We derived rain rate, cloud optical thickness and the effective radius in water clouds by a combined use of the Precipitation Radar and the Visible and Infrared Scanner onboard the Tropical Rainfall Measuring Mission. The derived data were used to study how cloud optical thickness relates to precipitation. In particular, we focused on the changes in cloud optical thickness resulted from changes in the size distributions of cloud droplets associated with precipitation. There were considerable scatter between cloud optical thickness and rain rate on a global scale. However, cloud optical thickness was found to increase with rain rate on average. The tendency to increase was mostly due to increases in liquid water path and depended on rain rate. For strong rain, relatively small increases in the optical thickness with rain rate were observed. Whereas, for weak rain, larger increases with rain rate were found, which is related to considerable changes in liquid water path and in the effective radius of cloud droplets. To study the effects of drop size variation, the relationships between cloud optical thickness and rain rate for same values of liquid water path were analyzed. Results show that there were no significant dependences of cloud optical thickness on rain rate for strong rain. For weak rain, cloud optical thickness was found to decrease with rain rate. In particular, significant differences of optical thickness were found between non-precipitating clouds and precipitating clouds: smaller cloud optical thickness was observed for precipitating clouds. Dispersion of cloud drop size was found in the rain formation process, which may relate to changes in the shape of drop size spectra and leads to the decreases in the cloud optical thickness for precipitating clouds.
Long-term variations of the Aleutian Low (AL) defined by the sea level pressure (SLP) minimum within the region of [30°N-60°N, 150°E-150°W] during winter (December-February) are investigated, using atmospheric reanalysis datasets. The intensity, latitudinal position, and longitudinal position of AL reveal different temporal variations: the longitudinal shift accompanies intensity variation with an interdecadal timescale (about 20 years), and the latitudinal shift does with a decadal timescale (about 10 years). The AL intensity variation and the longitudinal shift are related to activity of the Pacific/North American teleconnection pattern: in a strengthening (weakening) phase of AL, the AL shifts eastward (westward); westerlies strengthen (weaken), and both subtropical and subpolar gyres spin-up (spin-down) simultaneously. The latitudinal shift is associated with activity of the West Pacific teleconnection pattern. It is independent of the intensity variation of AL: when the AL shifts northward (southward), the westerlies correspondingly move northward (southward). Consequently, the gyre boundary, which is defined by the zero line of the Sverdrup stream function, also shifts northward (southward). The role of AL north-south shift on the upper oceanic variations is investigated by using a wind-driven hind-cast model. The oceanic Rossby wave formed as a result of the baroclinic response for the AL movement influences the sea surface temperature in the Kuroshio-Oyashio Extension region.
Azimuthal wavenumber-one asymmetric structures in the inner-core region of typhoon are examined primarily using the operational mesoscale analyses from the Japan Meteorological Agency (JMA) for the four typhoon seasons from 2004 to 2007. The results show that both the cold and high specific humidity anomalies at low-tomiddle-tropospheric levels tend to be located in the downshear-left quadrant (looking down at the direction of environmental vertical wind shear), consistent with some theoretical predictions. In addition, the vertical tilt of typhoon vortex is directed predominantly downshear to downshear-left with a magnitude being roughly proportional to shear strength, in agreement with previous numerical case studies. In the present study, the wavenumber-one structures of near-surface winds are also investigated using both theoretical and statistical approaches. In the theoretical approach, a set of analytical formulae that relate the asymmetries of radial and tangential winds to convective asymmetry in the eyewall region are derived by applying a scaling argument to the result from a numerical simulation of Typhoon Chaba (2004). The formulae predict that the maximum in storm-relative tangential wind occurs 90° azimuthally downwind of the enhanced updraft region. The statistical approach using the mesoscale analysis data reveals that the azimuthal location of tangential wind maximum relative to storm direction depends strongly on the directional difference between shear and storm motion, under relatively strong shear conditions, and the wind maximum, contrary to the majority of cases, tends to occur to the left of motion in cases where the directional difference is very small. Considering the strong dependence of convective asymmetry in the inner-core region on the shear, the results are in line with expectations from the analytic theory.
We have used an AGCM (atmospheric general circulation model)-based Chemistry Transport Model (ACTM) for the simulation of methane (CH4) in the height range of earth’s surface to about 90 km. The model simulations are compared with measurements at hourly, daily, monthly and interannual time scales by filtering or averaging all the timeseries appropriately. From this model-observation comparison, we conclude that the recent (1990-2006) trends in growth rate and seasonal cycle at most measurement sites can be fairly successfully modeled by using existing knowledge of CH4 flux trends and seasonality. A large part of the interannual variability (IAV) in CH4 growth rate is apparently controlled by IAV in atmospheric dynamics at the tropical sites and forest fires in the high latitude sites. The flux amplitudes are optimized with respect to the available hydroxyl radical (OH) distribution and model transport for successful reproduction of latitudinal and longitudinal distribution of observed CH4 mixing ratio at the earth’s surface. Estimated atmospheric CH4 lifetime in this setup is 8.6 years. We found a small impact (less than 0.5 ppb integrated over 1 year) of OH diurnal variation, due to temperature dependence of reaction rate coefficient, on CH4 simulation compared to the transport related variability (order of ±15 ppb at interannual timescales). Model-observation comparisons of seasonal cycles, synoptic variations and diurnal cycles are shown to be useful for validating regional flux distribution patterns and strengths. Our results, based on two emission scenarios, suggest reduced emissions from temperate and tropical Asia region (by 13, 5, 3 Tg-CH4 for India, China and Indonesia, respectively), and compensating increase (by 9, 9, 3 Tg-CH4 for Russia, United States and Canada, respectively) in the boreal Northern Hemisphere (NH) are required for improved model-observation agreement.
The Yamase is a cool easterly wind that is observed in summer along the eastern coast of the northern part of the main island (Honshu) of Japan. It usually accompanies a boundary layer cloud, the “Yamase cloud.” The origin of the Yamase is the cool polar maritime air mass that develops over the North Pacific, including the Bering Sea and the Sea of Okhotsk. The characteristics of the Yamase are controlled by air mass transformation over the western North Pacific. Campaign observations of the Yamase were performed using the marine vessel Koufu-maru of the Japan Meteorological Agency over the sea east of northern Honshu in summer, from 2001 to 2007. We studied two Yamase events and examined the heat flux along back trajectories as well as the heat and moisture budget. The vertical structure of the Yamase was strongly dependent on the history of the air over the ocean, despite the many factors that influence boundary layer clouds. In the June 2003 event, the stepwise upward development of Yamase clouds observed at the Koufu-maru site was related to the influence of Hokkaido Island and an oceanic front on Yamase flow trajectories. For the July 2006 event, the temperature profiles observed from the Koufu-maru changed from stable layer type to mixed layer type. Changes in the visibility and oceanic heating along Yamase air trajectories were also observed. Ocean heating increased when the trajectories changed from westward to southward across an oceanic front located to the east of 144°E in the Kuroshio-Oyashio extension. The meridional SST gradient was smaller over water off northern Honshu, where the Oyashio current prevailed. A heat and moisture budget analysis using aerological data observed by the Koufu-maru and three weather stations in northern Japan showed weak sensible heating and weak moisture sink when the Yamase wind prevailed. We ascribed the weak sensible heating to the small air-sea temperature difference, which was caused by the weak meridional SST gradient and offset by radiative cooling at the cloud top.
The interaction between the seasonal mean circulation and the transient eddies over the western North Pacific (WNP) during El Niño-Southern Oscillation (ENSO) warm and cold years was investigated by the three-dimensional eddy kinetic energy (EKE) and eddy available potential energy (EAPE) budget equations for total eddy, high-frequency (< 10 days) and low-frequency (20-70 days) components. Composites of the energy results indicate that low-level anomalous cyclonic circulation, westerly jet and ascending motion associated with the eastward extension of warm SST during warm ENSO years are favorable for eddy barotropic energy conversion (CK) and eddy baroclinic energy conversions (CE). The enhancement of CK and CE might provide kinetic energy for the growth of high- and low-frequency transient eddies including tropical storms (TSs) from the Philippine Sea to the date line over the tropical WNP during warm ENSO years. In contrast, high- and low-frequency eddies convert EKE to seasonal mean circulation over the subtropical and mid-latitude WNP during warm years. Enhanced eddy baroclinic energy conversion plays an important role in the maintenance and enhancement of the subsequent development of transient eddies including TSs as they propagate northward. The loss of EAPE to EKE due to the eddy baroclinic energy conversion is mainly supplemented by the generation of EAPE associated with eddy diabatic heating. However, the energy conversion from mean available potential energy (MAPE) to EAPE is also important due to the eddy vertical heat transport which is neglected in the two-dimensional EAPE budget equation. It is suggested that high- and low-frequency eddies including TSs may be self-development and intensify through their enhanced diabatic heating and vertical heat transport.
Characteristics of a flow in the boundary layer over a sequence of small localized urban canopies with various heights were studied by wind-tunnel experiment under neutral stratification, where “small” means that the length (fetch) is insufficient for the flow to fully adapt to the surface geometry. The sequence was composed of two taller building groups, such as commercial buildings, and two roughness areas of lower obstacles, such as houses. In the sequence, the flow passed over boundaries from the building groups to the roughness areas, named downward boundaries, and boundaries from the roughness areas to the building groups, named upward boundaries. The flow was quite different depending on the boundary types. The focus was placed on equilibrium-like states of the flow and it was found that the flow behind the boundaries were in the states in short fetches. Relating to the downward boundary, flow over the roughness areas was obtained in the range L/H = 7.5 to 63, where H is the height of the upstream array and L is the distance from the end of the upstream array to the measurement position. It was found that the profiles of the Reynolds stress exhibited double-layered structure separated by kinks. In the range L/H > 19, the profiles had constant forms above the kinks and the flow was in some sort of equilibrium state. It was discussed schematically and found that the double-layered structure was explained well based on the three sources of turbulence: turbulence transported from the flow over the upstream building group, turbulence caused by the downward boundary, and turbulence caused by the surface obstacles. Relating to the upward boundary, flow over the building group (cubic array with a frontal area index of 0.25) was obtained. Two adaptation processes to equilibrium state were found: adaptation of transversal average of the streamwise velocity (U) and adaptation of transversal shear of U. The latter was in balanced state in shorter fetch than the former.
This paper examines long-term change in the interannual variability in surface air temperature and its cause by using monthly data generated in climate change experiments (1851-2100) performed using two models. Regions north of 20°N are analyzed. Anomalies are defined as high-pass filtered values with a cutoff period of 30 years. Interannual variabilities of anomalies are expressed as the root mean square value for a 30 year period. Before global warming (around year 1900), the interannual variability in temperature is large over sea areas north of 50°N. This is because a large gradient of sea ice concentration brings about a large temperature gradient there. The interannual variability decreases generally in a cold season at high latitudes with global warming. In contrast to the general decrease of the interannual variability, there are some regions where it increases locally north of regions that show significant decrease around the Arctic. It can be understood that these phenomena are brought about by the decrease in the sea ice concentration gradient (i.e., the decrease in the temperature gradient) in the southern part of the polar region and the increase in the sea ice concentration gradient (i.e., the increase in the temperature gradient) in the northern part, due to the northward shift of sea ice edges. However, the degree of the decrease of the interannual variability is larger than that of the increase. This is because the global decrease of the temperature gradient strengthens the decrease of the interannual variability and weakens the increase. In addition, even over regions without sea ice, there are some areas where the interannual variability decreases. This is also because the temperature gradient decreases globally.
This study reveals a structure of line-shaped convective systems (LSCSs) observed around the Southwest Islands of Japan on June 10, 2006 using dual-Doppler analyses obtained from two X-band Doppler radars installed on Shimoji and Tarama Islands. Many LSCSs with lengths ranging from 20 km to 40 km form in a band-shaped precipitation system with a length exceeding 300 km along the Baiu front. The alignment directions of the Baiu front and the LSCSs are west-southwest to east-northeast and south-southwest to north-northeast, respectively. Thus, the alignment direction of the LSCSs is oblique to that of the Baiu front, and is parallel to the vertical wind shear vector between 0.5 km and 3 km in altitude. These LSCSs consist of several precipitation cells. The precipitation cells generate only to the south of the southern edge of each LSCS, and move northward and decayed at the northern edge. The pre-existing cells in each LSCS hardly affect the successive generation of new cells. The generation area of precipitation cells coincides with low-level convergence along the Baiu front. The successive generation mechanism of new cells in the observed LSCSs and their structure shown in this study correspond to those in a humid environment, suggested in a previous numerical study.
The effect of the horizontal component fH of the planetary vorticity on the symmetric stability of zonal flow is investigated using the linearized Boussinesq equations on the f -plane. It is shown that, as in the case of neglecting fH, the stability under full-component Coriolis force is determined by the sign of the potential vorticity. It is also revealed that even in such a generalized situation, the movement associated with the symmetric instability can be decomposed into two independent motions, i.e., the buoyancy oscillation (or instability) modified by the Coriolis force and the inertial oscillation (or instability) modified by the buoyancy. The squared product of their frequencies remains proportional to the potential vorticity of the zonal flow. Meanwhile, the horizontal component of the planetary vorticity is found to exhibit both stabilizing and destabilizing effects, although there is a wide range of stable regions that are not affected by fH. The existence of fH also causes an asymmetry such that the stability changes depending on the sign of the vertical shear of the zonal flow, even if the Richardson number and the dimensionless relative vorticity are maintained constant.
Reproducibility of the Pacific Decadal Oscillation (PDO) is evaluated in the sea surface temperature (SST) anomaly field in “the 20th century climate in coupled models” (20C3M) simulations of the 24 CMIP3 models. In this evaluation, we examine how well patterns of the PDO match between the observations and simulations by calculating a metric of the patterns that is a function of their spatial correlation and their standard deviation. Among the CMIP3 models, the models with the high PDO metric reproduce the decadal SST variability with opposing polarities between the central North Pacifc and the tropical Pacific. As observed, temporal correlation between the PDO and decadal-ENSO indices in those simulations are negatively correlated at the statistically-significant level. The sea level pressure and outgoing longwave radiation anomalies onto the decadal-ENSO index in those simulations are realistic both in the tropical Pacific and North Pacific, indicating that this tropicsextratropics linkage in the SST anomaly field is induced by atmospheric teleconnection. This notion is consistent with the previous studies for the natural climate variability. In contrast, the models with the low PDO metric fail to reproduce those characteristics. In the simulations under a middle-range IPCC greenhouse gas emissions scenario (A1B), the PDO indices during the 21st century still represent SST variations on the decadal timescales with superimposition on a linear warming trend. Several models which reproduce the observed PDO pattern in the 20th century record tend to simulate a similar pattern over the 21st century. This indicates that the models with the high PDO metric have their own properties that tend to simulate the natural climate variations with the observed pattern under the global warming condition.
Precipitation and high-level cloud (HLC) areas in association with the large-scale circulation over the tropical Pacific are analyzed for simulations of nineteen Coupled Model Intercomparison Project Phase 3 (CMIP3) models with observations for 16 years of 1984-1999. The distribution of rainfall and HLC areas are composited around the geographical center of tropospheric upper-level (200 hPa) divergence (DIV) along Intertropical Convergence Zone (ITCZ) using monthly anomaly data. Datasets with a finer temporal sampling than monthly means were not available for the present purposes. The most notable feature is that the horizontal spread of enhanced circulation and the related rainfall and HLC areas are all underestimated around the DIV center in the models compared to the observation. Particularly, the underestimation is pronounced in HLC, presumably owing to difficulties in the physical processes relevant to the spatial distribution of HLC area. In general, a model with a higher correlation between the large-scale circulation field and rainfall tends to have a wider spread of HLC area around the DIV center.
The simulated Madden-Julian oscillation (MJO) in the climate of the 20th Century (20C3M) experiment of 23 models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) is examined. The models having moisture-convergence-type convection schemes well simulate the MJO signal in precipitation. By analyzing the data from these models, we confirm that the MJO convection is active from the Indian Ocean to the western Pacific, consistent with the observations. However, analyses of the structure of the simulated MJO reveal that the convergence of the surface wind tends to be overestimated in the models. In addition, the peak of the MJO signal over the Indian Ocean is relatively weaker than that over the maritime continent and the western Pacific, and shifts westward in the models. These inconsistencies appear to arise from the distribution of sea-surface temperature (SST) bias. As a general tendency in all of the climate models studied, it is also demonstrated that the skill of the MJO simulation correlates positively with that of the climatological SST. Potential importance of the basic field and the air-sea coupling in the MJO simulation is suggested.
The 20th century simulations from the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset are examined statistically using numerical skills to capture the characteristics of the models which realistically simulate responses of the tropical western Pacific (TWP) precipitation to sea surface temperature (SST) variability over the Niño3 region [150°-90°W, 5°S-5°N]. The simultaneous correlation of precipitation anomaly over the TWP region of [90°-170°E, 20°S-20°N] with Niño3 SST variability is successfully reproduced with relatively high skills for June to August (JJA) as compared with those for December to February (DJF) and March to May. The high skill models have common characteristics of realistically simulating the observed largest precipitation response to Niño3 SST variability over the equatorial central Pacific east of the dateline. Furthermore, the realistic simulation of the climatological mean equatorial precipitation west of the dateline seems to be responsible for the realistic response of the TWP precipitation to Niño3 SST variability. A few of the models successfully simulate the delayed response of the JJA precipitation over the TWP region of [90°-170°E, 10°S-30°N] to the preceding DJF Niño3 SST variability with high skills. Those models reproduce the statistically observed features of subtropical northwestern Pacific SST anomaly, precipitation and SST anomalies over the Indian Ocean in JJA following DJF Niño3 SST variability. Another distinctive characteristic of those models is to reproduce almost null correlation of the equatorial central Pacific SST anomaly in JJA with the preceding DJF season Niño3 SST variability.
The reproducibility and future changes in the first baroclinic Rossby radius of the ocean in twenty atmosphere-ocean coupled general circulation models are investigated based on the Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset. For each of the models, the zonally averaged first baroclinic Rossby radius and the corresponding phase speed of long baroclinic Rossby waves simulated in the 20th century (20C3M) experiment are compared to their counterpart estimated from observed fields of temperature and salinity based on the World Ocean Atlas 2005. A tendency is found for these quantities to be better reproduced in higher-resolution models than in lower-resolution models. The response of the radius to the increasing atmospheric CO2 level is assessed for each of the models through comparison between the 20C3M and A1B experiments. In all the models, the zonally averaged first baroclinic Rossby radius increases by the late 21st century at almost all latitudes, but the magnitude of the projected increase varies from one model to another, for example, ranging from 2% to 20% as the relative change around 40°N. In the subtropics (equatorward of 45° latitude), the enhanced increase in the simulated first baroclinic Rossby radius due to the enhanced static stability in the upper ocean is attributable largely to the enhanced warming in the lower troposphere. At higher latitudes, in contrast, the enhanced static stability is more sensitive to the enhanced precipitation. Our result indicates that the propagation of oceanic Rossby waves is likely to be faster in the warmer climate, implying that the dominant time scale of extratropical low-frequency climate variability will likely shorten.
Characteristics of precipitation in the Meiyu-Baiu season in twenty-two 20th century climate simulations contributed to the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) are studied comparing with two observed precipitation data. The distribution of precipitation in the Meiyu-Baiu season is characterized by the narrow Meiyu and Baiu frontal precipitation zone (MFZ and BFZ) extending along the northern rim of the North Pacific subtropical anticyclone (NPSA), the maximum precipitation zone (S. Max zone) along the southern rim of the NPSA, and the minimum precipitation zone (S. Min zone) to the south of MFZ and BFZ. The latitude of these zones and the precipitation within these zones produced by the models are examined, using the 20-year (1980-1999) averaged values for May, June and July. The precipitation in these zones in each month obtained from the respective many-model ensemble average (MEA) coincides approximately with the observation data, except the western “S. Max zone” in May. However, the MEA simulates MFZ, BFZ and “S. Min zone” to the north of their observed latitude in May. MEA reasonably reproduce the latitude of both MFZ and BFZ for June, and the latitude of BFZ and “S. Min zone” for July. The latitude of the “S. Max zone” is approximately reproduced for MJJ. The standard deviation (STD) of the precipitation from the MEA does not change widely from zone to zone and from month to month. However, STD of the latitude of zones varies widely. The STD of the latitude of MFZ and BFZ became larger in June and July. The STD of the latitude of MFZ is larger than that of BFZ. The STD of the latitude for “S. Max zone” is especially large. The models show large deviation from the MEA around the western and southern rim of the NPSA. Low horizontal resolution models tend to yield the larger deviation, in regard to both the latitude and precipitation, as compared with models of medium resolution. However, a few high resolution models still produce large deviation from the MEA. It is hard to state conclusively the influence of the cumulus parameterization scheme on the intense precipitation zones.