Atusi Numaguti

Global three‐dimensional simulation of aerosol optical thickness distribution of various origins

A global three-dimensional model that can treat transportation of various species of aerosols in the atmosphere is developed using a framework of an atmospheric general circulation model (AGCM). Main aerosols in the troposphere, i.e., soil dust, carbonaceous (organic and black carbon), sulfate, and sea-salt aerosols, are introduced into this model. Prior to the model calculations the meteorological parameters are calculated by the AGCM with the nudging technique using reanalysis data. To evaluate aerosol effects on the climate system and to compare simulated results with observations, the optical thickness and Angstrom exponent are also calculated taking into account the size distribution and composition. The model results are validated by both measured surface aerosol concentrations and retrieved aerosol optical parameters from National Oceanic and Atmospheric Administration/Advanced Very High Resolution Radiometer. A general agreement is found between the simulated result and the observation globally and seasonally. One of the significant results is that the simulated relative contribution of anthropogenic carbonaceous aerosols to the total optical thickness is comparable to that of sulfate aerosols at midlatitudes of the Northern Hemisphere, which agrees with recent observations. This result leads to a conclusion that the radiative effect evaluation of aerosols on the climate system is necessary to be modified because optical properties of carbonaceous aerosols are different from those of sulfate aerosols. The other finding is that the seasonal shift off the west coast of North Africa observed by satellites, i.e., the latitude of the maximum optical thickness moves seasonally, is also reproduced in consideration of a mixed state of soil dust and carbonaceous aerosols.

Origin and recycling processes of precipitating water over the Eurasian continent: Experiments using an atmospheric general circulation model

By using an atmospheric general circulation model, origin and transport processes of water in the atmosphere-land system are examined. The water vapor and the land-surface water in the model are tagged according to the geographical regions of water input (evaporation) and are separately treated. The results are examined focusing on the water cycle over the Eurasian continent. In the winter season, most of the precipitating water over the Eurasian continent is supplied by evaporation from the oceans. On the other hand, the precipitating water in the summer is mostly supplied by the evaporation from the continental surface, indicating active recycling of water between the atmosphere and the land in this season. Considering that the water in the continental surface should be supplied from the oceans sometime before, the history of the water from its origin (evaporation from the oceans) is examined, by separately treating the components of the soil water and the snow according to the geographical regions of the origin. Two additional types of tracers are included in order to determine the timescale of the transport and the frequency of the recycling between the atmosphere and the land. The results show that the main origin of water in the northern part of the Eurasian continent is the Northern Atlantic Ocean and that in the southern part is the Northern Indian Ocean. In the southern part the mean age of the precipitating water since its origin is 1 month or shorter, and the mean count of recycling is less than one, indicating that the water comming directly from the oceans by atmospheric transport is dominant. In the northern inland part in summer, however, the mean age is 3 months or longer, and the mean count of recycling is above two. These results suggest that a significant portion of the precipitating water in inland Eurasia in the summer originals in the Atlantic Ocean in the previous winter and is transported eastward with a few recycling cycles between the atmosphere and the continental surface.

Tropical Atlantic air‐sea interaction and its influence on the NAO

An atmospheric general circulation model (AGCM) is forced with a prescribed SST dipole anomaly in the tropical Atlantic to investigate the cause of cross-equatorial SST gradient (CESG) variability and its teleconnection to the extratropics. The model response bears a striking resemblance to observations in both the tropics and extratropics. The tropical response is robust and can act to reinforce the prescribed SST anomalies through wind-induced evaporation. A new feedback mechanism involving low-level stratiform clouds in the subtropics is also identified in the model and observations. The tropical SST dipole forces a barotropic teleconnection into the extratropics that projects onto the North Atlantic Oscillation (NAO). It further induces the extratropical portion of the North Atlantic SST tripole when the AGCM is coupled with an ocean mixed layer model. CESG variability thus appears to be the centerpiece of a pan-Atlantic climate pattern observed to extend from the South Atlantic to Greenland.

Dynamics and Energy Balance of the Hadley Circulation and the Tropical Precipitation Zones: Significance of the Distribution of Evaporation

Abstract A series of numerical experiments is performed using a general circulation model with an idealistic ocean-covered boundary condition. The meridional structure of the Hadley circulation system, which is a combined structure of the Hadley circulation and the tropical precipitation zone, is examined from the standpoints of the water vapor and the energy budgets. Although the prescribed SST distribution has a broad peak centered at the equator, the distribution of the precipitation has two peaks straddling the equator. The distribution of the evaporation rate is revealed to be an important factor in the formation of this structure. The evaporation rate is smaller near the equator than in the subtropics because of its dependence upon the wind speed. If this dependence is removed from the parameterization of the evaporation, the latitudinal distribution of evaporation becomes flat and the precipitation concentrates at the equator to form a single band structure. Qualitatively similar results are obtain...

A Study of the Aerosol Effect on a Cloud Field with Simultaneous Use of GCM Modeling and Satellite Observation

The indirect effect of aerosols was simulated by a GCM for nonconvective water clouds and was compared with remote sensing results from the Advanced Very High Resolution Radiometer (AVHRR) satellite-borne sensor for January, April, July, and October of 1990. The simulated global distribution of cloud droplet radius showed a land‐sea contrast and a characteristic feature along the coastal region similar to the AVHRR results, although cloud droplet radii from GCM calculations and AVHRR retrievals were different over tropical marine regions due to a lack of calculation of cloud‐aerosol interaction for convective clouds in the present model and also due to a possible error in the satellite retrieval caused by cirrus and broken cloud contamination. The simulated dependence of the cloud properties on the column aerosol particle number was also consistent with the statistics obtained by the AVHRR remote sensing when a parameterization with the aerosol lifetime effect was incorporated in the simulation. The global average of the simulated liquid water path based on the parameterization with the aerosol lifetime effect showed an insignificant dependence on the aerosol particle number as a result of a global balance of the lifetime effect and the wash-out effect. This dependence was contrary to the results of simulations based on the Sundqvist’s parameterization without aerosol lifetime effect; that is, the simulated cloud liquid water path showed a decreasing tendency with the aerosol particle number reflecting only the wash-out effect.

Dynamics and Energy Balance of the Hadley Circulation and the Tropical Precipitation Zones. Part II: Sensitivity to Meridional SST Distribution

Abstract A series of GCM experiments is performed in order to examine the dynamics of the time-averaged distribution of precipitation and Hadley circulation in low-latitude areas. As an extension to Part I of this study, the sensitivity to the latitudinal distribution of the globally prescribed sea surface temperature (SST) is examined. When the peak of SST is not at the equator but nearby, a single precipitation peak appears on the opposite side of the equator. When the latitude of the SST peak becomes high enough (about 15°), an abrupt jump of the precipitation peak is observed. This behavior is completely different if the evaporation from the ocean is estimated independent of the surface wind speed. Factors controlling the latitude of the precipitation zone are discussed. It is found that one essential component is the convergent flow in the planetary boundary layer, which is driven by the SST gradient. Another important factor is the net moist static energy input into the atmosphere, which is the diff...

Role of Transients in the Dynamics of East Asian Summer Seasonal Mean Circulation Anomalies—A Study of 1993 and 1994

Abstract By using the National Centers for Environmental Prediction reanalysis data and a nonlinear barotropic model, the transient activities and the possible role of transient vorticity fluxes in the maintenance of east Asian summer seasonal mean circulation anomalies associated with Japan’s extremely cool and wet summer of 1993 and hot and dry summer of 1994 are examined. Data analysis shows that the summertime atmospheric circulation and the transient activity anomalies exhibit nearly opposite patterns over east Asia during these two years. Vorticity budget calculations indicate that the anomalies of transient activities in these two years produced strong transient vorticity flux forcing anomalies that are comparable in magnitude to the divergent forcing anomalies over east Asia. The nonlinear barotropic model, when forced by the anomalous divergence and transient vorticity flux forcing together, produces simulations that bear resemblance to the observed summertime seasonal mean circulation anomalies ...

Effect of aerosols on cloud field with satellite-derived and GCM simulation

Numerical experiment was performed using a general circulation model (GCM) including aerosol indirect effect into water cloud and the simulated global distribution of cloud droplet radii was compared with the global distribution of cloud effective radii retrieved from Advanced Very High Resolution Radiometer (AVHRR). Comparisons of GCM calculation with AVHRR retrieval showed that our GCM generally can simulate the global characteristics of cloud droplet radii such as a land-sea contrast associated with difference of aerosol abundance and coastal region features due to aerosol injection from adjacent continental area. AVHRR retrieval and GCM simulation, however, are turned out to show disagreement over tropical region. AVHRR retrieval may tend to overestimate droplet radii due to the contamination of signal by drizzles and ice particles, whereas our GCM does not treat aerosol indirect effect in deep convective clouds predominant over tropics. Over equatorial central Pacific, where satellite retrieval may suffer from statistical biases, satellite retrieval and GCM simulation are also found to be different.

Global three-dimensional simulation and radiative forcing of various aerosol species with GCM

A global three-dimensional transport model that can simultaneously treat main tropospheric aerosols, i.e., carbonaceous (organic and black carbons), sulfate, soil dust, and sea salt, is developed. It is coupled with a Center for Climate System Research (CCSR)/National Institute for Enviormental Studies (NIES) atmospheric general circulation model (AGCM), and the meteorological field of wind, temperature, and specific humidity can be nudged by reanalysis data. Simulated results are compared with not only observations for aerosol concentrations but also the optical thickness and Angstrom exponent retrieved from remote sensing data such as National Oceanic and Atmospheric Administration (NOAA)/Advanced Very High Resolution Radiometer (AVHRR) and Aerosol Robotic Network (AERONET). A general agreement is found between simulated results and observations spatially seasonally, and quantitatively. The present model is also coupled with the radiative process over both the solar and thermal regions. The annual and global mean radiative forcing by anthropogenic aerosols from fossil fuel sources is estimated to be -0.5 W m-2 over the clear sky for the direct effect and -2.0 W m-2 for the indirect effect.