Kazuaki Yasunaga

Mismo field experiment in the equatorial Indian Ocean

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...

Differences between more divergent and more rotational types of convectively coupled equatorial waves. Part I: Space-time spectral analyses

AbstractPrecipitation-related differences in different types of convectively coupled equatorial waves are examined here and in a companion paper. Here the authors show spectra and cross-spectra among tropical-belt time sections of satellite-derived surface rain, infrared brightness temperature Tb, precipitable water (PW), and Japan Meteorological Agency reanalysis of divergence and PW.Cross-spectra between rain and divergence at 1000- and 200-hPa levels show significant coherence peaks oriented along the dispersion curves of Kelvin, n = 1 equatorial Rossby (ERn1), mixed Rossby–gravity (MRG), n = 0 eastward inertial gravity (EIGn0), and n = 1 and n = 2 westward inertial gravity (WIG) waves, as well as the spectral signatures of the Madden–Julian oscillation (MJO) and tropical depression (TD)-type disturbances. Middle-troposphere divergence (indicative of stratiform rain and half-depth convection involvement in the coupling) is coherent with rain for the higher-frequency and more divergent wave types (Kelvi...

Observation of Moisture Tendencies Related to Shallow Convection

AbstractTropospheric 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 ...

Melting Layer Cloud Observed during R/V Mirai Cruise MR01-K05

Abstract In this study, cloud profiling radar and lidar were used to determine the frequency distribution of the base heights of cloudy layers with little (or no) falling condensate particles. The data were obtained from stationary observations conducted from Research Vessel Mirai over the tropical western Pacific (around 1.85°N, 138°E) from 9 November to 9 December 2001. The observed cloudy layers had base heights predominantly in the range of 4.5–6.5 km. Almost all cloudy layers with a base in the range of 4.5–6.5 km had thickness thinner than 500 m, and the frequency peak of the base heights of measured cloudy layers is considered to represent the common occurrence of midlevel thin clouds. Midlevel thin clouds were frequently observed even during the active phase of the Madden–Julian oscillation (MJO). Composite analysis of radiosonde-derived relative humidity and temperature lapse rate indicates that the midlevel thin cloud in the MJO active period is generated via melting within the stratiform cloud,...

Differences between More Divergent and More Rotational Types of Convectively Coupled Equatorial Waves. Part II: Composite Analysis based on Space-Time Filtering

AbstractThis paper describes an analysis of multiyear satellite datasets to characterize the modulations of convective versus stratiform rain, rain system size, and column water vapor by convectively coupled equatorial waves. Composites are built around space–time filtered equatorial-belt data from the Tropical Rainfall Measuring Mission (TRMM) 3B42 rainfall product, while TRMM Precipitation Radar (PR) and passive microwave data are the composited variables. The results are consistent with the more reanalysis-dependent findings in Part I, indicating that higher-frequency (or more divergent) waves, such as Kelvin and inertia–gravity families, modulate mesoscale convective systems and stratiform rain relatively more, whereas slower (or more rotational) types such as Rossby, mixed Rossby–gravity, and tropical depression (TD) or “easterly” waves primarily modulate convective rain and smaller-sized precipitation systems.Column water vapor composites indicate that the more rotational wave types modulate the moi...

Performance of Long-Term Integrations of the Japan Meteorological Agency Nonhydrostatic Model Using the Spectral Boundary Coupling Method

The spectral boundary coupling (SBC) method, which is an approach used to couple a limited-area model with a large-scale model, was introduced into a nonhydrostatic model. To investigate whether the SBC method works well in a long-term integration of a high-resolution nonhydrostatic model, two numerical experiments were conducted with a model having a horizontal grid interval of 5 km. In one experiment, the SBC method was employed, while it was not in the other experiment. The time integration in both experiments was over a 40-day period. The nonhydrostatic model was nested into objectively analyzed fields, instead of the forecasts from an extended-area model. Predicted patterns of sea level pressure and precipitation were compared with objective analyses, and data provided by the Global Precipitation Climatology Project (GPCP), respectively. The predicted rainfall amounts and surface temperature over the Japanese islands were statistically evaluated, making use of the analyzed rainfall and surface data observed by the Japan Meteorological Agency (JMA). All results examined in the present study exhibited better performances with use of the SBC method than those without the SBC method. It was found that the SBC method was highly useful in long-term simulations by a high-resolution nonhydrostatic model.

Characteristics of 3–4- and 6–8-Day Period Disturbances Observed over the Tropical Indian Ocean

Abstract A field observational campaign [i.e., the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO)] was conducted over the central equatorial Indian Ocean in October–December 2006. During MISMO, large-scale organized convection associated with a weak Madden–Julian oscillation (MJO) broke out, and some other notable variations were observed. Water vapor and precipitation data show a prominent 3–4-day-period cycle associated with meridional wind υ variations. Filtered υ anomalies at midlevels in reanalysis data [i.e., the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS)] show westward phase velocities, and the structure is consistent with mixed Rossby–gravity waves. Estimated equivalent depths are a few tens of meters, typical of convectively coupled waves. In the more rainy part of MISMO (16–26 November), the 3–4-day waves were coherent through the lower and midtroposphere, while in the less active early November period midlevel υ fluctuations appear le...

Differences between Faster versus Slower Components of Convectively Coupled Equatorial Waves

AbstractThis paper describes an analysis of multiyear satellite datasets that subdivide two halves (faster and slower) of the space–time spectral signal peaks corresponding to convectively coupled equatorial waves such as Kelvin and inertia–gravity waves [n = 0 eastward inertia–gravity wave (EIGn0 wave), and n = 1 and n = 2 westward inertia–gravity waves (WIGn1 and WIGn2 waves, respectively)]. The faster (slower) component of an equatorial wave is defined as that which has a spectral signal peak in the regions with deeper (shallower) equivalent depths. The data obtained from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (TRMM-PR) are composited around space–time-filtered equatorial-belt data from the TRMM-3B42 rainfall product to separately estimate the convective and stratiform rainfall modulations.Results indicate that the faster components of WIGn1 and WIGn2 waves modulate convective rain relatively more (and stratiform rain relatively less) than their slower counterparts. For Kelv...

Numerical Simulations of the Formation of Melting-Layer Cloud

Abstract A number of previously published observational studies have reported the common occurrence of cloudy layers at around 5-km elevation in the tropics. There are two candidate processes that are able to explain the occurrence of cloudy layers in the middle level: cloud detrainment promoted by the stable layer and enhanced condensation to compensate for melting cooling. In the present study, the authors used a cloud-resolving nonhydrostatic model and conducted numerical simulations of a squall line to clarify the process responsible for the formation of midlevel thin cloud, especially the cloud at the 0°C level. In a two-dimensional control experiment thin cloud was simulated in the middle level, and cloud coverage showed a notable peak just below the 0°C level for environments without a stable layer in the initial temperature profile. Enhanced and weakened stability layers simultaneously appeared above and below the peak level of the cloud coverage. The formation of midlevel thin cloud is associated...

Diurnal Cycle of Precipitation Observed in the Western Coastal Area of Sumatra Island: Offshore Preconditioning by Gravity Waves

AbstractThis 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 d...