Po-Hsiung Lin

A Report of the Field Operations and Early Results of the South China Sea Monsoon Experiment (SCSMEX)

Abstract The South China Sea Monsoon Experiment (SCSMEX) is an international field experiment with the objective to better understand the key physical processes for the onset and evolution of the summer monsoon over Southeast Asia and southern China aiming at improving monsoon predictions. In this article, a description of the major meteorological observation platforms during the intensive observing periods of SCSMEX is presented. In addition, highlights of early results and discussions of the role of SCSMEX in providing valuable in situ data for calibration of satellite rainfall estimates from the Tropical Rainfall Measuring Mission are provided. Preliminary results indicate that there are distinctive stages in the onset of the South China Sea monsoon including possibly strong influences from extratropical systems as well as from convection over the Indian Ocean and the Bay of Bengal. There is some tantalizing evidence of complex interactions between the supercloud cluster development over the Indian Oce...

Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR): An Overview

DOTSTAR (Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region) is an international research program conducted by meteorologists in Taiwan partnered with scientists at the Hurricane Research Division (HRD) and the National Centers for Environmental Prediction (NCEP) of the National Oceanic and Atmospheric Administration (NOAA). The experiment is based on successful surveillance missions conducted in the Atlantic with NOAAs Gulfstream-IV jet aircraft. During the experiment, GPS dropwindsondes are released from a jet aircraft flying above 42000 ft in and around tropical cyclones approaching Taiwan to collect critical meteorological data for improving the analysis and the prediction of typhoons. After one-year of training, development and installation of all the needed software and hardware in the aircraft, the DOTSTAR research team initiated typhoon surveillance in 2003. Two missions (in Typhoons Dujuan and Melor) were conducted successfully,and seven or eight missions are expected to be conducted annually during the 2004 and 2005 typhoon seasons. The current manuscript provides an overview of the scientific objectives of DOTSTAR including operational plans, organization, data management, and data archiving. Preliminary results of the two missions in the first season in 2003 are presented. The experiment marks the beginning of typhoon surveillance in the western North Pacific and is expected to yield impressive improvements in typhoon research, observations and forecasting.

The Impact of Dropwindsonde Data on Typhoon Track Forecasts in DOTSTAR

Abstract Starting from 2003, a new typhoon surveillance program, Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR), was launched. During 2004, 10 missions for eight typhoons were conducted successfully with 155 dropwindsondes deployed. In this study, the impact of these dropwindsonde data on tropical cyclone track forecasts has been evaluated with five models (four operational and one research models). All models, except the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model, show the positive impact that the dropwindsonde data have on tropical cyclone track forecasts. During the first 72 h, the mean track error reductions in the National Centers for Environmental Prediction’s (NCEP) Global Forecast System (GFS), the Navy Operational Global Atmospheric Prediction System (NOGAPS) of the Fleet Numerical Meteorology and Oceanography Center (FNMOC), and the Japanese Meteorological Agency (JMA) Global Spectral Model (GSM) are 14%, 14%, and 19%, respectively. The tra...

The Influence of Assimilating Dropsonde Data on Typhoon Track and Midlatitude Forecasts

A unique dataset of targeted dropsonde observations was collected during The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in the autumn of 2008. The campaign was supplemented by an enhancement of the operational Dropsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) program. For the first time, up to four different aircraft were available for typhoon observations and over 1500 additional soundings were collected. This study investigates the influence of assimilating additional observations during the two major typhoon events of T-PARC on the typhoon track forecast by the global models of the European Centre for MediumRange Weather Forecasts (ECMWF), the Japan Meteorological Agency (JMA), the National Centers for Environmental Prediction (NCEP), and the limited-area Weather Research and Forecasting (WRF) model. Additionally, the influence of T-PARC observations on ECMWF midlatitude forecasts is investigated. All models show an improving tendency of typhoon track forecasts, but the degree of improvement varied fromabout20%to40%inNCEPandWRFtoacomparablylowinfluenceinECMWFandJMA.Thisislikely related to lower track forecast errors without dropsondes in the latter two models, presumably caused by a more extensive use of satellite data and four-dimensional variational data assimilation (4D-Var) of ECMWF and JMA compared to three-dimensional variational data assimilation (3D-Var) of NCEP and WRF. The different behavior of the models emphasizes that the benefit gained strongly depends on the quality of the first-guess field and the assimilation system.

The Impact of Dropwindsonde Observations on Typhoon Track Forecasts in DOTSTAR and T-PARC

The typhoon surveillance program Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) has been conducted since 2003 to obtain dropwindsonde observations around tropical cyclones near Taiwan. In addition, an international field project The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in which dropwindsonde observations were obtained by both surveillance and reconnaissanceflights was conducted in summer 2008 in the same region. In this study, the impact of the dropwindsonde data on track forecasts is investigated for DOTSTAR (2003‐09) and T-PARC (2008) experiments. Two operational global models from NCEP and ECMWF are used to evaluate the impact of dropwindsonde data. In addition, the impact on the two-model mean is assessed. The impact of dropwindsonde data on track forecasts is different in the NCEP and ECMWF model systems. Using the NCEP system, the assimilation of dropwindsonde data leads to improvements in 1- to 5-day track forecasts in about 60% of the cases. The differences between track forecasts with and without the dropwindsonde data are generallylargerforcasesinwhichthedataimprovedtheforecaststhanincasesinwhichtheforecastsweredegraded. Overall, the mean 1- to 5-day track forecast error is reduced by about 10%‐20% for both DOTSTAR and T-PARC cases in the NCEP system. In the ECMWF system, the impact is not as beneficial as in the NCEP system, likely because of more extensive use of satellite data and more complex data assimilation used in the former, leading to betterperformanceevenwithoutdropwindsondedata. Thestronger impactsofthedropwindsonde dataare revealed forthe3-to5-dayforecastinthetwo-modelmeanoftheNCEPandECMWFsystemsthanforeachindividualmodel.

The Eyewall-Penetration Reconnaissance Observation of Typhoon Longwang (2005) with Unmanned Aerial Vehicle, Aerosonde

Abstract In this paper, a successful eye-penetration reconnaissance flight by an unmanned aerial vehicle, Aerosonde, into Typhoon Longwang (2005) and the preliminary analyses of the collected data are presented. The 10-h flight is diagnosed through four flight legs. The wind field measured along flight leg 1 provides the tangential and radial wind profiles from the outer perimeter into the eye of the typhoon at the 700-hPa layer. A vertical sounding was taken in the eye along flight leg 2 and the derived surface pressure in the eyewall is close to the estimates made by the local weather agencies. Along flight leg 3, the strongest winds during the whole flight mission were measured. These in situ wind measurements by Aerosonde are consistent with the winds observed by the Hua-lien Doppler weather radar. The maximum 10-min (1 min) wind along flight leg 3 when Aerosonde was flying around the eyewall region is 58.6 m s−1 (62 m s−1). The maximum sustained surface wind derived from this maximum wind speed is al...

Quality-Controlled Upper-Air Sounding Dataset for TiMREX/SoWMEX: Development and Corrections

Abstract During the Terrain-Influenced Monsoon Rainfall Experiment (TiMREX), which coincided with Taiwan’s Southwesterly Monsoon Experiment—2008 (SoWMEX-08), the upper-air sounding network over the Taiwan region was enhanced by increasing the radiosonde (“sonde”) frequency at its operational sites and by adding several additional sites (three that were land based and two that were ship based) and aircraft dropsondes. During the special observing period of TiMREX (from 15 May to 25 June 2008), 2330 radiosonde observations were successfully taken from the enhanced network. Part of the challenge of processing the data from the 13 upsonde sites is that four different sonde types (Vaisala RS80, Vaisala RS92, Meisei, and Graw) were used. Post–field phase analyses of the sonde data revealed a significant dry bias in many of the sondes—in particular, in the data from the Vaisala RS80 sondes that were used at four sites. In addition, contamination of the sonde data by the ship’s structure resulted in poor-quality ...

Parameterization of topographic effect on surface solar radiation

[1] Topographic impact on the surface solar (shortwave (SW)) radiation was investigated using a 40 m high-resolution digital topographic data model and a radiative transfer model. With applications to the complex terrain of Taiwan, we found that the spatial variation of the surface SW radiation has a strong diurnal cycle. The standard deviation of the SW radiation, σ f , over Taiwan is nearly constant for a given solar zenith angle (SZA). The maximum σ f occurs at SZA ∼55°, corresponding to midmorning (early morning) and midafternoon (late afternoon) in winter (summer). For a spatial resolution of 40 m, the σ f over Taiwan attains a maximum value of 300 W m -2 , which is ∼50% of the domain mean SW radiation. The topographic impact on the spatial variation of SW radiation increases with decreasing grid box size, Δx. In mesoscale land and atmospheric models, Δx is small, and the topographic impact is strong. Over Taiwan, the standard deviation of the difference between the daily mean SW radiation computed with a spatial resolution of 40 m and that computed with degraded spatial resolution reaches 25 W m -2 for Δx = 1 km and 8 W m -2 for Δx = 10 km, indicating the importance of topographic impact on the surface SW radiation in mesoscale land and atmospheric models. We developed a parameterization of the topographic impact on the surface SW radiation that scales the SW radiation computed for a flat surface without shading by surrounding terrains. The scaling is separately applied to the direct and diffuse radiation. It is applicable to all clear, aerosol-laden, and cloudy conditions and to all spatial resolutions with Δx > 40 m.

GIS-based Tests for Quality Control of Meteorological Data and Spatial Interpolation of Climate Data

Abstract Constructing climate layers is more difficult and important in mountainous areas as a result of sparse meteorological stations and complex topography. This requires a 2-stage process: quality control of meteorological data and spatial interpolation of climate data. For this article, unscreened metadata and observed data were collected from all stations in Taiwan for the period 1961–2002. A quality-control procedure based on a geographic information system (GIS) allowed us to reject 13.5% of stations because of missing or erroneous metadata and filter out 8.3% of the observed data because of extreme errors or unreasonable temporal sequence and spatial patterns. After applying the quality-control procedure, the monthly mean temperature and total monthly precipitation were calculated as spatial interpolation sampling points. We evaluated the performance of 6 kriging-based spatial interpolation methods with regard to their errors by cross-validation. For interpolating the monthly mean temperature, the strong relation between temperature and elevation led us to favor modified residual kriging. For interpolating the total monthly precipitation, log-transformed kriging was chosen for practical reasons (steadier and simpler). We compared our product layers with pre-existing climate layers. The overall spatial patterns of these layers were similar, except for certain extremes in the mountains. Consequently, the GIS-based approaches presented here could help in rapid construction of adequate climate layers for regions with unconfirmed data.

Potential Vorticity Diagnosis of the Factors Affecting the Track of Typhoon Sinlaku (2008) and the Impact from Dropwindsonde Data during T-PARC

AbstractIn 2008, abundant dropwindsonde data were collected during both reconnaissance and surveillance flights in and around tropical cyclones (TCs) in the western North Pacific basin under the framework of The Observing System Research and Predictability Experiment (THORPEX)–Pacific Asian Regional Campaign (T-PARC). The National Centers for Environmental Prediction Global Forecast System (GFS) showed significant track improvements for Typhoon Sinlaku (2008) after the assimilation of dropwindsonde data. For this particular typhoon, the potential vorticity (PV) diagnosis is adopted to understand the key factors affecting the track. A data denial run initialized at 0000 UTC 10 September is examined to evaluate how the extra data collected during T-PARC improve GFS track forecasts.A quantitative analysis of the steering flow based on the PV diagnosis indicates that the Pacific subtropical high to the east of Sinlaku is a primary factor that advects Sinlaku northwestward, while the monsoon trough plays a sec...