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Hirohiko Masunaga

Abstract

The Madden–Julian oscillation (MJO), Kelvin wave, and equatorial Rossby (ER) wave—collectively called intraseasonal oscillations (ISOs)—are investigated using a 25-yr record of outgoing longwave radiation (OLR) measurements as well as the associated dynamical fields. The ISO modes are detected by applying bandpass filters to the OLR data in the frequency–wavenumber space. An automated wave-tracking algorithm is applied to each ISO mode so that convection centers accompanied with the ISOs are traced in space and time in an objective fashion. The identified paths of the individual ISO modes are first examined and found strongly modulated regionally and seasonally. The dynamical structure is composited with respect to the convection centers of each ISO mode. A baroclinic mode of the combined Rossby and Kelvin structure is prominent for the MJO, consistent with existing work. The Kelvin wave exhibits a low-level wind field resembling the shallow-water solution, while a slight lead of low-level convergence over convection suggests the impact of frictional boundary layer convergence on Kelvin wave dynamics. A lagged composite analysis reveals that the MJO is accompanied with a Kelvin wave approaching from the west preceding the MJO convective maximum in austral summer. MJO activity then peaks as the Kelvin and ER waves constructively interfere to enhance off-equatorial boundary layer convergence. The MJO leaves a Kelvin wave emanating to the east once the peak phase is passed. The approaching Kelvin wave prior to the development of MJO convection is absent in boreal summer and fall. The composite ER wave, loosely concentrated around the MJO, is nearly stationary throughout. A possible scenario to physically translate the observed result is also discussed.

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Hirohiko Masunaga

Abstract

The thermodynamic variability associated with moist convection over tropical oceans is analyzed by making use of a variety of satellite sensors including radars, an infrared and microwave sounder unit, and a microwave radiometer and scatterometer aboard different platforms. Satellite measurements of atmospheric parameters including air temperature, water vapor, cumulus cloud cover, and surface wind are composited with respect to the temporal lead or lag from Tropical Rainfall Measuring Mission (TRMM)-detected convection to obtain statistically continuous time series on hourly to daily time scales. The Atmospheric Infrared Sounder (AIRS)-observed temperature and humidity profiles, representing cloud-cleared sounding, are combined with semitheoretical estimates of in-cloud temperature and humidity to construct the large-scale mean field. Those measurements are ingested to the moisture and thermal budget equations integrated vertically over each layer separated by cloud base. This strategy makes it possible to evaluate the free-tropospheric (FT) convergence of moisture and dry static energy and their vertical flux at cloud base from satellite observations alone. The main findings include the following: 1) vertical moisture transport at cloud base is the dominant source of FT moistening prior to isolated cumulus development while overwhelmed by horizontal moisture convergence for highly organized systems; 2) FT diabatic heating is largely offset on an instantaneous basis; and 3) FT moistening by convective eddies amounts to a half of the total cloud-base moisture flux in the background state, while large-scale mean updrafts modulate the variability of cloud-base flux when highly organized systems develop. The known correlation between congestus clouds and FT moisture before deep convection may be accounted for by large-scale mean moisture updraft rather than congestus eddy moistening.

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Hirohiko Masunaga

Abstract

Satellite data are analyzed to explore the thermodynamic evolution of tropical and subtropical atmospheres prior and subsequent to moist convection in order to offer an observational test bed for convective adjustment, which is central to the quasi-equilibrium hypothesis. Tropical Rainfall Measuring Mission (TRMM) and Aqua satellite measurements are projected onto a composite temporal sequence over an hourly to daily time scale by exploiting the temporal gap between the local satellite overpasses, which changes from one day to another. The atmospheric forcing and response to convection are investigated separately for deep convective and congestus clouds. In the deep tropics, systematic moisture transport from the atmospheric boundary layer (ABL) to the free troposphere is evident in association with deep convection. The quick ABL ventilation suggests a swift convective adjustment but is preceded by a steady buildup of ABL moisture, which does not imply continuous adjustment to equilibrium. The evolution of convective available potential energy (CAPE) is controlled not only by the ABL moisture but also largely by a coincident ABL cooling linked with a bipolar anomaly of tropospheric temperature. The ABL moisture and temperature effects together lead to a rapid drop of CAPE for 12 h preceding convection, followed by a restoring phase that emerges as the cool anomaly recovers for a day or two. When moist convection is brought by congestus clouds with no deep convection nearby, CAPE gently increases over a period of 1–2 days until congestus occurs and then declines as slowly, suggestive of no efficient convective adjustment. The subtropical atmosphere shows no sign of convective adjustment whether or not vigorous convection is present.

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Hirohiko Masunaga

Abstract

In this study the observed relationship of precipitation with column relative humidity (CRH), a metric of tropospheric humidity, is examined in order to address a known discrepancy inherent to past studies. A composite analysis of satellite data is carried out to explore the short-term (i.e., from hourly to daily) atmospheric variability for comparison with the climatology, hypothesizing that a primary cause for the discrepancy arises from a difference in the time scale of interest. The analysis is broken down into four classes on the basis of the degree of convective organization, ranging from unorganized shallow cumuli to highly organized convective systems. The CRH–precipitation relationship is found to be extremely nonlinear for the short-term variability, while the nonlinearity weakens to some degree when different convective systems in diverse humidity environments are averaged together into climatology. The weak exponential rise in the climatological CRH–precipitation curve occurs because highly organized convective systems become more frequent and intense and thus receive increasing weight in the climatological mean as the environment moistens.

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Hirohiko Masunaga and Teruyuki Nakajima

Abstract

The influence of broken clouds on radiative flux has provided a major source of uncertainty in radiative transfer models of the atmosphere because plane-parallel approximations are assumed in most of the current atmospheric models, where horizontal inhomogeneity cannot be adequately taken into account. In this paper, effects by cloud inhomogeneity on longwave radiation fields are investigated, using a simple model of a cloud array that consists of identical cuboids following some past studies. In contrast to past work that adopted simplified formulations of radiative transfer, multistream radiative transfer is considered to obtain the exact solutions of radiative flux, which enable us to consider semitransparent clouds as well as optically thick clouds in desirable accuracy. Applicability to semitransparent clouds is important because cirrus clouds, which are considered to play significant roles for longwave radiation, are often semitransparent to infrared radiation.

The computational results show that the empirical formula previously derived by Harshvardhan and Weinman systematically underestimates the effective cloud fraction. An alternative formula is proposed for the effective cloud fraction to supply a better fit to the exact solution of radiative flux. Furthermore, new formulas are derived to approximate the exact solutions including the dependence on the optical thickness of clouds. They are useful to convert plane-parallel flux to 3D flux passing through broken clouds, either for optically thick or thin clouds.

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Yukari Sumi and Hirohiko Masunaga

Abstract

A moist static energy (MSE) budget analysis is applied to quasi-2-day waves to examine the effects of thermodynamic processes on the wave propagation mechanism. The 2-day waves are defined as westward inertia–gravity (WIG) modes identified with filtered geostationary infrared measurements, and the thermodynamic parameters and MSE budget variables computed from reanalysis data are composited with respect to the WIG peaks. The composite horizontal and vertical MSE structures are overall as theoretically expected from WIG wave dynamics. A prominent horizontal MSE advection is found to exist, although the wave dynamics is mainly regulated by vertical advection. The vertical advection decreases MSE around the times of the convective peak, plausibly resulting from the first baroclinic mode associated with deep convection. Normalized gross moist stability (NGMS) is used to examine the thermodynamic processes involving the large-scale dynamics and convective heating. NGMS gradually decreases to zero before deep convection and reaches a maximum after the convection peak, where low (high) NGMS leads (lags) deep convection. The decrease in NGMS toward zero before the occurrence of active convection suggests an increasingly efficient conversion from convective heating to large-scale dynamics as the wave comes in, while the increase afterward signifies that this linkage swiftly dies out after the peak.

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Hirohiko Masunaga and Sandrine Bony

Abstract

This work seeks evidence for convective–radiative interactions in satellite measurements, with a focus on the variability over the life cycle of tropical convection in search of the underlying processes at a fundamental level of the convective dynamics. To this end, the vertical profiles of cloud cover and radiative heating from the CloudSatCALIPSO products are sorted into a composite time series around the hour of convective occurrence identified by the TRMM PR. The findings are summarized as follows. Cirrus cloud cover begins to increase, accompanied by a notable reduction of longwave cooling, in moist atmospheres even 1–2 days before deep convection is invigorated. In contrast, longwave cooling stays efficient and clouds remain shallow where the ambient air is very dry. To separate the radiative effects by the preceding cirrus clouds on convection from the direct effects of moisture, the observations with enhanced cirrus cover are isolated from those with suppressed cirrus under a moisture environment being nearly equal. It is found that rain rate is distinctly higher if the upper troposphere is cloudier regardless of moisture, suggesting that the cirrus radiative effects may be linked with the subsequent growth of convection. A possible mechanism to support this observational implication is discussed using a simple conceptual model. The model suggests that the preceding cirrus clouds could radiatively promote the moistening with the aid of the congestus-mode dynamics within a short period of time (about 2 days) as observed.

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Kaya Kanemaru and Hirohiko Masunaga

Abstract

The known characteristics of the relationship between sea surface temperature (SST) and column water vapor (CWV) are reevaluated with recent satellite observations over tropical and subtropical oceans. Satellite data acquired by the Aqua Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E), Atmospheric Infrared Sounder (AIRS)/Advanced Microwave Sounder Unit (AMSU) suite, the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR), and the Quick Scatterometer (QuikSCAT) SeaWinds are analyzed together for 7 years from October 2002 to September 2009. CWV is decomposed into surface humidity, presumably coupled closely to SST, and the water vapor scale height as an index of vertical moisture gradient between the boundary layer and the free troposphere. Surface relative humidity is climatologically homogeneous across tropical and subtropical oceans, while the dependence of CWV on SST varies from one region to another. SST mainly accounts for the variation of CWV with the water vapor scale height, which is virtually invariant over subtropical oceans. On the other hand, over tropical oceans, the variability of CWV is explained not only by SST but also by a systematic change of the water vapor scale height. The regional contrast between tropical and subtropical oceans is discussed in the context of the regional moisture budget including vertical moisture transport through convection.

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Yukari Sumi and Hirohiko Masunaga

Abstract

A quasi-2-day wave is known as a convectively coupled westward inertia–gravity (WIG) wave with a shallower equivalent depth (or slower phase speed) than the dry counterpart. This study investigates the relationship between the phase speed of quasi-2-day waves and effective static stability in terms of a vertical mode perspective. By using WIG filters with different equivalent depths, different phases of the 2-day wave are identified by filtering brightness temperature data obtained from geostationary satellites. The composite time series and the vertical modes in the tropical atmosphere are calculated from reanalysis data. The large-scale dynamical fields of the composite WIG waves are explained by the superposition of the first four baroclinic modes. Phase speed of the moist vertical mode is computed by applying the Radon transform to the mode transform coefficient. Different vertical modes share a common phase speed, which is slower than its dry counterpart, implying that the wave is not dispersive. To address the question of what slows the vertical modes, the effective static stability is evaluated by defining the degree of cancellation between diabatic heating and adiabatic cooling due to the ascent. This cancellation is confirmed to be almost complete for the first baroclinic mode as expected theoretically. The effective static stability is found to be higher for a higher vertical mode, but this change over different vertical modes is not as rapid as predicted from nondispersiveness. Possible reasons for this disagreement are discussed herein.

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Kaya Kanemaru and Hirohiko Masunaga

Abstract

The current study is aimed at exploring the potential roles of the seasonally altering background surface wind in the seasonality of the intraseasonal oscillations (ISOs) with a focus on the sea surface temperature (SST) variability. A composite analysis of the ocean mixed layer heat budget in term of ISO phases with various satellite data is performed for boreal winter and summer. The scalar wind is found to be a dominant factor that accounts for the ocean surface heat budget, implying that the background surface wind as well as its anomaly is important for the SST variability. An easterly anomaly to the east of convection diminishes scalar wind, and thus latent heat flux, when superposed onto a background westerly wind, implying that the presence of basic westerly wind is important for the development of a warm SST anomaly ahead of the ISO convection. On the other hand, an easterly anomaly in combination with basic easterly wind magnifies scalar wind and latent heat flux and cancels out the shortwave heat flux anomaly. The seasonal migration of the background westerly wind, which is confined to a southern equatorial belt in boreal winter but spread across the northern Indian Ocean in boreal summer, may offer a mechanism that partly accounts for the seasonal characteristics of ISO propagation. The northward propagation of the SST variability associated with the boreal summer ISO is found to also involve a similar mechanism with the meridional wind modulation of scalar wind.

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