This study examines the dynamic and thermodynamic characteristics associated with the evolution of the SCSSM of 1998 using the data from the South China Sea Monsoon Experiment (SCSMEX). The objective is to identify the physical processes that determined the active and inactive periods of the 1998 SCSSM. The intraseasonal variations in the activity of the 1998 SCSSM are found to be mainly controlled by the 30-60-day oscillation, but modified by the 10-20-day mode. Definition of the active and inactive period is thus based on the OLR anomaly and its 30-60-day filtered component, giving two active (25-30 May, 23 June-5 July) and two inactive periods (6-18 June, 13-26 July). An examination of the circulation features suggests that the two modes operated very differently. After the onset of the SCSSM, a northward propagation of the 30-60-day mode into the SCS dictates the active and inactive phases of the SCSSM. During an active (inactive) phase, positive (negative) relative vorticity and convergence (divergence) associated with a low-level trough (ridge) propagated into the SCS, initiating (suppressing) the convection. The divergence field at the upper level was in phase with the low-level relative vorticity. However, the direction of propagation associated with the 10-20-day mode was different. A westward propagation of this mode into the SCS caused changes in the convection over the region during the first half of the 30-60-day mode. A second oscillation apparently propagated from the midlatitudes into the SCS to initiate or suppress convection. The dynamic conditions in terms of the vorticity and divergence are similar to those for the 30-60-day mode. The atmosphere over the SCS was also found to be generally favourable for the development of deep convection after the onset of the SCSSM.
A short-wave train emanating from the Southeast Asian tropics which links interannual variations of the winter climate systems in both East Asia and North America was identified recently. This shortwave train may affect the activity of cold-surge disturbances over the eastern seaboard of East Asia, and the northwestern Pacific. The interaction between cold surges and the planetary-scale winter monsoon circulation in East Asia, was extensively explored after the Winter Monsoon Experiment (WMONEX). However, the finding of the North-Pacific short-wave train motivates us to revisit three aspects of the East-Asian circulation related to cold surges, with an emphasis on the effect of the wave activity in East Asia. First, the well-developed local Hadley circulation coupled with the East-Asian stationary waves and tropical troughs facilitates the interaction between cold surges, and planetary-scale circulation in East Asia. Second, the intensification of the East-Asian jet following cold surges is attributed to the ridge amplification/trough deepening of the East-Asian stationary waves by the cold-surge disturbances. Finally, the downward branch of the local Hadley circulation in East Asia provides the large-scale vortex compression, which enhances the development of cold-surge disturbances along the east coast of Northeast Asia.
This paper studies the evolution of the South China Sea (SCS) monsoon during May-June 1998, toelucidate relationships among the large scale circulation, organization of convection, cloud structures,and fluctuations of the regional water cycle of the SCS. Primary data used include field observationsfrom the South China Sea Monsoon Experiment (SCSMEX), and the satellite rain products from theTropical Rainfall Measuring Mission (TRMM). Prior to the onset of the SCS monsoon, enhanced convectiveactivities associated with the Madden and Julian Oscillation were detected over the equatorialIndian Ocean in early May while the SCS was under the influence of the West Pacific Anticyclone withprevailing low level easterlies and suppressed convection. Subsquently, a bifurcation of the MJO convectionnear 90°E led to the development of strong convection over the Bay of Bengal, which spawnedlow-level westerlies across Indo-China and contributed to the initial build-up of moisture and convectiveavailable potential energy over the northern SCS. The onset of the SCS monsoon occurred around May18-20, and appeared to be triggered by the equatorward penetration of extratropical frontal disturbances,originating from the continental regions of East Asia. Analysis of TRMM microwave and precipitation radar data revealed that during the onset phase,convection over the northern SCS consisted of squall-type rain cells embedded in meso-scale complexessimilar to extratropical systems. The radar Z-factor intensity indicated that SCS clouds possessed abimodal distribution, with a pronounced signal (>30 dBz) at a height of 2-3 km, and another one(>25 dBz) at the 8-10 km level, separated by a well-defined melting level signaled by a bright band ataround 5-km level. The most convectively active phase of the SCS monsoon, as measured by the abundanceof convective and stratiform hydrometeor types, inferred from the radar vertical profile, was foundto occur when the large scale vertical wind shear was weakest. The fluctuation of the water cycle over the northern SCS was found to be closely linked to the largescaledynamical and SST forcings. Before onset and during the break, the northern SCS was relativelywarm and served as a moisture source (E - P > 0) to the overlying atmosphere. During the active phase,the northern SCS was cooled, providing a strong sink (E - P « 0) for atmospheric moisture, with theprimary source of moisture coming from regions further west over Indo-China and the eastern IndianOcean. Vigorous water recycling by convective systems in the northern SCS occurred during the maturephase of the SCS monsoon, with precipitation efficiency (defined as the ratio of the surface precipitationto the sum of large scale moisture convergence and surface evaporation from the ocean) approaching96%. Westward transport of moisture from Indo-China into, and northward transport out of, the northernSCS provided the main source of moisture for the torrential rain over the YRV in mid-June 1998.The present results suggest that the SCS may play an important role in regulating the SCS monsoonactive and break cycles through charge and discharge of moisture, and convective available potentialenergy.
The spatial and temporal structures of atmospheric circulation and SST associated with the Indian monsoon rainfall variability on quasi-biennial (2-3 year) and lower-frequency (3-7 year) time scales were investigated, using the domain-averaged Indian rainfall, the NCAR/NCEP reanalysis and Reynolds SST data. We took both time-filtering and composite analysis approaches. The results indicate that physical processes that determine the monsoon rainfall variation on the 2-3 year and 3-7 year time scales are different. The quasi-biennial variability of the monsoon is primarily determined by local processes in the Indian Ocean. Both local SST and moisture flux convergence anomalies are highly correlated with the monsoon at a lagged time of 3-6 months. It is argued that a positive SST anomaly in the Indian Ocean increases local moisture due to enhanced surface evaporation. The accumulation of these moistures leads to a strong monsoon through anomalous moisture advection by summer mean flows. The lower-frequency variability of the monsoon is primarily attributed to remote forcing mechanisms. Three possible processes may contribute to the monsoon variability on the 3-7 year time scale. The first is through the change of large-scale, east-west circulation induced by the eastern Pacific SST anomaly. The second is attributed to the effect of the SST anomaly in the Northwestern Pacific through enhanced (or suppressed) convective activity along the monsoon trough. The third is attributed to the tropicalmidlatitude teleconnection—a strong north-south, land-ocean thermal contrast occurs six months prior to a wet monsoon, and it persists from the preceding winter to summer and is responsible for the monsoon intensity change.
Effect of mountain uplift on climate is investigated by a global coupled ocean-atmosphere general circulation model with an emphasis on surface temperature changes. Results of the no-mountain run (NM) are compared with those of the control run with the present-day orography (M). When the lapserate effect is eliminated, continent interior becomes warmer with mountain uplift because clouds become fewer and the surface drier due to decreased moisture transport. On the other hand, South Asia and East Asia become cooler because summer monsoon precipitation is stronger, which makes the land surface wetter and increases clouds. Over the ocean, the existence of orography has a role to reduce sea surface temperatures (SST), particularly over the subtropical eastern oceans. This occurs because evaporation is larger due to stronger trade winds and also less solar radiation reaches the surface due to more low-level clouds, both associated with stronger subtropical anticyclones in M. The subtropical gyre is stronger in M than in NM, and therefore, the Kuroshio Current is stronger in M. When the effect of the ocean general circulation is not included, the SST over the western north Pacific becomes much lower in M than in NM because of stronger cold air outbreak from Siberia in winter in M. Thus, the ocean circulation changes act to reduce the SST changes by heat transport.
Observations of brightness temperature, Tb, made over land regions by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) radiometer are analyzed with the help of nearly simultaneous measurements of the vertical profiles of reflectivity factor, Z, made by the Precipitation Radar (PR) onboard the TRMM satellite. Furthermore, this analysis is done separately over convective and stratiform rain regions. This examination reveals a clear relationship between TMI and PR data. Possible explanation for this relationship is explored with the help of radiative transfer calculations. With this approach, we demonstrate that the 85 GHz observations of TMI can be simulated crudely from the observations of Z. However, the 37 and 19 GHz observations are not as well simulated, possibly because of horizontal non-uniformity in the hydrometeor distribution in the broad footprints of these channels and contamination introduced by land-surface emissivity. On the other hand, from TMI and PR observations, we find that the brightness temperature difference (T19-T37) minimizes these sources of error. Our simulations of (T19-T37) over convective rain regions are in reasonable agreement with this finding. This investigation indicates that the TMI 85 GHz channel yields the best information about rain over tropical land, because it has minimal surface contamination, strong extinction, and a fine footprint. The brightness temperature difference (T19-T37) can supplement the information given by the 85 GHz channel.
Atlantic decadal climate variations are studied using marine meteorological observations. To remove artificial interhemispheric correlation, we perform empirical orthogonal function (EOF) analysis of sea surface temperature (SST) variability separately for the North and South Atlantic. The first EOF for the North (South) Atlantic in the decadal (8-16 years) band features a meridional tripole (dipole). In the tropics, the northern and southern leading EOFs form a meridional dipole with a center of action at 15° on either side of the equator. The leading sea level pressure (SLP) EOFs for the North and South Atlantic each feature a center of action that is displaced poleward of the tropical SST extreme, at 30° latitude. The SLP center of action in the North Atlantic has a barotropic structure and contributes significantly to surface wind variability in the tropics. Despite being derived from statistically independent data samples, the principle components for the leading SST and SLP EOFs (four in total) are signifi-cantly correlated with one another, indicative of the existence of an interhemispheric mode spanning the entire Atlantic Ocean. The same analysis for a longer SST record suggests that this pan-Atlantic decadal variability exists throughout the 20th century. In the North Atlantic, composite analysis of wind velocity and heat fluxes based on the PCs of the leading SST modes indicates that wind-induced latent heat flux is the major forcing for decadal SST variability. In the South Atlantic, by contrast, wind anomalies are neither organized in space nor in geostrophic balance with SLP, a problem likely due to poor sampling there as indicated by a comparison with well-sampled satellite measurements. Spatially coherent anomalies of low-level cloud cover are found to be associated with the tropical Atlantic dipole, with increased (decreased) cloudiness over the cold (warm) lobe. These low-level cloud anomalies do not appear to be associated with significant surface wind convergence, unlike the deep convective clouds near the equator. By shielding solar radiation, these low-level cloud anomalies act to reinforce the underlying SST anomalies, reducing the Newtonian cooling rate for SST by as much as 30%.
An intense long-lived, quasi-stationary convection band that occurred on 5-6 August 1998 over the middle of the Korean peninsula, was studied using both observation and a numerical model. The convection band persisted for more than 10 hours, and produced heavy rain along the band with a maximum of 619 mm in 15 hours at a coastal station (Kanghwa). Radar observation indicates that the convection band is 20-30 km wide, and about 300 km long at its mature stage. It consists of several long-lived precipitation cells along the band. Some of the precipitation cells develop on the west coast, and move northeastward along the band. The convection band occurred over the middle of the Korean peninsula, where a converging airflow pattern is found between the mid latitude cyclone to the north, and the western Pacific subtropical high to the south. The simulated convection band forms and evolves in a consistent manner with the observation, although it shows notable differences from the observation in the onset time, lifetime and rainfall intensity. Formation of the simulated band proceeds in the following manner. First, new convection cells continuously form on the west coast and move northeastward forming a line of long-lived convection cells. Second, a line of low-level convergence forms somewhat rapidly in the upwind side of the convection initiation point, and then convective cells develop and move along this line. Some cells develop into deep convection and last several hours. These processes result in a long convection band. The favorable largescale condition (especially the converging airflow), and its interaction with convection, seem to be the important elements for the development of the present convection band. Surface latent heat flux is found to play a crucial role in establishing a convectively unstable environment for the onset of convection. It is also found that orographic effect is not an essential factor for the formation of the present convection band.
A radar echo composite chart (RECC) displays precipitation intensity levels in 5 × 5-km grids for a composite region of digitized radars covering more than 2 × 106 km2. Each continuous precipitation cluster in a RECC is considered a precipitating cloud (p-cloud). At around 400 km, the distribution of the horizontal scale (S) for p-clouds changes. At scales smaller than 400 km, the distribution decreases smoothly with scale. In the distributions of S and maximum precipitation intensity (A) for p-clouds, there is a peak at between 400 and 1,000 km and about 45 mm/h throughout the year. This peak results from large-scale p-clouds accompanying macro-β and meso-α-scale disturbances. From June to September over land, precipitation is concentrated at scales between 15 and 200 km and intensities exceeding 8 mm/h. This distribution results from cumulus-scale p-clouds, which include a variety of convective clouds initiated by boundary-layer processes. A third type of p-cloud, caused by air-sea interactions, appears with the cold northwesterly flow that follows the passage of extratropical cyclones, especially in winter. This type dominates the weaker intensity side of the cumulusscale p-clouds between 5 and 200 km in the precipitation distribution.
Large-scale precipitation activity in the subtropics and the mid-latitudes, in addition to seasonal variation, were studied using new climatological precipitation datasets: GPCP rain gauge-satellite combined data, and TRMM level-3 data derived from TRMM-PR observations. We found significant large-scale precipitation zones extending between the subtropics and the midlatitudes, referred to as SMPZs (Subtropical Mid-latitude Precipitation Zones) in this study. Most portions of the SMPZs existed over the oceans. Meridional variation of zonal averaged precipitation was quite different between land and the oceans. Mid-latitude peaks at ∼40° in zonal averaged precipitation were observed only over the oceans, because the SMPZs over the ocean formed the 40° peaks. The midlatitude peaks significantly intensified in the fall and winter when the SMPZs zonally extended in the mid-latitudes and kept precipitation as substantial as in summer. In summer, the SMPZs extended diagonally across the subtropics from the active tropical monsoon rainfall areas. Precipitation over the subtropical oceans thus increased in summer. Over land, mid-latitude peaks were not found. Precipitation intensified in summer, both in the subtropics and in the mid-latitudes. The mid-latitude portions of the SMPZs extended near the storm tracks defined by high baroclinic wave activity. However, many portions of the SMPZs shifted to the lower latitudes from the storm tracks in fall and winter. In summer, baroclinic wave activity was weak for the subtropical portions of the SMPZs, which were characterized by convergence and frontal zones with weak baroclinicity. Vertical structure of precipitation observed by PR suggested that deep (shallow) stratiform and convective precipitation largely contributed to the active rainfall along the SMPZs in summer (fall and winter). A possible relationship between the shallow precipitation and the displacement of the SMPZs from the storm tracks in fall and winter was discussed.
A very fine mesh dispersion model was developed for volcanic gas over Miyake Island. The model consisted of a meteorological prediction part and a dispersion part. The multi-nesting method was used for the meteorological prediction part to express the field, depending on synoptic weather pattern in this study. The grid size of the innermost model was 100 m. The days when the observed SO2 concentration was comparatively high were chosen for the verification of the model. Four cases of 24-hour numerical simulations were conducted. We try to reproduce the SO2 concentration in each situation. Following results were derived. The effect of the advection was dominant as compared with other physics, so the volcanic gas moved leeward. Even the slight change of the wind direction largely varied the concentration. The volcanic gas spread widely, in case of weak wind at night, because of the mountain breeze. Although there were some cases of time-lags of the peaks, and some degrees of overestimation, or underestimation, each situation was almost well reproduced.
The upper ocean, atmosphere and their interaction over the North Pacific exhibit pronounced decadal to interdecadal variations. A diagnostic equation for analyzing the heat budget for decadal variability in winter sea surface temperature (SST) is derived that can properly account for subsurface geostrophic advection, and strong seasonal cycle in the depth and temperature of the ocean mixed layer. A modelassimilated ocean dataset, partially validated for the period of the TOPEX/Poseidon mission, is used to evaluate the relative importance of subsurface advection and surface forcing due to wind-induced turbulent heat flux and Ekman advection. For our analysis, two key regions are chosen where decadal SST variance reaches local maxima, centered at 170°E, 42°N (Region A) and 155°W, 35°N (Region B), respectively. Region B is under the direct influence of the Aleutian Low, where the surface effects are dominant. Region A is part of the Kuroshio-Oyashio Extension, where the winter mixed layer is deep and the subsurface geostrophic advection contributes significantly to low-frequency winter SST variations. Our analysis suggests that anomalous geostrophic advection changes signs north and south 38°N, presumably as a result of ocean gyre circulation adjustment to wind changes to the east. The surface forcing shows a larger-scale structure covering the entire mid-latitude North Pacific, in response to basin-wide changes in atmospheric circulation.
There exists a firm relation between the rotational updraught (and/or downdraught) of a convective storm and the storm-relative environmental helicity. The already-known formula, which was derived with linear approximation and on a form-preserving assumption, describes the relation qualitatively well. In this note, a similar formula is derived without the linear approximation, although the formpreserving assumption is retained. The nonlinear terms generate the acceleration of propagation and/or the temporal variation of growth rate of the storm, which are absent from the linear formula. These nonlinear effects enter the derived formula.