The hierarchical structures of eastward-moving tropical super cloud clusters (SCC) and embedded westward-moving quasi-periodic cloud clusters (QPCC) are successfully simulated by a simple longitude-height two-dimensional model with simple moist processes. The model results clearly show a life cycle of cloud activity. The cloud area starts as a low-level shallow cloud, develops into deep convection, becomes a top-heavy cloud, and decays. Gravity-wave packets are excited by the growth and decay of this cloud and propagate both eastward and westward. The westward-propagating gravity waves are coupled with cloud activity and form westward-moving QPCC. On the other hand, the eastward propagating waves are not immediately coupled with deep convective activity. A deep convective cloud develops only after the low and middle troposphere is sufficiently moistened and cooled. The quasi-periodic emergence of the new convective cloud to the east results in the eastward movement of the envelope of QPCC, forming an eastward-moving SCC. It is suggested that the excitation of gravity waves of two vertical modes by grow and decay of the heating with top-heavy vertical profile is essential to this hierarchical structure. Especially, the net upward parcel displacement due to the shallow gravity-wave cell have a important role in the generation of new QPCC. Although both eastward-moving SCC and westward-moving SCC are possible in a non-rotating atmosphere with no external origins of east-west asymmetry, eastward-moving SCC can be selected under existence of some external asymmetry, such as the beta effect, mean zonal wind shear, or asymmetry of latent heat flux due to WISHE (wind-induced surface heat exchange) effect.
Latent heating profiles associated with three TOGA COARE active convective episodes (December 10-17 1992; December 19-27 1992; and February 9-13 1993) are examined using the two-dimensional version of the Goddard Cumulus Ensemble (GCE) Model, and retrieved by using the Goddard Convective and Stratiform Heating (CSH) algorithm. The following sources of rainfall information are input into the CSH algorithm: Special Sensor Microwave Imager (SSM/I), shipborne radars and the GCE model. Diagnostically determined latent heating profiles are calculated using 6 hourly soundings used for validation. The GCE model simulated rainfall and latent heating profiles are in excellent agreement with those estimated by soundings. In addition, the typical convective and stratiform heating structures (or shapes) are well captured by the GCE model. Radar measured rainfall is smaller than that estimated by the GCE model and SSM/I in both December convective episodes. SSM/I derived rainfall is more than the GCE model simulated for the December 19-27 and February 9-13 periods, but it is in excellent agreement with the GCE model for the December 10-17 period. The GCE model estimated stratiform amount is about 50% for December 19-27, 42% for December 11-17 and 56% for the February 9-13 case. These results are consistent with large-scale analyses. Accurate estimates of stratiform amount are needed for good latent heating retrieval. A higher (lower) percentage of stratiform rain can imply a maximum heating rate at a higher (lower) altitude. The GCE model always simulates more stratiform rain (10 to 20%) than the radar for all three convective episodes. The SSM/I derived stratiform amount is about 37% for December 19-27, 48% for December 11-17 and 41% for the February 9-13 case. Temporal variability of CSH algorithm retrieved latent heating profiles using either the GCE model simulated or radar estimated rainfall and stratiform amount is in good agreement with that diagnostically determined for all three periods. However, less rainfall and a smaller stratiform percentage estimated by radar resulted in a weaker (underestimated) latent heating profile, and a lower maximum latent heating level compared to those determined diagnostically. Rainfall information from SSM/I can not retrieve individual convective events due to poor temporal sampling. Nevertheless, this study suggests that a good rainfall retrieval from SSM/I for a convective event can lead to a good latent heating retrieval. Sensitivity testing has been performed and the results indicate that the SSM/I derived time averaged stratiform amount may be underestimated for December 19-27. Time averaged heating profiles derived from SSM/I, however, agree well with those derived by soundings for the December 10-17 convective period. The heating retrievals may be more accurate for longer time scales, provided there is no bias in the sampling. An appropriate selection of latent heating profiles from the CSH look-up table is important. Sensitivity tests addressing this issue have been performed.
Sets of numerical hindcast experiments were carried out to study the excessive rain that happened over China in 1998 by using an atmospheric general circulation model. The monthly sea surface temperatures for 1998 were prescribed as the model boundary conditions. The initial atmospheric conditions for each of the 30 member simulations were obtained from the daily reanalysis data for 00 UTC from April 1 to April 30, 1998. The initial conditions for snow mass, soil temperature, and soil wetness were prescribed as those of the model climatology. The ensemble averages of the 30 member hindcast experiments captured the positive rainfall anomaly occurred over China in the summer of 1998, with 5 degrees of northward shift. The observed patterns of summer geopotential anomalies were qualitatively reproduced as well. It was revealed that initial atmospheric anomalies in April have apparent impacts on the simulated flow patterns over Eurasia and the North Pacific, and rainfall anomalies over China during the summer of 1998. However, the overall results suggest that tropical sea surface temperature anomaly played a key role in the heavy rainfall over China in 1998.
The South China Sea (SCS) Monsoon Experiment was conducted in May and June 1998 to study various aspects of the SCS summer monsoon (SCSSM). This paper presents results of a preliminary study of the dynamic and thermodynamic characteristics associated with the SCSSM onset. The objective is to determine the mechanism that triggered the onset, which is defined based on the shift of the 850-hPa zonal winds from easterly to westerly, and a sudden increase in the observed daily rainfall over the SCS. Based on those criteria, the onset day of the 1998 SCSSM is May 25. The results show that the summertime meridional temperature gradient, the meridional circulation as well as high relative humidity were already established in early May over the SCS. On the other hand, reversals of the meridional mean-sea-level pressure gradient and the zonal wind circulation occurred only near the onset. One day before the onset, the vertical gradient of the regional mean saturated equivalent potential temperature between 850 and 500hPa reached a (negative) maximum over the entire SCS, which implies that the atmosphere was very unstable. A tropical cyclone in the Bay of Bengal apparently provided a conduit for the transport of moisture to the SCS. In addition, the subtropical high over the northwest Pacific withdrew eastward, and a cross-equatorial flow developed at 105°E in association with an equatorialbuffer zone between 105 and 125°E. These resulted in the setting in of a low-level westerly flow. All of the above processes provided the necessary conditions for the SCSSM onset. When a frontal cyclone generated at the northeastern part of the Tibetan Plateau moved equatorward into the SCS, rising motion was strongly enhanced in situ and led to the development of heavy rainfall. This was when the monsoon onset occurred over the entire SCS.
Two longitudinal-mode snowbands (bands I and II) were observed over the Ishikari Bay, Hokkaido, Japan during a wintertime cold-air outbreak. The three-dimensional kinematic structure of a snowband (band II) was examined in detail using dual-Doppler radar data. Band II noticeably developed over the Ishikari Bay. A high-reflectivity (approximately 35 dBZ at the maximum) zone was formed along the band axis and characterized the radar-echo structure of band II. The high-reflectivity zone of band II had the airflow structure dominated by circulations in vertical cross sections perpendicular to the band axis. The interactions between the two snowbands were discussed. Interestingly, it was found that radarecho bridges existed at the low levels between the two snowbands. The radar-echo bridges were formed in association with low-level outflows from the meso-γ-scale convective cloud systems composing band I. The low-level outflows moved toward band II with time and penetrated into band II. This caused strong low-level convergence and the enhancement of updrafts in band II. Consequently, stronger radar-echoes were formed in band II and band II rapidly developed. Ice/snow particles were transported from band I into band II by the low-level outflows. It was considered that the rapid growth of these particles in the enhanced updrafts in band II would have contributed to the rapid development of band II.
The changes of a sea breeze and a daytime heat island due to land-use alteration during an 85 year period (1900-1985) have been numerically simulated. The domain of interest is the Kanto Plain (15000km2), including the Tokyo metropolitan area. This urban area is located in the southern part of the plain and consists of many cities in Tokyo and its suburbs. The horizontal scale of the area is about 40km and has increased by a factor of four during the 85 year period. The simulations were conducted under a summer synoptic condition with weak gradient wind and almost clear sky. The model is based on the three-dimensional anelastic equations, taking into account the hydrostatic assumption. First, it was confirmed that the simulated wind field and temperature distribution with using the land-use data for 1985, agreed with observed data. The simulations were then conducted using the land-use data for 1950 and 1900. From comparison among the three simulations, the following two major conclusions were obtained: (1) Land-use alteration modified the wind system over the Kanto Plain. In particular, the simulated sea breeze front in 1985 was more clearly defined around the northern end of the Tokyo metropolitan area. The time required for the sea breezes to reach inland areas increased by two hours. (2) The warming due to land-use alteration is found over the Tokyo metropolitan area and the northwestern part of the Kanto Plain. In particular, the area of the most prominent warming is found in the northern end of the Tokyo metropolitan area. Intensity of daytime heat island in the area were estimated as 3-4°C and 2-3°C during the 85 year period, and latest 35 years respectively. The above warming is confirmed to result from the enhanced sensible heat flux and the change of interaction between the boundary layer heating and sea breeze front.
The possible impact of anthropogenic climate change on the Asian summer monsoon is investigated in several time-slice experiments using prescribed sea-surface temperature (SST) and sea-ice anomalies. The study is carried out with four different atmospheric general circulation models (GCMs), each being involved in two pairs of experiments differing only by the treatment of the land surface hydrology. The objective is to assess the robustness of the simulated climate change, and its possible sensitivity to the land surface scheme. Despite the use of identical SST anomalies, the four GCMs do not predict similar monsoon responses on the regional scale. All models produce a stronger warming over the Asian continent than over the Indian Ocean, but this warming is not a good predictor of the monsoon response to increased CO2 level. There is a significant spread in the summer precipitation anomalies despite a general weakening of the monsoon circulation, showing that the response of the monsoon rainfall is not solely related to the changes in the large-scale dynamics. In a warmer climate, the monsoon precipitation can increase despite a weakening of the monsoon flow, due to an increase in the atmospheric water content. For decades to come, the increase in the atmospheric water content could be more important than the increase in the land-sea thermal gradient for understanding the evolution of the monsoon precipitation. Though it does not represent a major source of uncertainty, the treatment of the surface hydrology is liable to affect significantly the regional response of the monsoon to CO2 doubling. A slight change in evapotranspiration is enough to induce a significant change in precipitation. A simple analysis of the regional water budget indicates that this sensitivity is not only related to changes in the horizontal transport of water vapor, but also to changes in the precipitation efficiency, which depends on the treatement of the land surface hydrology.
Toward a physical understanding of the decadal oscillation found in midlatitude sea surface temperatures (SSTs), a numerical study has been carried out using a simple atmosphere-ocean coupled model. The ocean model consists of the linearized shallow-water, quasi-geostrophic equation and the mixed-layer temperature equation, while the atmospheric perturbations are expressed by steady equations which employ an empirical relationship between SST and surface wind anomalies. In the model, the empirical relation manifests the positive wind-evaporation feedback with so small magnitude that cannot overcome the thermal damping in the heat fluxes. The coupled solution in this model was sought with the time integration, to explore the dependence on such parameters as the mean and anomalous current strengths (γc and γt), coupling coefficients (γh), and magnitude in the local thermal damping (γd). Numerical solutions with standard parameters yield oscillations with periods 13 and 16 years in the North Atlantic and the North Pacific, respectively. However, the modal structures are hardly identified because the local damping is too strong. When the damping is weakened, temporal evolutions of the temperature anomalies in these oscillations have a similarity to observed decadal changes. As a prototype of the simulated decadal oscillations, two modes were found with different parameter sets: advectioe mode driven by the advection by the mean gyre and Rossby wave mode in which the geostrophic current anomaly induced by the long Rossby wave is responsible for the scillation. The former generally has larger decay than the latter due to direct influence by the thermal damping. An essential feature of these types of oscillation is exemplified by conceptual models. Stochastic weather noise was introduced into the system to clarify the role of atmosphere-ocean coupling. When the ocean model is forced by the noise alone, the temperature spectrum exhibits the red noise spectra consistent with the stochastic theory. The noise applied to the coupled system increases the low-frequency variance with distinct decadal peaks, which are well separated from the red noise. However, the decadal peaks disappear if the geostrophic advection terms were eliminated. These results indicate that the decadal modes inherent in the coupled system are crucial in generating the decadal spectral peak in the presence of the noise, even if the modes themselves are strongly damped oscillations.
The diurnal variation of precipitation over the Indo-China Peninsula is investigated using a two-dimensional, nonhydrostatic, and cloud-resolving numerical model. The model is initialized by the climatological monthly mean vertical profiles of zonal flow, temperature and humidity at the center of the Indo-China Peninsula in June. The model successfully simulates the diurnal variation of precipitation. The simulated diurnal variation is as follows: convection is activated at the lee-side foot of two mountainous regions located at the west and middle of Thailand in the evening of each day; activated clouds then are organized into squall lines that travel eastward during the night at about 5-10m s-1. These squall lines weaken around midnight. It is concluded that the solar-synchronized life cycle of the squall lines and their eastward movement cause the nighttime maximum of the precipitation over the inland area of the Indo-China Peninsula. High resolution analysis of a convective activity index calculated from the GMS IR data over the Peninsula supports this conclusion.
A limited area model has to be integrated over a long period to simulate a regional climate. The spectral boundary coupling (SBC) method is used for a regional climate model to adjust the phase shift of disturbances between the outer coarse mesh model and the inner fine mesh model, and to suppress the noise which is inevitably developing near the lateral boundary in our model. In this study, the MRI regional climate model (MRI/RCM) with a horizontal transformed grid size of 40km (at 60°) is nested in the Global Analysis data set (GANL), which was produced by the Numerical Prediction Division of the Japan Meteorological Agency (NPD/JMA). One-month long integrations of the model are performed for January and August 1994 with the following four different boundary conditions; (i) the conventional lateral boundary condition, namely, the boundary relaxation method without using the SBC method (CASE A), (ii) the SBC method used for all layers and without the boundary relaxation method (CASE B), (iii) the SBC method used only above the σ=0.5 level and without the boundary relaxation method (CASE C), and (iv) the boundary relaxation method at all layers and the SBC method used only above the σ=0.5 level (CASE D). The simulated results are verified in comparison with observations using some statistical methods. The results of the statistical evaluation show that when the SBC method is not used, the Root Mean Square Error (RMSE) of the height at the 850hPa level near the lateral boundary is smaller than that around the center of the calculation domain. In contrast, when the SBC method is used, the RMSE of it around the center is smaller than that near the lateral boundary. In the summer case, the height at the 850hPa level is improved by using the SBC method. However, using the SBC method for all layers is not good for the scores of surface air temperature and precipitation. Almost all the scores are improved by using the SBC method only above the σ=0.5 level. In the winter case, the boundary relaxation method suppresses noise effectively, while the SBC method shows a little improvement. The best score, however, is obtained from the combination of both the methods. Almost all the statistical values show that the SBC method for only above the σ=0.5 is useful for the regional climate model throughout both the seasons, particularly in the summer case.
The linear stability of a two-layer front wherein the potential vorticity is uniform inside each layer is investigated as the most natural frontal model formed by two air masses, and also as the most fundamental frontal model. Although there is no pure Rossby mode in each layer owing to the uniformity of the potential vorticity, Kelvin mode with a small wavenumber, where it possesses a Rossby-wave-like feature, plays an important role for causing the instability of this front. In particular, if the basic state is characterized by the most plausible parameter for which the basic velocity outside the frontal zone vanishes, a mode classified as a kind of baroclinic instability is the fastest growing. Rossby-gravity instability is not significant in this model, which is consistent with the result in the continuously stratified model.
The longitudinal structure of interannual variability (IAV) of sea surface temperature (SST) is examined diagnostically for the period 1949-97 (49 years) in the entire equatorial oceans (5°N-5°S). In the centraleastern Pacific to the east of 170°E, the mean SST is lower than 29°C, the variance of SST IAV is large, and the air-sea coupling on the IAV timescale is strong. There further appear two variance maxima of the SST IAV around 160°W and 90-100°W. The variability in the 38-47-month band (LF1) is dominant at 90-100°W, while both LF1 and the 51-68-month variability (LF2) are large at 160°W. The contribution of 18-32-month variability (QB) is also large but not as that of LF1 and LF2. The detailed propagation direction is changed according to the periodicity in this region, i. e., eastward in the LF2, but is westward in the QB and LF1. Strong air-sea coupling is identified in all LF1, LF2, and QB. In the Indian Ocean-western Pacific sector to the west of 170°E, the mean SST is higher than 28°C, the variance of SST JAY is small, and the air-sea coupling is weak on the IAV timescale. The SST IAVs in the dominant QB, LF1, and LF2 exhibit a common phase structure in the Indian Ocean-Pacific sector. These SST IAVs are in phase in the central-eastern Pacific, but are out of phase in the western Pacific with a node around 170°E indicating a standing seesaw-like oscillation in the Pacific. In the Indian Ocean, the SST IAVs lag from a few to several months behind those in the central-eastern Pacific. In the Atlantic sector, the longitudinal change in the mean and variance fields are similar to that appearing in the Indian Ocean-Pacific sector. The IAV of SST tends to lag behind that in the centraleastern Pacific in general, but does not show the other common phase structure in the QB, LF1, and LF2 in detail. The air-sea coupling is weak on the IAV timescale in the equatorial Atlantic.