Gin-Rong Liu

An image-based retrieval algorithm of aerosol characteristics and surface reflectance for satellite images

Abstract An image-based retrieval algorithm of aerosol characteristics and surface reflectance for satellite images is proposed. By assuming the Junge size distributed aerosol in the atmosphere and feeding back the new Junge parameter, not only the aerosol optical depth but also the Junge parameter, single-scattering albedo and phase function can be iteratively derived and converged from digital counts of dense dark vegetation in the green and red bands of SPOT satellite image. These retrieved aerosol characteristics are considered to be more physically related to the image itself than estimated. To prevent over-estimating aerosol optical depth, multiple scattering of path radiance for aerosol is taken into consideration. Although in lack of field measurement, the algorithm is evaluated and proved to be useful by simulation. Its sensitivities to assumed dense dark vegetation reflectances, log-normal size distribution, initial assumed power of Junge size distribution, refractive index and radius range are ...

Profiling transboundary aerosols over Taiwan and assessing their radiative effects

A synergistic process was developed to study the vertical distributions of aerosol optical properties and their effects on solar heating using data retrieved from ground-based radiation measurements and radiative transfer simulations. Continuous MPLNET and AERONET observations were made at a rural site in northern Taiwan from 2005 to 2007. The aerosol vertical extinction profiles retrieved from ground-based lidar measurements were categorized into near-surface, mixed, and two-layer transport types, representing 76% of all cases. Fine-mode (Angstrom exponent, alpha, approx.1.4) and moderate-absorbing aerosols (columnar single-scattering albedo approx.0.93, asymmetry factor approx.0.73 at 440 nm wavelength) dominated in this region. The column-integrated aerosol optical thickness at 500 nm (tau(sub 500nm)) ranges from 0.1 to 0.6 for the near-surface transport type, but can be doubled in the presence of upper-layer aerosol transport. We utilize aerosol radiative efficiency (ARE; the impact on solar radiation per unit change of tau(sub 500nm)) to quantify the radiative effects due to different vertical distributions of aerosols. Our results show that the ARE at the top-of-atmosphere (-23 W/ sq m) is weakly sensitive to aerosol vertical distributions confined in the lower troposphere. On the other hand, values of the ARE at the surface are -44.3, -40.6 and -39.7 W/sq m 38 for near-surface, mixed, and two-layer transport types, respectively. Further analyses show that the impact of aerosols on the vertical profile of solar heating is larger for the near-surface transport type than that of two-layer transport type. The impacts of aerosol on the surface radiation and the solar heating profiles have implications for the stability and convection in the lower troposphere.

Applying SPOT data to estimate the aerosol optical depth and air quality

Abstract The improvement in the structure function method for retrieving aerosol optical depth (AOD) with SPOT HRV data and its application in air quality monitoring are highlighted in this paper. Generally speaking, estimation of the aerosol optical depth will be affected by the temporal change of surface canopy, observation geometry and terrain effect when applying the contrast reduction method to the multi-temporal satellite image set. In order to reduce the errors induced by such effects, the single-directional structure function is replaced by the multi-directional mode, which can describe the real characteristics of the surface structure more completely. Comparison of the results with in-site observations show a significant improvement in the accuracy of the retrieved AOD. Furthermore, due to the linear relationship between aerosol optical depth and turbidity coefficient, satellite images can be employed for monitoring air quality. Application of the method is demonstrated with a case study situated around the northern Taiwan area.

Estimation of the Upper-Layer Rotation and Maximum Wind Speed of Tropical Cyclones via Satellite Imagery

Abstract The movement of convective rainbands embedded in a tropical cyclone (TC) is usually derived from satellite images via the atmospheric motion vector (AMV) method or through the calculation of a radar’s echo track. In estimating the rotation speed of a TC rainband, however, the land-based radar can only detect approaching tropical cyclones within the vicinity. The AMV method is unable to fully account for the TC eyewall movement, thus making it difficult to estimate the TC intensity. The widely used method in estimating the TC maximum wind speed is the Dvorak technique in which the cloud pattern is extracted from only one image. In this study, the rainband rotation speeds are computed via satellite imagery and further applied in estimating the TC maximum wind speed. In contrast to previous research, this study adopts an innovative method by using two subsequent geostationary satellite images. The TC spin rates observed by weather satellites could often be seen to be positively related to the TC int...

Meteorology-hydrology study targets Typhoon Nari and Taipei flood

Typhoon Nari struck Taiwan on 16 September 2001, taking 92 lives. Analysis reveals that the storms heavy rains were due to warmer ocean temperatures, Naris unique track and slow-moving speed, and the terrain of Taiwan. Analysis further suggests that the heavy rains in Nari contained many small raindrops. The typhoon rains overwhelmed existing flood protection capacities downstream of the Chi-Lung River in a part of Taipei that has no regulatory reservoirs, resulting in major flooding. Preliminary findings underscore several key issues for future study, the goal of which will be to improve quantitative precipitation estimation/prediction, hydrologic modeling, and flood prediction.

Applying Satellite-Estimated Storm Rotation Speed to Improve Typhoon Rainfall Potential Technique

Abstract Heavy rainfall from typhoons or tropical cyclones often causes inland flooding and mudslides that threaten lives and property. In this study, Special Sensor Microwave Imager (SSM/I) satellite data observed from 2000 to 2004 were used to calculate the rainfall rates of different typhoons in the northwestern Pacific. Geostationary weather satellite infrared images were also applied to estimate the typhoon rotation speed via the maximum cross-correlation technique. By including such information in the tropical rainfall potential (TRaP) technique, an improved typhoon rainfall potential technique can be constructed. Considering the fact that a typhoon’s spiral rainbands move constantly, half-hourly or hourly infrared data observed from geostationary weather satellites were used to calculate the revolving speed, which was subsequently used to predict the rainband movement over the next hour. After comparing the predicted rainfall potential with the rain gauge data of Taiwan’s small offshore islands, it...

Rainfall Intensity Estimation by Ground-Based Dual-Frequency Microwave Radiometers

Abstract Many investigators have used satellite data to derive rainfall intensity and to compare them with rain gauge data. However, there has always been a problem: what is the optimal time period for the two different types of data? A set of well-controlled data collected by ground-based dual-frequency microwave radiometers at the National Central University (24.9°N, 121.1°E) in Taiwan between January of 1996 and December of 1997 was used to find the answer. The results show that a 1-h interval would be the optimal time period and that hourly data will provide a better accuracy than other options (5, 10, or 30 min or 2 h). Two algorithms, the differential and the brightness temperature, were established to estimate rainfall intensity using ground-based dual-frequency microwave brightness temperature and rain gauge data. The results show that the root-mean-square error and the correlation coefficient are 0.63 mm h−1 and 0.88, respectively, for the differential method, and 0.91 mm h−1 and 0.71 for the bri...

A contrast and comparison of near-sea surface air temperature/humidity from GMS and SSM/I data with an improved algorithm

With data sets gained from Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager (SSM/I) microwave channels, Geostationary Meteorological Satellite (GMS-5) infrared channels, and ship-measured data, the statistical algorithms to estimate sea surface temperature and near-sea surface air humidity around Taiwan and the South China Sea areas are developed. Then a new, improved method to estimate near-sea surface air temperature based on the algorithm proposed by Konda et al. (1996) is established in this study. The results estimated with SSM/I data show that the root mean square error (RMSE) of SST, near-sea surface air humidity and air temperature over the oceans around Taiwan and the South China Sea are 1.2 K,1.43 g/kg, and 1.6 K, respectively. The results with GMS data are 1.7 K,1.71 g/kg and 1.7 K, respectively. The results also show that the improvements in the algorithm of Konda et al. simplify the computation scheme, improve the accuracy, and match the regional ocean-atmosphere properties in retrieving near-sea surface air temperature. The estimate produced using SSM/I and GMS data also show good consistency between them, both in temporal and spatial variations. Basically, the accuracy of this result implies strong potential for application of satellite data to relative studies and operational work in the ocean-atmosphere interaction.

Using Surface Stations to Improve Sounding Retrievals from Hyperspectral Infrared Instruments

Having an accurate atmospheric thermodynamic state is critical for environmental research, particularly the vertical temperature and moisture profiles within the atmospheric boundary layer. This paper investigates the synergistic use of spaceborne hyperspectral infrared radiance measurement and traditional surface observation to conduct the best estimation of atmospheric temperature and water vapor profiles. Comparing the retrieval results from the original spaceborne observation stand-alone algorithm, atmospheric boundary layer temperature and moisture retrievals appear to be improved through the inclusion of the surface observation in the new developed algorithm. The statistics of retrieval performance by comparing with radiosonde observation suggest that the improvement is not only at the lowest surface level but also within the planetary boundary layer. This implies the benefit of surface observation in the atmospheric sounding retrieval algorithm, and the boundary layer thermodynamic structure could be retrieved optimally from the use of both spaceborne and ground-based observations.

Modified Bowen ratio method in near-sea-surface air temperature estimation by using satellite data

The near-sea-surface air temperature can be derived through Special Sensor Microwave/Imager (SSM/I) microwave and Geostationary Meteorological Satellite (GMS-5) infrared data by using a so-called modified Bowen ratio method (MBRM). A specified value (/spl kappa/) that was defined to be a ratio between bulk coefficients c/sub e/ and c/sub h/ was viewed as a constant and was input into this retrieval model for the derivation of near-sea-surface air temperature. However, there is insufficient evidence to prove the hypothesis why the specified value can be viewed as a constant and used in this model. This study strives in providing an answer. By using the iterative technique, the fact that the optimal /spl kappa/ value oscillated merely within a small range for present experimental area covering the area from 10/spl deg/N to 30/spl deg/N and 105/spl deg/E to 135/spl deg/E is shown clearly. The small oscillation does not strongly affect the value of the derived near-sea-surface air temperature. Therefore, the optimal /spl kappa/ value could be considered as a constant both temporally and spatially. Moreover, the results show that the rmse of the estimated near-sea-surface air temperature from SSM/I and GMS-5 data are 1.46 and 1.69 K, respectively. Therefore, the MBRM may be a good approach to estimate this parameter. However, the optimal /spl kappa/ proposed in this study may be suitable only to present experimental area. Generally speaking, this is due to the fact that the /spl kappa/ values are usually larger further to the north (in higher latitudes) than to the south (in lower latitudes) and also during the cold season than in the warm season.