Climate System Research/National Institute For Environmental Studies Research Articles

Observing System Simulation Experiment to Reproduce Kelvin Wave in the Venus Atmosphere

Planetary-scale 4-day Kelvin-type waves at the cloud top of the Venus atmosphere have been reported from the 1980s, and their significance for atmospheric dynamics has been pointed out. However, these waves have not been reproduced in Venus atmospheric general circulation models (VGCMs). Recently, horizontal winds associated with the planetary-scale waves at the cloud top have been obtained from cloud images taken by cameras onboard Venus orbiters, which could enable us to clarify the structure and roles of Kelvin-type waves. In order to examine this possibility, our team carried out an idealized observing system simulation experiment (OSSE) with a data assimilation system which we developed. The wind velocity data provided by a CCSR/NIES (Center for Climate System Research/National Institute for Environmental Studies) VGCM where equatorial Kelvin-type waves were assumed below the cloud bottom was used as idealized observations. Results show that 4-day planetary-scale Kelvin-type waves are successfully reproduced if the wind velocity between 15° S and 15° N latitudes is assimilated every 6 h at 70 km altitude. It is strongly suggested that the Kelvin-type waves could be reproduced and investigated by the data assimilation with the horizontal wind data derived from Akatsuki ultraviolet images. The present results also contribute to planning future missions for understanding planetary atmospheres.

A study on the formation and trend of the Brewer-Dobson circulation

[1] The Brewer-Dobson circulation (BDC) is approximately expressed by the residual circulation (RC) and considered to be driven by the body force induced by the breaking and/or dissipation of atmospheric waves. The contribution of different types of waves to the RC in the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) Chemistry Climate Model (CCM) is diagnosed using the “downward control principle (DC).” Gravity wave drag (GWD) including orographic gravity wave drag (OGWD) has a great influence on the RC in the low and middle latitudes of the lower stratosphere. In particular, the summer hemispheric low-latitude part of winter circulation is mainly formed by the GWD. These results are consistent with the estimates of the GWD contribution using reanalysis data by subtracting the resolved wave contribution from the RC with DC principle. In addition, it is seen that the net upward mass flux on the 70 hPa surface is strengthened during the 21st century because of the upward shift of the OGWD, which is consistent with previous studies. These conclusions indicate that gravity waves play an important role in maintaining the BDC.

Ozone and temperature response of a chemistry climate model to the solar cycle and sea surface temperature

The results from a three‐dimensional chemistry climate model (CCM) of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) were analyzed for 1980–2000 to detect variations due to the 11 year solar cycle. Multiple regression analysis, including the constant, linear trend, solar, quasi‐biennial oscillation, volcanic, and sea surface temperature (SST) terms, was applied for ozone and temperature of the CCM for the REF1 scenario of CCM Validation. Using observed SST as a lower boundary condition, the results suggest that the contribution of solar cycle forcing in the CCSR/NIES CCM is about 1% in ozone volume mixing ratio and 0.2 K in temperature per 100 units of 10.7 cm solar microwave flux in the tropical lower stratosphere for the period 1980–2000. A change in ozone transport may be the main factor for the solar signal of ozone concentration in the lower stratosphere. Another sensitivity experiment excluding solar cycle forcing shows a small solar response in the lower stratosphere, suggesting that the interannual variability of the SST could contribute to the solar term in the lower stratosphere through troposphere‐stratosphere processes and/or have an effect as an artifact of interference due to the insufficient period for analysis.

A CCM simulation of the breakup of the Antarctic polar vortex in the years 1980–2004 under the CCMVal scenarios

The changes in breakup time of the Antarctic polar vortex in the years 1980–2004 are examined using the output of chemistry climate model (CCM) calculations, data from the National Centers for Environmental Prediction/the National Center for Atmospheric Research (NCEP/NCAR) Reanalysis, and data from the European Center for Medium‐Range Weather Forecasts Reanalysis (ERA40). The CCM used in this study is from the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES). The CCM calculations are performed with the two ensemble members for REF1 scenario of the chemistry climate model validation (CCMVal) and the one ensemble member for the REF2 scenario. CCM simulates the development of the ozone hole from 1982 to 2000, as observed with a total ozone mapping spectrometer (TOMS), although the year‐to‐year variation is different from the observation owing to the internal variability of CCM and the ozone decreasing trends of CCM ozone in the two ensemble members of REF1 are underestimated. The trends in temperature and zonal mean zonal wind are analyzed and compared with the observations. There is consistency among the trends in zonal mean temperature, zonal mean zonal wind, and total ozone, but they differ among the ensemble members and observations. The diabatic heating rates and Eliassen‐Palm flux fields are investigated in order to explain the differences. A delay trend in the breakup time of the Antarctic polar vortex is obtained for the period of 1980–1999 in the NCEP/NCAR and ERA40 data. A similar trend is also obtained from the CCM simulations, with statistical significance in one ensemble member of REF1 and REF2. Because the trends of the observations in the EP flux from the troposphere and its deposition in the lower stratosphere are consistent with an advanced breakup date of the polar vortex and because the trends of the CCM simulations are very small, it is likely that the Antarctic ozone depletion had some effect on the delay during the period 1980–1999. From 2000 to 2004, the NCEP/NCAR data show a large variation in breakup time, which makes the delay trend much less important. It is likely that the large variation in wave flux masked the effects of the ozone loss during that period. The two ensemble members of the REF1 simulation do not show such a dramatic change in the trend for the period 2000–2004, whereas REF2 shows a change in the trend for that period.

Midlatitude and high‐latitude N2O distributions in the Northern Hemisphere in early and late Arctic polar vortex breakup years

Simulated N2O distributions at midlatitudes and high latitudes in the Northern Hemisphere are analyzed in early and late vortex breakup years with the probability distribution function (PDF) technique. The data are from a Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) nudging chemical transport model (CTM) for 24 years from 1979 to 2002. Results show that there is a large difference in midlatitude and high‐latitude N2O concentrations on the 600 K isentrope between early and late vortex breakup years. In the early breakup years, the N2O concentration with the maximum area shows low values in the lower stratosphere in the springtime after the vortex breakup. In the late breakup years, the maximum area concentration shows constant high values from the winter to the summer. Our analyses show that the winter and springtime meridional circulation is a main factor for these differences in N2O concentration. In the early breakup years, a larger eddy heat flux causes a stronger winter meridional circulation and a stronger downward advection of low‐N2O concentration air at higher altitudes to the lower stratosphere, which leads to the low values of N2O concentration in the lower stratosphere in late winter and early spring. Inside the Arctic vortex, however, the importance of vertical advection is smaller than or comparable to other processes such as horizontal divergence and subgrid‐scale motions. These results are consistent with the previous studies on tracer distribution, which showed that not only the vertical advection but also the horizontal eddy transport are important for tracer concentration tendency in the polar vortex.

Ozone decrease outside Arctic polar vortex due to polar vortex processing in 1997

We examine the effect of polar vortex processing on ozone concentrations outside the 1997 Arctic polar vortex. The Arctic vortex in this year was well isolated, cold, and circumpolar, and it broke up unusually late. However, time threshold diagnostics (TTD) analysis using a middle vortex boundary defined by the first derivative of the equivalent latitude gradient of potential vorticity and calculations using the nudging chemical transport model (CTM) of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) show that there were intermittently several relatively large transport events from the vortex to the outside region in the lower stratosphere, with timescales and spatial scales that can be resolved at T42 CTM horizontal resolution (2.8° by 2.8° grid). These intermittent outflow events of polar air are also identified in TTD analysis using an outer vortex boundary defined by the second derivative of potential vorticity and a boundary defined by the N2O concentration. These intermittent events had a significant effect on the ozone concentration outside the vortex near the boundary in this year. A CTM calculation with a polar chemical ozone tracer shows that the effect on the ozone concentration outside the polar vortex near the vortex boundary in the equivalent latitude band of 55°–65°N and 450 K is 0.3 ppmv (15–20% of the ozone concentration at this height) and that on the total ozone is 12–15 Dobson units (1 DU = 0.001 atm cm) (3–4% of the total ozone) by the end of April just before the final vortex breakup. The effect in the equivalent latitude band of 30°–60°N is much smaller, with a reduction of 2 DU at the end of March and 4 DU by the end of April (less than 1% of the total ozone). The effect is about the half if we use the inner boundary or a boundary of 73°N equivalent latitude for the polar tracer calculations. The CTM calculations also show that these polar vortex processing effects might be masked at midlatitudes by the local gas phase chemical ozone production/loss reactions after mid‐April at 450 K and earlier than those at 500 K.

Isotopic composition and origin of snow over Siberia

The spatial isotopic distribution of the snowpack over Siberia, where winter temperatures are as cold as those of the polar regions, was observed by the Trans‐Siberian Snow Survey (TSSS) and Trans‐Verkhoyansk Snow Survey Expedition (TVSSE) in March 2000 and March 2001. The results show inland δD depletion and a slightly increasing deuterium excess value, d, in the snowpack over Siberia. To explore the relationship between source region variability and the isotopic composition of snow, a model simulation was performed that reproduced the observed isotopic composition of snow. Moisture sources for Siberian precipitation were estimated using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (AGCM). A simple isotopic model was used to evaluate the total isotopic changes during transport from the designated source region to the region of precipitation. The results showed that the variability of the contribution of each source to the snow results in large isotopic variability, and the fact that the model reproduced the observed inland depletion of δD in snowpack suggests that GCM‐predicted source contributions were verified by observed values. However, the modeled d values did not match observed d values over Siberia. Observations of d values in precipitation show an increase during autumn toward a maximum in late autumn and then a decrease during winter; however, the modeled d value reached a maximum in early autumn and decreased toward a minimum in winter. The simple isotope model does not consider additional moisture evaporation joining an airmass moving from a source region. Therefore the disagreement between the modeled and observed d values of snow suggests that moisture supplied from the land surface during transportation significantly contributes to autumn snow. The increased d values of Siberian snow show that evaporation from open water or from the soil surface, which are accompanied by isotopic fractionation, are more important than transpiration flux, which does not change the isotopic content. The contribution of land‐derived moisture that has evaporated from open water plays an important role in eastern Siberian snow.

Ozone perturbations in the Arctic summer lower stratosphere as a reflection of NOX chemistry and planetary scale wave activity

Ozone concentration perturbations in the high‐latitude lower stratosphere in the Northern Hemisphere were observed by Improved Limb Atmospheric Spectrometer (ILAS) after the polar vortex breakdown at the beginning of May 1997 and until the end of June of that same year. Simulations and a passive tracer experiment using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) nudging chemical transport model (CTM) show that the low‐ozone perturbations observed in May were caused by the Arctic polar vortex debris, while those after the end of May resulted from a dynamical elongation due to zonal wave number 2 planetary waves of the low‐ozone region in the summer polar stratosphere, which had been developed by the catalytic ozone destruction cycle of NOX. These low‐O3 air masses of different origin were advected or elongated from the polar region to the ILAS measurement points. An episodic event of a dynamical O3 perturbation in June 1997 on a chemically induced meridional O3 gradient is described. These results show that a timing of the polar vortex breakdown and activity of planetary waves after the breakdown may affect the O3 background gradient in the summer lower stratosphere at middle and high latitudes.

Analysis of year‐to‐year ozone variation over the subtropical western Pacific region using EP_TOMS data and CCSR/NIES nudging CTM

The year‐to‐year ozone variation over the subtropical western Pacific region is studied, especially the ozone lows in the 1996/1997, 1998/1999, and 2001/2002 winters, using the Earth Probe Total Ozone Mapping Spectrometer (EP_TOMS) ozone data from August 1996 to July 2002. Regression analyses show that dynamical signals, such as the quasi‐biennial oscillation, play an important role in determining total ozone variation. A nudging chemical transport model (CTM) is used to simulate the year‐to‐year ozone variation and explain the mechanism for producing ozone lows in a three‐dimensional distribution of ozone. The CTM was developed using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model and introducing a nudging process for temperature and horizontal wind velocity. The year‐to‐year ozone variation, especially the winter ozone low, is well simulated by the model excluding heterogeneous reaction processes between 45°S and 45°N latitude. Results show that the year‐to‐year ozone variation is mainly controlled by dynamical transport processes.

全球化学天気予報と可視化

A quasi-realtime calculation system for the global distribution of O3, NOx, HOx, and NMHCs by using a chemical AGCM for study of atmospheric environment and radiative forcing (CHASER) has been developed. The model is based on the version 5.4g of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) AGCM. A7 days'forecast is executed once a day by using the NCEP reanalysed and forecasted dynamical field, and the model result is available on the WWW.

大気大循環モデルでシミュレートされた北極振動の解析

The annular mode in the Northern Hemisphere (NH) extratropical circulation, which has an equivalent barotoropic structure from the surface to the lower stratosphere, is called the Arctic Oscillation (AO) by Thompson and Wallace (1998). The AO is the leading empirical orthogonal function (EOF) mode of the sea level pressure field, and it can be seen throughout the year. It is more dominant in winter. It is characterized as a seesaw pattern of mass, between the Arctic region and the midlatitude belt. It is also a seesaw of the mean zonal wind, between the region poleward of 40°N and that equatorward of it In this study, an analysis of the simulated AO is made, to confirm that the AO is an atmospheric interna. mode, and for studying the transition mechanism to the high/low polarity of the AO. The seasonal and perpetual February runs were made by using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (AGCM). In both runs. the AO signature is dominant. Its magnitude is similar to the observed one. Thus, it is confirmed that the AO is an internal mode of the atmosphere. Using data from a perpetual February integration, a composite analysis of the transition to the high/low polarity of the AO is made. Based upon the transformed Eulerian mean equation for the mean zonal wind, it is investigated which term is responsible for the transition. The results indicate that the wave forcing term contributes to the transition and the term of the residual meridional circulation acts to restore the transition. A decomposition of the wave forcing to zoual wave component indicates that planetary-scale wavenumbers 2 and 3 contribute most to the transition. The synoptic-scale waves contribute partly to the low-latitude wind change. In the perpetual February run, a slowly propagating AO-related stratosphere-troposphere coupled mode, is detected. Its oscillation period is 4-6 months. The anomaly of the zonal-mean, zonal wind first appears in the subtropical stratosphere and propagates poleward. Once the anomaly reaches high latitude, it becomes large and propagates into the troposphere.

The first simulation of an ozone QBO in a general circulation model

The quasi‐biennial oscillation (QBO) of ozone has been simulated for the first time in a general circulation model (GCM). The model employed is the first version GCM developed at the Center for Climate System Research / National Institute for Environmental Studies (CCSR/NIES). The model has a horizontal resolution of T21 and 60 layers in the vertical. Modified Chapman reactions of ozone photochemistry are included in the model, allowing the ozone to be fully interactive with the radiative and dynamical fields. The simulation is, for the most part, successful except that a deficiency of about 40 DU exists in the total ozone in comparison with observations. The simulated zonal wind QBO has a rather short period (about 1.5 years) and somewhat weak easterlies. The lower stratospheric ozone QBO over the equator is qualitatively similar to that observed, although the simulated amplitude of about 0.3 ppmv is somewhat less than that observed. The phase relationship between the equatorial upper and lower stratospheric ozone QBOs is successfully reproduced, although the phase of the modeled ozone QBO slightly precedes the observed phase in the upper stratosphere. The mid‐latitude ozone QBO is out of phase with the equatorial ozone QBO. The phase propagation of planetary‐scale structure in ozone from the mid‐latitudes to equatorial regions is more pronounced during the westerly than in the easterly phase of the stratospheric QBO, especially in the Northern Hemisphere winter.

準2年振動を再現した大循環モデルにおける赤道波について

The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere has been well simulated in the first version of the Center for Climate System Research / National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (Takahashi, 1996). The model horizontal resolution is T21 with 60 layers of vertical resolution of about 500 m in the upper troposphere and lower stratosphere, with a moist convective adjustment scheme. The period of the simulated oscillation is 1.5 years, which is slightly shorter than that observed. The equatorial wave behavior in the model is examined using the spectral analysis method. The westerly acceleration phase of the QBO-like oscillation in the model is due to Kelvin waves and eastward-propagating gravity waves. The easterly acceleration phase of the oscillation is due to westward-propagating n = 1 equatorial gravity waves, random westward-propagating gravity waves and Rossby waves propagating from the mid-latitudes in the northern winter hemisphere to the equatorial region. The Rossby-gravity wave has a role in the easterly acceleration of the QBO-like oscillation in the present general circulation model.

Simulation of the stratospheric Quasi‐Biennial Oscillation using a general circulation model

The quasi‐biennial oscillation is fairly well simulated by use of a T21 60‐layer general circulation model with moist convective adjustment. The model is the first version of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) general circulation model. The vertical resolution is approximately 500m in the upper troposphere and lower stratosphere. The damping time for the maximum wavenumber in the fourth‐order horizontal diffusion is 20 days, being about one‐order of magnitude less than that in standard general circulation models. The period of the oscillation is 1.5 years, which is slightly shorter than that of the observed quasi‐biennial oscillation. The simulated oscillation occurs in the middle stratosphere, being slightly higher than that observed in the equatorial lower stratosphere. The amplitude of the zonal‐wind associated with the oscillation is 20 ms−1 which is comparable to the observed value.

3次元セクターモデルに於ける赤道波の振舞い

Equatorial waves in a three-dimensional sector (1/5) model with moist convective adjustment are examined. The model is derived from a preliminary version of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) general circulation model. The model produces a QBO-like oscillation in the equatorial upper stratosphere (Takahashi and Shiobara, 1995).There is a strong signal in the zonal wavenumber s=1 (global zonal wavenumber 5, since the model is a 1/5 sector), meridional mode number n=1 having a period of 2.5 days, corresponding to a westward-propagating equatorial gravity wave. The wave produces the dominant easterly phase of the QBO-like oscillation. The signal is also clearly seen in the precipitation. The eastward-propagating waves which produce the westerlies in the QBO-like oscillation are rather complicated. The signals are difficult to identify with equatorial waves, for example the Kelvin wave or n=1 equatorial gravity wave. The wave structures are like an ensemble of random gravity waves. These gravity waves produce the QBO-like oscillation in the sector model.A lower horizontal resolution experiment, using the T21 global model, is also performed to compare equatorial wave behavior. Kelvin and Rossby-gravity waves are simulated in the T21 general circulation model. Simulated gravity waves are, however, weak and as a result, the T21 model does not produce a QBO-like oscillation, but rather a steady state of easterlies in the equatorial upper stratosphere with westerlies in the middle stratosphere.

1/5セクター・3次元モデルに現れたQBO的な振動について

A QBO-like oscillation has been obtained in a simulation using the 1/5 sector three-dimensional model, with moist convective adjustment, derived from a preliminary version of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) general circulation model. The basic structure of the oscillation is similar to that of the observed quasi-biennial oscillation (QBO). The amplitude of the oscillation is rather strong, about 30ms-1, which is comparable to the observed oscillation (25ms-1). However, the period of the oscillation is about 400 days, or half as long as that of the observed QBO in the lower equatorial stratosphere. Further, the height of the simulated oscillation is about 15km higher than that of the observed QBO.From a preliminary analysis of the wave behavior, it is suggested that gravity waves are important in the simulation of the QBO-like oscillation.