Atusi Numaguti

Bio: Atusi Numaguti is an academic researcher from Hokkaido University. The author has contributed to research in topics: Precipitation & Aerosol. The author has an hindex of 19, co-authored 33 publications receiving 2064 citations. Previous affiliations of Atusi Numaguti include University of Tokyo.

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

Abstract: 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

Abstract: 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.

Modeling of the radiative process in an atmospheric general circulation model

Abstract: A new radiation scheme has been developed for dynamic general-circulation modeling An automatic determination of k-distribution parameters and a treatment of solar–terrestrial radiation interacting with gaseous and particulate matter are incorporated into the scheme by a technique that combines discrete ordinate and matrix operator methods An accelerated scheme for cloud overlap is developed and tested The resultant accuracy of the scheme is ±05 K/day to a 70-km height in clear sky better than that of the line-by-line calculation method

Future climate change: Implications for Indian summer monsoon and its variability

Abstract: The broad climatological features associated with the Asian monsoon circulation, including its mean state and intraseasonal and interannual variability over the Indian subcontinent, as simulated in the CCSR/NIES coupled A–O GCM in its control experiment are presented in this paper. The model reproduces the seasonal cycle as well as basic observed patterns of key climatic parameters, in spite of some limitations in simulation of the monsoon rainfall. While the seasonality in rainfall over the region is well simulated and the simulated area-averaged monsoon rainfall is only marginally higher than the observed rainfall, the peak rainfall is simulated to be about two-thirds of the observed precipitation intensity over central India. . The transient experiments performed with the model following the four SRES ‘Marker’ emission scenarios, which include revised trends for all the principal anthropogenic forcing agents for the future, suggest an annual mean area-averaged surface warming over the Indian subcontinent to range between 3.5 and 5.5°C over the region during 2080s. During winter, India may experience between 5 and 25% decline in rainfall. The decline in wintertime-rainfall over India is likely to be significant and may lead to droughts during the dry summer months. Only a 10 to 15% increase is projected in area-averaged summer monsoon rainfall over the Indian subcontinent. The date of onset of summer monsoon over India could become more variable in future.

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

Abstract: 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.

The Changing Character of Precipitation

Abstract: From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital....

Global indirect aerosol effects: a review

Abstract: . Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei, they may inhibit freezing and they could have an influence on the hydrological cycle. While the cloud albedo enhancement (Twomey effect) of warm clouds received most attention so far and traditionally is the only indirect aerosol forcing considered in transient climate simulations, here we discuss the multitude of effects. Different approaches how the climatic implications of these aerosol effects can be estimated globally as well as improvements that are needed in global climate models in order to better represent indirect aerosol effects are discussed in this paper.