network (MPLNET)]. However, these essential observations are incomplete due to the unique properties of the data constituted unilaterally on either the spatial (snapshot global coverage) or temporal (long-term point sites) dimension. Comprehensive modeling is required to bridge these spatial and temporal observations, and to serve as an integrator for our understanding of the effects of dust's physical, optical, and radiative properties on various forcing, response, and feedback processes occurring in the Earth–atmosphere system.

Recently, many field experiments (e.g. international ACE-Asia and regional follow-on campaigns) have been conducted to shed light on characterizing the compelling variability of dust aerosols on spatial and temporal scales, especially near the source and downwind regions. As a result of synergizing satellite, aircraft, and surface observations, our understanding of the distributions and properties of airborne dust aerosols has advanced significantly. It is our goal/hope to continue combining observational and theoretical studies to investigate in depth the changes of regional climate, hydrological budget, tropospheric chemistry, wind erosion, and dust properties in Asia. These regional changes (e.g. aerosol loading, cloud amount, precipitation rate) constitute a vital part of global change, and our success or failure in developing reliable predictions of, as well as adequate responses to, the changes will determine the prospective course for sustainable civilization. Consequently, the lessons learned will help strengthen our ability to issue early warnings of Asian dust storms and minimize further desertification in the future.

Satellite observations are the only means of obtaining a continuous survey of cloud properties on global scales. Many climate and weather forecasting models have been evaluated using satellite-derived products. For clouds, the evaluation has been limited mainly to cloud amount, in part because conventional satellite remote sensing techniques cannot provide detailed information on cloud vertical structure, and in part because conventional satellite retrieval algorithms assume a single cloud layer in their retrievals. Increasing attention is being paid to the vertical structure of cloud fields. However, observations of cloud layer data on global scales are scarce and unreliable for model evaluation. The single-layer assumption does not emcompass the overlapping of cloud layers or the inhomogeneity of cloud vertical structure. In nature, overlapped upper-level cirrus and low-level stratus clouds occur frequently. As such, comparisons of cloud vertical structures derived from the satellite measurements and models remain a challenge. This article discusses some issues related to the single-layer assumption used in current satellite cloud remote sensing techniques. The assumption represents one major deficiency in satellite observations of cloud vertical structure. Some differences caused by different satellite retrieval algorithms and some improvements on enhanced satellite retrieval algorithms are discussed based on the research work by the author. Credible global cloud and radiation measurements are needed in order to evaluate and improve the performance of global climate and weather forecast models. The recent launch of the CloudSat radar and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission in a formation flight of the “A-Train” satellite constellation is expected to provide large-scale spatial and temporal observations on the vertical profile of cloud properties.

In this paper, we summarize our recent research results from ice processes in the atmosphere from storm- to microscale. First, we tested the sensitivity of ice processes in a simulated US High Plains supercell storm using a 3-dimensional cloud model by specifying three different ice physics schemes; namely, the control run, the all-liquid run with normal latent heats, and the all-liquid run with latent heat of sublimation. We showed that the absence of ice processes would result in a substantially shorter lifespan of the storm. Furthermore, we showed that this impact is due to the microphysical properties of ice rather than the thermodynamics of the cloud due to latent heat release.

Secondly, we tested the sensitivity of ice crystal habits on the development of thin cirrus clouds using a 2-dimensional cirrus model with detailed microphysics. Four different ice crystal habits were studied: plates, columns, rosettes and spheres. The results show that the cirrus development is indeed greatly influenced by the habit of ice crystals in the cloud. The largest differences exist between cirrus consisting of rosettes and that consisting of spheres. The largest impact is in the long-wave heating rates where the peak heating rates between these two cases differ by more than 6 times. Other cloud properties are also significantly influenced by the different habits.

Finally, calculations of the ice crystal capacitance for three ice habits: rosettes, solid columns and hollow columns were performed using finite element techniques. The results show that the homomorphic solid and hollow columns of same dimensions have nearly the same capacitance, implying that the mass growth rates of the two are nearly the same but the hollow column will have greater linear growth rate due to its hollowness. The results for rosettes show that the capacitance of rosettes is a nonlinear function of the number of lobes, and hence previous assumptions that their capacitances can be approximated by spheres or prolate/oblate spheroids of the same diameter may result in substantial errors. The computed capacitances can be used in the calculations of crystal growth rates in ice clouds.

In this work, we review the formation of high levels of ozone in megacities of East Asia and their potential impact on the increasing trends of tropospheric ozone concentration. A series of intensive observation experiments were conducted in Kaohsiung, Taipei and Taichung by scientists of the Research Center for Environmental Changes (RCEC) of Academia Sinica, in collaboration with colleagues from the National Taiwan University and the National Central University, to study the ozone formation and the strategy for its control in 2003–2005. In addition, RCEC scientists participated in three large-scale international experiments led by Peking University in the Pearl River Delta (PRD) and in Beijing in 2004 and 2006. A one-dimensional model and an observation-based method (OBM) were used to analyze data from these experiments to examine the photochemical processes of ozone formation and the relationship of ozone to its precursors. We found that the O₃ production rate was NMHCs-limited, and that controlling NMHCs was more efficient than controlling NO_x in reducing ozone levels in all the megacities studied.

The increasing trends of ozone formation at the background stations in Taiwan, Hong Kong and Japan from the 1980s to 2005 strongly suggest that photochemical production of ozone in Asia has been increasing due to anthropogenic emissions of ozone precursors. In addition, we believe that the increasing trend of ozone concentration in Mauna Loa (4.1% per decade between 1973 and 2004) could be a good indicator/measure and be useful for inverse-modeling the trend of the background ozone level of the entire Asia and even the Northern Hemisphere.

The Formosa Satellite Mission #3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC), hereafter referred to as COSMIC, is a Taiwan–US mission launched in April 2006. COSMIC consists of six small satellites that employ the Global Positioning System (GPS) radio occultation (RO) technique to sound the neutral atmosphere and ionosphere with uniform global coverage. As of January 2008, COSMIC provides approximately 2,000 RO soundings per day to support the research and operational communities. For East Asian countries, the COSMIC soundings are particularly valuable for the study of typhoons and Mei-yu convective systems, as they provide observations over the data-sparse Western Pacific Ocean and the South China Sea.

In this study, we assimilate COSMIC GPSRO soundings and examine their impact on the prediction of Typhoon Shanshan (2006). The assimilation is first carried out using the WRF-Var (3D-Var) system. We find that in order for COSMIC GPSRO soundings to have an impact, it is critical to perform continuous assimilation through cycling. With one-hour cycling over a one-day period, COSMIC GPSRO soundings significantly improve the track forecast. However, the assimilation of only seven COSMIC GPSRO soundings in a cold-start experiment produces virtually no impact. The continuous cycling assimilation is able to incorporate 110 GPSRO soundings, and has a profound impact. We also find that the assimilation of typhoon bogus soundings improves the typhoon intensity and track forecast, particularly during the first two days.

To assess the impact of data-assimilation systems, we compare the performance of the WRF 3D-Var system with the WRF/DART ensemble filter system for the assimilation of COSMIC GPSRO soundings. The results show that the WRF/DART ensemble filter system can assimilate the GPSRO data more effectively than the WRF 3D-Var method. In particular, the WRF/DART ensemble filter system is able to produce a storm with more coherent typhoon structure after one day of continuous assimilation, while a much weaker and less coherent storm is produced by WRF 3D-Var.

In addition to Typhoon Shanshan (2006), we assimilate GPSRO soundings from COSMIC during the two-week period of 1–14 June 2007, associated with a Mei-yu system, using the WRF/DART ensemble filter data-assimilation system. We find that the assimilation of COSMIC data significantly strengthens the Western Pacific Subtropical High, and consequently improves the prediction of Mei-yu precipitation over southern China and Taiwan.

The following sections are included:

• Author Index