Search Results

You are looking at 1 - 10 of 16 items for

  • Author or Editor: Masaki Katsumata x
  • Refine by Access: All Content x
Clear All Modify Search
Biao Geng and Masaki Katsumata

Abstract

In this study, we examined the variations of precipitation morphology and rainfall in relation to the simultaneous passages of a Madden–Julian oscillation (MJO) event and convectively coupled equatorial waves (CCEWs) observed during the Years of the Maritime Continent pilot study. We utilized globally merged infrared brightness temperature data and the radiosonde and radar data observed aboard the Research Vessel Mirai at 4°4′S, 101°54′E. As well as the observed MJO event, equatorial Rossby waves (ERWs), Kelvin waves (KWs), and mixed Rossby–gravity waves (MRGWs) were identified. The radar data exhibited high-frequency variation, mainly caused by KWs and MRGWs, and low-frequency variation, mainly caused by the MJO and ERWs. The MRGWs predominantly modulated convective echo areas and both convective and stratiform volumetric rainfall. In contrast, the MJO event had little influence on the variance of convective echoes. Moreover, stratiform echo areas and volumetric rainfall were more strongly modulated by the combined effects of the MJO, ERWs, KWs, and MRGWs than their convective counterparts. The intense development of stratiform echo areas and volumetric rainfall was coherent with the superimposition of the active phases of the MJO event and all the analyzed CCEWs. The strongest development and a significant reduction of convective echo-top heights before and after the peak MJO date, respectively, were coherent with the passages of ERWs and MRGWs, which were the dominant wave types in modulating echo-top heights. Thus, it appears that the superimposition of the CCEWs on the MJO event exerted complex modulations on the convective activities within the MJO event.

Open access
Satoru Yokoi and Masaki Katsumata

Abstract

Moist static energy (MSE) in the atmospheric boundary layer (BL) is one of the essential parameters determining convective activity over tropical oceanic areas. It is thus important to quantitatively understand BL MSE budget processes and their variability. Among these processes, only few studies have evaluated contributions of entrainment across the BL top and convective downdraft. This study aims to estimate these contributions by analyzing upper-air and surface meteorological observations obtained using research vessel Mirai over the tropical western Pacific in June 2008. Daily mean downward mass fluxes due to the two processes are calculated using BL dry static energy and moisture budget equations under the BL quasi-equilibrium approximation. Estimated mass fluxes are consistent with convective activity observed by a shipborne weather radar and a ceilometer. This study further examines how the mass fluxes and budget processes are modulated when a convectively active phase of boreal summer intraseasonal oscillation arrives at the observation area in the second half of the month. It is found that, while the contribution of the entrainment does not change significantly, the convective downdraft mass flux and the resultant BL MSE export increase 5 times and 3 times, respectively, in the convectively active period compared with those in the preactive period. Furthermore, ∼1/4 of the increase in the convective downdraft mass flux is attributable to the increase in MSE of convective downdraft air associated with midtropospheric moistening.

Open access
Masaki Katsumata, Hiroyuki Yamada, Hisayuki Kubota, Qoosaku Moteki, and Ryuichi Shirooka

Abstract

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.

Full access
Masaki Katsumata, Richard H. Johnson, and Paul E. Ciesielski

Abstract

A case study of an intraseasonal oscillation (ISO) is investigated in the period leading up to its convectively active phase during the Mirai Indian Ocean Cruise for the Study of the MJO-Convection Onset (MISMO), which was conducted during boreal autumn 2006. Detailed observations, including apparent heat and moisture analyses, reveal that synoptic-scale variability of heat and moisture sources and sinks associated with the passage of three eastward-propagating cloud systems (EPCSs) was prominent during this period. These systems with periods of ∼6 days propagated through the MISMO domain, priming the atmosphere for a convectively active phase of the ISO. The prominent shallow heating during this period may explain the rather slow (8 m s−1) propagation speed for EPCSs. The zonal structure and sign of the frictional convergence show that these EPCSs have common characteristics to the frictional Kelvin mode studied by Ohuchi and Yamasaki. With the analyses of the period-averaged vertical profiles, the EPCSs were identified as the principal mechanism to moisten the atmosphere prior to the ISO convectively active phase.

Full access
Masaki Katsumata, Hiroshi Uyeda, Koyuru Iwanami, and Guosheng Liu

Abstract

The first study in both observing and modeling radiative properties of snow clouds in the microwave frequencies is described in this paper. Snow clouds over ocean were observed simultaneously using an airborne microwave radiometer and an X-band Doppler radar. Results show that brightness temperatures at 36- and 89-GHz microwave channels responded well to the horizontal variations of precipitation particles and to the cloud dynamic structures determined by the Doppler radar, which reflect the development stages of convective cells. For the quantitative validation, physical retrievals of liquid water and snow water amounts were performed using a radiative transfer model. The retrieved snow water amount agrees well with the observed snow water amount that was converted from observed radar reflectivity. In the retrieval method, the model-simulated brightness temperatures were able to match the observed values within 3 K per channel for the most part. The ambiguities of the retrieved parameters that depend on some assumptions are also examined.

Full access
Hiroyuki Yamada, Kunio Yoneyama, Masaki Katsumata, and Ryuichi Shirooka

Abstract

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.

Full access
Masaki Katsumata, Paul E. Ciesielski, and Richard H. Johnson

Abstract

Results of sounding-derived heat and moisture budgets for the Mirai Indian Ocean Cruise for the Study of the MJO Onset (MISMO) project in 2006 are evaluated using observational and simulated datasets. Estimated rainfall rates from the budget analyses agree well with both the satellite-derived products and in situ rain gauge measurements in the period leading up to the convectively active phase of the MJO. During the active phase, however, large discrepancies are found in the temporal variation of the rainfall estimates on the time scale of a few to several days. Utilizing MJO circulations simulated by the two numerical models, a linear model and a high-resolution AGCM, it is revealed that this discrepancy may result from inability of the MISMO triangular network to properly capture the divergence associated with the Rossby and/or inertia–gravity wave components of the circulation. A rectangular array is demonstrated to be superior at capturing the wind associated with these wave disturbances. Possible effects of the equatorial waves and sampling issues, which could act to enhance or suppress the error in the present case, are discussed. The reliability of the budget analyses during the preactive period is also supported by both model simulations.

Full access
Weixin Xu, Steven A. Rutledge, Courtney Schumacher, and Masaki Katsumata

Abstract

This study investigates the evolution, structure, and spatial variability of Madden–Julian oscillation (MJO) convection observed during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign. Generally, the C-band radars located in the near-equatorial Indian Ocean—Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) on Addu Atoll (Gan) and NASA TOGA on the R/V Roger Revelle (Revelle)—observed similar trends in echo-top heights, stratiform rain fraction, and precipitation feature size across the MJO life cycle. These trends are closely related to changes in mid- to upper-tropospheric moisture, sea surface temperature (SST), zonal wind, and diagnosed vertical air motions. However, the evolution of convection, moisture, and vertical air motion at the R/V Mirai (Mirai), located in the intertropical convergence zone (ITCZ) at 8°S, exhibited a pattern nearly opposite to Gan and Revelle. When the MJO was active over the equator, convection was suppressed around Mirai owing to induced subsidence by the strong upward motion to the north. SST and zonal winds near Mirai were nearly invariant across the MJO life cycle, indicating little influence from the MJO in these fields. Compared to Gan and Revelle, Mirai had a significant amount of precipitation that fell from shallow and isolated convection. There were subtle differences in the evolution and properties of the convection observed between Gan and Revelle. Deep convection occurred slightly earlier at Gan compared to Revelle, consistent with the west-to-east progression of the MJO in the central Indian Ocean. Furthermore, convective deepening was more gradual over Revelle compared to Gan, especially during the October MJO event.

Full access
Satoru Yokoi, Shuichi Mori, Masaki Katsumata, Biao Geng, Kazuaki Yasunaga, Fadli Syamsudin, Nurhayati, and Kunio Yoneyama

Abstract

This study analyzes data obtained by intensive observation during a pilot field campaign of the Years of the Maritime Continent Project (Pre-YMC) to investigate the diurnal cycle of precipitation in the western coastal area of Sumatra Island. The diurnal cycle during the campaign period (November–December 2015) is found to have a number of similarities with statistical behavior of the diurnal cycle as revealed by previous studies, such as afternoon precipitation over land, nighttime offshore migration of the precipitation zone, and dependency on Madden–Julian oscillation (MJO) phase. Composite analyses of radiosonde soundings from the Research Vessel (R/V) Mirai, deployed about 50 km off the coast, demonstrate that the lower free troposphere starts cooling in late afternoon (a couple of hours earlier than the cooling in the boundary layer), making the lower troposphere more unstable just before precipitation starts to increase. As the nighttime offshore precipitation tends to be more vigorous on days when the cooling in the lower free troposphere is larger, it is possible that the destabilization due to the cooling contributes to the offshore migration of the precipitation zone via enhancement of convective activity. Comparison of potential temperature and water vapor mixing ratio tendencies suggests that this cooling is substantially due to vertical advection by an ascent motion, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves emanating from convective systems over land play a significant role in the offshore migration of the precipitation zone.

Full access
Yasu-Masa Kodama, Masaki Katsumata, Shuichi Mori, Sinsuke Satoh, Yuki Hirose, and Hiroaki Ueda

Abstract

The large-scale distribution of precipitation and latent heating (LH) profiles in the tropics, subtropics, and part of the midlatitudes was studied using a 9-yr dataset derived from Tropical Rainfall Measuring Mission precipitation radar observations, with emphasis on the contribution of warm rain. The distribution of warm rain showed features unique from those of rain in other categories and those of outgoing longwave radiation. Warm rain was weak over land but widely distributed over oceans, especially along the intertropical convergence zone (ITCZ) and the western part of the subtropical oceans. The observed amount of warm rain depended on the rainfall intensity rather than on the frequency of warm rain events. The amount of warm rain over ocean was positively correlated with sea surface temperature (SST); this dependency was found in the tropics, subtropics, and part of the midlatitudes, whereas dependency of SST on total rain was confined to the tropics. Both total rain and warm rain were concentrated in the ITCZ, which elongated along the local SST maximum. Small amounts of warm rain were found along subtropical convergence zones (the baiu frontal zone and subtropical portions of the South Pacific convergence zone and the South Atlantic convergence zone) with ample total rainfall. However, larger amounts of warm rain were observed at the lower-latitude sides of these zones in the upstream portions of low-level moisture flow toward the zones. Warm rain may cultivate the subtropical convergence zones by deepening the moist boundary layer and increasing moisture flux toward the zones. The statistical relationship between warm rain and low-level cloudiness showed that the warm rain amount was large when low-level cloudiness was 20%–30% and small when low-level cloudiness was greater than 40%. This indicates that intense warm rain is provided by convective clouds, not by stratiform clouds, in conditions of substantial cloudiness. Despite the small contribution to total rain, warm rain maintained positive LH values over most of the tropical and subtropical oceans. The LH by warm rain masked low-level cooling observed in stratiform rain and maintained positive LH in the lower atmosphere below the melting layer. Because warm rain was confined to oceans, a strong LH contrast was maintained along the coast; this contrast reached values of 1–2 K day−1 in certain places and may affect local and monsoonal circulation across continental coasts.

Full access