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Observations of temperature and wind velocity in the 30–40-km altitude layer have been sparse since elimination of the standard rocketsonde sounding network in the 1990s. In an effort to extend the vertical range of radiosonde observations into the upper stratosphere, experiments were conducted with a 3-kg balloon at Tsukuba, Japan, on 5 November 2019. Using this relatively inexpensive balloon technology, four radiosondes were launched, with two reaching above 40-km altitude. These profiles were compared with satellite and reanalysis data in the 30–40-km layer, which showed an overall good agreement and an ability of radiosondes to capture shorter vertical-scale variations. The ability to quantify gravity wave parameters from the data is described, with application to wave events detected near 38–40 km. This type of balloon will be deployed extensively in an upcoming intensive observation campaign over the Maritime Continent, which will contribute toward achieving standard radiosonde observations in the 30–40-km altitude range. This system extends the ability to provide information regarding gravity wave and planetary wave activity upward to ∼40 km.

This paper explores the characteristics of raindrop spectra in terms of raindrop size distribution (RSD) using 4 years of Joss–Waldvogel disdrometer data over Palau islands (7 o 20′ N, 134 o 28′ E) in Western Tropical Pacific ocean. The RSD characteristics are studied in two seasons (easterly monsoon—EM and westerly monsoon—WM) using three (stratiform, deep convection, and shallow convection) rain types identified from collocated 1290-MHz wind profiler radar (WPR). In addition to the ground-based sensors observations, TRMM and MODIS satellite-derived rain parameters and atmospheric parameters are utilized to study RSD characteristics. RSD characteristics stratified on the basis of rainrate show that the mean values of raindrop concentrations of small (medium and larger) drops are same (more) in WM compared to EM season. Normalized gamma distribution of RSD shows that the mean value of mass-weighted mean diameter, D m (normalized intercept parameter, log 10 N w ), is higher (lower) in WM than the EM season. In addition, the mean value of D m (log 10 N w ) is higher (lower) in deep convective precipitation as compared to the other two types of precipitation (stratiform and shallow convection) in both monsoon periods. In conjunction with the remote sensing data (MODIS & TRMM), RSD shows that the presences of cold clouds which extend to deeper altitudes are responsible for the higher D m during WM season. The immediate significance of the present work is that (1) it contributes to our understanding of seasonal variations of RSD and distribution of different rain types, and (2) it provides information which is useful for quantitative estimation of rainfall from weather radar observations.

The seasonal cycle of the meridional sea-surface temperature (SST) gradient in the upstream Kuroshio Extension (KE) region was examined using satellite observation data and model simulations. In general, the meridional SST gradient is small in summer. However, in early summer (June and July), the SST front is sustained or intensified on the northern side of the KE near the coast of eastern Japan. This observed seasonal cycle was successfully simulated in the North Pacific Ocean model for the Earth Simulator (NP-OFES). Analysis of the forecast data revealed that the vertical profiles of temperature and salinity are shifted upward along the KE in early summer. As a result, the permanent thermocline depth is shallowest during summer, causing a relatively small SST tendency. In addition, significant cyclonic vorticity in the lower atmosphere related to the southwesterly sea-surface wind was found to the south of the KE, associated with the Baiu frontal zone (BFZ). It was inferred that the positive vorticity causes Ekman upwelling over the KE region, resulting in suppressed SST warming on the northern side of the KE. These results suggest that the BFZ contributes to maintaining or strengthening the SST front.

Tropospheric moisture is a key factor controlling the global climate and its variability. For instance, moistening of the lower troposphere is necessary to trigger the convective phase of a Madden–Julian oscillation (MJO). However, the relative importance of the processes controlling this moistening has yet to be quantified. Among these processes, the importance of the moistening by shallow convection is still debated. The authors use high-frequency observations of humidity and convection from the Research Vessel (R/V) Mirai that was located in the Indian Ocean ITCZ during the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the MJO (CINDY/DYNAMO) campaign. This study is an initial attempt to directly link shallow convection to moisture variations within the lowest 4 km of the atmosphere from the convective scale to the mesoscale. Within a few tens of minutes and near shallow convection occurrences, moisture anomalies of 0.25–0.5 g kg−1 that correspond to tendencies on the order of 10–20 g kg−1 day−1 between 1 and 4 km are observed and are attributed to shallow convective clouds. On the scale of a few hours, shallow convection is associated with anomalies of 0.5–1 g kg−1 that correspond to tendencies on the order of 1–4 g kg−1 day−1 according to two independent datasets: lidar and soundings. This can be interpreted as the resultant mesoscale effect of the population of shallow convective clouds. Large-scale advective tendencies can be stronger than the moistening by shallow convection; however, the latter is a steady moisture supply whose importance can increase with the time scale. This evaluation of the moistening tendency related to shallow convection is ultimately important to develop and constrain numerical models.

In this paper, four years (April, 2003–March, 2007) of wind profiler radar (WPR) observations, Tropical Rainfall Measuring Mission measurements, radiosonde and automatic weather station data are utilized to characterize melting layer/bright band (caused due to the melting of hydrometeors when the precipitation has a significant stratiform cloud contribution) variability during easterly and westerly monsoon periods over Palau in Western Pacific Ocean. Four years observational results show that presence of stratiform precipitation is more in Westerly monsoon compared to Easterly monsoon. However, bright band height (BBH) is lower in Westerly monsoon. It is also observed that the BBH during the year 2006 is higher than 2005 possibly due to El Nino effect.

This study examined the synoptic evolution and internal structure of a monsoon trough in association with the deep equatorward intrusion of a midlatitude upper trough in the western North Pacific Ocean in June 2008. The study was based on data from routine synoptic observations and intensive observations conducted on board the research vessel Mirai at 12°N, 135°E. The monsoon trough was first observed to extend southeastward from the center of a tropical depression. It then moved northward, with its eastern edge moving faster and approaching a surface low pressure cell induced by the upper trough. The distinct northward migration caused the monsoon trough to become oriented from the southwest to the northeast. The monsoon trough merged with the surface low pressure cell and extended broadly northeastward. The passage of the monsoon trough over the Mirai was accompanied by lower pressure, higher air and sea surface temperature, and minimal rainfall. The monsoon trough extended upward to nearly 500 hPa and sloped southward with height. It was overlain by northwesterly winds, negative geopotential height and temperature anomalies, and extremely dry air in the upper troposphere. Precipitation systems were weak and scattered near the monsoon trough but were intense and extensive south of the surface monsoon trough, where intense low-level convergence and upper-level divergence caused deep and vigorous upward motion. It appears that the upper trough exerted important impacts on the development of both the monsoon trough and associated precipitation, which are discussed according to the observational results.

[1] The cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) was conducted to capture atmospheric and oceanic characteristics of the Madden-Julian Oscillation (MJO) in the central Indian Ocean from late 2011 to early 2012. During CINDY2011, the research vessel (R/V) MIRAI stayed at 8°S, 80.5°E for two months during the special observing period (SOP). Intraseasonal convection associated with the MJO was organized in the central Indian Ocean in late October and late November during the SOP. In the middle of November, both sea surface temperature (SST) and mixed layer temperature decreased suddenly when cold low salinity water intruded into the upper layer around the R/V MIRAI. This intrusion was accompanied by a surface current change from southwestward to westward/west-northwestward associated with the passage of the annual oceanic downwelling Rossby wave. The mixed layer heat budget analysis shows that horizontal advection plays an important role in the abrupt cooling whereas the net surface heat flux cannot account for the cooling. This is an interesting result because the associated downwelling Rossby wave is usually considered to increase SST through a reduction of entrainment cooling. In addition, for the second MJO event convection was activated around 20 November over the central north and equatorial Indian Ocean but not in the south. It is suggested that the cooler surface waters (as seen at the location of the R/V MIRAI) tended to suppress the initial atmospheric convection, resulting in the lagged convective onset in the end of November over the central south Indian Ocean.

[1] This study examined the thermodynamic structure of the atmospheric mixed layer and its evolution in different periods during the onset of the western North Pacific summer monsoon by using data observed on board the research vessel Mirai at 12°N and 135°E during 6–24 June 2008. The overall mean depth of the mixed layer was 482 m, and the mixed layer tended to shrink after the monsoon onset. The mixed layer was relatively warm and moist during the early onset period. However, the mixed layer became very cool and dry during the latter onset period, when intense and sustained rainfall was observed. The surface buoyancy flux, environmental subsidence over the mixed layer, and upward motion at cloud base intensified after the monsoon onset. All of these factors were negatively correlated with the depth of the mixed layer, which implies that the upward motion at cloud base and compensating environmental subsidence act to suppress the growth of the mixed layer driven by buoyancy forcing and to shrink the mixed layer after the monsoon onset. A significant positive correlation existed between the surface buoyancy flux and the upward motion at cloud base after the monsoon onset. Such positive feedback between the sea surface and cumulus clouds facilitates the formation of the warmer and moister mixed layer during the early onset period, thereby benefiting the further development of the monsoon onset.

This report describes the in situ observed evolution of the atmospheric profile during an event of the boreal summer intraseasonal variation (BSISV) in the tropical western Pacific Ocean. The convectively active region of the BSISV proceeded northward over the sounding and radar network. Over the array, the situation changed from a convectively inactive period to an active period. Inspection of the sounding data revealed the gradual moistening of the lower troposphere during the convectively inactive period. The sounding-derived heat and moisture budget analyses indicated that both the convective- and large-scale processes caused moistening of the lower and middle troposphere where the radar echo tops were observed most frequently. This study is the first to identify such a “preconditioning” process for the BSISV in the western Pacific using detailed in situ observational data. During the preconditioning, an increase in CAPE was observed, as in previous studies of the MJO. An increase of moisture in the boundary layer was responsible for the increase of CAPE. The large-scale horizontal convergence in the boundary layer may be a key factor to moisten the boundary layer through the convective-scale processes, as well as through the large-scale processes to moisten the lower and middle troposphere.

Raindrop size distribution (RSD) characteristics in summer season rainfall of two observational sites [Taiwan (24o 58’ N, 121o 10’ E) and Palau (7o 20’ N, 134o 28’ E)] in western Pacific are studied by using five years of impact type disdrometer data. In addition to disdrometer data, TRMM, MODIS, and ERA-Interim data sets are used to illustrate the dynamical and microphysical characteristics associated with summer season rainfall of Taiwan and Palau. Taiwan and Palau's raindrop spectra showed a significant difference, with a higher concentration of mid and large drops in Taiwan than Palau rainfall. RSD stratified on the basis of rain rate showed a higher mass-weighted mean diameter (Dm) and a lower normalized intercept parameter (log10Nw) in Taiwan than Palau rainfall. Precipitation classification into stratiform and convective regimes showed higher Dm values in Taiwan than Palau. Furthermore, for both the locations, the convective precipitation has a higher Dm value than stratiform precipitation. The radar reflectivity - rain rate relations (Z= A*Rb) of Taiwan and Palau showed a clear variation in the coefficient and a less variation in exponent values. Terrain-influenced clouds extended to higher altitudes over Taiwan resulted with higher Dm and lower log10Nw values as compared to Palau.

This paper describes the propagation of the influence of radiosonde observations made during MISMO (Mirai Indian Ocean cruise for the Study of the Madden–Julian oscillation-convection Onset), which was conducted over the Indian Ocean in the boreal autumn of 2006. The impact of these observations was assessed using an experimental reanalysis called ‘ALERA’ that was produced by the local ensemble transform Kalman filter with the atmospheric global circulation model for the Earth Simulator. The difference of the analysis ensemble mean between the analysis cycles with and without the MISMO observations was used to quantify the influence of these observations on the analysis field, which we call impact signals. Since the impact signals were contaminated by noise, probably due to the model's truncated spectral basis, a significance test was performed using the analysis ensemble spread, and the numerical noise was then successfully eliminated. The results indicated that convectively coupled equatorial waves propagated the impact signals to the central Pacific to the east and to the African continent to the west. In particular, the analysis fields around typhoons Cimaron, Chebi, and Durian over the tropical Western Pacific were significantly modified by the propagation of the impact signals through the Kelvin waves. Here two factors played important roles: (1) the meridional positions of the typhoons; and (2) the duration of the observations. The equatorial Rossby waves also made a significant contribution to the propagation of the impact signals. Such influences through the waves resulted in the reduction of the analysis spread. The shape of the region of reduced spread resembled the Matsuno–Gill pattern with an east-west width of more than 20000 km. Copyright © 2011 Royal Meteorological Society

The future changes in the Madden-Julian Oscillation (MJO) and its extratropical teleconnection in East Asia during the boreal winter are examined by analyzing 12 climate models with good simulation skills of the MJO convective signal in the Coupled Model Intercomparison Project phase 3 (CMIP3). The MJO convection increases over the western to central Indian Ocean (IO) under the warming climate in seven models (MJO-plus models). However, it decreases in the other five models (MJO-minus models). The wintertime sea surface temperature (SST) exhibits more El Nino-like warming in the MJO-plus models. Furthermore, the predicted increase of the MJO amplitude over the IO significantly correlates with that of the amplitude in the El Nino-Southern Oscillation. In East Asia, the convective activity increases (decreases) in the MJO-plus (MJO-minus) models associated with the enhancement (weakening) of the MJO over the western to central IO. The MJO-plus models predict the intensified extratropical circulations as a wave train along the subtropical jet, and increased low-level warm-moist air transport into East Asia related to the intensified Kelvin-Rossby response. Such changes are not detected in the MJO-minus models. It is suggested that proper simulations of the change in the MJO and ENSO are required in order to predict the wintertime climate change in East Asia.

This paper has investigated the structure, propagation, orientation, and rainfall of precipitation systems and environmental wind and thermodynamic structures observed by the Doppler radar and the radiosonde system on board the R/V Mirai around a fixed site (12°N, 135°E) during the onset of the western North Pacific summer monsoon in 2008. The monsoon onset occurred in mid-June and was accompanied by a remarkable change in large-scale circulation over the observational area.Statistical analyses indicate that precipitation systems evolved deeply and widely upon the monsoon onset. The development of mesoscale convective systems contributed to the formation of much more rainfall. Precipitation systems tended to move along with low-level winds during the pre-onset and the early-onset periods and with mid-level winds during the latter-onset period. The increase in the height and the area of precipitation systems possessed much stronger correlation with the evolution of the low-level shear and the mid-level humidity than with other environmental variables.The internal structure of two precipitation systems that occurred in the environment with the distinct low-level shear and mid-level humidity has also been analyzed. The shear-parallel precipitation system during the early-onset period occurred in the environment with the dry middle troposphere and the weak low-level shear and was characterized by the descending rear inflow and weaker updrafts. On the other hand, the shear-normal precipitation system during the latter-onset period occurred in the environment with the moist middle troposphere and the strong low-level shear and was featured with the elevated rear inflow and stronger updrafts.According to the observational results, possible interaction processes between precipitation systems and their environment during the monsoon onset are discussed.

The variability of the oceanic surface and subsurface layers associated with the northward-propagating intraseasonal variability (ISV) in northern summer were examined in the western North Pacific (WNP) warm pool, using satellite and in-situ observation data. Analyses of satellite observation data revealed that the SST decreases when a convection maximum of the ISV passes. The sea-surface temperature (SST) difference (maximum minus minimum) was about 1.5 to 2.0 °C. In the southern part of the warm pool, the SST abruptly decreased just before the convection maximum. Examination of Argo float data revealed that the temperature profile shifted upward before the convection peak. During this period, the mixed-layer depth (MLD) did not largely change despite strong sea-surface wind and diabatic cooling. According to the sea-surface wind data, a large cyclonic wind-stress curl was identified just north of the convection center associated with a small Coriolis parameter. The upward Ekman pumping appears to contribute to the maintenance of a shallow mixed layer and the abrupt cooling of SST. After the rapid cooling, the SST was almost constant although the mixed layer was deepened. However, in the northern part where the Ekman pumping was relatively small, the SST decreased in accordance with the increase of MLD throughout the cooling period. It was suggested that the air-sea coupling in the off-equatorial region differs from that on the equator.

The multiscale structure of a super cloud cluster (SCC) over the equatorial Indian Ocean, observed in November and December 2006, was investigated using data from satellite microwave sensors and surface-based radars. The smaller-scale structure of this SCC was marked by a complicated relationship between rainfall systems and upper-tropospheric cloud shields, which moved eastward and westward, respectively, with a cycle of 2–4 days. In the analyses, attention was given to the structure of slow eastward-propagating (5–11 m s−1) precipitating systems and related synoptic-scale (∼2000 km) disturbances. A case study of one of the systems revealed that it consisted of several lines of convective cells with a depth that was usually shallower than 10 km unless the cells encountered the westward-moving cloud shields. The environment of the convective lines was characterized by persistent unstable conditions with an increase of the westerly flow in the lower troposphere, suggesting the existence of a synoptic-scale upward motion. Composite analyses revealed that each rainfall system formed in a region of zonal flow convergence near the surface and divergence near 300 hPa. The vertical temperature structure tilted westward with height below this pressure level and eastward aloft, similar to that of a convectively coupled Kelvin wave. These results suggest that a SCC involves a group of synoptic-scale shallow waves propagating eastward. An additional analysis over the western Pacific also showed the predominance of eastward propagation in a SCC, demonstrating the advantage of satellite microwave sensors over infrared ones in monitoring the multiscale structure of tropical convection.

The tropical-extratropical interaction associated with the Madden-Julian Oscillation (MJO) and El Nino-Southern Oscillation (ENSO) in boreal winter is examined. When MJO convection is activated over the Indian Ocean, an anomalous extratropical high appears over the north Pacific. In this study, a zonal shift of the high is found to depend on ENSO phases, and the high shifts westward in the El Nino developing (EV) phases. Northeasterly trade surges that originate from the high intrude the tropical western north Pacific only during EV phases and moisturize the area through convergence. Although convective activities are centered south of the equator in boreal winter, the moisturization by the surges results in the activation of MJO convection not only south of the equator but also north of the equator, facilitating the formation of the twin cyclonic disturbances straddling the equator. During the other ENSO phases, on the other hand, extratropical fluctuations and MJO activity do not interact each other. It has been shown in previous studies that these twin cyclonic disturbances produce westerly wind bursts, which can trigger El Nino. These results suggest that tropical-extratropical interaction, between the MJO and the north Pacific high accompanied by the trade surges, occurs preferentially during EV phases, and feeds back to the development of El Nino.

Cloud-top height (CTH) variability in the tropical tropopause layer (TTL) in association with equatorial Kelvin waves is investigated using a new CTH dataset based on MTSAT-1R geostationary satellite measurements with a statistical look-up table constructed based on CloudSat measurements. We focus on a case in the tropical Indian Ocean during October-December 2006, when shipboard radiosonde, TTL water vapor, and 95-GHz cloud radar measurements were taken during the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO) field campaign. At 10-15 km, the satellite-based CTH data agree well with the radar echo top heights from shipboard radar reflectivity data. During the MISMO campaign, cloud frequency was suppressed in the warm phase of equatorial Kelvin waves propagating in the TTL. The suppressed-cloud region moves eastward to the western Pacific together with Kelvin waves. We found that changes in CTH occurrence frequency over the vessel in association with Kelvin waves are much greater than those associated with the diurnal cycle. It is expected that the phase of equatorial Kelvin waves is important for the intraseasonal variabilities of both the radiative budget of the tropical atmosphere and water vapor transport in the TTL.

The simulated Madden-Julian oscillation (MJO) in the climate of the 20th Century (20C3M) experiment of 23 models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) is examined. The models having moisture-convergence-type convection schemes well simulate the MJO signal in precipitation. By analyzing the data from these models, we confirm that the MJO convection is active from the Indian Ocean to the western Pacific, consistent with the observations. However, analyses of the structure of the simulated MJO reveal that the convergence of the surface wind tends to be overestimated in the models. In addition, the peak of the MJO signal over the Indian Ocean is relatively weaker than that over the maritime continent and the western Pacific, and shifts westward in the models. These inconsistencies appear to arise from the distribution of sea-surface temperature (SST) bias. As a general tendency in all of the climate models studied, it is also demonstrated that the skill of the MJO simulation correlates positively with that of the climatological SST. Potential importance of the basic field and the air-sea coupling in the MJO simulation is suggested.

The Mirai Indian Ocean cruise for the Study of the Madden-Julian oscillation (MJO)-convection Onset (MISMO) was a field experiment that took place in the central equatorial Indian Ocean during October–December 2006, using the research vessel Mirai, a moored buoy array, and landbased sites at the Maldive Islands. The aim of MISMO was to capture atmospheric and oceanic features in the equatorial Indian Ocean when convection in the MJO was initiated. This article describes details of the experiment as well as some selected early results. Intensive observations using Doppler radar, radiosonde, surface meteorological measurements, and other instruments were conducted at 0°, 80.5°E, after deploying an array of surface and subsurface moorings around this site. The Mirai stayed within this buoy array area from 24 October through 25 November. After a period of stationary observations, underway meteorological measurements were continued from the Maldives to the eastern Indian Ocean in early December. All observatio...

The Hydrometeorological ARray for Isv-Monsoon AUtomonitoring (HARIMAU), a 5-year project under the Japan EOS Promotion Program (JEPP) contributing to the Global Earth Observation System of Systems (GEOSS), has begun in 2005 to set up a radar-profiler network for observing the world's most active convective activities over the Indonesian Maritime Continent (IMC). Rainfall and wind distributions are displayed in nearly real time on the internet. Both scientific understanding and practical concepts on intraseasonal variations (ISVs) interacting with larger (seasonal and interannual) and smaller (diurnal or local) scale phenomena will be established. These are expected to contribute greatly and directly to climatic disaster prevention over the IMC and to global climate change assessment through studies on the global effects of the IMC-induced variations such as El Nino, and through construction of the first climatic database over the IMC.

The effect of the assimilation of dropsonde data over the tropical western Pacific was investigated in an objective analysis. In June 2005, 30 dropsondes were released on four separate flight days. The data impact was assessed using the objective analysis dataset of “ALERA.” The analysis of the zonal wind field over the tropical western Pacific in ALERA revealed large errors corresponding to active convection. These errors were reduced by 1-3 m s-1 due to the assimilation of the dropsondes. The impact signal due to the assimilation of the dropsondes propagated northward and appeared significantly around Japan. The phase and group speeds of the impact signal at 700 hPa were approximately 3 m s-1 and 12 m s-1, respectively. The former speed was consistent with the mean meridional wind speed, and the latter speed roughly corresponded to the meridional group speed of Rossby waves with wavelengths of a few thousand kilometers.

The structures of deep convections and their environment were examined during the latter half of the active phase of the Madden-Julian oscillation (MJO), using fine temporal resolution of research vessel observation data. During the intensive observation period, the MJO associated with deep convections passed through the observation area. A strong westerly wind also appeared during the active phase of the MJO. Vertical structures associated with convective activity were detected using lag correlations of atmospheric parameters. Convective activity included deep convections, with a nocturnal maximum and another peak 12 to 18 hours earlier. Active convections associated with the first peak developed in the afternoon and decayed within a few hours. Those active convections transported water vapor to the lower to middle troposphere, making the environment favorable for the more intensive deep convections that developed during the night. Westerly winds were intensified in the lower layers during the deep convections.

In this study, the authors focused on the seasonal variations of precipitation properties over the western Pacific, particularly those associated with the wind direction of the monsoon. An observational project over Peleliu Island in the Republic of Palau was carried out, and data on precipitation, equivalent cloud amount, and precipitable water were collected from 28 June 2001 to 30 April 2002. First, the monsoon season over Palau was defined as a period with 850-hPa zonal-wind sounding data with sustained winds exceeding 5 m s−1. The westerly wind regime continued until 25 November 2001, and the next westerly wind regime began on 18 May 2002. The equivalent cloud amount increased during the period when the westerly wind intensified. The precipitation had a diurnal variation in the active phase of the westerly wind regime, increasing from nighttime to early morning and decreasing in the afternoon. The diurnal variation was weak in the inactive phase and had a lesser afternoon maximum. Precipitation intensity was high and its duration was short during the westerly wind regime. The precipitable water decreased during the easterly wind regime when a dry period appeared, and precipitation was also suppressed during those days. However, there was little difference between the precipitation amounts of the westerly and easterly wind regimes. The equivalent cloud amount did not decrease as the zonal-wind direction changed to easterlies during the easterly wind regime. The authors noticed no diurnal variation of precipitation during the easterly wind regime. These differences in the precipitation properties during westerlies and easterlies may be related to the seasonal variation of humidity in the environment.

During the GAME-Tibet Intensive Observation Period (IOP), the precipitation radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) satellite detected a diurnal cycle of rainfall in Tibet. Much rainfall was brought both by convection in the daytime and by stratiform precipitation over a wide rain area in the evening and night. Two case studies were conducted to clarify the structure of stratiform rainfall in the nighttime. In Case 1 (7 and 8 July 1998), stratiform precipitation was observed in the evening and night. On 7 July, a synoptic convergence area developed in the southern part of the Plateau in the evening. Southwesterly wind dominated above the 6 km ASL, but the wind direction below 6 km ASL was variable during the rainfall. When the mesoscale convergence strengthened, the rainfall amount became large. On 8 July, a convergence zone between southwesterly wind and northerly wind was detected and passed over the radar site in the evening. A sudden wind direction change from westerly to northwesterly was observed below 8 km ASL from the vertical profiles at the radar site. The boundary between the two wind directions may correspond to the cold frontal surface. However, the precipitation with the frontal passage was stratiform and had a low echo top. The variation of rainfall on the two days was consistent with the diurnal variation revealed by TRMM PR. In Case 2 (1 and 2 August 1998), there was no remarkable disturbance in the synoptic field, but much rainfall (exceeding 4 mm/h) was observed in the midnight. A 90 km-diameter stratiform echo stayed over the radar site for seven hours. Below 6 km ASL, the wind direction varied from northerly to northeasterly, then to easterly. When northeasterly wind dominated, the convergence in the lower layer strengthened and stratiform precipitation was intensified. Mesoscale convergence of moist air in the lower layer effectively contributes to development and maintenance of stratiform rainfall.

In order to reveal the characteristics of convective clouds over the Tibetan Plateau, we carried out Doppler radar observations of the clouds in the suburbs of Naqu city (4500 m ASL), which is located in the central part of the Tibetan Plateau, from 27 May to 19 September 1998, during the intensive observation period of the GEWEX Asian Monsoon Experiment in Tibet. Diurnal variation of the radar echo are and echo-top height were remarkable throughout the intensive observation period indicating the formation and development of convective clouds in the daytime and their decaying at night. The echo-top height and convective area, extracted objectively from grid data of radar reflectivity, were larger after the monsoon onset than before the onset. The maximum echo-top height of convective clouds (> 10 dBZ) was about 17 km ASL. Furthermore, many vortices accompanied by the clouds, identified on Doppler velocity fields, were observed in the daytime. The vortices generated in nearly all regions of the radar observation area (r = 64 km). The strongest vorticity of a given day with vortices during the observation period showed a positive correlation with the appearance of the maximum echo-top height (> 30 dBZ) of the convective clouds. The daily maximum vorticity after the monsoon onset was larger than that of the pre-monsoon period. A comparison of features of radar reflectivity and vorticity of convective clouds with sounding data indicates the presence of the following processes: formation of a strong updraft by solar heating, descending of westerly wind in the upper troposphere to the lower altitude, increase in strength of the vertical shear in low altitudes, tilting of horizontal vortex tube, and intensification of vortex in the clouds. Convective clouds developed above the melting layer at Naqu on the Tibetan Plateau were characterized by their rapid development from solar heating, by the formation of graupel particles and by the association of a strong vortex in the clouds.

The exchange speed (CHU) for sensible heat flux is one of the most important parameters for estimating sensible and latent heat fluxes by the bulk transfer method. CHU is also important for estimating soil temperature by using the heat balance model combined with the bulk transfer method. CHU is usually treated as a function of wind speed; however, if wind speed data is not available, it is not easy to estimate an accurate and representative value using measured wind speed from another location due to the effect of the surrounding environment. In this study, we investigated the accuracy of estimating the sensible heat flux over a bare soil surface and its daily mean soil temperature by the bulk transfer method using daily mean meteorological data without wind speed. This means that CHU is treated as a constant value. We also investigated a method of parameterization of CHU on a bare soil without observed flux data. The estimation of the daily mean sensible heat flux without wind speed data has a large root mean square error (RMSE). RMSE without wind speed was 18.3 W m-2, whereas that with wind speed was 10.5 W m-2. However, the accuracy of the estimation of cumulative monthly sensible heat flux without wind speed data was almost equal to that with wind speed data. The two values of RMSE of monthly estimations with and without wind speed data were about 5 W m-2. The estimated cumulative sensible heat flux without using wind speed data, over a period of more than several days, agreed with observed sensible heat flux values. With theoretical analysis and the parameterization of CHU on a bare soil without using observed flux data, the results indicate that cumulative sensible heat flux can be estimated from routine meteorological data without wind speed. The daily mean soil temperature was estimated by the method of Hirota et al. (1995) with and without wind speed data. The accuracy of the estimation of the daily mean soil temperature without wind speed data was almost equal to that with wind speed data. The result indicates that even if wind speed data is not used, the model provides a good estimation of the daily mean soil temperature.

Return periods of continuous snow cover duration at the 24 observation points in Hokkaido were investigated using the Weibull distribution method. The mean value of continuous snow cover duration was in the region of 120 days to 170 days. The differences between the mean value and the value of return periods of continuous snow cover duration at 3, 5, 10 and 30 year intervals were about 7, 14, 21 and 30 days, respectively. The differences between the mean value and the first date of the return periods of continuous snow cover duration at 3, 5, 10 and 30 year intervals were about 6, 10, 15 and 23 days. In the same, the last date of continuous snow cover duration at 3, 5, 10 and 30 year intervals were about 4, 8, 10 and 14 days, respectively. The variation of the return periods of last date was small compared with the first date.

We observed global solar radiation changes through two months during the IOP of TOGA-COARE from 19 November 1992 to 19 January 1993 on Manus Island, Papua New Guinea, in the western tropical Pacific. The whole observation period was divided into four stages through consideration of the daily variations of solar radiation. Significant features of each period were also presented in terms of the activity of cloud convection derived from TBB data of GMS and zonal wind changes observed by a wind profiler. Surface winds were changeable throughout the observation period; however, an episodic strong westerly wind was observed from the end of December to early January. The most active stage of cloud convection, which was characterized by a decrease of solar radiation in the afternoon, was observed before the westerly wind episode. Morning convection was, however, prominent during the period of the strong westerly wind. A diurnal change of solar radiation was also compared with the change of latent and sensible heat fluxes obtained using the eddy correlation method. Energy fluxes on a ground surface indicated a quick response within about 30 minutes to solar radiation change. Latent and sensible heat fluxes were almost zero at nighttime, but the maximum value of latent and sensible heat fluxes in the daytime were about 270 and 200 Wm -2 , respectively.

A simple model to estimate the daily mean soil temperature using routine meteorological data has been developed. The model is based on a heat budget equation connected with the Force-Restore Model, in which interdiurnal variation of the soil temperature at the surface layer is estimated. The model has some advantages. First, it is not necessary to set up soil thermometric parameters and initial values at the deep soil layer. Second, sensitivities of the soil thermometric parameters are low compared to the case of estimating diurnal variation of soil temperatures. Finally, the boundary condition is easy to determine because annual mean soil temperatures at different depths are nearly equal.The model was applied for a bare soil field and verified using the observed data by the heat balance method. Estimated values of daily mean soil temperature agreed well with the observed value, even if the soil thermometric parameters were set as approximated values. To adapt the model for any case, a way to estimate the initial value of soil temperature was examined.Although the soil moisture availability was required to estimate the evaporation in the model, the average value of long-time period gave a good result for estimation of soil temperature. In case the value of the actual soil moisture is unavailable, the model gave a good estimation of cumulative evaporation using routine meteorological data and soil temperature at the surface layer. The model could be used to evaluate the soil moisture availability.

Observations of tropical convection and cloud clusters with two X-band Doppler radars were carried out on Manus Island, Papua New Guinea for two and a half months from 12 November 1992 during the Intensive Observation Period (IOP) of the TOGA-COARE. Several different types of clouds were observed by Doppler radars during the period, such as squall lines, convective clouds and stratiform clouds associated with cloud clusters, isolated convective clouds over the island in the day time, etc. Outlines of the observations on Manus Island and brief reviews of the results of these observations are described. Preliminary summaries of the analyses on the observations are: 1) A warm rain process dominates at the beginning stage of radar echo development and the maximum radar reflectivity is recognized at this stage, 2) The maximum echo top height is observed within 3 hours from the first echo, 3) The maximum echo area within the radar range was analyzed a few to several hours later than the time of maximum echo top height corresponding to the size of cloud cluster, 4) In the long-lasting stratiform echo, updrafts independent from those below the melting layer were identified above the melting layer. It is concluded that further studies utilizing these observational data may reveal the structure and the evolution mechanism of tropical cloud clusters.

[1] Equatorial westerly wind bursts (WWBs) and their relationship with El Nino–Southern Oscillation (ENSO) in the 18 climate models presented in the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3) are examined. Some models depict a realistic eastward shift of collective occurrences of WWBs over the Pacific as the warm pool expands eastward. These models that depict the frequent western Pacific WWBs preceding El Nino peak, known to trigger or enhance El Nino, tend to reproduce westerly background states and ENSO more accurately. Thus the reproducibility of the westerly background states is suggested to be fundamental for WWB occurrences as well as the following El Nino. Although WWBs generate with active convection in most of the models as observations, various kinds of intraseasonal disturbances that cause the active convection are found. It is suggested that organized convection is essential for the WWB generation but is prepared by each model's own dominant mode in the tropics. Under global warming, WWBs tend to increase over the eastern Pacific and decrease over the Indian Ocean whereas the total number of WWBs does not change consistently. This might arise from an increase of short-period convective disturbances over the eastern Pacific due to a sea surface temperature increase. Although there is a weak relationship between changes in the ENSO amplitude and the eastern Pacific WWBs in general, good models reproducing the WWB-ENSO relationship in the current climate tend to show consistent changes, suggesting the possibility of the eastern Pacific WWBs to intensify ENSO.

The present study investigates modulation by the Madden-Julian Oscillation (MJO) of storm track activity (STA) over the North Pacific (NP) during boreal winter for El Nino and La Nina periods. STA defined by vertically averaged synoptic eddy kinetic energy (EKE) greatly intensifies over the western North Pacific (WNP) and central eastern North Pacific during La Nina and El Nino years, respectively, when the MJO convection is located over the central Indian Ocean (IO)-Maritime Continent. When the MJO moves into the western central Pacific, the STA in La Nina years is suppressed (enhanced) at higher (lower) latitudes than in El Nino years. Diagnoses of EKE and eddy available potential energy budgets indicate that the difference in STA over the WNP for the MJO phases between El Nino–Southern Oscillation (ENSO) years is mainly contributed by baroclinic energy conversion and potential energy conversion between background and eddy (BCPE). We reveal that BCPE is mainly attributed to intraseasonal baroclinicity and eddy heat flux (EHF) anomalies and their interactions with strong winter mean baroclinic fields in the WNP. Through the EHF, synoptic eddies act to counterbalance an intraseasonal temperature that is primarily caused by the anomalous horizontal advection of mean temperature by MJO-related flow. The intraseasonal circulation and associated temperature and EHF anomalies dominate in northwest (southeast) portion of the NP during La Nina (El Nino), leading to BCPE difference. Changes in the spatial pattern and strength of the NP circulation and STA are caused by dominance of strong MJO amplification over the IO (central Pacific) during La Nina (El Nino).

[1] We used a three-year dataset from an impact disdrometer to study the diurnal to interannual variation of the raindrop size distribution (DSD) over Palau in the Western Pacific warm pool. The DSD variability was primarily seasonal, with a secondary contribution due to the El Nino Southern Oscillation (ENSO) cycle. We did not find evidence of a diurnal cycle. In the seasonal cycle, mean drop diameters tended to increase during the westerly monsoon period. Within this period, drop diameters generally increased during the El Nino year and decreased during the La Nina year. However, within the easterly monsoon period, they generally increased in the winter following La Nina, and decreased in the winter following El Nino. The amplitude of the seasonal variation in mean drop diameter was approximately the same as that in Singapore and Sumatra, and about one third of that in India.

[1] This paper describes the structure and evolution of northward propagating mesoscale convective systems (MCSs) observed over the tropical western Pacific on 15 June 2005. A wedge-shaped convective cloud area, consisting of three groups of MCSs, was generated near the equator to the north of New Guinea at around 0000 LST on 15 June, and one of the groups of MCSs propagated toward Palau (7°–8°N). Dropsonde analysis revealed that the MCSs developed at the northern edge of a cold air mass that had a horizontal scale of more than 400 km. Global objective analysis data from the Japan Meteorological Agency showed that the cold air mass observed by the dropsonde analysis originated in a land breeze from New Guinea. The cold air mass was initially formed by cold advection from the south and was maintained by cooling with the MCSs. When the initial MCSs were generated along the land-breeze front, an easterly wave, which was accompanied by low-level southerly wind and a low-pressure area, was located near Palau. The cold air mass associated with the land breeze between 132° and 137°E was locally extended toward the center of the low-pressure area. The results suggest that the easterly wave located around Palau helped locally promote northward cold advection from New Guinea, inducing the long-distance northward propagation of MCSs generated along the land-breeze front.

[1] We examined the sea surface temperature (SST) distribution in the western North Pacific warm pool when a typhoon is generated in northern summer. Analyzing satellite observation data revealed higher (lower) SST north (south) of the typhoon. As a result, the meridional SST gradient became especially steep, and the SST was not homogeneously high, when a typhoon is generated. In-situ observation data in the oceanic surface layer suggested that the low SST was caused by vertical mixing related to the strong southwesterly monsoon. In contrast, the hot spots north of the typhoon were associated with weak sea surface wind and vertical mixing. According to satellite observation, the maxima of surface heat flux and water vapor shifted northward corresponding to the high SST. They may produce an asymmetry of the thermodynamic structure of a typhoon.

[1] We examined the convective activity over the tropical Indian Ocean in boreal autumn using satellite-observation data. In November and December, we detected an intertropical convergence zone-like (ITCZ-like) precipitation zone between Maldives and Somalia that is unobserved in the other periods of the year. It accompanied a high sea-surface temperature (SST) north of the equator, and a cold tongue near the equator, similar to the ITCZs over the Pacific and Atlantic Oceans. However, the formation mechanism of the SST anomalies over the Indian Ocean is not the same as that in the Pacific and Atlantic Oceans. The high-SST region forms in early November corresponding to weak sea-surface wind during the monsoon transition. The low SST along the equator appears, associated with the upwelling of the subsurface water.

[1] Satellite imagery and C-band radar on the R/V Mirai detected an isolated convective cloud band over the western tropical Pacific, and dropsondes observed the surrounding atmospheric environment. The reasonable development mechanism of this unusual cloud band is investigated using observational and numerical simulation data. Although theoretically, the surrounding atmospheric profile supported convective rolls, only a single cloud band was developed. Numerical simulations revealed a single cloud band and roll structure without condensate in the boundary layer. A westerly jet just above the cloud band influenced the environment with strong vertical shear supporting convective rolls; this was maintained by a typhoon at a distance of 1500 km away. A large-scale convergence in the meridional wind was also accompanied with this westerly jet. This special environment made by a typhoon could develop the unusual single cloud band.

[1] Cirrus cloud variability associated with n = 0 eastward inertio-gravity equatorial waves and equatorial Kelvin waves (both with the period of ~4 days) and equatorial Kelvin wave with another periodicity (~16 days) were observed in the tropical Indian Ocean (8.0°S, 80.5°E) in November of 2011 during the Cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011)/Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign using balloon-borne cryogenic frostpoint hygrometers, Vaisala radiosondes, and a shipborne high spectral resolution lidar system. During early to mid-November, the cirrus cloud appearance corresponded primarily with high supersaturation and high relative humidity caused by the temperature disturbances associated with the ~4 day waves between 12 km altitude and the cold-point tropopause. The cirrus clouds disappeared under the unfavorable (downward wind and dry) conditions that were caused by the ~4 day waves, although the ~16 day wave was generating favorable conditions. Our multi-instrument cirrus measurements revealed that we must consider the phases of various overlapping waves when estimating dehydration efficiency caused by cirrus clouds around the cold-point tropopause.

[1] We performed synergy observation of subvisual cirrus clouds (SVC) with 1064-nm lidar, 95-GHz cloud radar and radiosondes launched every 3 hours at 2°N, 138°E for one month from 9 November to 9 December 2001. The estimated effective radius reff of SVC is 9–16 μm (5–9 μm in mode radius). This assumes that the shape of cloud particles is spherical and the number concentration of the particles is 105 m−3. SVC disappear when temperature anomalies exceed +1.5°C. The fall velocity of SVC is same as the downward velocity of the phase propagation of the Kelvin wave, and the terminal velocity of hexagonal particles. Thus sedimentation and phase propagation are important for dissipation of SVC. In addition, it was noted that SVC tended to disappear at midday during the observational period except on 27 November 2001.