Jau-Ming Chen

The 10–20-Day Mode of the 1979 Indian Monsoon: Its Relation with the Time Variation of Monsoon Rainfall

Abstract The synoptic structure of the 10–20-day monsoon mode and this intraseasonal monsoon modes relationship with the Indian monsoon rainfall are examined with the 1979 summer First GARP Global Experiment IIIb data of the European Centre for Medium-Range Weather Forecasts and the daily 1° × 1° rainfall estimates retrieved from the satellite data by the Goddard Laboratory for Atmospheres. The major findings of this study are as follows. 1) The 10–20-day monsoon mode exhibits a double-cell (either double-high or double-low) structure; one cell is centered at about 15°–20°N and the other at the equator. 2) Both cells of the 10–20-day monsoon mode propagate coherently westward along the Indian monsoon trough and along the equator, respectively. 3) Based upon the zonal wind and local Hadley circulation, the vertical structure of the 10–20-day monsoon mode does not exhibit a phase change. 4) A significant rainfall occurs around low centers of the 10–20-day monsoon mode through the modulation of this monsoon...

An Observational Study of the South China Sea Monsoon during the 1979 Summer: Onset and Life Cycle

Abstract The onset and life cycle of the 1979 South China Sea summer monsoon were examined in the context of the 30–60- and 12–24-day monsoon modes. The former intraseasonal mode formed the northward-migrating monsoon trough/ridge, while the latter intraseasonal mode propagated westward in the South China Sea. The monsoon in this region exhibited three cycles over the summer (May–August), with the onset taking place about one cycle ahead of the onset of the Indian and Japanese monsoons. Climatologically, a summer trough line radiated out from the Indian monsoon trough region, across Indochina, to the northern South China Sea. The monsoon onset occurred when the 30–60-day monsoon trough and the 12–24-day low center arrived simultaneously at the northern South China Sea, close to the climatological summer trough line, in the middle of May. The breaks occurred when the 30–60-day monsoon ridge lines and the 12–24-day high center met near 15°–20°N in the northern South China Sea. The South China Sea monsoon wa...

Interdecadal Variation in U.S. Pacific Coast Precipitation over the Past Four Decades

Abstract The interdecadal variation of precipitation along the U.S. Pacific coast over the past 45 winters (1950–94) was examined with station observations. An interdecadal decreasing (minor increasing) trend appeared north (south) of 36°N. This interdecadal trend in precipitation is related to the development of an anomalous dipole structure (a major anomalous high centered over the Pacific coast at British Columbia and a minor anomalous low centered over northern Baja California) throughout the past several decades. The correlation coefficient patterns between the Pacific coast precipitation and upper-air geopotential heights, and the difference charts of upper-air geopotential heights between the first (1950–59) and last (1985–94) 10 winters revealed that the interdecadal variation of the western U.S. circulation is a part of that in the Northern Hemisphere wintertime circulation. In turn, the interdecadal variation of the Pacific coast precipitation is a consequence of the interdecadal variation of th...

Low-Frequency Variations in the Atmospheric Branch of the Global Hydrological Cycle

Abstract According to the atmospheric water balance equation, the divergence of the water vapor flux is responsible for the exchange of water vapor between its source and sink regions. Because the water vapor flux divergence is primarily determined by the divergent circulation, time variations of the global hydrological cycle reflect the pronounced low-frequency modes of the global divergent circulation, that is, the annual and intraseasonal (30–60 day) modes. The annual variation of the hydrological cycle is illustrated in terms of hemispheric-mean hydrological variables for the Northern and Southern Hemispheres, while the intraseasonal variations of the global hydrological cycle are illustrated with mean values over two hemispheres that form an east-west partition of the globe. This partition is defined by the 60°E–120°W great circle and was chosen so that the mean precipitation difference and the divergent water vapor transport between the two hemispheres was maximized. Two years (1979–80) of daily pre...

On the Relationship between the Streamfunction and Velocity Potential of the Madden–Julian Oscillation

Abstract The streamfunction and velocity potential variances associated with the Madden–Julian oscillation are explained substantially by their zonal wavenumber-1 components, which are used to measure quantitatively the phase relationship between the two variables. There exists a quarter-cycle phase difference between them. A simplified streamfunction budget equation was used to demonstrate analytically that this spatial quadrature relationship is maintained through the counteraction between streamfunction tendencies induced by the horizontal advection of planetary vorticity and the divergence of planetary vorticity flux.

The 12-24-day mode of global precipitation

Abstract Global precipitation estimates derived from satellite data at the Goddard Laboratory for Atmospheres for 1979–80 were used to explore time variations in global precipitation. Time series of the area-averaged precipitation [P] over the Asian-Australian (AA) monsoon (60°E–12°W), and the extra-AA monsoon (120°W–60°E) hemispheres were used in describing the variations. A distinct seesawlike intraseasonal variation of precipitation between these two hemispheres emerges from the two time series. Two intraseasonal (30–60 and 12–24 day) modes stand our in die spectral analysis of the two [P] time series. The 30–60 day mode is well known, while the 12–24-day mode is identified here for the first time. Using data generated by the Global Data Assimilation System of the National Meteorological Center, an effort was made to investigate the characteristics of the 12–24-day mode of global precipitation via potential functions for the 200-mb wind, water vapour transport, and precipitation. It is found that the 1...

On the Maintenance of Stationary Eddies in Terms of the Streamfunction Budget Analysis

Abstract The upper-level seasonal-mean eddy streamfunction (ψE) is often used to portray atmospheric stationary eddies. A budget analysis of the 200 mb ψE field simulated by the NCAR Community Climate Model revealed that a quadrature relationship exists between ψE and the ψE tendency induced by either the vorticity advection associated with rotational flow (χψ) or by the vorticity source with divergent flow (χD); that χψ and χD counterbalance each other to a large extent; and that physical processes not included in χψ and χD are generally insignificant, as can be inferred from the residual between χψ and χD. Apparently, ψE is maintained primarily by the counterbalance between χψ and χD. To better illuminate the aforementioned quadrature relationship it must be explained that the ψE budget was formulated explicitly with zonal and eddy flow components. The detailed ψE budget analysis indicated that the distinct low- and high-latitude ψE regimes am maintained in different manners.

Variability of the global precipitable water with a timescale of 90–150 days

A 90-150 day signal is identified in the global precipitable water field generated by the Global Data Assimilation Systems (GDAS) of the Goddard Laboratory for Atmospheres (GLA), National Meteorological Center, and European Centre for Medium-Range Weather Forecasts. The finding of this intraseasonal signal in global precipitable water is significant for two reasons : (1) it suggests that there is 90-150 day intraseasonal variability in the atmospheric branch of the global hydrological cycle and (2) it provides a useful parameter to test the sensitivity of the GDAS-generated hydrological data. This newly identified intraseasonal signal in the global precipitable water was verified with Special Sensor Microwave/Imager precipitable water data over oceans and station-mixing ratio data over the continental United States. Based upon some simple statistical analyses and global and regional composite charts, it was found that the 90-150 day low-frequency oscillations contained in different GDAS data sets are more coherent with each other in regions with good data coverage but are poorly correlated over the data-sparse areas. Furthermore, the GLA GDAS provides the most realistic representation of this intraseasonal global precipitable water signal.

The Effect of Horizontal Resolution on Systematic Errors of the GLA Forecast Model

Abstract Systematic prediction errors of the Goddard Laboratory for Atmospheres (GLA) forecast system are reduced when the higher-resolution (2° × 2.5°) model version is used. Based on a budget analysis of the 200-mb eddy streamfunction, the improvement of stationary eddy forecasting is seen to be caused by the following mechanism: By increasing the horizontal spatial resolution of the forecast model, atmospheric diabatic heating over the three tropical continents is changed in a way that intensifies the planetary-scale divergent circulations associated with the three pairs of divergent-convergent centers over these continents. The intensified divergent circulation results in an enhancement of vorticity sources in the Northern Hemisphere. The additional vorticity is advected eastward by a stationary wave train along 30°N, thereby reducing systematic errors in the lower-resolution (4° × 5°) GLA forecast model.

The semiannual variation in the atmospheric hydrologic processes

Semiannual variations of several atmospheric hydrological processes are identified from two sets of data: (1) the monthly mean precipitation data generated by the Global Precipitation Climate Project (GPCP) and (2) the monthly mean upper air wind and moisture data generated by the Climate Data Assimilation System of National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project for the period 1986–1993. Three major findings of this study follow. First is that the existence of semiannual variations in precipitation and water vapor flux are revealed from (1) time series of area-mean precipitation and divergence of water vapor flux over the Asian-Australian (AA) monsoon (60°E-120°W) and extra-AA monsoon (120°W-60°E) hemispheres and (2) the power spectra of that time series. Second is that the semiannual precipitation variation exhibits an east-west seesaw oscillation. In January and July, positive precipitation anomalies concentrate in the AA monsoon hemisphere, while major negative precipitation anomalies exist in the extra-AA monsoon hemisphere. Opposite spatial patterns occur in April and October. Third is that to maintain the semiannual variation of precipitation, water vapor diverges out of the extra-AA (AA) monsoon hemisphere and converges toward the AA monsoon (extra-AA monsoon) hemisphere in January and July (April and October).