In this research, a case study is conducted on the formation process of the Mei-yu frontal disturbance in the eastern foot of the Tibetan Plateau. The target period is middle-late June of 1992, during which the Mei-yu front is re-intensified after the decaying phase observed in middle June. The re-intensification process of the Mei-yu front occurs in accordance with the approach of the migrating upper level trough to the north of the Plateau. When the upper level trough is situated to the northwest or north of the Plateau, the lower level high pressure area becomes apparent. Subsequently, the low pressure area on the northeast of the Plateau deepens. At the same time, a shallow cold air mass observed below the 700 hPa level formed in the southeast of the low pressure area. The appearance of the cold air mass might be related to the development of low and high pressure systems to the north-northeast of the Plateau. Concurrently, a lower level strong westerly wind appears along the northern periphery of the Plateau, and turns into northwesterly or northerly wind along the eastern periphery of the Plateau. A shear line formed in the northeastern or eastern foot of the Plateau between this northwesterly wind and the southerly wind prevailing over the North-Middle China Plain. This shear line in the lower layer changed into the Mei-yu frontal disturbance after∼:00UTC 21 June. Note that the wind system along the northern-eastern periphery of the Plateau mentioned above is considered to be an ageostrophic wind system, accompanied by the transient small scale low and high pressure systems that migrate clockwise along the northern-eastern periphery of the Plateau. The synchronic appearance of the cold air mass and the shear line is considered to be an effective trigger for the formation of the initial Mei-yu frontal disturbance. Further, the coupling of the upper level migrating trough and the lower level shear line also can be important for the evolution of the Meiyu frontal disturbance.
A dominant impact of the El Niño-Southern Oscillation (ENSO) on the South Asian summer monsoon interannual variability is identified using the National Centers for Environmental Prediction /National Center for Atmospheric Research reanalysis aided by an ocean general circulation model. It has an equatorially symmetric structure and is most pronounced at the growth phase of ENSO in late summer during the period from the 1960s to mid-1970s when tropospheric biennial oscillation (TBO)-like ENSO dominates. As ENSO develops, an anomalous Walker circulation system over the tropical Indian and Pacific oceans changes from a single-cell regime in summer to a double-cell regime in fall. Meanwhile, rainfall anomalies of an equatorially symmetric structure are induced over the tropical Indian Ocean, accompanied by north-south twin circulation anomalies in the lower troposphere. The northern circulation is dynamically linked with anomalous monsoon rainfall over India especially in late summer. This research suggests that a combination of the wind-evaporation feedback in the Indian Ocean and ocean dynamics in the tropical Pacific is crucial for the regime transition of the anomalous Walker circulation system associated with the TBO-like ENSO. As the seasonality of the ENSO cycle changes before and after the late 1970s, the difference in ENSO impacts between its growth and decay phases may have influenced the ENSO-monsoon relationship and caused its interdecadal change.
Precipitation radar and microwave radiometer data from the Tropical Rainfall Measuring Mission (TRMM) satellite during the spring and summer months of 1998 are used to study the precipitation characteristics in the mid-latitude East Asia region (20°-40°N, 100°-140°E), with particular emphasis on comparing the differences between stratiform and convective rains, and between rains during spring and summer. The characteristics in the mid-latitude are also compared with those in the tropics. The two-season mean results for the mid-latitudes show that the mean rainfall rate for stratiform clouds is ∼2 mm h-1, at least 6 times lower than its convective counterpart. The mean convective rainfall rate is higher over land than over ocean, and higher in the mid-latitude than in the tropics. Generally, convective clouds generate more rain total for both mid-latitudes and tropics although they cover less area than stratiform clouds. Comparing between mid-latitude and tropical regions, the contribution of stratiform rains is larger in the mid-latitudes than in the tropics. There are distinct differences between stratiform and convective precipitation profiles, as well as noticeable seasonal variations of these profiles. The mean vertical profiles showed that stratiform rains have a quasi-constant rainfall rate below freezing level and a sharp drop-off above. Convective profiles, on the other hand, often have a maximum rainfall rate somewhere below the freezing level. Above the freezing level, precipitation layer for convective rains is thicker than for stratiform rains. For convective rains, significant differences are found between profiles over land and ocean, with a significant deeper layer for land convections given the same surface rainfall rate. Consistently, microwave scattering signature over land, expressed by polarization-corrected temperature at 85 GHz, is nearly twice as strong as that over ocean for the same surface rainfall rate. Precipitation profiles are similar in shape and depth between mid-latitude and tropical regions during summer when the freezing level heights are also similar between the two regions. However, compared to being no noticeable variation in the tropics during a year, there is a significant seasonal change for the precipitation profiles in mid-latitudes. The seasonal difference of precipitation profiles arises from the deeper precipitation layer in the summer than in the spring for a given surface rainfall rate, being in consistent with the seasonal variation of the freezing level height. This change in precipitation depth applies to both stratiform and convective rains, and to rains over both land and ocean. It should be mentioned that all the aforementioned results, except for the 85 GHz scattering signature, are derived from TRMM precipitation radar data.
The intertropical convergence zone (ITCZ) is the rising branch of the global Hadley circulation and often considered as the climatic axis of symmetry. A full-physics atmospheric general circulation model is coupled with an intermediate ocean model to investigate the effect of ITCZ's meridional configuration on the space-time structure of climate variability. In the control experiment where the model settles into a north-south symmetric climatology, strong interhemispheric interaction takes place and sea surface temperature (SST) anomalies are organized into an anti-symmetric dipole pattern with the equator as the nodal line. The trade winds intensify (weaken) over the anomalously cold (warm) side of the equator, indicative of a positive feedback between surface wind, evaporation and SST (WES). When the mean ITCZ is displaced into the Northern Hemisphere by perturbing the shape of continents, SST variability is significantly reduced at low-frequencies, especially with periods greater than 5 years, as a result of reduced interhemispheric interaction. The nodal line now coincides with the northward-displaced ITCZ, and the SST correlation across this nodal line is greatly reduced to a statistically insignificant level. Calculations with a simple baroclinic model of the atmosphere indicate that the departure of the climatic axis of symmetry from the geographic equator weakens the WES feedback and hence interhemispheric interaction. Implications for tropical Atlantic variability are discussed. In particular, our result of ITCZ’s modulation of interhemispheric interaction is consistent with the seasonality of cross-equatorial SST gradient variability: it peaks in boreal spring when the mean ITCZ is nearly symmetric about the equator and is significantly reduced in other seasons when the Atlantic ITCZ is displaced into the Northern Hemisphere.
The present paper studies features of the Baiu front and associated precipitation systems simulated in the climatological SST run by an AGCM T106L52 (primitive equation spectral model, the maximum wave-number of 106, with 52 vertical levels) in comparison with the features found in observational studies. The Baiu front is properly simulated in 1-20 June Y07 (the 7th year after the spin-up integration). In this “Baiu phase”, the large-scale circulation systems, such as the upper cold lows and blocking ridge in the northern latitudes, westward extending Pacific subtropical anticyclone, monsoon westerly, and subtropical jet stream, are simultaneously maintained. Under this condition, the synoptic- and meso-ascale variations of the Baiu front are simulated. The southward shift of the front occurs when a synopticscale cyclone develops in association with a westerly trough. When westerly troughs are inactive, weak subsynoptic-scale depression is formed in the frontal zone. A few meso-α-scale precipitation systems are generated in the trailing portion of the preceding depression, and form a “Baiu precipitation system family” with a length of∼2000 km. This indicates that the simulation of the realistic Baiu front depends on the maintenance of proper large- and synoptic-scale circulation systems. However, the Baiu precipitation zone disappears in the early July, as the west-east elongating barotropic anticyclone, which is usually seen over Japan in August in the real atmosphere, is situated over ∼37°N.
Dual Doppler radar observations were carried out from November 1992 to January 1993 during the TOGA-COARE IOP (Tropical Ocean and Global Atmosphere-Coupled Ocean Atmosphere Response Experiment, Intensive Observation Phase) at Manus Island in Papua New Guinea. The heating profiles of six stratiform and eight convective cloud systems were calculated using three-dimensional wind field and hydrometeor distribution in a same way as Roux and Sun (1990). Relations between the heating profiles and the thermodynamic properties of environment were studied. Convective and stratiform cloud systems both showed structures consistent with past studies. The heating level varied significantly with environmental conditions that had been linked to the phase of Madden Julian Oscillation (MJO). Specifically, at the beginning of the active phase of MJO, convective cloud systems dominated and the heating level was high (7 km). These cloud systems heated and moistened the middle to upper troposphere and stabilized the atmosphere. In the middle of the active phase of MJO, stratiform cloud systems dominated and the heating level lowered (5 km). At the end of the active phase of the MJO, convective cloud systems again dominated. A composite profile for the entire observation period was calculated using the representative profiles and satellite data, and was compared with that derived by Lin and Johnson (1996) using radiosonde observation. The derived maximum heating rate coincided with their results, however, the heating rates of lower and upper troposphere were smaller and the level of maximum heating (5 km) was slightly lower.
A statistical analysis is conducted of the covariance between summer precipitation in Japan and concurrent variations of sea surface temperature (SST) in the Western North Pacific (WNP; 120-160°E, 0-60°N). To this end, a singular value decomposition (SVD) is applied to observed monthly anomalies of the two fields for the period 1961-1998 (38 summer seasons). The first coupled mode of the SVD explains as much as 71% of the total cross-covariance between WNP SST and Baiu precipitation. It is characterized by a zonally elongated dipole in the SST anomaly with opposite loadings on either side of about 30°N and a precipitation anomaly with its maximum in southern Japan. The patterns indicate a negative correlation between Japan's summer precipitation and the sea surface temperature in the East China Sea, Japan Sea and Kuroshio-Oyashio extension. Simpler composite analyses confirm the robustness of the coupling. In a linear regression analysis, the score of the WNP SST explains 30% of the domain mean summer precipitation variance, which is larger than the variance accounted for by lagged and concurrent ENSO-indices. Composites related to the mode's precipitation score were found to support both directions of the atmosphere—ocean interaction. On the one hand, the anomalies in radiation and evaporation at the sea surface would be sufficiently large to induce the observed temperature variations in the northern pole of the SST pattern. On the other hand, the possibility for an atmospheric response to the SST is supported by the consistency of anomalies in lower-tropospheric baroclinicity and moisture flux divergence with the underlying SST distribution.
Lidar observations of polar stratospheric clouds (PSCs) were made during three winter campaigns from 1994/95 to 1996/97 at Ny-Aalesund, Svalbard. PSCs composed mainly of solid particles were found at the beginning of most PSC events. They appeared when the temperature became lower than the assumed equilibrium temperature of nitric acid trihydrate (TNAT) but sometimes appeared even at temperature a little higher than TNAT. As the stratospheric temperature became very low, close to the frost point of ice (Tice), PSCs composed mainly of liquid particles appeared. The behaviors of liquid PSCs observed are consistent with the expected ones, which were estimated based on the current formation theory of super cooled ternary solution (STS) particles. Results of backward trajectory analysis show that some solid PSCs experienced the temperature below TNAT at least for more than 10 hours after passing through the melting point of sulfuric acid tetrahydrate. The solid PSCs which appeared at temperatures higher than TNAT experienced the temperature below TNAT for more than 1 day in the past. Almost no PSCs experienced the temperature lower than Tice during 10 days prior to the observation. Results of trajectory analysis also suggest that the increase of the depolarization ratio depends strongly on the degree to which air parcels of PSCs cooled below TNAT and on the period during which the air parcels experienced the temperature lower than the condensation point, but not on the experience of lower temperature than Tice. Some liquid PSCs with a large scattering ratio (4-5) and a low depolarization ratio (0-0.005) appeared after they experienced the temperature close to Tice for a few days prior to the observation.
Concentrations of carbonyl sulfide (COS) were measured in whole air samples collected between East Asia (Japan) and the Arctic (Spitsbergen) up to an altitude of 12 km during the Arctic Airborne Measurement Program 98 (AAMP98). Continuously measured O3 and CO2 concentrations were used to interpret the distribution of COS concentrations. A latitudinal gradient to lower concentrations of COS was observed poleward of 70°N; concentrations of COS were 421±27 pptv poleward of 70°N and 461±30 pptv at 60-70°N. The relationship between COS concentration and the O3/CO2 ratio indicates that the concentrations of COS near the ozonopause were almost equal to those in the troposphere, and then decreased as the O3/CO2 ratio increased. Tropospheric influence was observed up to potential temperatures of 350 K in this study. According to the AAMP98 observations, on the basis of the trajectory analysis and the O3/O2 ratio, distribution of COS in the Arctic lowermost stratosphere is certainly effected by the downward transport of an arctic stratospheric air mass and by the horizontal transport of a tropospheric air mass across the tropopause over midlatitudes. The sulfate aerosol produced by COS oxidation is estimated to be very small at the observed altitudes between East Asia and the Arctic in late winter.
In our recent paper “Parameterization of the effect of cloud condensation nuclei on optical properties of a non-precipitating water layer cloud” (Kuba et al. 2003), we developed an approximation to predict the cloud droplet concentration as a function of the CCN concentration and the updraft velocity at the cloud base. This study improves on the approximation for convenience. In addition, this study proposes an approximation for higher updraft velocities than those in the previous paper.
The Kosa event was observed widely in Japan on 12 November 2002. Measurements of size-separated aerosol number concentrations and precipitation chemistry were performed in Toyama during the autumn and early winter of 2002. The number of aerosol particles larger than 3.0 μm in diameter dramatically increased during the Kosa event, whereas the number concentrations of finer particles hardly changed. The concentration of non-sea-salt calcium and pH were high in the precipitation sampled during the Kosa event.
In this paper, the evolution of the cloud activities for six typhoons in 1997 is examined, utilizing TBB (equivalent Black Body temperature) data of GMS (Geostationary Meteorological Satellite)-5 split window channels. As the result, periods of 24-hour and about 10-hour are found in the time variation of the cloud areas whose cloud top temperature is lower. Diurnal variation is found most distinctly in the time series of the cloud area of TBB lower than -40°C, which corresponds to the one whose cloud top gets at the upper level of the troposphere. The maximum and the minimum peaks of this variation appear around at 1500 JST (Japan Standard Time) and 0800 JST. And, the maximum appears corresponding to vigorous convections at 0600-1000 JST of all the vigorous convections. About 10-hour period time variation is found most distinctly in the time series of the cloud areas of TBB lower than -60°C, which corresponds to the one whose cloud top gets at the upper level of tropopause or over. The maximum of this variation appears corresponding to a preceding vigorous convective activity which occurs in the wide region of the typhoon a few hours before.