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Hui Liu and Jun Li

Abstract

Hyperspectral infrared (IR) sounders, such as the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI), provide unprecedented global atmospheric temperature and moisture soundings with high vertical resolution and accuracy. In this paper, the authors investigate whether advanced IR soundings of water vapor and temperature observations can improve the analysis of a tropical cyclone vortex and the forecast of rapid intensification of a tropical cyclone. Both the IR water vapor and temperature soundings significantly improve the typhoon vortex in the analysis and the forecast of the rapid intensification of Typhoon Sinlaku (2008). The typhoon track forecast is also substantially improved when the full spatial resolution AIRS soundings are assimilated. This study demonstrates the potential important application of high spatial and hyperspectral IR soundings in forecasting tropical cyclones.

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Yi-Leng Chen and Jun Li

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The large-scale processes responsible for development of heavy precipitation during 20–23 May 1987 along the southeastern China coast are studied. There are two distinct rainfall peaks around 0000 UTC 20 May and 0000 UTC 22 May. Prior to the heavy rain event, strong low-level southwesterly flow developed ahead of a 850-hPa trough and transported warm, moist air from the south into southeastern China to generate the convective instability. For the first rainfall period, positive vorticity advection by thermal winds was observed ahead of the 850-hPa trough. Upper-level divergence as a result of the imbalance between winds and geopotential height fields was found in the different airflow region on the northeastern quadrant of the south. Asian anticyclone prior to the first rainfall period. For the second rainfall period, an upper-level trough deepened in the lee side of the Tibetan Plateau and moved to the southeastern China coast. Strong upper-level frontogenesis caused by horizontal deformation was found along the trough axis. The heaviest precipitation occurred along the low-level warm, moist tongue when the low-level baroclinic forcing was coupled with the upper-level one as the upper-level trough approached. These results are in contrast to a CISK (conditional instability of the second kind) process proposed by Chen and Chang for a Mei-Yu case in mid-June over southern China.

Analyses of the large-scale heat and moisture budgets show a vertical separation of Q 1 and Q 2 peaks during the first rain period, suggesting that the rainfall is convective in nature. For the heaviest rain period, the Q 1 peak increases and shifts upward. The Q 2 profile shows a double-peak structure with the upper-level peak coinciding with the Q 1 peak but with a much smaller magnitude, indicating that the precipitation mainly falls from convective clouds and the anvils associated with them.

Over the Taiwan area, heavy rain did not develop as forecasted, because of the weakening of both the upper-level and low-level troughs and the lack of coupling between the upper-level and low-level forcings. When the low-level southwesterly flow interacted with the island of Taiwan, the low-level flow moved around the island with a windward ridge-leeside trough pressure pattern. Along the western coast, the wind component parallel to the central mountain range increased northward down the pressure gradient and had a maximum value (≥14 m s−1) at approximately 1.5 km above sea level over the northwestern coast. With the turning of the large-scale low-level flow from south-southwest to west-southwest and the strengthening of the southwesterly flow, the surface pressure ridge became stronger and extended northward along the western coast of Taiwan. The deflection and flow deceleration upstream were also more significant.

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Yi-Leng Chen and Jun Li

Abstract

Principal component (PC) analysis was used to study the wind and sea level pressure patterns over the island of Taiwan during the early summer rainy season. An eight-point, running-mean, low-pass filter was applied to the 3-h surface winds and sea level pressure deviations from its areal average to remove signals with a time scale of 1 day or less. The first and the second PC modes of the low-pass-filtered wind data account for 66% and 13% of the total variance, respectively. The first, the second, and the third PC modes of the low-pass-filtered pressure deviations account for 63%, 22%, and 10% of the total variance, respectively.

Composite analysis of low-pass-filtered surface variables was made during a 3-day period for each TAMEX (Taiwan Area Mesoscale Experiment) frontal passage episode, using the timing of the maximum PC scores of the dominant PC modes to arrive at a general picture of the evolution of surface airflow and pressure patterns. The prefrontal southwesterly flow reached the maximum intensity when the large-scale surface pressure trough arrived. At this time, the pressure pattern over Taiwan was dominated by a windward ridge along the southwestern coast and a leeside trough. A case study of the low-level airflow at this stage was made based on the wind data from aircraft and soundings. Below the 1000-m level, the airflow moved around the topography. The northern branch of the splitting flow had a cross-isobar wind component down the windward pressure ridge and accelerated downstream. Notable westerly flow crossing the southern portion of the central mountain range was observed above the 1200-m level. As an upper-level trough approached, the mesolow over southeastern Taiwan developed. It reached the maximum intensity during the time the westerly wind aloft was the largest. With the arrival of the surface front and the decrease of the westerly wind aloft, the mesolow weakened. After the frontal passage, northeasterlies prevailed over the Taiwan area with a increasing easterly wind component. The shallow cold northeasterlies interacted with the central mountain range, resulting in a pressure ridge along the northeastern coast and a Iceside trough. This pressure pattern was most significant after the migratory high moved off the southeastern China coast.

Principal component analysis was also applied to the bandpass-filtered diurnal winds and sea level pressure. The land—sea-breeze circulations were more pronounced over the southwestern plain with a slightly larger amplitude in diurnal sea level pressure than most areas. There were no significant differences in the diurnal cycle between the southwest and northeast flow regimes. In general, the winds turned clockwise (counterclockwise) during the diurnal cycle for the stations along the western (eastern) coast. The mesolow over southeastern Taiwan was slightly stronger during the day than at night.

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Jun Li and Yi-Leng Chen

Abstract

A barrier jet is frequently found along the northwestern coast of Taiwan in the prefrontal southwesterly flow regime during the Taiwan Area Mesoscale Experiment (TAMEX). It has a maximum wind speed of 14 m s−1 at approximately 1 km above the surface with a vertical wind shear approximately 10 × 10−3 s−1 below and 4 × 10−3 s−1 above that altitude. During TAMEX, the southwesterly monsoon flow strengthens over Taiwan when the low-level pressure trough/surface front moves toward the southeastern China coast. The barrier jet is a result of the stably stratified airflow past an island obstacle under a small–Froude number [<O(1)] flow regime. During the occurrence of a barrier jet, a windward pressure ridge is observed along the southwestern coast due to island blocking. In low levels, the incoming southwesterly flow decelerates off the southwestern coast and moves around the island. Along the western coast, the deflected airflow accelerates northward with a large cross-contour wind component down the pressure gradient along the western coast, resulting in a barrier jet over northwestern Taiwan. The barrier jet is the strongest when the surface windward ridge–leeside trough pressure pattern is most significant and weakens after a surface front arrives.

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Jun Li and Yi-Leng Chen

Abstract

Nocturnal rain showers over the windward side of the island of Hawaii were investigated from the late afternoon of 2 August to the early morning of 3 August 1990 during the Hawaiian Rainband Project (HaRP). Three types of rainbands produce rainfall peaks over the lowland/coastal region during the evening, after midnight, and along the coast in the early morning.

In the early evening, an inland rainband develops over the lower slopes as a result of orographic lifting and low-level forcing along the drainage front. As the drainage front progresses toward the coast, new rain cells continue to develop along the drainage front. These cells move westward with the trade winds aloft and dissipate. After the drainage front moves to the Hilo coast, new cell generation along the drainage front ceases. It appears that in the absence of orographic lifting, the shallow (∼0.2 km) katabatic flow offshore is not deep enough to lift the low-level air to saturation. Thus, the rainband weakens and dissipates. It lasts about 6 h and produces the heaviest rainfall over the lowland/coastal region during the analysis period.

During the evening transition, rain evaporative cooling aloft deepens the cool pool behind the drainage front over land. After the drainage front moves off the coast around 2300 HST, cold air continues to move from lower slopes and lowlands toward the coast. The offshore flow gradually deepens. By ∼0207 HST, the depth of the katabatic flow over the coastal region reaches 0.48 km, which is above the level of free convection. A rainband develops along the drainage front offshore. As the rainband moves westward over the deepened katabatic flow, the katabatic flow is replaced by easterly winds. The rainband weakens and dissipates over the lower slopes.

During the early morning, two groups of trade wind rain showers move into the coastal region. They are enhanced offshore in the convergent zone between the offshore flow and the trade winds. When the first group of rain showers moves over the katabatic flow, the outflow associated with the rain showers deepens the offshore flow (∼0.6 km) east of the rain showers. The second group of rain showers is more intense than the first group as the rain showers move into the convergent zone and interact with the deep offshore flow. Nevertheless, they weaken rapidly over the deep offshore flow and produce little rainfall along the coast.

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Wenjun Tang, Kun Yang, Jun Qin, Jun Li, and Jiangang Ye

Abstract

Surface solar radiation (SSR) over the ocean is essential for studies of ocean–atmosphere interactions and marine ecology, and satellite remote sensing is a major way to obtain the SSR over ocean. A new high-resolution (10 km; 3 h) SSR product has recently been developed, mainly based on the newly released cloud product of the International Satellite Cloud Climatology Project H series (ISCCP-HXG), and is available for the period from July 1983 to December 2018. In this study, we compared this SSR product with in situ observations from 70 buoy sites in the Global Tropical Moored Buoy Array (GTMBA) and also compared it with another well-known satellite-derived SSR product from the Clouds and the Earth’s Radiant Energy System (CERES; edition 4.1), which has a spatial resolution of approximately 100 km. The results show that the ISCCP-HXG SSR product is generally more accurate than the CERES SSR product for both ocean and land surfaces. We also found that the accuracy of both satellite-derived SSR products (ISCCP-HXG and CRERS) was higher over ocean than over land and that the accuracy of ISCCP-HXG SSR improves greatly when the spatial resolution of the product is coarsened to ≥ 30 km.

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Qingyong Li, Weitao Lu, and Jun Yang

Abstract

Cloud detection is the precondition for deriving other information (e.g., cloud cover) in ground-based sky imager applications. This paper puts forward an effective cloud detection approach, the Hybrid Thresholding Algorithm (HYTA) that fully exploits the benefits of the combination of fixed and adaptive thresholding methods. First, HYTA transforms an input color cloud image into a normalized blue/red channel ratio image that can keep a distinct contrast, even with noise and outliers. Then, HYTA identifies the ratio image as either unimodal or bimodal according to its standard deviation, and the unimodal and bimodal images are handled by fixed and minimum cross entropy (MCE) thresholding algorithms, respectively. The experimental results demonstrate that HYTA shows an accuracy of 88.53%, which is far higher than those of either fixed or MCE thresholding alone. Moreover, HYTA is also verified to outperform other state-of-the-art cloud detection approaches.

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Zhigang Yao, Jun Li, Jinlong Li, and Hong Zhang

Abstract

An accurate land surface emissivity (LSE) is critical for the retrieval of atmospheric temperature and moisture profiles along with land surface temperature from hyperspectral infrared (IR) sounder radiances; it is also critical to assimilating IR radiances in numerical weather prediction models over land. To investigate the impact of different LSE datasets on Atmospheric Infrared Sounder (AIRS) sounding retrievals, experiments are conducted by using a one-dimensional variational (1DVAR) retrieval algorithm. Sounding retrievals using constant LSE, the LSE dataset from the Infrared Atmospheric Sounding Interferometer (IASI), and the baseline fit dataset from the Moderate Resolution Imaging Spectroradiometer (MODIS) are performed. AIRS observations over northern Africa on 1–7 January and 1–7 July 2007 are used in the experiments. From the limited regional comparisons presented here, it is revealed that the LSE from the IASI obtained the best agreement between the retrieval results and the ECMWF reanalysis, whereas the constant LSE gets the worst results when the emissivities are fixed in the retrieval process. The results also confirm that the simultaneous retrieval of atmospheric profile and surface parameters could reduce the dependence of soundings on the LSE choice and finally improve sounding accuracy when the emissivities are adjusted in the iterative retrieval. In addition, emissivity angle dependence is investigated with AIRS radiance measurements. The retrieved emissivity spectra from AIRS over the ocean reveal weak angle dependence, which is consistent with that from an ocean emissivity model. This result demonstrates the reliability of the 1DVAR simultaneous algorithm for emissivity retrieval from hyperspectral IR radiance measurements.

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Yufei Ai, Wanbiao Li, Zhiyong Meng, and Jun Li

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By combining high temporal and spatial resolution Multifunctional Transport Satellite-1R (MTSAT-1R) infrared (IR) images and precipitation data from the Climate Prediction Center morphing technique (CMORPH), this study tracked mesoscale convective systems (MCSs) from May to August in 2008 and 2009 in the middle of east China with an automatic tracking algorithm based on an areal overlapping methodology. This methodology is adjusted to include those MCSs with a relative weak intensity before formation. The unique advantage of combining high temporal and spatial resolution geostationary satellite brightness temperature images and the precipitation measurements for tracking MCSs is that the cloud-top height along with the coverage and the precipitation intensity can be well identified. Results showed that the MCSs formed most frequently in the southwest Henan Province and at the border of four provinces—Shandong, Henan, Anhui, and Jiangsu—which is east of the convergence zone near the terrain’s edge. Locations of the highest cloud tops and of the heaviest precipitation rates did not always match. In addition, the MCSs in the study region tended to first reach the maximum precipitation rate, followed soon by the minimum brightness temperature, then the maximum associated precipitation area, and finally the maximum in system area.

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Yong-Keun Lee, Zhenglong Li, Jun Li, and Timothy J. Schmit

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A physical retrieval algorithm has been developed for deriving the legacy atmospheric profile (LAP) product from infrared radiances of the Advanced Baseline Imager (ABI) on board the next-generation Geostationary Operational Environmental Satellite (GOES-R) series. In this study, the GOES-R ABI LAP retrieval algorithm is applied to the GOES-13 sounder radiance measurements (termed the GOES-13 LAP retrieval algorithm in this study) for its validation as well as for potential transition of the GOES-13 LAP retrieval algorithm for the operational processing of GOES sounder data. The GOES-13 LAP retrievals are compared with five different truth measurements: radiosonde observation (raob) and microwave radiometer–measured total precipitable water (TPW) at the Atmospheric Radiation Measurement Cloud and Radiation Testbed site, conventional raob, TPW measurements from the global positioning system–integrated precipitable water NOAA network, and TPW measurements from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E). The results show that with the GOES-R ABI LAP retrieval algorithm, the GOES-13 sounder provides better water vapor profiles than the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) forecast fields at the levels between 300 and 700 hPa. The root-mean-square error (RMSE) and standard deviation (STD) of the GOES-13 sounder TPW are consistently reduced from those of the GFS forecast no matter which measurements are used as the truth. These substantial improvements indicate that the GOES-R ABI LAP retrieval algorithm is well prepared to provide continuity of quality to some of the current GOES sounder products, and the algorithm can be transferred to process the current GOES sounder measurements for operational product generation.

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