This paper studies the cloud systems in the stationary Meiyu-Baiu front observed in 1-10 July 1991, using the Geostationary Meteorological Satellite IR data and ECMWF re-analysis data. The Meiyu-Baiu frontal cloud zone consists of a few trains of cloud systems with a length of ∼2000 km, which is recognized as the “cloud system family”. Each family consists of a preceding subsynoptic-scale cloud system, and a few meso-α-scale cloud systems aligned along the trailing portion of the sub-synoptic-scale cloud system. During the 10-day period, 4 sub-synoptic-scale cloud systems, accompanied by Meiyu-Baiu frontal depressions, are formed with an interval of 2-3 days, in association with the short-wave westerly troughs. The sub-synoptic-scale depressions indicate the peak value of vorticity more than 6× 10-5 s-1, at both 850 and 500 hPa. The sub-synoptic-scale cloud system consists of convective clouds in its southern portion, and wide spread clouds in the northeastern portion. A zone of potential vorticity appears in the trailing portion of the sub-synoptic-scale depression. The cold and dry advection occurring to the back of the depression sustain thermal and moisture gradient in the narrow frontal zone after the passage of the depression. A few meso-α-scale cloud systems develop along the trailing portion of the sub-synoptic-scale depression. The main part of the meso-α-scale cloud system consists of the active convective clouds. Some of the cloud systems show features of oval shaped cloud cluster, while others indicate features of leaf shaped cloud system with a cloud band in its trailing portion. These meso-a-scale cloud systems are categorized into three groups based on the horizontal extension of the cloud area, duration and the magnitude of the relative vorticity associated with the cloud system. Type-A meso-α-scale cloud systems are identified for∼2 days, and are accompanied by positive vorticity of ∼4 × 10-5 s-1 in the lower and middle troposphere. Type-B meso-a-scale cloud systems are identified only for∼1 day, and are accompanied by positive vorticity of ∼2 × 10-5 s-1 mainly in the lower troposphere. Type-C meso-α-scale cloud systems are generated during the daytime and decay during the nighttime. The wavelength estimated by Eddy’s baroclinic instability model for the unsaturated frontal zone coincides with the wavelength of the observed sub-synoptic-scale cloud system, while that for the moist frontal zone is consistent with the scale of Type-A meso-a-scale cloud systems. The cloud system families are most clearly observed in 110-140°E, where the Pacific subtropical anticyclone, and the cold low in the northern latitude, provide favorable conditions for the development of cloud systems.
A general structure of Typhoon 9426 (Orchid) up to the lower stratosphere including the inner coreregion was observed by the MU (middle and upper atmosphere) radar on 29-30 September 1994. In thispaper, a meso-a-scale wind field and meso-β and -γ scale features of precipitating clouds in the typhoonwere investigated.The kinematic structure in the front and rear of the typhoon was quite different because of its transitionfrom mature to decaying stages and an asymmetric distribution of cloud and precipitation. In frontof the typhoon, the meso-α-scale wind field was characterized by a cyclonic rotation with the maximum atlow level, outflow regions tilted outward with height, and vertical motions affected by convection andtopography. In the vicinity of the typhoon center, the tangential wind had a vertical spiral structure forthe center, considered to be the result from deformation of the center of the decaying typhoon. In the rearof the typhoon without precipitating clouds, the cyclonic wind became weak, and the outflows and verticalmotions as seen in the front were not detected. In front of the typhoon, an eyewall and wide rainband were observed. In the eyewall, a meso-γ-scaleremarkable updraft associated with an outflow region, considered to be a part of vertical circulation, wasfound in the upper troposhere. The outflow region tilted outward was originated from the area of maximumradial shear of the low-level cyclonic wind. It was extended to an updraft region within an upperlevelband-shaped cloud, located far from the typhoon center. The wide rainband (50-70 km width) waslocated at the outer edge of the band-shaped cloud, and it lasted in the development of the upper-levelcloud. It was accompanied by the tilted outflow region to 6 km altitude, the bottom of which was locatedat the maximum of the cyclonic wind. A narrow rainband (20-40 km width), that a lifetime was quiteshort (1.5 hours), was also observed by the boundary layer radar (BLR) at another site. An outflow regionwith the tilted structure was present in the outer part (convective portion) of the rainband, and its bottomwas not associated with strong cyclonic wind as seen in the wide rainband.
A method for classifying precipitation is proposed to clarify the complicated properties of rainfallduring the Baiu season. With three-dimensional radar reflectivity data in the Kanto area of Japanthrough the Baiu period of 1995, the Bright Band Fraction (BBF) classification method, which focuseson the vertical structure of a radar echo, was improved and applied to the Baiu rainfall. As a result ofthis improvement, main rainfall types in the Baiu period were divided into three categories: convective,stratiform, and mixed. To grasp the comprehensive properties of precipitation, four diagnostic parameters were introduced.By a fuzzy-clustering analysis, which uses the parameters as characteristics of rainfall samples, theclassification of the main rainfall types based on the improved BBF was verified to be reasonable, anddetailed patterns within the three types were further distinguished. Stratiform rain can barely be dividedinto patterns, which shows the consistency of its properties. In contrast, convective rain can be dividedinto a few patterns, one of which always produces the heaviest rain intensity at each of the selectedAutomated Meteorological Data Acquisition System stations in the Kanto area. Mixed rain usually has afew patterns that show the variation of its properties. Applying the improved method of BBF to a rainfall case observed off the western coast of the NagasakiPrefecture in the Baiu season of 1997 with dual Doppler radar, the kinematic properties of therainfall of the three types were shown to correspond with their given definitions. The mixed type is recognizedas one of the important types of rainfall during the Baiu period, because it contains shallowconvective shape with relatively large rainfall amount.
An ice core-drilling program was carried out at the accumulation area of Dasuopu glacier (28°23'N, 85°43'E, 7100 m a.s.l.) in the central Himalayas in 1997. The ice core was analyzed continuously for stable isotopes (δ18O), and major ions throughout the core. Cycles indicated by δ18O, cations were identified and counted as seasonal fluctuations as annual increment from maximum to maximum values. Reconstructed 300-year annual net accumulation (water equivalent) from the core, with a good correlation to Indian monsoon, reflects a major precipitation trend in the central Himalayas. The accumulation trend, separated from the time series, shows a strong negative correlation to Northern Hemisphere temperature. Generally, as northern hemisphere temperature increases 0.1°C, the accumulation decreases about 80 mm, reflecting monsoon rainfall in the central Himalayas has decreased over the past decades in the condition of global warming.
The onset processes of the zonally asymmetric anomalies of convection and sea surface temperature(SST) over the tropical Indian Ocean are investigated with considering seasonal evolution, and interannualvariability, of the large-scale convection anomalies in the Asian summer monsoon, using outgoinglongwave radiation (OLR), SST, and NCEP/NCAR reanalysis data. This asymmetric pattern ofthe convection anomalies is particularly dominated in boreal autumn. Some recent studies have notedthat these anomalies, based on the atmosphere-ocean coupling phenomenon, can be developed and maintainedby itself. The time evolution shows that the eastern part of the zonally asymmetric anomalies over the IndianOcean lead the western part of those. In July, the negative SST anomalies and positive OLR anomaliesfirst appeared off the Sumatra coast, and southeasterly wind anomaly accelerated the climatologicalsoutheasterly wind along the west coast of Sumatra. This southeasterly wind acceleration provide a SSTcooling over the southeastern Indian Ocean, and play a role in triggering of the zonally asymmetricanomalies in the following autumn. It is suggested that this southeasterly wind acceleration over thesoutheastern Indian Ocean is closely linked to the meridionally asymmetric anomalies of convection,between the maritime continent and the South China Sea/ Philippine Sea (SCS/PS). That is, the intensificationof the local Hadley circulation over the western Pacific associated with the enhanced convectionover the SCS/PS, and suppressed convection over the maritime continent, is found to be a clearprecursory signal of the zonally asymmetric anomalies over the Indian Ocean. It has also been notedthat the convection anomalies over the southern and northern parts of the meridionally asymmetricanomalies over the western Pacific are not always the opposite sign, and seem to have different interannualvariability respectively. It is likely that the former might be strongly influenced by the ENSO,through the Walker circulation anomalies and the latter might be affected by the modulation of the intraseasonalvariation of the Asian summer monsoon. The seasonality of the zonally asymmetric anomaliesis also suggested from the occurrence of the intensification of the local Hadley circulation in borealsummer.
In this study, a parameterization method based on NOAA-14/AVHRR data and field observations is described and tested for deriving the regional land surface variables, vegetation variables and land surface heat fluxes over a heterogeneous landscape. As a case study, the method was applied to the Tibetan Plateau area. The regional distribution maps of surface reflectance, MSAVI, vegetation coverage, surface temperature, net radiation, soil heat flux, sensible heat flux and latent heat flux were determined over the Tibetan Plateau area. The derived results were validated by using the “ground truth”. The results show that the more reasonable regional distributions and their seasonal variations of land surface variables (surface reflectance, surface temperature), vegetation variables (MSAVI and vegetation coverage), net radiation, soil heat flux and sensible heat flux can be obtained by using the method proposed in this study. However, the approach of deriving regional latent heat flux, and their seasonal variation as the residual of the energy budget, may not be a good method due to the unbalance of energy and the strong advection over the study area. Further improvement of the method was also discussed.
In 1999/2000, as many as 51 Electrochemical Concentration Cell (ECC) ozonesondes were launched from December to March at the Canadian Arctic Eureka observatory (80°N, 86°W), one of the most northern stations in the Arctic, and the temporal evolution of the vertical ozone profiles was obtained in detail. During the winter, monthly average total ozone and temperature at 50 hPa, both observed with the ECC ozonesondes, were substantially lower than normal over Eureka since 1993. From December to March, Eureka was most of the time inside the polar vortex in the lower stratosphere (475 K isentropic surface level), except for early March. When Eureka was inside the polar vortex, very low temperatures were observed in the lower stratosphere, in accordance with the detection of Polar Stratospheric Clouds (PSCs) by Mie lidar between the 17 and 22 km level in January. However, PSCs were not observed over Eureka around the same altitude in February. Together with the observations of large HNO3-containing particles by the ER-2, and depleted HNO3 by the UARS Microwave Limb Sounder, the lidar observations suggest the denitrification in the lower stratosphere in late winter inside the vortex. Under these conditions, the intravortex ozone mixing ratio on the 475 K isentrope decreased by about 2.2 ppmv from 3.1 ppmv on the 497 K isentrope (4 February), to 0.9 ppmv on the 475 K isentrope (29 March) in late winter to early spring, following the diabatic descent. In this period, the loss rate inside the vortex was found to be 0.041±0.008 ppmv/day (90% confidence interval). These findings and results are consistent with other observations (THESEO 2000/SOLVE campaign), and suggest that significant chemical ozone loss did occur in the lower stratosphere inside the polar vortex during the winter of 1999/2000.
The hypothesis that the steady state of the climate is constrained to maximize its dissipation is investigated by using a one-dimensional radiative-convective model where the convective fluxes are parameterized following the mixing length theory. The eddy heat diffusivity is chosen to produce the maximum energy dissipation caused by convection (maximum dissipation principle). The state of maximum dissipation due to heat diffusion in a model atmosphere with a fixed vertical profile of specific humidity is obtained for a value of the eddy heat diffusivity, which is in close agreement with that expected for air in stirred conditions. By including the temperature—opacity feedback (i.e., condensable absorbing gas in the infrared with a fixed vertical profile of relative humidity), the state of maximum dissipation is attained for a value of the eddy heat diffusivity that lies within the range of the expected values. In this case, the vertical temperature profile is convectively stable and does not differ in excess from the observed one. Thus, it is suggested that the results here reported represent an ‘empirical’ support to the maximum dissipation principle.
In this paper, we study the characteristics of moisture variability over the tropical western Pacific warm pool region using sounding data obtained through the use of research vessels. Data used here were collected in both boreal summer and winter seasons over a nine-year period from 1993 through 2001. Sounding data are categorized into five zonal wind vertical shear modes, and then examined for common and singular features of moisture variability. The results show frequent occurrences of high relative humidity centered around 945 hPa, and 560 hPa, for any wind shear conditions, as previously found in the Tropical Ocean and Global Atmosphere/ Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) studies. We also found another small peak at 845 hPa in low-level easterly conditions. The degree of these features is modulated by wind shear mode differences. An interesting feature is that the variation of total precipitable water is well represented by the synoptic scale mid-tropospheric moisture variation. However, a close relation between the total moisture budget and the moisture variation in the boundary layer is also obtained for a time scale of less than a few days (especially the diurnal cycle). We investigated extremely dry air conditions as one factor modulating mid-tropospheric moisture. Backward air parcel trajectories show that there are two major routes for the intrusion of dry air over the warm pool. In the first, the air parcel could be traced back to higher latitudes and the upper troposphere. For this type, anticyclonic equatorward flow associated with developing mid-latitude baroclinic waves, such as Rossby wave breaking, is responsible for injecting subtropical dry air into the Tropics—as proposed by Yoneyama and Parsons for TOGA COARE dry events. In the second type, the air parcel could be traced back to the upper troposphere, but within lower latitudes in the central-eastern Pacific. For this case, the air parcel originates from the trade winds that passed over the region where dry air was injected by the anticyclonic equatorward flow associated with the baroclinic wave development in the subtropical region. The relationship between dry air conditions and large-scale features is also discussed.
During the Mei-Yu season, when the surface front approaches and pre-frontal southwesterly flow intensifies, shallow and short-lived mesolows often form to the southeast of Taiwan on the leeside of the Central Mountain Range (CMR). Although they develop under similar synoptic settings, cases observed during the Taiwan Area Mesoscale Experiment (TAMEX) in 1987 still exhibited some case-to-case variations, particularly in the upstream Froude number (Fr) and the amount of windward-side precipitation. The present study selects two cases with more contrasting mesoscale characteristics for detailed examination, and differences found in their flow structure and associated features are discussed and compared with theoretical aspects. Results indicate that as long as Fr exceeds a certain value, two possible scenarios associated with relatively higher and lower Fr values can lead to formation of leeside mesolow through slightly different combination of processes. The case with a larger Fr (>0.5) on June 78 had a flow structure relatively more in favor of the “flow-over” regime under stronger prevailing winds. The airflow produced a large amount of precipitation at the windward side then sunk at the leeside together with parcels from higher levels. The adiabatic warming led to formation of the mesolow, which had no corresponding mesovortex most of the time with a higher Rossby number (Ro≈2), but small and localized vortices independent from the low might appear at other favorable spots near the terrain. The second TAMEX case (June 2) had smaller Fr (<0.2) and Ro (<1) values. Due to the strongerCoriolis deflection and upstream blocking effect, the flow showed more characteristics of the “flowaround”regime. Only a relatively shallow layer below the crest of the CMR was able to climb over, andsubsequently nearly no precipitation was produced. However, the stronger blocking allowed air parcelsabove the mountain crest to sink directly at the lee for greater depths as a transient response, and toproduce mesolow at comparable strength even without the contribution from latent heat release. In thisscenario with smaller Ro, the low possessed a closed circulation for longer periods during its life span.Two schematic illustrations are presented to depict the major processes found in these two mesolowcases, which show general agreement with theoretical aspects and exemplify variations in flow structureunder slightly different Fr and Ro values in the real atmosphere from an observational standpoint.Cases with characteristics in between the two scenarios are also expected.
This paper focuses on two cases of a local front that formed ahead of a synoptic cold front under a southwesterly inflow in the Kanto region. Vertical sounding data provided by an observation network of the Tsukuba Area Precipitation Studies (TAPS) enabled a detailed investigation of the mesoscale structure in the vicinity of the local front from its formation through dissipation. Despite some differences in synoptic features, the local front exhibited essential similarities in vertical structure between the two cases, as well as in horizontal structure. The local front, with WSW-ENE orientation, was well defined even in the daytime. A stable layer with a thickness of about 400 m formed on the cold side of the developed local front. As in the upper layer of warm inflow, southwesterly winds prevailed in this stable layer, except for the lowest 50-200 m. The maintenance mechanism of the local front is discussed. The evolution of the local front largely depends on the development of the stable layer on its cold side. In both cases, frontal formation began in the nighttime. A nocturnal cooling on the land surface contributed to forming a cold air mass on the northern side of the local front. In addition, the southwesterly inflow caused warming just above the surface-based cooling layer, which intensified the stratification. Furthermore, the observed vertical structure suggests that the locally modified circulation affected the frontal evolution. Two orographic effects on the circulation, 1) intensification of the inflow over the eastern part, and 2) downslope wind over the western part, both related to the mountains in the west of the Kanto region are expected. Especially, the downslope wind can reasonably explain the daytime persistence of the local front through the enhancement of stratification by adiabatic warming, and resultant influences on the mass transport.
A new method is proposed to predict the optical thickness, effective radius, and concentration of cloud droplets in water layer clouds by using the spectrum of cloud condensation nuclei (CCN), ascent velocity at cloud base, and liquid water path (LWP). A retrieval method is also proposed to predict CCN number concentration by using independent observational data of ascent velocity at the cloud base, the optical thickness and LWP of clouds. For this purpose, a newly developed cloud microphysical model that relates cloud droplet size distributions to the updraft velocity, and to CCN constituents and size distribution is used. Cloud droplet growth is calculated, with special care being taken to avoid non-physical numerical diffusion of the droplet spectrum. Near the cloud base, CCN activation and subsequent cloud droplet growth are calculated in a Lagrangian framework to model the effect of CCN on growth by condensation more accurately. In the middle and upper parts of the cloud, an Eulerian framework is used to estimate growth by coalescence for cloud droplet size distributions. Simulated vertical profiles of droplet size distributions, and a solu-tion to the radiative transfer equation using the discrete ordinate method, with no parameterizations, yield the optical properties of the cloud for short-wavelength radiation. Using the approximation equation proposed in this study, the maximum value of supersaturation in a cloud (Smax) is predicted by observing cumulative activated CCN number concentration at 0.075% supersaturation Nc(075%)), and the ascent velocity at the cloud base. Using this Smax, we can estimate Nc(Smax), which is the cumulative number concentration of CCN, whose critical supersaturations are lower than Smax. Nc(Smax) is used to predict the concentration of cloud droplets at the middle altitude of layer cloud (Nd). Conventionally Nd is assumed to be Nc(Smax). However, our parameterization can show that Nd is smaller than Nc(Smax), when Nc(Smax) is large. For the fixed liquid water path, optical thickness and effective radius can be expressed as a function of Nd, unless drizzle is falling from the cloud. The parameterizations developed in this paper are based on the U.S. Standard Atmosphere 1976. If necessary, the parameterizations for the extremely different atmosphere from that used here can be developed in the same way.
The regime shifts previously identified by the authors over the Northern Hemisphere during the period from the 1910s to the 1990s is investigated in relationship with tropical sea surface temperature (SST) variation. It is found that SST of the Niño 3.4 region, which is known to be a good indicator of El Niño/Southern Oscillation, shows coherent changes with the regime shifts. The regression maps based on Niño 3.4 SST show typical El Niño condition. By subtracting the variations linearly correlated with Niño 3.4 SST from the raw SST field, the residual SST field is obtained. An empirical orthogonal function (EOF) analysis shows a dominant mode of variations in the residual field that is confined to the mid- and high-latitude North Pacific, which is known as the North Pacific mode. Another dominant mode of variation corresponds to the Arctic Oscillation. All the regime shifts are detectable in the residual SST field. Most of the regime shifts (the 1925/26, 1945/46, 1957/58, 1970/71, and 1976/77 shifts) took place concurrently with the two aforementioned EOF modes, and changes in the Niño 3.4 SST, while the 1988/89 shift was not associated with the Niño 3.4 SST changes. This indicates that the regime shifts can be divided into two groups: one is closely linked with the tropical Pacific and the Indian Ocean variations, and the other is independent of these tropical variations.
The Communications Research Laboratory is studying spaceborne cloud-profiling radar with Doppler measurement capability. This study uses pulse-pair operation, based on the polarization-diversity technique, to measure Doppler velocity. The error in the measured Doppler velocity was analyzed using simulation technique as a parameter of pulse-pair intervals at several signal to noise ratios. The optimal pulse-pair interval (TS) is investigated in this paper because it is a key parameter in determining the dynamic range of the Doppler velocity and its accuracy. Moreover, it was found that the terminal fall velocity of cloud drops in still air could be measured. Two TS’s (10 μsec and 60 msec) were used as the optimal values. The dynamic range of Doppler velocity for TS = 60 msec is 13.3 m/sec. If this dynamic range for Doppler velocity is enough to avoid the aliasing of Doppler velocity, TS = 60 msec will be suitable for spaceborne cloud-profiling radar because it effectively reduces the error of mean Doppler velocity (<υ>). If the accuracy of the beam-alignment of the antenna is not within 0.1 degrees as a general requirement, a wider dynamic range of Doppler velocity is required to avoid the aliasing of Doppler velocity. It was found that TS of 10 msec gives a wide dynamic range of Doppler velocity. It is thus expected that use of TS = 10 μsec is an optional way for spaceborne cloud-profiling radar, though it does not give good accuracy. Selecting TS ranging from 10 to 60 μsec is also a possible way of optimizing the expansion of the dynamic range of Doppler velocity and reducing the error of <υ>.