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A significant enhancement of precipitation can result from the interplay between two independent, large-scale phenomena: an atmospheric river (AR) and a cutoff low. An AR is a long, narrow region with a deep moist layer. A cutoff low is an upper-level cyclonic eddy isolated from the meandering upper-level westerly jet. Herein, we construct composites of cutoff lows both close to an AR (AR-close category) and distant from an AR (AR-distant category) over a 14-yr period across the western North Pacific region. A comparison between the two categories shows an enhanced precipitation area to the northwest of the cutoff low and to the south of the AR axis in the AR-close category. The horizontal formation among the AR, cutoff low, and enhanced precipitation area in the composite coincides with that in a disastrous flood event that occurred in Hiroshima, Japan, in 2014. The deep moist layer associated with the AR, and the destabilization and isentropic up-gliding effect associated with the cutoff low are also observed in both the composite and the Hiroshima cases. We further evaluate the distribution of quasigeostrophic forcing (Q vector) for vertical motion. This shows that warm air advection associated with the AR overcomes the descending forcing inherent in the northwest of the cutoff low and makes the instability and up-gliding effect in that region more effective. These results indicate that the interplay between ARs and cutoff lows is a common mechanism in the enhancement of precipitation and the Hiroshima case is an extreme precipitation event caused by this interplay.

The lack of westerly wind bursts (WWBs) when atmospheric intraseasonal variability (ISV) events occur from boreal spring to autumn is investigated by comparing two types of El Niño years with unmaterialized El Niño (UEN) years. Although high ocean heat content buildup and several ISV events propagating eastward are observed in all three types of years, few WWBs accompany these in the UEN years. The eddy kinetic energy budget analysis based on ISV shows that mean westerly winds in the lower troposphere facilitate the development of eddy disturbances, including WWBs, through convergence and meridional shear of zonal winds. In the UEN years, these westerly winds are retracted westward and do not reach the equatorial central Pacific mainly as a result of interannual components. In addition, positive sea surface temperature anomalies in the western Pacific, which are conducive to active convection, spread widely in a meridional direction centered on 15°N. Both westward-retracted mean westerlies and off-equatorial warming enhance off-equatorial eddies, which result in a reduction in equatorial eddies such as WWBs. The characteristics of the UEN years are significantly different from those observed during the eastern Pacific El Niño (EP-EN) years, which are characterized by anomalous cooling (warming) and suppressed (enhanced) convective eddies in the off-equatorial (equatorial) western Pacific. The central Pacific El Niño years show mixed features during both EP-EN and UEN years. Different background states not only in the equatorial region but also in the off-equatorial region can be a reason for the lack of WWBs in the UEN years.

A quasi-stationary front, called the baiu front, often appears during the early-summer rainy season in East Asia (baiu in Japan). The present study examines how precipitation characteristics during the baiu season are determined by the large-scale environment, using satellite observation three-dimensional precipitation data. Emphasis is placed on the effect of subtropical jet (STJ) and lower-tropospheric convective instability (LCI). A rainband appears together with a deep moisture convergence to the south of the STJ. Two types of mesoscale rainfall events (REs; contiguous rainfall areas), which are grouped by the stratiform precipitation ratio (SPR; stratiform precipitation over total precipitation), are identified: moderately stratiform REs (SPR of 0%–80%) representing tropical organized precipitation systems and highly stratiform REs (SPR of 80%–100%) representing midlatitude precipitation systems associated with extratropical cyclones. As the STJ becomes strong, rainfall from both types of mesoscale precipitation systems increases, with a distinct eastward extension of a midtropospheric moist region. In contrast, small systems appear regardless of the STJ, with high dependency on the LCI. The results indicate that the STJ plays a role in moistening the midtroposphere owing to ascent associated with secondary circulation to the south of the STJ, producing environments favorable for organized precipitation systems in the southern part of the rainband. The horizontal moisture flux convergence may also contribute to precipitation just along the STJ. On the other hand, the LCI plays a role in generating shallow convection. In high-LCI conditions, deep convection can occur without the aid of mesoscale organization.

We demonstrate that the rate of development of cumulus clouds, as inferred from the so-called geostationary satellite “rapid-scan” measurements, is a good proxy for convective cloud top vertical velocity related to deep convective clouds. Convective cloud top vertical velocity is estimated from the decreasing rate of infrared brightness temperature observed by the Multi-functional Transport SATellite-1R (MTSAT-1R) over the ocean south of Japan during boreal summer. The frequency distribution of the estimated convective cloud top vertical velocity at each height is shown to distribute lognormally, and it is consistent with the statistical characteristics of direct measurements acquired in previous studies.

Reproducibility of summer precipitation over northern Eurasia in climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) is evaluated in comparison with several observational and reanalysis datasets. All CMIP5 models under- and overestimate precipitation over western and eastern Eurasia, respectively, and the reproducibility measured using the Taylor skill score is largely determined by the severity of these west–east precipitation biases. The following are the two possible causes for the precipitation biases: very little cloud cover and very strong local evaporation–precipitation coupling. The models underestimate cloud cover over Eurasia, allowing too much sunshine and leading to a warm bias at the surface. The associated cyclonic circulation biases in the lower troposphere weaken the modeled moisture transport from the Atlantic to western Eurasia and enhance the northward moisture flux along the eastern coast. Once the dry west and wet east biases appear in the models, they become amplified because of stronger evaporation–precipitation coupling. The CMIP5 models reproduce precipitation events well over a time scale of several days, including the associated low pressure systems and local convection. However, the modeled precipitation events are relatively weaker over western Eurasia and stronger over eastern Eurasia compared to the observations, and these are consistent with the biases found in the seasonal average fields.

This paper demonstrates the impact of the enhancement in detectability by the dual-frequency precipitation radar (DPR) on board the Global Precipitation Measurement (GPM) core observatory. By setting two minimum detectable reflectivities—12 and 18 dBZ—artificially to 6 months of GPM DPR measurements, the precipitation occurrence and volume increase by ~21.1% and ~1.9%, respectively, between 40°S and 40°N.GPM DPR is found to be able to detect light precipitation, which mainly consists of two distinct types. One type is shallow precipitation, which is most significant for convective precipitation over eastern parts of subtropical oceans, where deep convection is typically suppressed. The other type is probably associated with lower parts of anvil clouds associated with organized precipitation systems.While these echoes have lower reflectivities than the official value of the minimum detectable reflectivity, they are found to mostly consist of true precipitation signals, suggesting that the official value may be too conservative for some sort of meteorological analyses. These results are expected to further the understanding of both global energy and water budgets and the diabatic heating distribution.

Precipitation in excess of 100 mm h−1 in Hiroshima, Japan, on 19 August 2014, caused a flash flood that resulted in 75 deaths and destroyed 330 houses. This study examined the meteorological background of this fatal flood. During this event, considerable filamentary transport of water vapor from the Indochina Peninsula to the Japanese islands occurred, forming a so-called atmospheric river (AR). This AR had a deep structure with an amount of free tropospheric moisture comparable with that of the boundary layer. Furthermore, a cutoff low (COL), detached from the subtropical jet over the central Pacific, moved northwestward to the Japanese islands. Instability associated with the cold core of the COL and dynamical ascent induced in front of it, interacted with the free tropospheric moisture of the AR, which caused the considerable precipitation in Hiroshima. Moreover, the mountains of the Japanese islands played a role in localizing the precipitation in Hiroshima. These roles were separately evaluated on the basis of sensitivity experiments with a cloud-resolving model.

Latent heating (LH) from precipitation systems with different sizes, depths, and convective intensities is quantified with 15 years of LH retrievals from version 7 Precipitation Radar (PR) products of the Tropical Rainfall Measuring Mission (TRMM). Organized precipitation systems, such as mesoscale convective systems (MCSs; precipitation area > 2000 km2), contribute to 88% of the LH above 7 km over tropical land and 95% over tropical oceans. LH over tropical land is mainly from convective precipitation, and has one vertical mode with a peak from 4 to 7 km. There are two vertical modes of LH over tropical oceans. The shallow mode from about 1 to 4 km results from small, shallow, and weak precipitation systems, and partially from congestus clouds with radar echo top between 5 and 8 km. The deep mode from 5 to 9 km is mainly from stratiform precipitation in MCSs. MCSs of different regions and seasons have different LH vertical structure mainly due to the different proportion of stratiform precipitation. MCSs over ocean have a larger fraction of stratiform precipitation and a top-heavy LH structure. MCSs over land have a higher percentage of convective versus stratiform precipitation, which results in a relatively lower-level peak in LH compared to MCSs over the ocean. MCSs during monsoons have properties of LH in between those typical land and oceanic MCSs. Consistent with the diurnal variation of precipitation, tropical land has a stronger LH diurnal variation than tropical oceans with peak LH in the late afternoon. Over tropical oceans in the early morning, the shallow mode of LH peaks slightly earlier than the deep mode. There are almost no diurnal changes of MCSs LH over oceans. However, the small convective systems over land contribute a significant amount of LH at all vertical levels in the afternoon, when the contribution of MCSs is small.

The authors demonstrate that an appropriate treatment of convective entrainment is essential for determining spatial distributions of and temporal variations in precipitation. Four numerical experiments are performed using atmospheric models with different entrainment characteristics: a control experiment (Ctl), a no-entrainment experiment (NoEnt), an original Arakawa–Schubert experiment (AS), and an AS experiment with a simple empirical suppression of convection depending on cloud-layer humidity (ASRH). The fractional entrainment rates of AS and ASRH are constant for each cloud type and are very small in the lower troposphere compared with those in the Ctl, in which half of the buoyancy-generated energy is consumed by entrainment. Spatial and temporal variations in the observed precipitation are satisfactorily reproduced in the Ctl, but their amplitudes are underestimated with a so-called double intertropical convergence zone bias in the NoEnt and AS. The spatial variation is larger in the Ctl because convection is more active over humid ascending regions and more suppressed over dry subsidence regions. Feedback processes involving convection, the large-scale circulation, free tropospheric moistening by congestus, and radiation enhance the variations. The temporal evolution of precipitation events is also more realistic in the Ctl, because congestus moistens the midtroposphere, and large precipitation events occur once sufficient moisture is available. The large entrainment in the lower troposphere, increasing free tropospheric moistening by congestus and enhancing the coupling of convection to free tropospheric humidity, is suggested to be important for the realistic spatial and temporal variations.

Characteristics and global distribution of regional extreme rainfall are presented using 12 yr of the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) measurements. By considering each rainfall event as a set of contiguous PR rainy pixels, characteristic values for each event are obtained. Regional extreme rainfall events are defined as those in which maximum near-surface rainfall rates are higher than the corresponding 99.9th percentile on a 2.5° × 2.5° horizontal-resolution grid. The geographical distribution of extreme rainfall rates shows clear regional differences. The size and volumetric rainfall of extreme events also show clear regional differences. Extreme rainfall rates show good correlations with the corresponding rain-top heights and event sizes over oceans but marginal or no correlation over land. The time of maximum occurrence of extreme rainfall events tends to be during 0000–1200 LT over oceans, whereas it has a distinct afternoon peak over land. There are also clear seasonal differences in which the occurrence over land is largely coincident with insolation. Regional extreme rainfall is classified by extreme rainfall rate (intensity) and the corresponding event size (extensity). Regions of “intense and extensive” extreme rainfall are found mainly over oceans near coastal areas and are likely associated with tropical cyclones and convective systems associated with the establishment of monsoons. Regions of “intense but less extensive” extreme rainfall are distributed widely over land and maritime continents, probably related to afternoon showers and mesoscale convective systems. Regions of “extensive but less intense” extreme rainfall are found almost exclusively over oceans, likely associated with well-organized mesoscale convective systems and extratropical cyclones.

Using 10-yr high-resolution satellite and reanalysis data, the synoptic-scale dual structure of precipitable water (PW), in which the southern and northern bands straddled at the ITCZ produce zonally propagating meridional dipoles, is observed over the eastern Pacific (EP) during boreal summer and fall. Composites indicate that the PW dipole, concurrent with the dipole-like filtered divergence, has a shift to the west of the anomalously cyclonic circulation. The vertical structure of filtered meridional wind is characterized by a wavenumber-1 baroclinic mode, and the vertical motion has two peaks situated at 850 and 300 hPa, respectively. To the east of the PW dipole, the shallow convection is embedded within the deep convection, forming a multilevel structure of meridional wind on the ITCZ equatorward side. To the west of the PW dipole, the deep convection tends to be suppressed because of the invasion of midlevel dry air advected by northerly flows. The generation and propagation of the dual PW band can be attributed to the divergence and advection terms related to specific humidity and three-dimensional wind. By comparison, the PW anomalies over the western North Pacific, only exhibiting a single band, coincide with the centers of synoptic disturbances with a barotropic vertical structure. Because of the weakening of lower-level divergence, the vertical motion, and the horizontal gradient of PW, the synoptic-scale PW signal is reduced significantly. The typical cases and statistics confirm that the strong meridional dipoles and westward-propagating disturbances are closely associated with the distortion and breakdown of ITCZ over the EP.

Contrasts in precipitation characteristics across the baiu front are examined with Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data near Japan during June–July (1998–2011). The vertical structure of atmospheric stratification differs between the tropics and midlatitudes. On an average, the baiu front is found around the latitude that roughly divides the midlatitude atmosphere from the tropical atmosphere. Precipitation characteristics are compared between the southern and northern sides of the reference latitude of the baiu front, which is detected with equivalent potential temperature at 1000 hPa of 345 K in terms of the boundary between the tropics and midlatitudes. The results show that there are obvious differences in precipitation characteristics between the southern and northern sides. In the south, convective rainfall ratios (CRRs) are 40%–60%, which are larger than those in the north (20%–40%). Greater rainfall intensity and taller/deeper precipitation are also observed in the south. Moreover, the characteristics of precipitation features (PFs), which are contiguous areas of nonzero rainfall, differ between the southern and northern sides. In the north, wide stratiform precipitation systems with CRRs of 0%–40% and heights of 8–11 km are dominant. In the south, organized precipitation systems with heights of 12–14 km and CRRs of 30%–50% and those with very large heights (14–17 km) and CRRs of 50%–80% are dominant in addition to wide stratiform precipitation systems. These results suggest that the mechanisms to bring rainfall are different between the southern and northern regions of the baiu front.

Reproducibility of precipitation distribution over the tropical oceans for the recent dataset of the Coupled Model Intercomparison Project phase 5 (CMIP5) is investigated and compared to CMIP3. The Taylor skill score for the reproducibility of the CMIP5 multi-model ensemble mean (0.64) is slightly higher than that of CMIP3 (0.60), but the difference is not statistically significant. Still, there is some evidences that the double intertropical convergence zone (ITCZ) bias is mitigated from CMIP3 to CMIP5, whereas the cold tongue bias remains similar. An inter-model empirical orthogonal function analysis shows that these two biases are closely related to the dominant inter-model discrepancies of precipitation patterns. The two biases are attributed to two factors, respectively. In the CMIP5 models with the prominent double ITCZ, the deep convection is not sensitive enough to environmental air humidity at the lower-mid troposphere, as is in CMIP3. Thus, the deep convection is not suppressed even over the dry subsidence region of the southeastern Pacific, forming the double ITCZ bias. Conversely, models with the severe cold tongue bias have lower ocean model resolution with too strong equatorial trades. Therefore, proper representation of the sensitivity of deep convection to humidity and higher resolution of the ocean models with better equatorial trades are important for reducing the double ITCZ and the cold tongue biases.

Differences in the characteristics of rain systems in the eastern Pacific (EP) intertropical convergence zone (ITCZ) and the western Pacific (WP) warm pool are quantitatively examined in relation to the large-scale environment. This study mainly uses precipitation feature (PF) data observed by the precipitation radar (PR) on board the Tropical Rainfall Measuring Mission (TRMM). The PFs are classified into four types according to their areas and maximum heights. Rain from tall unorganized systems and very tall organized systems tends to be dominant in high-SST regions such as the WP. On the other hand, the EP has more rain from congestus and organized systems with moderate heights than the WP. It is shown that shallow rain from congestus and moderately deep rain from organized systems are highly correlated with shallow (1000–925 hPa) convergence fields with coefficients of 0.75 and 0.66, respectively. These relationships between characteristics of rain systems and the large-scale environment are robust through all seasons.

Synoptic-scale westward-propagating disturbances over the eastern Pacific (EP) are analyzed in boreal autumn, utilizing spectral analysis, composite analysis, and energy budget analysis. The results are compared with those over the western Pacific (WP). Spectral peaks of total precipitable water (TPW) and vertical velocity at 850 hPa (ω850), and outgoing longwave radiation (OLR) are detected at periods of ~3–7 days over the EP. Meanwhile over the WP, a spectral peak of OLR is pronounced, but peaks of TPW and ω850 are not detected. Composite analysis reveals that disturbances that have a coupled structure, with a vortex at its center near ~9°N and a mixed Rossby–gravity (MRG) wave–type disturbance, frequently exist over the EP. At the same time, the disturbances have a double-deck structure associated with divergence both in the upper and in the middle to lower troposphere. These disturbances are associated with both deep convection and congestus, which generate kinetic energy of the disturbance in the upper and in the lower troposphere, respectively. Examining diabatic heating in relation to the coupled disturbances, deep heating with the peak at the height of ~7.5 km is greatest in the northeastern part of the vortex. The coupled MRG wave–type disturbance provides a relatively deep cross-equatorial southerly flow into the northeastern part of the vortex. It is suggested that deep rain is maintained with the existence of deep convergence produced by the coupled disturbances over the EP, where a very shallow convergence field exists on average.

The convective momentum transport (CMT) properties of 13 215 rainbands within a Madden–Julian oscillation (MJO) event simulated by a global nonhydrostatic model are examined. CMT vectors, which represent horizontal accelerations to the mean winds due to momentum flux convergences of deviation winds, are derived for each rainband. The CMT vectors are composited according to their locations relative to the MJO center. While a similar number of rainbands are detected in the eastern and western halves of the MJO convective envelope, CMT vectors with large zonal components are most plentiful between 0° and 20° to the west of the MJO center. The zonal components of the CMT vectors exhibit a coherent directionality and have a well-organized three-layer structure: positive near the surface, negative in the low to midtroposphere, and positive in the upper troposphere. In the low to midtroposphere, where the longitudinal difference in the mean zonal wind across the MJO is 10 m s−1 on average, the net acceleration due to CMT contributes about −16 m s−1. Possible roles of the CMT are proposed. First, the CMT delays the eastward progress of the low- to midtroposphere westerly wind, hence delaying the eastward migration of the convectively favorable region and reducing the propagation speed of the entire MJO. Second, the CMT tilts the MJO flow structure westward with height. Furthermore, the CMT counteracts the momentum transport due to large-scale flows that result from the tilted structure.

The Madden-Julian Oscillation (MJO) is the dominant intraseasonal signal in the global tropical atmosphere. Almost all numerical climate models have difficulty to simulate realistic MJO. Four TRMM datasets of latent heating were diagnosed for signals in the MJO. In all four datasets, vertical structures of latent heating are dominated by two components, one deep with its peak above the melting level and one shallow with its peak below. Profiles of the two components are nearly ubiquitous in longitude, allowing a separation of the vertical and zonal/temporal variations when the latitudinal dependence is not considered. All four datasets exhibit robust MJO spectral signals in the deep component as eastward propagating spectral peaks centered at period of 50 days and zonal wavenumber 1, well distinguished from lower- and higher-frequency power and much stronger than the corresponding westward power. The shallow component shows similar but slightly less robust MJO spectral peaks. MJO signals were further extracted from a combination of band-pass (30 - 90 day) filtered deep and shallow components. Largest amplitudes of both deep and shallow components of the MJO are confined to the Indian and western Pacific Oceans. There is a local minimum in the deep components over the Maritime Continent. The shallow components of the MJO differ substantially among the four TRMM datasets in their detailed zonal distributions in the eastern hemisphere. In composites of the heating evolution through the life cycle of the MJO, the shallow components lead the deep ones in some datasets and at certain longitudes. In many respects, the four TRMM datasets agree well in their deep components, but not in their shallow components and the phase relations between the deep and shallow components. These results indicate that caution must be exercised in applications of these latent heating data.

The role of air–sea interaction in the diurnal variations of convective activity during the suppressed and developing stages of an intraseasonal convective event is analyzed using in situ observations from the Mirai Indian Ocean cruise for the Study of the Madden–Julian oscillation (MJO)-convection Onset (MISMO) experiment. For the whole period, convection shows a clear average diurnal cycle with a primary maximum in the early morning and a secondary one in the afternoon. Episodes of large diurnal sea surface temperature (SST) variations are observed because of diurnal warm layer (DWL) formation. When no DWL is observed, convection exhibits a diurnal cycle characterized by a maximum in the early morning, whereas when DWL forms, convection increases around noon and peaks in the afternoon. Boundary layer processes are found to control the diurnal evolution of convection. In particular, when DWL forms, the change in surface heat fluxes can explain the decrease of convective inhibition and the intensification of the convection during the early afternoon.

The global hydrological cycle is central to the Earth's climate system, with rainfall and the physics of its formation acting as the key links in the cycle. Two-thirds of global rainfall occurs in the Tropics. Associated with this rainfall is a vast amount of heat, which is known as latent heat. It arises mainly due to the phase change of water vapor condensing into liquid droplets; three-fourths of the total heat energy available to the Earth's atmosphere comes from tropical rainfall. In addition, fresh water provided by tropical rainfall and its variability exerts a large impact upon the structure and motions of the upper ocean layer. Three-dimensional distributions of latent heating estimated from Tropical Rainfall Measuring Mission Precipitation Radar (TRMM PR)utilizing the Spectral Latent Heating (SLH) algorithm are analyzed. Mass-weighted and vertically integrated latent heating averaged over the tropical oceans is estimated as approx.72.6 J/s (approx.2.51 mm/day), and that over tropical land is approx.73.7 J/s (approx.2.55 mm/day), for 30degN-30degS. It is shown that non-drizzle precipitation over tropical and subtropical oceans consists of two dominant modes of rainfall systems, deep systems and congestus. A rough estimate of shallow mode contribution against the total heating is about 46.7 % for the average tropical oceans, which is substantially larger than 23.7 % over tropical land. While cumulus congestus heating linearly correlates with the SST, deep mode is dynamically bounded by large-scale subsidence. It is notable that substantial amount of rain, as large as 2.38 mm day-1 in average, is brought from congestus clouds under the large-scale subsiding circulation. It is also notable that even in the region with SST warmer than 28 oC, large-scale subsidence effectively suppresses the deep convection, remaining the heating by congestus clouds. Our results support that the entrainment of mid-to-lower-tropospheric dry air, which accompanies the large-scale subsidence is the major factor suppressing the deep convection. Therefore, representation of the realistic entrainment is very important for proper reproduction of precipitation distribution and resultant large-scale circulation.

The relationship among surface rainfall, its intensity, and its associated stratiform amount is established by examining observed precipitation data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). The results show that for moderate–high stratiform fractions, rain probabilities are strongly skewed toward light rain intensities. For convective-type rain, the peak probability of occurrence shifts to higher intensities but is still significantly skewed toward weaker rain rates. The main differences between the distributions for oceanic and continental rain are for heavily convective rain. The peak occurrence, as well as the tail of the distribution containing the extreme events, is shifted to higher intensities for continental rain. For rainy areas sampled at 0.5° horizontal resolution, the occurrence of conditional rain rates over 100 mm day−1 is significantly higher over land. Distributions of rain intensity versus stratiform fraction for simulated precipitation data obtained from cloud-resolving model (CRM) simulations are quite similar to those from the satellite, providing a basis for mapping simulated cloud quantities to the satellite observations. An improved convective–stratiform heating (CSH) algorithm is developed based on two sources of information: gridded rainfall quantities (i.e., the conditional intensity and the stratiform fraction) observed from the TRMM PR and synthetic cloud process data (i.e., latent heating, eddy heat flux convergence, and radiative heating/cooling) obtained from CRM simulations of convective cloud systems. The new CSH algorithm-derived heating has a noticeably different heating structure over both ocean and land regions compared to the previous CSH algorithm. Major differences between the new and old algorithms include a significant increase in the amount of low- and midlevel heating, a downward emphasis in the level of maximum cloud heating by about 1 km, and a larger variance between land and ocean in the new CSH algorithm.

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.

Oceanic responses to relatively strong Madden-Julian Oscillations (MJOs) and background winds controlled by El Nino-Southern Oscillation (ENSO) are examined. The MJO’s arrival excites dominant downwelling and upwelling Kelvin waves during El Nino developing (pre-El Nino: PEN) and other (non-PEN) phases, respectively. These opposite signals come from background wind directions under different ENSO phases and exert opposite impacts on SST. In addition, MJO convection itself develops accompanied by larger surface wind variations during PEN phases, which can be related to the interactive amplifications of synoptic- and planetary-scale disturbances when westerly wind bursts occur. Consequently, the strength of westerly forcing and its oceanic response during PEN phases are larger than that of the corresponding easterly forcing and its response during non-PEN phases. These results suggest that modulations of MJO amplitude and structure under the background westerly and easterly winds associated with ENSO phases exert opposite but asymmetric impacts on the ocean.

Three-dimensional rain characteristics of tropical cyclones (TCs) are statistically quantified, using Tropical Rainfall Measuring Mission (TRMM) data from December 1997 to December 2003. Tropical cyclones are classified into four maximum intensity classes (

High-frequency atmospheric variability depends on the phase of El Nino/Southern Oscillation (ENSO). Recently, there is increasing evidence that state-dependent high-frequency atmospheric variability significantly modulates ENSO characteristics. Hence, in this study, we examine the model simulations of high-frequency atmospheric variability and, further, its dependency on the El Nino phase, using atmospheric and coupled GCMs (AGCM and CGCM). We use two versions of physical packages here—with and without convective momentum transport (CMT)—in both models. We found that the CMT simulation gives rise to a large climatological zonal wind difference over the Pacific. Also, both the climate models show a significantly improved performance in simulating the state-dependent noise when the CMT parameterization is implemented. We demonstrate that the better simulation of the state-dependent noise results from a better representation of anomalous, as well as climatological, zonal wind. Our further comparisons between the simulations, demonstrates that low-frequency wind is a crucial factor in determining the state-dependency of high-frequency wind variability. Therefore, it is suggested that the so-called state-dependent noise is directly induced by the low-frequency wind anomaly, which is caused by SST associated with ENSO.

The present paper investigates the relationship between the skin sea surface temperature (SSTskin) and the precipitable water (PW) observed over the tropical Indian Ocean. PW is derived from a shipborne Global Positioning System (GPS). Composite diurnal variations indicate that the increase of PW and radar echo coverage (rainfall) in the daytime correspond to the large SSTskin rise during the undisturbed period (The PW increase is statistically significant at 90% level). The surface fluxes calculated using the bulk flux algorithm are too small to account for the observed increase of PW, while the bulk flux agrees with the directly measured eddy flux.

Realistic large-scale diurnal marches of precipitation in various regions in the global tropics over both land and ocean were successfully simulated in an atmospheric general circulation model (AGCM) developed in collaboration by the Center for Climate System Research, the National Institute for Environmental Studies, and the Frontier Research Center for Global Climate (CCSR/NIES/FRCGC), with a resolution of T106, and was forced with prescribed sea surface temperatures. This is an outstanding performance of the diurnal cycle simulated in the AGCM with similar resolution.Comparison analyses with the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Precipitation Radar (PR) data revealed that an implementation of the relative humidity (RH) threshold to the prognostic Arakawa-Schubert (AS) cumulus parameterization significantly improved the diurnal variation of precipitation especially in its large-scale marches. It was suggested that the cloud-layer RH threshold lead to a tighter coupling between free-tropospheric gravity waves and cumulus convection compared to the original prognositic AS, with which convection is more obedient to the boundary-layer warming.

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.

We statistically analyzed both the reproducibility of the present climate, and future climate projections in the Asian monsoon region, using two Regional Climate Models (RCMs), nested into the MRI-CGCM2.2 to assess regional climate projections associated with global warming. Both GCM-RCM systems reproduced the present regional surface air temperature well. Also, they indicated about the same temperature increases as that of GCM for all regions over the Asian continent. The reproducibility of the present-climate precipitation amounts, in the lower-latitude regions was not as good as that of the surface air temperature, although it was better simulated in the higher-latitude regions. The future precipitation increase was not statistically significant. It was also statistically revealed that precipitation in future projections, with GCM-RCM systems, tended to converge in regions where the model biases were small. This result suggests the importance of an accurate reproduction of the present regional climate using physically based dynamical models, in order to analyze regional climate changes.

Precipitation observation by the Tropical Rainfall Measuring Mission's (TRMM's) Precipitation Radar (PR) lasted for almost 17 years. Homogeneity of long-term PR data will be essential to study the water cycle change related to the interannual variability and the decadal change. In this study, we aim to develop a precipitation climate data from 17-year PR data. In this paper, PR data are adjusted to mitigate the discontinuity of the PR hardware (H/W) change. An obvious discontinuity of storm top height caused by the PR H/W change is mitigated creating the adjusted data. Semi-global (35S-35N) precipitation derived from the adjusted data is decreased by 0.98 % as compared with the original data.

Rainfall characteristics of the Madden–Julian oscillation (MJO) are analyzed primarily using tropical rainfall measuring mission (TRMM) precipitation radar (PR), TRMM microwave imager (TMI) and lighting imaging sensor (LIS) data. Latent heating structure is also examined using latent heating data estimated with the spectral latent heating (SLH) algorithm. The zonal structure, time evolution, and characteristic stages of the MJO precipitation system are described. Stratiform rain fraction increases with the cloud activity, and the amplitude of stratiform rain variation associated with the MJO is larger than that of convective rain by a factor of 1.7. Maximum peaks of both convective rain and stratiform rain precede the minimum peak of the outgoing longwave radiation (OLR) anomaly which is often used as a proxy for the MJO convection. Stratiform rain remains longer than convective rain until ∼4000 km behind the peak of the mature phase. The stratiform rain contribution results in the top-heavy heating profile of the MJO. Associated with the MJO, there are tri-pole convective rain top heights (RTH) at 10–11, ∼7 and ∼3 km, corresponding to the dominance of afternoon showers, organized systems, and shallow convections, respectively. The stratiform rain is basically organized with convective rain, having similar but slightly lower RTH and slightly lags the convective rain maximum. It is notable that relatively moderate (∼7 km) RTH is dominant in the mature phase of the MJO, while very tall rainfall with RTH over 10 km and lightning frequency increase in the suppressed phase. The rain-yield-per flash (RPF) varies about 20–100% of the mean value of ∼2–10 × 10 9 kg fl −1 over the tropical warm ocean and that of ∼2–5 × 10 9 kg fl −1 over the equatorial Islands

In this study, we focused on the difference in appearances of the convectively coupled equatorial waves (CCEWs) in a simulation with the CCSR/NIES/FRCGC AGCM, between two experiments, one with and the other without implementation of the convective suppression scheme (CSS) in the prognostic Arakawa–Schubert cumulus parameterization. Realistic CCEW modes, i.e., Kelvin, Rossby, mixed Rossby-gravity (MRG), and n = 0 eastward inertio-gravity (EIG) wave modes, were reproduced in the with-CSS experiment, while only Rossby-wave-like signals appeared in the without-CSS experiment. By comparing the structures of the Kelvin wave mode and the Rossby wave mode in two runs, it was suggested that the structural difference between these two modes in conjunction with the difference in the controlling factor of cumulus convection determines the CCEW features. The CSS implemented here is such that cumulus convection is suppressed until the cloud-layer-averaged relative humidity exceeds the threshold of 80%. In the without-CSS model, only Rossby wave modes are coupled with the convection. This is because CAPE controls cumulus convection in this model, and the larger frictional convergence of Rossby wave mode prepares CAPE to generate favorable condition for cumulus convection. In the case of the with-CSS model, on the other hand, cumulus convection is largely controlled by the humidity in the free atmosphere. The convergence associated with the equatorial waves can produce the moisture anomaly to overcome the relative humidity threshold, and maintains the favorable condition for cumulus convection once it starts. In this case, not only Rossby waves but also Kelvin, MRG, and n = 0 EIG waves are reproduced more realistically. It is suggested that inclusion of some kind of mechanism connecting the free tropospheric moisture with the convection under the condition of abundant convective available potential energy could be a key factor for realistic coupling between large-scale atmospheric waves and convection.

Rainfall is a fundamental process within the Earth's hydrological cycle because it represents a principal forcing term in surface water budgets, while its energetics corollary, latent heating, is the principal source of atmospheric diabatic heating well into the middle latitudes. Latent heat production itself is a consequence of phase changes between the vapor, liquid, and frozen states of water. The properties of the vertical distribution of latent heat release modulate large-scale meridional and zonal circulations within the Tropics, as well as modify the energetic efficiencies of midlatitude weather systems. This paper highlights the retrieval of latent heating from satellite measurements generated by the Tropical Rainfall Measuring Mission (TRMM) satellite observatory, which was launched in November 1997 as a joint American–Japanese space endeavor. Since then, TRMM measurements have been providing credible four-dimensional accounts of rainfall over the global Tropics and subtropics, information that can be used to estimate the space–time structure of latent heating across the Earth's low latitudes. A set of algorithm methodologies for estimating latent heating based on precipitation-rate profile retrievals obtained from TRMM measurements has been under continuous development since the advent of the mission s research program. These algorithms are briefly described, followed by a discussion of the latent heating products that they generate. The paper then provides an overview of how TRMM-derived latent heating information is currently being used in conjunction with global weather and climate models, concluding with remarks intended to stimulate further research on latent heating retrieval from satellites.

In order to describe the connection from an event of MJO to the next in the boreal winter, the eastward propagation of MJO is studied, focusing on that over the western hemisphere. Propagation signal is identified by EEOF analysis, performed on the bandpass filtered OLR for the period of 1979-2000. Besides NOAA OLR, total precipitable water (TPW), and surface winds from Special Sensor Microwave/ Imager (SSM/I), precipitation observed from Microwave Sounding Unit (MSU), and reanalysis and operational analysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF), are utilized for the composite. Positive TPW anomalies are found, synchronizing with tropospheric and surface zonal wind anomalies. They propagate eastward all around the equator in the boreal winter. They propagate at a speed of about 6 ms-1, with a Kelvin-Rossby coupled mode structure in the eastern hemisphere, and at about 20 ms-1 as an envelope of a radiating response in the western hemisphere. Within the envelope in the western hemisphere, faster propagating signals corresponding to 30-40 ms-1 exist in the fields of TPW, zonal wind at 200 and 700 hPa, surface zonal wind. It is especially clear in the geopotential anomalies at 1000 hPa. This fast propagation speed of 30-40 ms-1 is consistent with a first-baroclinic dry Kelvin wave mode recently rediscovered by Milliff and Madden (1996), and Bantzer and Wallace (1996). TPW increases under surface easterly anomalies along the equator. After the preceding TPW accumulation for 5-7.5 days, convective anomalies begin to occur as a part of the next cycle of the MJO from the eastern Atlantic to the western Indian Ocean. These results suggest a following conceptual model for propagations and event-to-event connections of MJO. Equatorial Kelvin wave generated by convection of the MJO propagates eastward emanating from a warm pool region at a faster speed (30-40 ms-1) in the western hemisphere. Elevated topography of the South American and African continent, blocks the wave propagation. After being blocked several days by topography, they continue to proceed. As a result, the signal propagates at 20 ms-1 on average. Frictional convergence with lower easterlies of the dry Kelvin wave results in the associated propagation of TPW positive anomaly. Although it does not induce deep convections over large-scale subsidence regions, once it enters over the warm water in the western Indian Ocean, it helps to induce active convections for the next cycle of MJO.

In order to estimate the annual surface shortwave forcing by clouds+aerosols and aerosols, the shortwave flux from pyrheliometer and pyranometer measurements, atmospheric profiles from the radiosonde measurements, and aerosol optical properties retrieved from sky radiometer measurements were integrated with high-accuracy transfer model calculations. Clear-sky flux was defined from transfer calculations for a pure Rayleigh-scattering atmosphere, with measured temperature and humidity profiles by radiosonde observations. Monthly variation of the clear-sky flux due to the temperature and water vapor variation was 10-30 W m -2 . Cloud+aerosol forcing was defined by the difference between the observed flux and the clear-sky flux (positive downward). The annual mean values of the cloud+aerosol surface shortwave forcing was estimated as -81 W m -2 , which corresponds to about 24 % of the insolation. The aerosol-sky flux is defined with the transfer calculation using the aerosol optical depth retrieved from the sky radiometer measurements. Aerosol forcing was obtained from the differences between the clear-sky flux and the aerosol-sky flux. The mean direct aerosol forcing for 1996, except for March and April, was estimated as -18 W m -2 , about 6 % of the insolation. We also performed a sensitivity study of the aerosol-sky flux by varying the weight fraction of soot in aerosols. Among the selected soot fraction, the best estimates were obtained as 10 % for January, February and July, 20 % for October through December, 5 % for May, June and August, and 0 % for September. These values are close to the measured seasonal variations of soot fraction in previous studies. Surface flux calculation with the retrieved aerosol size distributions performed no better than those with the LOWTRAN 7 urban model size distribution, especially in the summer months when the water vapor column amount was large. The necessity of further examination of retrieval methods of aerosol optical properties, using sky radiometer measurements, was suggested.

Diurnal variations in atmospheric convection, dynamic/thermodynamic fields, and heat/moisture budgets over the equatorial Pacific warm pool region are analyzed based on data collected from different observation platforms during the Intensive Observation Period of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). Results reveal that the diurnal variations in rainfall/convection over the TOGA COARE region can be classified into three distinct stages: warm morning cumulus, afternoon convective showers, and nocturnal convective systems. Afternoon rainfall comes mostly from convective cells, but the nocturnal rainfall is derived from deeper convective cells and large areas of stratiform clouds. Results further show that afternoon convective showers are more evident in the large-scale undisturbed periods when the diurnal SST cycle is strong, but the nocturnal convective systems and morning cumulus are more enhanced in the disturbed periods when more moisture is available. The primary cause of the nocturnal rainfall maximum is suggested to be associated with more (less) available precipitable water in the night (day) due to the diurnal radiative cooling/heating cycle and the resultant change in tropospheric relative humidity.

The Precipitation Radar (PR) on the TRMM satellite has observed tropical rainfall for more than 15 years. The long-term rainfall data from TRMM PR have been very useful for scientific studies. However, several studies have shown that the amount of weak convective rainfall decreased in the PR product due to the TRMM orbit boost from 350 km to 402.5 km in August 2001. In this study, the impact of the sensitivity degradation on the rainfall amounts is estimated by adjusting the pre-boost data with a simulation of reduced sensitivity by the orbit boost. A PR precipitation product with reduced artificial discontinuity at the timing of the orbit boost is produced for climate studies. The result shows that the decrease of the global rainfall amount due to the sensitivity is about 2.1% on average. However, the decreases of rainfall amount due to the sensitivity degradation vary spatially and they are larger, in particular, in subtropical ocean.

[1] In this study, ten years (1998–2007) of the Tropical Rainfall Measuring Mission (TRMM)-derivative radar precipitation feature (PF) product are analyzed in order to determine the differences between tropical cyclone-related precipitation characteristics compared with those of the tropical Pacific warm pool (10 to 30°N and 130 to 150°E) in general. The PF data, from the University of Utah's archive, are based on the TRMM precipitation radar's 2A25 product, where one PF consists of a single or group of contiguous pixels with near surface rainfall greater than zero. Using the PF database, the height and area of tropical background PFs versus those within 500 km of tropical cyclones (TCs) are compared. It is found that TC-related PFs are markedly more frequent from 5 to 10 km altitude than are background tropical PFs. The enhanced midlevel TC-related populations not only exist in stratiform precipitation around the melting level at 5.5 km, but also from 6 to 9 km in stratiform regions, in convective precipitation, and when culling the smallest features from the data set. This increase in congestus-like echoes in TC environments aligns well with observations regarding mesoscale convective systems (MCSs), in which the enhanced ice present in MCS (and TC) environments creates a stabilized melting layer through cooling immediately below 0°C. This stable layer appears to enhance the detrainment of convective PFs in our TC data set at and for a few kilometers above the melting level.

[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.

[1] The diurnal variations in summer rainfall over the South China Sea (110°E−120°E, 10°N−20°N) are examined for active and inactive intraseasonal oscillation (ISO) periods that are characterized by strong and weak 850-hPa zonal winds, respectively. By analyzing the rainfall retrievals from the Tropical Rainfall Measuring Mission Precipitation Radar for the period 1999–2006, it is found that the peak rainfall during the active and inactive ISO periods occurs in the late morning and late evening, respectively. The morning peak in the active ISO period arises from the organized oceanic convective systems associated with the local convergence along the west coast of the Philippines. The evening peak in the inactive ISO period originates from a stratiform morphology that is initiated by land-based convective systems owing to the increased thermodynamic instability over the Philippines during daytime.

[1] Rain-yields per flash (RPF) over the entire tropics were calculated from 3 years of data collected by a Lightning Imaging Sensor (LIS) and a Precipitation Radar (PR) housed onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The results confirm that RPF is a reliable indicator of precipitation regimes, with a marked land–ocean contrast and intermediate values over monsoonal regions and continental oceans. Mean RPF values averaged over the entire TRMM region are 3.94 × 108 kg fl−1 over land and 1.96 × 109 kg fl−1 over oceans. A good (−0.55) correlation between RPF and the Tall Convective Rain Contribution with a threshold of −20degC (TCRC–20C) was found especially over land. This result indicates that large amounts of tall convective rain are fundamentally associated with intense updrafts that are able to sustain vigorous lightning activity. The correlation is weaker over ocean, except for that over continental oceans.

[1] We constructed a composite lifecycle of the Madden-Julian oscillation (MJO) with cloud-top information and atmospheric temperature and moisture profiles. Japanese Geostationary Meteorological Satellite (GMS/IR TBB) histograms and upper-soundings from the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) intensive observation period (IOP) were analyzed around Ujung Pandang (119°E, 5°S) and Kupang (123°E, 10°S), which encountered the mature phase of MJO from January to February, 1993. Under a large-scale boundary layer (BL) convergence, convection developed in stages:1) ‘suppressed stage’ with suppressed clouds, 2) ‘shallow convection stage’ (2–3 days) with clouds confined under the trade inversion level, 3) ‘developing stage’ (3–4 days) with most clouds stopped at the mid-troposphere but some tall convective clouds developing 4) ‘mature stage’ (4–5 days) with large anvil cloud shields, and 5) ‘decaying stage’ with decaying anvils. These stages of MJO were related to three stable layers; trade inversion, 0°C level, and tropopause. It is suggested that staged convective development associated with the MJO was strongly affected by the interaction among convection, stable layers, and atmospheric moistening.

[1] A summary of rainfall statistics over the equatorial area (10°N–10°S), observed by a space-borne precipitation radar of the Tropical Rainfall Measuring Mission (TRMM), is presented utilizing all nadir data of PR2a25 version 5 for the period of 1998–1999. Spectral representation of rain profiles is introduced, and convective and stratiform rain characteristics over land and over ocean are examined. With 0.3 mm hr−1 rain-top threshold, convective: stratiform rain ratios at 2–4 km over land and over ocean are, 61:39 and 50:50 in amount, 20:80 and 17:83 in area, respectively. For diurnal variations over ocean, two types of rain vary almost synchronously, with an early-morning maximum in 03–06 LT, indicating that direct radiative effect is the dominant diurnal control. Over land, convective rain has a distinct maximum of afternoon showers in 15–18 LT, while stratiform rain has a midnight (24–03LT) maximum. Also shown is that afternoon showers are more frequent but nighttime rain is stronger, suggesting contributions from large-scale disturbances.