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Alfred T. C. Chang and Long S. Chiu

Abstract

About 10 yr (July 1987–December 1997 with December 1987 missing) of oceanic monthly rainfall based on data taken by the Special Sensor Microwave/Imager (SSM/I) on board the Defense Meteorological Satellite Program satellites have been computed. The technique, based on the work of Wilheit et al., includes improved parameterization of the beam-filling correction, a refined land mask and sea ice filter. Monthly means are calculated for both 5° and 2.5° latitude–longitude boxes.

Monthly means over the latitude band of 50°N–50°S and error statistics are presented. The time-averaged rain rate is 3.09 mm day−1 (std dev of 0.15 mm day−1) with an error of 38.0% (std dev of 3.0%) for the 5° monthly means over the 10-yr period. These statistics compare favorably with 3.00 mm day−1 (std dev of 0.19 mm day−1) and 46.7% (std dev of 3.4%) computed from the 2.5° monthly means for the period January 1992–December 1994. Examination of the different rain rate categories shows no distinct discontinuity, except for months with a large number of missing SSM/I data.

An independent estimate of the error using observations from two satellites shows an error of 31% (std dev of 2.7%), consistent with the 38% estimated using (a.m. and p.m.) data from one satellite alone. Error estimates (31%) based on the 5° means by averaging four neighboring 2.5° boxes are larger than those (23%) estimated by assuming the means for these neighboring boxes are independent, thus suggesting spatial dependence of the 2.5° means.

Multiple regression analyses show that the error varies inversely as the square root of the number of samples but exhibits a somewhat weaker dependence on the mean rain rate. Regression analyses show a power law dependence of −0.255 to −0.265 on the rain rate for the 5° monthly means using data from a single satellite and a dependence of −0.366 for the 5° monthly means and −0.337 for the 2.5° monthly means based on two satellite measurements. The latter estimate is consistent with that obtained by Bell et al. using a different rainfall retrieval technique.

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C-P. Chang and George T-J. Chen

Abstract

The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea. This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF data for 1981–86 are used to study the tropical circulations associated with the development of low-level westerlies during both events.

Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and nine June surges, respectively, indicate that both events occur with the approach of a midlatitude trough–front system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring, this is the only Asian monscon region where midlatitude fronts can move into the Tropics without having to overcome significant terrain barriers.

Following the two events, opposite teleconnection-like patterns develop in the Tropics in both hemispheres. During the May onsets, the arrival of the midlatitude trough/front appears to lead to a southwestward extension of a cyclogenesis zone into the equatorial Indian Ocean. Along this zone, cyclonic vortices develop over.the Andaman Sea, the Bay of Bengal, and perhaps the southern equatorial Indian Ocean, and increased deep convection is indicated by the OLR composites. During the June surges, a pair of anticyclones develop straddling the equator at the longitudes of Indochina. This anticyclonic couplet is associated with decreased deep convection and propagates westward to dominate the flow changes over.the Bay of Bengal and the southern Indian Ocean. The steady 4–5 m s−1 westward speed and near-perfect symmetry with respect to the equator indicate the possibility of an equatorial Rossby wave generation in conjunction with the June westerly surges in the northern South China Sea.

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Alfred T. C. Chang, Long S. Chiu, and Thomas T. Wilheit

Abstract

Global averages and random errors associated with the monthly oceanic rain rates derived from the Special Sensor Microwave/Imager (SSM/I) data using the technique developed by Wilheit et al. are computed. Accounting for the beam-filling bias, a global annual average rain rate of 1.26 m is computed. The error estimation scheme is based on the existence of independent (morning and afternoon) estimates of the monthly mean. Calculations show overall random errors of about 50%–60% for each 5° × 5° box. The results are insensitive to different sampling strategy (odd and even days of the month). Comparison of the SSM/I estimates with raingage data collected at the Pacific atoll stations showed a low bias of about 8%, a correlation of 0.7, and an rms difference of 55%.

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Awdhesh K. Sharma, Alfred T. C. Chang, and Thomas T. Wilheit

Abstract

A study of differences between the morning and evening monthly rainfall for 5° × 5° cells over the oceans from the SSM/I data has been conducted. The monthly rainfalls are estimated from the technique given by Wilheit et al. The difference between the morning and evening monthly rainfall arises due to the various random errors involved in the retrieval process, the sampling error in the observations, and the diurnal component of oceanic rainfall. The diurnal component is weak but clearly visible when averaged over large areas and for long time periods. The analysis shows that morning rainfall is consistently greater than evening rainfall. The Northern Hemisphere seems to have a larger diurnal variation than does the Southern Hemisphere. The maximum ratio between the morning and evening monthly rainfall is 1.7 while 1.2 is the more typical value.

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L. C. Chou, C-P. Chang, and R. T. Williams

Abstract

A two-dimensional frontal model was used to study the structure and behavior of the Mei-Yu front over East Asia. The Mei-Yu front is characterized by mixed midlatitude-baroclinic and tropical-convective properties, with frequent occurrence of a low-level jet (LLJ) that is highly correlated with heavy convective rainfall.

The quasi-steady state responses to a large-scale stretching deformation forcing were obtained by integrating the perturbation equations from an initial state of seasonal-mean zonal flow. Two major sets of experiments were conducted to simulate different midlatitude and subtropical conditions. The midlatitude front extends deeply into the upper troposphere with a strong poleward tilt, whereas the subtropical front is confined to the lower troposphere with less tilt, in good agreement with observations. Along the sloping front, slantwise updrafts develop with a multiband structure. This updraft is more evident in the subtropical cases and in the more moist midlatitude cases.

For the subtropical cases, concurrent development of upper-level easterlies and low-level westerlies equatorward of the front is observed. The low-level westerly maximum at z=3–4 km resembles a LLJ, whose intensity increases when more moisture is included. The concurrent development suggests that the LIJ may be the result of a thermally direct secondary circulation that resembles a “reversed Hadley” cell. This circulation is revealed by a meridional–vertical streamfunction, with a strong lower branch return flow coinciding with the development of a LLJ in the more moist, subtropical cases. The Coriolis torque of the meridional circulation can develop and maintain the upper easterlies and the LLJ. Importance of cumulus convection and especially a slant-wise structure in developing the reversed Hadley cell and the LLJ is suggested.

These conclusions are consistent with the observed intense convection and heavy rainfall in the Mei-Yu front, and a sinking region south of the Baiu front as revealed by Matsumoto's moisture analysis.

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C-P. Chang, T-C. Yeh, and J. M. Chen

Abstract

The island of Taiwan is situated in the main path of western North Pacific typhoons. Its dominant central mountain range (CMR), with a hoizontal scale comparable to the radius of a typhoon, often produces significant distortions in the typhoon circulation. A 20-year dataset from 22 surface stations is used to describe the effects of the Taiwan terrain on the surface structure of typhoons.

Empirical orthogonal function analysis on the pressure field is used to identify the primary structure modes. The first mode is a uniform-sign anomaly pattern portraying the decrease in pressure as a typhoon is approaching. The second mode represents the strong terrain-induced west-east pressure gradient that is normal to the main axis of the CMR. The third mode results mainly from the west-cast pressure gradient arising from the relative location of the typhoon center to the east or west of Taiwan, but it also contains a weak south-north pressure gradient that can he attributed to the terrain. A regression technique is then used to determine the surface wind, temperature, relative humidity, and hourly rainfall associated with each pressure mode. In all cases, them fields are consistent, showing the effects of the terrain blocking or deflection and their consequent ascending and descending motions.

The relative importance of each mode depends strongly on the location of the typhoon center. No dependence on the direction or speed of motion is discernible when all cases are considered. When different, persistently smooth tracks are identified, the variations due to motion direction can be recognized because the terrain effect is affected by the mean steering flow. Only two types of smooth tracks that represent clearly different steering flows intersect in an area. At the intersection, a subsequent difference in storm structure over Taiwan exists that can be explained by the difference in the steering flows associated with the two track types.

The leeside secondary low that was often observed on the west coast of Taiwan is found to consist of at least two basic modes. It develops only when the typhoon center is in southeastern Taiwan or an ocean area to the east-southeast. The observed scale of this low is significantly smaller than that which can be produced by an interaction of the mean steering flow and the CMR. This smaller scale is due to a local buildup of the surface pressure south of the lee vortex, which results from the against-mountain return flow of the cyclonic circulation.

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C. P. Chang, F. T. Jacobs, and B. B. Edwards

Abstract

A diagnostic model is proposed to use digitized satellite cloud brightness data to estimate objectively the large-scale flow patterns over data-void tropical regions. The model utilizes a linear barotropic vorticity equation with two primary assumptions: 1) that the area-averaged cloud brightness is positively correlated with large-scale divergence in the tropical upper troposphere; and 2) that the large-scale tropical flow is quasi-barotropic and quasi-non-divergent. It is designed to be used at any upper tropospheric level where divergence is important in determining the vorticity field. Three types of information are required: 1) boundary conditions determined from surrounding wind reports, 2) a mean zonal flow determined from climatology, and 3) an equivalent divergence forcing function constructed empirically from the brightness data.

The model is tested daily over a western North Pacific region for July-August 1971. Results for an 8-day representative period are presented and discussed. In general for 25% of the days tested, the model produces a flow field which accurately resembles the major features of the streamfunction field analyzed by the National Meteorological Center. In another 30% of the days it provides some valuable information about the flow patterns which would be difficult to obtain from boundary information alone. Experiments are also performed for two days in which the brightness data are enhanced by time-interpolated satellite infrared data. The resultant flow fields bear better resemblance to the NMC analysis. It is thus suggested that improved results may be expected when infrared and other types of advanced satellite data are available.

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C-P. Chang, J. E. Millard, and G. T. J. Chen

Abstract

The surface pressure, temperature, dew point and wind data over the South China Sea and vicinity during Winter MONEX are examined to determine the timing of the passage of cold surges at various reporting stations. It is found that for more than half of the surge cases during Winter MONEX, a surge occurs in two stages separated by a time interval of several hours to approximately one day. The first stage is often characterized by a significant rise in pressure, while the second stage by a sharp decrease in dew point temperature. Freshening of surface winds may accompany either or both stages. The time separation between the two stages is relatively short at the upstream stations but generally long at the downstream stations. The second stage is associated with a frontal passage. On the other hand, the first stage is not clearly associated with any significant synoptic events. From its very fast propagation speed which increases with an inferred depth scale, and an increase in the local cross-isobar angle of the surface wind during passage, the first stage is identified with a synoptic-scale gravity wave type motion.

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G. T. J. Chen, Y. J. Wang, and C-P. Chang

Abstract

This study compares the systematic errors of 36-h surface cyclone and anticyclone forecasts for two operational numerical weather prediction models over East Asia and the western North Pacific Ocean: the U.S. Navy's Operational Global Atmospheric Prediction System (NOGAPS), and Japan Meteorological Agency's Fine-mesh Limited Area Model (JFLM). The study is carried out for the 1983 Mei-Yu season (May–July), which is the wettest season over East Asia based on nontyphoon-produced rainfall. All available 0000 and 1200 GMT forecast runs are evaluated against an independent dataset of subjective analysis produced operationally by the Central Weather Bureau, Taipei. The mean position errors, mean central pressure errors and forecast skill indices for both cyclones and anticyclones in the NOGAPS and JFLM models are examined.

Both NOGAPS and JFLM models are more likely to underforecast than to overforecast the existence and/or genesis of both cyclones and anticyclones. However, over the Tibetan Plateau and its vicinity, both models tend to overforecast the existence and/or genesis of cyclones. They also forecast both cyclones and anticyclones too slow and too far to the north.

Diurnal variations in central pressure errors suggest that the error source is the lack of radiation processes in the JFLM and too strong a diurnal cycle of radiation processes in NOGAPS. Also, the failure to treat adequately the bulk effects of cumulus convection seems to be primarily responsible for the poor forecasts of oceanic cyclone development.

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A. T. C. Chang, L. S. Chiu, and G. Yang

Abstract

Four and a half years of the global monthly oceanic rain rates derived from the DMSP (Defense Meteorological Satellite Program) F-8 SSM/I (Special Sensor Microwave/Imager) data are used to study the diurnal cycles. Annual mean rainfall maps based on the SSM/I morning and evening observations are presented, and their differences are examined using a paired t test. The morning estimates are larger than the afternoon estimates by about 20% over the oceanic region between 50°S and 50°N, with significant differences located mainly along the intertropical convergence zone region. Using the measurements from two satellites, either DMSP F-8 and F-10 or DMSP F-10 and F-11, amplitudes and phases of the 24-h harmonic are estimated. The diurnal cycle shows a nocturnal or early morning maximum in 35%–40% of the oceanic regions. Monte Carlo simulations show that the rms errors associated with the estimated amplitude and phase are about 100% and 2 h, respectively, mainly due to the large random errors (50%) associated with the present rainfall estimates and the nonoptimal separation times of the DMSP satellite sampling.

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