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Ka-Ming Lau and P. H. Chan

Abstract

Intraseasonal variability Of tropical convection over the Indian Ocean/Pacific region during northern summer is studied using outgoing longwave radiation (OLR). OLR anomalies are found to propagate eastward along the equator from the Indian 0cean to the western Pacific and northward towards the Indian subcontinent and southern China. It is found that the dominant mode of tropical convection consists of a dipole with centers located over the Indian 0cean and the western Pacific/South China Set. This dipole undergoes complex structural changes over a broad period range centered around 40–50 days. During a typical oscillation, an anomaly first develops over the equatorial Indian Ocean. This anomaly then extends eastward to the equatorial western and central Pacific to form an elongated convection zone, while its center is displaced progressively northward from the Indian Ocean into the Indian subcontinent by an anomaly of the opposite sign. The elongated convection zone over the western Pacific then propagates northwestward over the South China Sea into southern China. The simultaneous northward propagation of the dipole centers ova India and South China suggests that the two components of the summer monsoon are subject to the same large-scale control.

The 40–50 day oscillation is also found to be phase-locked to the monsoon onset over India, the Mei-yu regime over South China, and in general the seasonal variation of convection over the equatorial Indian-west, Pacific Ocean. However, the eastward propagation along the equator over the Indian Ocean appears to be present all year round. This suggests that the eastward propagation may be a basic property of equatorially trapped wave modes and that the meridional propagation arises as a result of the interaction between these equatorial modes and the monsoon circulations.

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K-M. Lau, C-P. Chang, and P. H. Chan

Abstract

Short-term teleconnections over the Pacific sector of the tropics and the midlatitudes in relation to monsoonal surges during the winter of 1978–79 are investigated using 850 and 200 mb wind data from FGGE/Winter-MONEX. Results show that the intensification of the 200 mb jet streak over Japan is a precursor of cold surges and subsequent downstream development over the central Pacific. A large part of the transient variation of the jet at the entrance region near northeastern China is controlled by the upper level ageostrophic flow transverse to the jet while elsewhere the contribution by momentum flux convergence is more important. It is found that the interaction between cold surges and tropical convection over the maritime continent of Borneo and Indonesia basically agrees with the model of Chang and Lau (1980) and Part I of this paper, except the intensity of the interaction was much weaker during Winter-MONEX.

A new feature observed during the Winter-MONEX period is that the main convective heat source was found over the equatorial central Pacific and was enhanced 3–4 days after the surge onset. The enhancement coincided with the occurrence of a subtropical jet stream over Hawaii and a pronounced upper level trough-ridge system extending from the equatorial central Pacific to the west coast of North America.

It is also hypothesized that the abnormally weak surges during Winter-MONEX may be a result of the reduced positive feedback between the tropical convection over the maritime continent and midlatitude disturbances near the east coast of China due to the remoteness of the primary tropical heat source from the surge area. The apparent downstream teleconnection observed during the post-surge periods suggests enhanced midlatitude-tropical coupling over the central Pacific which may have strong influence on synoptic developments over the west coast of the United States. Notwithstanding possible uncertainties for our results in the tropics, the study suggests that midlatitude systems are capable of forcing tropical convection which in turn may give rise to midlatitude remote responses. Comparison of this study with Part I and other related work, also reveals some interesting aspects of the interannual variability of short-term planetary-scale interactions.

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K. M. Lau and P. H. Chan

The tropical ocean-atmosphere exhibits two prominent modes of low-frequency oscillations, i.e., the “40–50” day oscillation and the El Niño/Southern Oscillation (ENSO). The two phenomena are viewed in the same perspective from 10 years of satellite-derived outgoing-longwave-radiation data. Results reveal some interesting features that may lead to new insights into the understanding of the two phenomena.

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K. M. Kwong, Max H. Y. Wong, James N. K. Liu, and P. W. Chan

Abstract

Current research based on various approaches including the use of numerical weather prediction models, statistical models, and machine learning models have provided some encouraging results in the area of long-term weather forecasting. But at the level of mesoscale and even microscale severe weather phenomena (involving very short-term chaotic perturbations) such as turbulence and wind shear phenomena, these approaches have not been so successful. This research focuses on the use of chaotic oscillatory-based neural networks for the study of a mesoscale weather phenomenon, namely, wind shear, a challenging and complex meteorological problem that has a vital impact on aviation safety. Using lidar data collected at the Hong Kong International Airport via the Hong Kong Observatory, it is possible to forecast the Doppler velocities with satisfactory accuracy and validate the prediction model with the potential to generate the wind shear alert. Experimental results are found to be comparable to the actual measurement. Moreover, the selected testing cases and results show that the value of correlation coefficient between the predicted and lidar-measured wind velocities exceeds 0.9 with various window sizes ranging from 1 to 3 h. These provide areas for further research of the proposed model and lidar technology for turbulence and wind shear forecasts.

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Marco Y.-T. Leung, Wen Zhou, Dongxiao Wang, P. W. Chan, S. M. Lee, and H. W. Tong

Abstract

In this study, remote influence originating from the tropical western Indian Ocean on June precipitation in South China and the Indochina Peninsula is documented. Based on numerical simulation and statistical analysis, it is noted that the warm anomaly in the tropical western Indian Ocean can induce a weaker-than-normal Walker circulation across the tropical Indian Ocean and western Pacific Ocean. This further leads to a northeast–southwest-oriented western North Pacific subtropical high and a weaker-than-normal monsoon trough in the South China Sea. In addition, the weak monsoon trough is concurrent with an anomalous rising motion in South China and a sinking motion in the Indochina Peninsula. This enhances precipitation in South China and suppresses precipitation in the Indochina Peninsula on an interannual time scale. On the other hand, the warming trend in the tropical western Indian Ocean also supports the long-term trends of precipitation in the two regions.

Open access
P. Monti, H. J. S. Fernando, M. Princevac, W. C. Chan, T. A. Kowalewski, and E. R. Pardyjak

Abstract

Measurements were conducted on an eastern slope of the Salt Lake Basin (SLB) as a part of the Vertical Transport and Mixing Experiment (VTMX) conducted in October 2000. Of interest was the nocturnal boundary layer on a slope (in particular, katabatic flows) in the absence of significant synoptic influence. Extensive measurements of mean flow, turbulence, temperature, and solar radiation were made, from which circulation patterns on the slope and the nature of stratified turbulence in katabatic winds were inferred. The results show that near the surface (<25–50 m) the nocturnal flow is highly stratified and directed downslope, but at higher levels winds strongly vary in magnitude and direction with height and time, implying the domination of upper levels by air intrusions. These intrusions may peel off from different slopes surrounding the SLB, have different densities, and flow at their equilibrium density levels. The turbulence was generally weak and continuous, but sudden increases of turbulence levels were detected as the mean gradient Richardson number (Rig) dropped to about unity. With a short timescale Rig fluctuated on the order of a few tens of seconds while modulating with a longer (along-slope internal waves sloshing) timescale of about half an hour. The mixing efficiency (or the flux Richardson number) of the flow was found to be a strong function of Rig, similar to that found in laboratory experiments with inhomogeneous stratified shear flows. The eddy diffusivities of momentum and heat were evaluated, and they showed a systematic variation with Rig when scaled with the shear length scale and the rms vertical velocity of turbulence.

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Andreas Colliander, Thomas J. Jackson, Aaron Berg, D. D. Bosch, Todd Caldwell, Steven Chan, Michael H. Cosh, C. Holifield Collins, Jose Martínez-Fernández, Heather McNairn, J. H. Prueger, P. J. Starks, Jeffrey P. Walker, and Simon H. Yueh

Abstract

Soil moisture retrieval is particularly challenging during and immediately after precipitation events because of the transient movement of water in the shallow subsurface. Conventional L-band microwave radiometer–based soil moisture products use algorithms that assume a static state and a constant vertical soil moisture distribution. This study assessed the retrieval performance of a SMAP radiometer-based soil moisture product during and immediately after rain events. The removal of the rain event samples systematically improved the unbiased root-mean-square error (ubRMSE) from 0.037 (all measurements) to 0.028 m3 m−3 (transitory measurements screened out), while the magnitude of the bias became larger (from −0.005 to −0.014 m3 m−3); RMSE improved from 0.047 to 0.042 m3 m−3, and the Pearson correlation saw a minor positive change from 0.813 to 0.824. The results indicate that removing samples during the transitional period causes the comparison to improve, but also suggests that the true bias may be larger than the one estimated using all the samples. Furthermore, the results revealed that the effect was stronger for areas with high clay content. An assessment of the performance of the product during the rain events (overpass within 3 h from the start of the rain) showed that the ubRMSE degraded from the benchmarked 0.036 m3 m−3 (during no rain events at all) to 0.043 m3 m−3 (during rain). The results also showed that the bias became wetter, which is expected because SMAP sensed the water on the surface before propagating to the in situ sensors. SMAP maintains its soil moisture sensitivity even during rain events and screening of rain events may not be necessary to ensure sufficient soil moisture retrieval quality.

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Masashi Nagata, Lance Leslie, Yoshio Kurihara, Russell L. Elsberry, Masanori Yamasaki, Hirotaka Kamahori, Robert Abbey Jr., Kotaro Bessho, Javier Calvo, Johnny C. L. Chan, Peter Clark, Michel Desgagne, Song-You Hong, Detlev Majewski, Piero Malguzzi, John McGregor, Hiroshi Mino, Akihiko Murata, Jason Nachamkin, Michel Roch, and Clive Wilson

The Third Comparison of Mesoscale Prediction and Research Experiment (COMPARE) workshop was held in Tokyo, Japan, on 13–15 December 1999, cosponsored by the Japan Meteorological Agency (JMA), Japan Science and Technology Agency, and the World Meteorological Organization. The third case of COMPARE focuses on an event of explosive tropical cyclone [Typhoon Flo (9019)] development that occurred during the cooperative three field experiments, the Tropical Cyclone Motion experiment 1990, Special Experiment Concerning Recurvature and Unusual Motion, and TYPHOON-90, conducted in the western North Pacific in August and September 1990. Fourteen models from nine countries have participated in at least a part of a set of experiments using a combination of four initial conditions provided and three horizontal resolutions. The resultant forecasts were collected, processed, and verified with analyses and observational data at JMA. Archived datasets have been prepared to be distributed to participating members for use in further evaluation studies.

In the workshop, preliminary conclusions from the evaluation study were presented and discussed in the light of initiatives of the experiment and from the viewpoints of tropical cyclone experts. Initial conditions, depending on both large-scale analyses and vortex bogusing, have a large impact on tropical cyclone intensity predictions. Some models succeeded in predicting the explosive deepening of the target typhoon at least qualitatively in terms of the time evolution of central pressure. Horizontal grid spacing has a very large impact on tropical cyclone intensity prediction, while the impact of vertical resolution is less clear, with some models being very sensitive and others less so. The structure of and processes in the eyewall clouds with subsidence inside as well as boundary layer and moist physical processes are considered important in the explosive development of tropical cyclones. Follow-up research activities in this case were proposed to examine possible working hypotheses related to the explosive development.

New strategies for selection of future COMPARE cases were worked out, including seven suitability requirements to be met by candidate cases. The VORTEX95 case was withdrawn as a candidate, and two other possible cases were presented and discussed.

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Jhoon Kim, Ukkyo Jeong, Myoung-Hwan Ahn, Jae H. Kim, Rokjin J. Park, Hanlim Lee, Chul Han Song, Yong-Sang Choi, Kwon-Ho Lee, Jung-Moon Yoo, Myeong-Jae Jeong, Seon Ki Park, Kwang-Mog Lee, Chang-Keun Song, Sang-Woo Kim, Young Joon Kim, Si-Wan Kim, Mijin Kim, Sujung Go, Xiong Liu, Kelly Chance, Christopher Chan Miller, Jay Al-Saadi, Ben Veihelmann, Pawan K. Bhartia, Omar Torres, Gonzalo González Abad, David P. Haffner, Dai Ho Ko, Seung Hoon Lee, Jung-Hun Woo, Heesung Chong, Sang Seo Park, Dennis Nicks, Won Jun Choi, Kyung-Jung Moon, Ara Cho, Jongmin Yoon, Sang-kyun Kim, Hyunkee Hong, Kyunghwa Lee, Hana Lee, Seoyoung Lee, Myungje Choi, Pepijn Veefkind, Pieternel F. Levelt, David P. Edwards, Mina Kang, Mijin Eo, Juseon Bak, Kanghyun Baek, Hyeong-Ahn Kwon, Jiwon Yang, Junsung Park, Kyung Man Han, Bo-Ram Kim, Hee-Woo Shin, Haklim Choi, Ebony Lee, Jihyo Chong, Yesol Cha, Ja-Ho Koo, Hitoshi Irie, Sachiko Hayashida, Yasko Kasai, Yugo Kanaya, Cheng Liu, Jintai Lin, James H. Crawford, Gregory R. Carmichael, Michael J. Newchurch, Barry L. Lefer, Jay R. Herman, Robert J. Swap, Alexis K. H. Lau, Thomas P. Kurosu, Glen Jaross, Berit Ahlers, Marcel Dobber, C. Thomas McElroy, and Yunsoo Choi

Abstract

The Geostationary Environment Monitoring Spectrometer (GEMS) is scheduled for launch in February 2020 to monitor air quality (AQ) at an unprecedented spatial and temporal resolution from a geostationary Earth orbit (GEO) for the first time. With the development of UV–visible spectrometers at sub-nm spectral resolution and sophisticated retrieval algorithms, estimates of the column amounts of atmospheric pollutants (O3, NO2, SO2, HCHO, CHOCHO, and aerosols) can be obtained. To date, all the UV–visible satellite missions monitoring air quality have been in low Earth orbit (LEO), allowing one to two observations per day. With UV–visible instruments on GEO platforms, the diurnal variations of these pollutants can now be determined. Details of the GEMS mission are presented, including instrumentation, scientific algorithms, predicted performance, and applications for air quality forecasts through data assimilation. GEMS will be on board the Geostationary Korea Multi-Purpose Satellite 2 (GEO-KOMPSAT-2) satellite series, which also hosts the Advanced Meteorological Imager (AMI) and Geostationary Ocean Color Imager 2 (GOCI-2). These three instruments will provide synergistic science products to better understand air quality, meteorology, the long-range transport of air pollutants, emission source distributions, and chemical processes. Faster sampling rates at higher spatial resolution will increase the probability of finding cloud-free pixels, leading to more observations of aerosols and trace gases than is possible from LEO. GEMS will be joined by NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO) and ESA’s Sentinel-4 to form a GEO AQ satellite constellation in early 2020s, coordinated by the Committee on Earth Observation Satellites (CEOS).

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