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Abraham Zangvil and Michio Yanai

Abstract

The relationship between the large-scale transient wave disturbances at the 200 mb level and the associated cloud field is studied in the latitude belt 20°S to 40°N based on wind and satellite-measured brightness data for the period June–August 1967. Space-time cross-spectral analysis is used to examine the degree of association of the wind and cloud fields in the wavenumber-frequency domain. The total time variance of the brightness field has a well-defined maximum near 10°N corresponding to a belt of sea surface temperature maximum. The divergence and relative vorticity as well as the brightness have a pronounced spatial scale of zonal wavenumber s ≈ 10 at this latitude. The major component contributing to this scale is westward moving disturbances with a period ∼ 5 days. There is high coherence between the brightness and divergence fields at these scales. In addition, high coherence is found for s = 3–6 and a period ∼ 5 days of westward moving disturbances corresponding to the wavenumber and period of the mixed Rossby-gravity waves. Some evidence suggesting the connection of long-period eastward moving waves with the large-scale cloud field is presented also.

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Huibang Luo and Michio Yanai

Abstract

The large-scale heat and moisture budgets over the Tibetan Plateau and surrounding area during a 40-day period from late May to early July 1979 are studied using the FGGE Level II-b data. During this period the general circulation over East Asia underwent a distinct seasonal change characterizing the onset of the summer monsoon circulation.

The analyses of the horizontal distributions of the vertically integrated heat source and moisture sink reveal the major heat source regions and their different degrees of association with precipitation. The 40-day mean distributions show intense heat sources of 150–300 W m−2 with moisture sinks of nearly equal magnitude over the Assam–Bengal region and in a broad belt extending over the China Plain along the Mei-yü front. The heat source of ∼100–150 W m−21 over the eastern Tibetan Plateau is accompanied by a moisture sink with a magnitude about half as large. The heat sources over the western Plateau and the Takla Makan Desert are not accompanied by appreciable moisture sinks. The heat sources over the Plateau are pronounced in the upper troposphere. The mean heating rate of ∼3 K day−1m in the 200–500 mb layer above the eastern Plateau is as intense as that over the Assam–Bengal region.

Examination of the daily vertical profiles of the areal mean heat source and moisture sink shows distinct differences in the heating process and its time change over the western Plateau and adjacent areas (Region I), the eastern Plateau (Region II), the Yangtze River (Region III) and the Assam–Bengal region (Region IV). In Regions I and II heating in the troposphere is seen prior to the onset of the summer rains. The heating in Region II intensifies with the progress of the rain season. The heating in Region III is primarily due to frontal rains and that in Region IV is due to highly convective monsoonal rains. In Region I the mean sensible heat flux at the surface during the pre-onset period is ∼170 W m−2, while in Region II the mean sensible heat flux for the total period is ∼105 W m−2. The corresponding values of the condensation heating are ∼10 and ∼70 W m−2, respectively. The estimated mean evaporation rates on the Plateau for the total period are ∼1 mm day−1. These estimates compare well with the June mean values obtained by Yeh and Gao from surface observations.

There is a pronounced diurnal change of the surface air temperature on the Plateau, and a deep mixed layer with very high potential temperature is observed in the evening (1200 GMT). It is suggested that dry thermal convection originating near the heated surface in the afternoon hours is responsible for the deep tropospheric heating over the Plateau during the pre-onset phase. Above the eastern Plateau this heating mechanism is replaced by the condensation heating associated with cumulus convection after the onset of the summer rains.

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Michio Yanai and Tomohiko Tomita

Abstract

Using the National Centers for Environmental Predictions (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis, distributions of the heat source Q 1 and moisture sink Q 2 between 50°N and 50°S are determined for a 15-yr period from 1980 to 1994. Heating mechanisms operating in various parts of the world are examined by comparing the horizontal distributions of the vertically integrated heat source 〈Q 1〉 with those of the vertically integrated moisture sink 〈Q 2〉 and outgoing longwave radiation (OLR) flux and by comparing the vertical distributions of Q 1 with those of Q 2.

In northern winter, the major heat sources are located (i) in a broad zone connecting the tropical Indian Ocean, Indonesia, and the South Pacific convergence zone (SPCZ); (ii) over the Congo and Amazon Basins; and (iii) off the east coasts of Asia and North America. In northern summer, the major heat sources are over (i) the Bay of Bengal coast, (ii) the western tropical Pacific, and (iii) Central America. Throughout the year, the South Indian Ocean, eastern parts of the North and South Pacific Oceans, and eastern parts of the North and South Atlantic Oceans remain to be heat sinks. The desert regions such as the Sahara are characterized by large sensible heating near the surface and intense radiative cooling aloft. Over the tropical oceans, heat released by condensation with deep cumulus convection provides the major heat source. The radiative cooling and moistening due to evaporation are dominant features over the subtropical oceans where subsidence prevails. Over the Tibetan Plateau, the profiles of Q 1 and Q 2 show the importance of sensible heating in spring and contributions from the release of latent heat of condensation in summer. Off the east coast of Japan, intense sensible and latent heat fluxes heat and moisten the lower troposphere during winter.

Heat sources in various regions exhibit strong interannual variability. A long (4–5 yr) periodicity corresponding to the variations in OLR and sea surface temperature (SST) is dominant in the equatorial eastern and central Pacific Ocean, while a shorter-period oscillation is superimposed upon the long-period variation over the equatorial Indian Ocean. The interannual variations of 〈Q 1〉, OLR, and SST are strongly coupled in the eastern and central equatorial Pacific. However, the coupling between the interannual variations of 〈Q 1〉 and OLR with those of SST is weak in the equatorial western Pacific and Indian Ocean, suggesting that factors other than the local SST are also at work in controlling the variations of atmospheric convection in these regions.

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Michio Yanai and Chengfeng Li

Abstract

The structure of the boundary layer and the mechanism of heating over the Tibetan Plateau are examined using the data obtained from the First GARP (Global Atmospheric Research Program) Global Experiment and the Chinese Qinghai–Xizang (Tibet) Plateau Meteorological Experiment from May to August 1979. The meteorological elements near the plateau surface exhibit pronounced diurnal variations. There is a large ground–air temperature difference during the daytime generating a thin layer of superadiabatic lapse rates near the surface. The horizontal wind speed attains the minimum in the morning and the maximum in the evening. A deep and well-mixed layer of potential temperature is observed over the western and central plateau in the evening (1200 UTC). However, moisture is not well mixed vertically and water vapor mixing ratio is larger in the morning (0000 UTC) than in the evening. The mixed layer becomes shallower from the western to the central plateau and it disappears over the eastern plateau. There is a deep layer of large-scale ascent over the plateau. The upward motion and convective activity over the plateau are more intense in the evening than in the morning.

Before the onset of summer rains, sensible heat flux from the surface is the major source of heating on the plateau. The analysis of the ground–air temperature difference supports the hypothesis that dry convective elements rising from the heated surface provide the mechanism for tropospheric heating during this period. After the onset of the summer rains, however, the heat released by condensation is the primary source of heating over the eastern plateau. During the dry preonset period, the diabatic heating in the boundary layer is nearly balanced with the cold horizontal advection due to the prevailing westerly wind above the plateau. After the onset, the adiabatic cooling due to strong upward motions nearly balances with the heat released by condensation. The dry horizontal advection plays an important role in the moisture balance of the boundary layer.

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Xiaoqing Wu and Michio Yanai

Abstract

Dynamical effects of organized cumulus convection on its environment with vertical wind shear are studied. Analyses of the wind field and momentum budget residual for mesoscale convective systems observed during SESAME and PRE-STORM reveal systematic differences in the vertical transport of horizontal momentum between mesoscale convective complex (MCC) and squall line cases. In the MCC, a distinct minimum in wind speed appears over the area of intense convection and the momentum budget residual acts to decelerate the environmental wind and to reduce the upper-level vertical shear. Therefore, the inferred vertical transport of momentum in the MCC is downgradient in the upper layer. On the other hand, in the squall line, there is no wind speed minimum and the upper-level vertical shear of the line-normall component of the environmental wind increases as convection develops. Thus, the vertical transport of momentum normal to the squall line is upgradient in the upper layer, although the transport of momentum parallel to the line is downgradient.

The effects of cumulus convection on the environmental flow are through the subsidence of environmental air that compensates the cloud mass flux, the detrainment of momentum from clouds, and the convective-scale horizontal pressure gradient force acting on the environment. A cumulus momentum parameterization including the convective-scale pressure gradient force is formulated. The pressure gradient force is related to the vertical wind shear, cloud mass flux, and orientation of organized convection. The parameterization is capable of reproducing both the upgradient and downgradient transports of horizontal momentum, provided the mode of organization of cumulus convection is known.

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Chengfeng Li and Michio Yanai

Abstract

The onset and interannual variability of the Asian summer monsoon in relation to land–sea thermal contrast and its contributing factors are studied using a 14-yr (1979–1992) dataset. The onset of the Asian summer monsoon is concurrent with the reversal of meridional temperature gradient in the upper troposphere south of the Tibetan Plateau. The reversal is the result of large temperature increases in May to June over Eurasia centered on the Plateau with no appreciable temperature change over the Indian Ocean. In spring the Tibetan Plateau is a heat source that is distinctly separate from the heat source associated with the rain belt in the equatorial Indian ocean. The Tibetan heat source is mainly contributed by sensible heat flux from the ground surface, while the oceanic heat source is due to the release of latent heat of condensation. It is the sensible heating over the Plateau region in spring that leads to the reversal of meridional temperature gradient. Despite its intensity the condensational heating over the Indian Ocean does not result in tropospheric warming because it is offset by the adiabatic cooling of ascending air.

A monsoon intensity index, based on the magnitude of the summer mean vertical shear of zonal wind over the North Indian Ocean, is used to compare the years of strong and weak Asian summer monsoon circulation. The strong (weak) Asian summer monsoon years are associated with (a) positive (negative) tropospheric temperature anomalies over Eurasia, but negative (positive) temperature anomalies over the Indian Ocean and the eastern Pacific; (b) negative (positive) SST anomalies in the equatorial eastern Pacific, Arabian Sea, Bay of Bengal. and South China Sea, but positive (negative) SST anomalies in the equatorial western Pacific; and (c) strong (weak) heating and cumulus convection over the Asian monsoon region and the western Pacific, but weaker (stronger) heating and convection in the equatorial Pacific.

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Víctor Magaña and Michio Yanai

Abstract

The relationship between the low-frequency oscillation of convective activity in the tropics and the intensity of planetary-scale upper-tropospheric (200 mb) systems in the subtropics during the Northern Hemisphere (NH) summer is studied using data obtained in the First GARP Global Experiment (FGGE). The mid-Pacific trough and the South Asian (Tibetan) and Mexican anticyclones undergo cycles of amplification and decay with the 30–60 day fluctuation of convective activities in the Indonesia-western Pacific (IWP) region and in the intertropical covergence zone (ITCZ) over Central America. The thermal contrast between the Asian continent and the North Pacific and the resulting east-west circulation show similar time variations. This circulation regulates the intensity of the South Asian anticyclone and the mid-Pacific trough. Divergent circulation associated with convection over Central America maintains the Mexican anticyclone.

The low-frequency transients in the upper troposphere around the mid-Pacific trough play a dominant role in the northward transport of westerly momentum. Their effect is recognizable in the variation of zonally averaged relative angular momentum, which originates near 10°N and then propagates northward. At the same time, the westerlies that develop southeast of the trough form an equatorial westerly “duct,” through which wave energy can propagate into the tropics from midlatitudes. Thus, the mid-Pacific trough acts as a two-way link between the tropics and midlatitudes.

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Huibang Luo and Michio Yanai

Abstract

The time evolution of the large-scale precipitation, low-level (850 mb) wind, moisture and vertical motion fields over the Tibetan Plateau and surrounding areas during a 40-day period from late May to early July 1979 is studied based on the objectively analyzed FGGE Level II-b data set. During this period the general circulation over East Asia undergoes a distinct change characterizing the onset of the summer monsoon circulation.

The Tibetan Plateau exerts profound orographic and thermal influences upon the low-level wind field. The inflow towards the eastern part of the Plateau with a marked diurnal change in its intensity is the most prominent feature of the low-level wind field. The areas of organized precipitation are well related to synoptic systems seen in the 850 mb flow: the quasi-stationary Burma-India trough, the disturbances forming on the trough, the “Mei-yü (Baiu)” front, and the “transverse trough” extending eastward from the Plateau. There is a sharp contrast between the western and eastern Plateau in terms of precipitation and moisture distributions. The eastern Plateau acts as a huge chimney funneling water vapor from the lower to the upper troposphere. Maxima of 40-day mean upward velocities are located above the eastern Plateau, above the Assam-Bengal region and along the Mei-yü frontal zone. The vertical motions above the Plateau are more upward in the evening than in the morning. The reverse is true in the surrounding areas. Detailed examinations of daily values of the areal mean vertical p-velocity, mixing ratio and rainfall are made for four heat source regions (the western Plateau and adjacent areas, the eastern Plateau, the Yangzi River, the Assam-Bengal region) as a preliminary to the discussion of heating mechanisms operating in these regions.

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Abraham Zangvil and Michio Yanai

Abstract

Space and time spectra of large-scale wave disturbances at the 200 mb level in the latitude belt 20°S to 40°N are studied based on wind data for the period June–August 1967. The kinetic energy of the transient waves shows a minimum near 10°N where convective activity attains a maximum. The transient eddies in the zonal wind component have dominant scales of zonal wavenumbers s = 3–6 in most latitudes, while those in the meridional wind component have dominant scales of s = 6–8.

By decomposing the wind data into symmetric and antisymmetric components with respect to the equator, three prototype equatorial wave modes are detected: 1) Kelvin waves of zonal wavenumber s = 1 and 2 and a period of 7 days, and s = 1 and periods longer than 20 days; 2) mixed Rossby-gravity waves of s = 4 and a period of 5 days; and 3) Rossby waves (of the lowest meridional nodal number) of s = 2 and a period near 12 days. Westward moving short waves (s = 7–15) gain kinetic energy from the mean easterly flow. Eastward moving waves in the middle latitudes do not propagate into the tropics because of the absorption at critical latitudes. Westward moving long waves of s ≈ 4 and periods near 5 days accompany a distinct peak in the equatorward wave energy flux, suggesting the origin of the observed mixed Rossby-gravity waves.

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Victor Magaña and Michio Yanai

Abstract

The mechanisms associated with the excitation of mixed Rossby-gravity waves (MRGWs) in the upper troposphere are studied using wind and outgoing longwave radiation (OLR) data from 1979 to 1991. The largest anomalies in meridional wind associated with MRGWs at 200 mb generally appear in the Northern Hemisphere summer–fall periods and they are pronounced in the central/eastern Pacific where equatorial westerlies form. The OLR field in the intertropical convergence zone shows a spectral peak with time and space scales similar to those of MRGWs at 200 mb. However, tropical convective activity does not show a clear contrast between years of strong and weak 200-mb MRGW activity.

During the Northern Hemisphere summer, weak easterlies or westerlies often form over the equatorial central/eastern Pacific allowing disturbances to propagate from the Southern Hemisphere midlatitudes into the deep Tropics. Some of these disturbances that possess spatial and temporal scales similar to those of the observed MRGWs (zonal wavenumber 4–6 and period 5–7 days) appear to project onto MRGWs. MRGWs are then intensified when the flow associated with them, and that with extratropical disturbances, are favorably superposed. The extratropical disturbances propagating into the Tropics possess a baroclinic vertical structure in the midlatitude troposphere. As they approach the Tropics, the disturbances appear to confine themselves to the upper troposphere under the effect of the “easterly dome.”

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