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Kazuaki Yasunaga and Masashi Tomochika

Abstract

Long-term changes in the monthly precipitation along the coastal areas of the Sea of Japan are examined using monthly operational observation data from the Japan Meteorological Agency. The monthly precipitation in December significantly increased from the mid-1980s to 2015, even though no remarkable changes were found in January, February, or November. Significant positive trends in the December precipitation extend widely over the coastal areas of the Sea of Japan, and the amounts increase to approximately 50% of the climatological precipitation in December at most observational stations. The interannual variations in other variables, including the monthly accumulated actual sunshine duration, days with lightning detection, and satellite-retrieved outgoing longwave radiation, also show significant trends that are consistent with the precipitation increase in December. The effect of the sea surface temperature (SST) on precipitation change is discussed based on correlation and regression analyses. The interannual variations in the December precipitation averaged over the observational stations near the Sea of Japan are significantly correlated with the SSTs in the prior month (November). However, the SST increase in November is insufficient to account for the increase in precipitation. In addition, it was found that the satellite-retrieved surface wind speed in December has grown stronger in recent years over the Sea of Japan. It is suggested that the stronger wind corresponds to the enhanced monsoonal flow and is the primary cause of the precipitation increase in December.

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Kazuaki Yasunaga and Brian Mapes

Abstract

This 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 Kelvin and EIGn0 waves, however, there is no significant difference in the rainfall modulation between their faster and slower components. A space–time cospectral analysis of the satellite-retrieved rainfall and moisture shows that in the spectral regions corresponding to WIGn1 and WIGn2 waves, precipitation is significantly correlated with low-level moisture but not with midlevel moisture. In contrast, significant coherence between rainfall and moisture at these levels is found in the spectral regions corresponding to the Kelvin and EIGn0 waves. These results may bear on different convection–wave coupling mechanisms for these “divergent” waves (stratiform instability versus moisture–stratiform instability).

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Kazuaki Yasunaga and Brian Mapes

Abstract

This 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 moisture field more pronouncedly than do the divergent waves, leading the authors to speculate that the slow/rotational versus fast/wavelike distinction in precipitation characteristics may correspond to the different balances of two main convective coupling mechanisms: moisture control of cumulus cells versus convective inhibition control (via low-level density waves) of mesoscale convective systems.

The Madden–Julian oscillation (MJO) is unique in that it exhibits prominent modulation of both stratiform precipitation (like the fast divergent waves) and small-sized precipitation features, convective rainfall, and moisture (like the other low-frequency, rotational waves). A composite of other waves’ amplitudes as a function of MJO amplitude and phase shows that divergent waves are more active in the developing phase and rotational waves are more active in the decaying rather than developing phase of the MJO.

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Kazuaki Yasunaga and Brian Mapes

Abstract

Precipitation-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 (Kelvin, EIGn0, WIG) but shows little coherence with rain for more rotational disturbance types (ERn1, MRG, TD). These two broad families also exhibit different rain–PW phase lags, a result supportive of the notion that stratiform rain (which occurs in dry conditions after peak PW and rain) is more involved in the more divergent wave types.

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Kazuaki Yasunaga, Akihiro Hashimoto, and Masanori Yoshizaki

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 with intensified condensation to compensate for strong cooling due to the melting of ice particles. The enhancement of condensation continues until ice is no longer provided to the cloud at the melting level. This means that the cloud survives for a longer period than cloud at other levels.

To investigate the influence of the commonly observed tropical stable layer on the occurrence of midlevel thin cloud, the authors performed three sensitivity tests in which a warm rain microphysics scheme was employed and/or the initial temperature profile had enhanced and weakened stability layers in the middle level. Comparisons among the control and sensitivity experiments revealed that intensified condensation related to melting cooling plays a critical role in the formation of midlevel thin cloud, although the stable layer is associated with the inhibition of convection growth in the middle level. A three-dimensional experiment under more realistic conditions simulated cloud formation at the 0°C level, although the peak of the cloud coverage was less prominent than those in the two-dimensional experiments.

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Kuniaki Inoue, Ángel F. Adames, and Kazuaki Yasunaga

Abstract

A new diagnostic framework is developed and applied to ERA-Interim to quantitatively assess vertical velocity (omega) profiles in the wavenumber–frequency domain. Two quantities are defined with the first two EOF–PC pairs of omega profiles in the tropical ocean: a top-heaviness ratio and a tilt ratio. The top-heaviness and tilt ratios are defined, respectively, as the cospectrum and quadrature spectrum of PC1 and PC2 divided by the power spectrum of PC1. They represent how top-heavy an omega profile is at the convective maximum, and how much tilt omega profiles contain in the spatiotemporal evolution of a wave. The top-heaviness ratio reveals that omega profiles become more top-heavy as the time scale (spatial scale) becomes longer (larger). The MJO has the most top-heavy profile while the eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves have the most bottom-heavy profiles. The tilt ratio reveals that the Kelvin, WIG, EIG, and mixed Rossby–gravity (MRG) waves, categorized as convectively coupled gravity waves, have significant tilt in the omega profiles, while the equatorial Rossby (ER) wave and MJO, categorized as slow-moving moisture modes, have less tilt. The gross moist stability (GMS), cloud–radiation feedback, and effective GMS were also computed for each wave. The MJO with the most top-heavy omega profile exhibits high GMS, but has negative effective GMS due to strong cloud–radiation feedbacks. Similarly, the ER wave also exhibits negative effective GMS with a top-heavy omega profile. These results may indicate that top-heavy omega profiles build up more moist static energy via strong cloud–radiation feedbacks, and as a result, are more preferable for the moisture mode instability.

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Kazuaki Yasunaga, Satoru Yokoi, Kuniaki Inoue, and Brian E. Mapes

Abstract

The budget of column-integrated moist static energy (MSE) is examined in wavenumber–frequency transforms of longitude–time sections over the tropical belt. Cross-spectra with satellite-derived precipitation (TRMM-3B42) are used to emphasize precipitation-coherent signals in reanalysis [ERA-Interim (ERAI)] estimates of each term in the budget equation. Results reveal different budget balances in convectively coupled equatorial waves (CCEWs) as well as in the Madden–Julian oscillation (MJO) and tropical depression (TD)-type disturbances. The real component (expressing amplification or damping of amplitude) for horizontal advection is modest for most wave types but substantially damps the MJO. Its imaginary component is hugely positive (it acts to advance phase) in TD-type disturbances and is positive for MJO and equatorial Rossby (ERn1) wave disturbances (almost negligible for the other CCEWs). The real component of vertical advection is negatively correlated (damping effect) with precipitation with a magnitude of approximately 10% of total latent heat release for all disturbances except for TD-type disturbance. This effect is overestimated by a factor of 2 or more if advection is computed using the time–zonal mean MSE, suggesting that nonlinear correlations between ascent and humidity would be positive (amplification effect). ERAI-estimated radiative heating has a positive real part, reinforcing precipitation-correlated MSE excursions. The magnitude is up to 14% of latent heating for the MJO and much less for other waves. ERAI-estimated surface flux has a small effect but acts to amplify MJO and ERn1 waves. The imaginary component of budget residuals is large and systematically positive, suggesting that the reanalysis model’s physical MSE sources would not act to propagate the precipitation-associated MSE anomalies properly.

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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.

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Kazuaki Yasunaga, Kunio Yoneyama, Qoosaku Moteki, Mikiko Fujita, Yukari N. Takayabu, Junko Suzuki, Tomoki Ushiyama, and Brian Mapes

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 less connected to those at the surface.

SST diurnal variations were enhanced in light-wind and clear conditions. These coincided with westerly anomalies in prominent 6–8-day zonal wind variations with a deep nearly barotropic structure through the troposphere. Westward propagation and structure of time-filtered winds suggest n = 1 equatorial Rossby waves, but with estimated equivalent depth greater than is common for convectively coupled waves, although sheared background flow complicates the estimation somewhat.

An ensemble reanalysis [i.e., the AGCM for the Earth Simulator (AFES) Local Ensemble Transform Kalman Filter (LETKF) Experimental Reanalysis (ALERA)] shows enhanced spread among the ensemble members in the zonal confluence phase of these deep Rossby waves, suggesting that assimilating them excites rapidly growing differences among ensemble members.

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Kazuaki Yasunaga, Kunio Yoneyama, Hisayuki Kubota, Hajime Okamoto, Atsushi Shimizu, Hiroshi Kumagai, Masaki Katsumata, Nobuo Sugimoto, and Ichiro Matsui

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, rather than by detrainment of surface-based convection.

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