A regional atmospheric model was used to clarify the mechanisms that drive the diurnal cycle of tropical convection over the western Pacific Ocean. The diurnal cycle of convection was simulated using data from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Mechanisms suggested by previous investigators were confirmed through comparison experiments. The diurnal variation in precipitation was simulated without the effect of horizontal differences in radiative cooling between cloudy and clear regions. Horizontal differences of radiative cooling are not the main factor driving diurnal variation. Direct radiative cooling, and cloud radiative cooling effects destabilize the atmosphere during the night. Numerical experiments that reduced cloud radiative cooling demonstrated the importance of the cloud radiative cooling effect rather than the direct radiative cooling effect. Moreover, in the standard experiment, strong cloud radiative cooling appeared at both upper levels and at the top of the boundary layer. The experiment demonstrated that both the upper level cloud radiative cooling, and the boundary layer cloud radiative cooling, are important in driving the diurnal variation of convection. Boundary-layer cloud radiative cooling destabilizes the low layer atmosphere and promotes the development of shallow convection to deep convection. Diurnal variation of water vapor also plays a role in driving diurnal variation of convection. Moisture is transported upward from the boundary layer at night when radiative cooling of the boundary-layer cloud is intensified.
Non-geostrophic non-hydrostatic baroclinic instability with and without convective heating is reexamined with an emphasis on the structure and energy budget of growing modes. When the convective heating is absent, only a baroclinic instability mode has a positive growth rate for Ri = 2, where Ri is the Richardson number. The growth rate for a mode with meridional wavenumber l (l > 0) is proved to be exactly equal to that with -l. However, the energy budgets for the modes with ±l are found to be quite different: the mode with l (&> 0) gains its eddy kinetic energy (EKE) from both eddy available potential energy (EAPE) and mean kinetic energy (MKE), while the mode with -l gains EKE from EAPE, but loses it to MKE. When the convective heating is included, at least two different growing modes are found to exist for Ri = 2: One is a baroclinic instability mode (B-mode), modified by the convective heating, and the other a symmetric instability mode (S-mode) destabilized by the convective heating. The convective heating increases the growth rates of B-mode with l (> 0) more remarkably than that with -l. Energy budget analysis shows that the energy conversion between MKE and EKE is little affected by the convective heating, but that from EAPE to EKE increases more for the mode with l than for that with -l, resulting in a larger growth rate for the mode with l. For the most unstable wavenumber, B-mode has an eastward tilt of the trough with increasing height in the lower layer and a cold air to the east of the surface trough. The thermodynamic analysis shows that this cold air is caused by adiabatic cooling at the low-levels due to the updraft driven by the convective heating at the mid- and upper-levels. The present results give a useful basis for understanding the dynamics of meso-a-scale disturbances along the Baiu front and polar lows.
In this study, recent extreme events over the Northern Hemisphere are quantified in terms of an energy norm of the anomaly of the state variables. Since persistent low-frequency variabilities are characterized, in most cases, by their barotropic structure, the energy norm of the anomaly is measured for the barotropic component of the atmosphere. The norm is then normalized by its climatology to assess the abnormality of the extreme events. In this study the norm is evaluated in the framework of the 3-D spectral primitive equation model to assess the external forcing as well as the state variabiles. According to the analysis of the monthly mean anomaly data for 50 years from 1953 to 2002, the most abnormal months appear to be Apr. 1997, Jan. 1963, Jan. 1977, Mar. 1983, Apr. 1967, Feb. 1989, and Jan. 1989. Those are well known abnormal months in the past studies. In this study, the top 3% of the extreme events are listed as the abnormal months. The quantification of the abnormality is further extended to the external forcing of the barotropic component of the atmosphere and also to the SST anomaly. It is found that only 3 cases of the abnormal months (Jan. 1963, Feb. 1989 and Jan. 1989) are associated with the abnormal external forcing, and the rest of the abnormal months are associated with the non-abnormal external forcing. Likewise, it is found that most of the abnormal external forcing result in a non-abnormal month. The SST forcing anomaly is not directly related to the external forcing. It is concluded from the result that more than 80% of the abnormal months are induced by the natural variability of the barotropic component of the atmosphere under the non-abnormal external forcing for the last 50 years. For the monthly time scale, the chaotic nonlinear behavior is quantitatively larger than a linear response to the external forcing or that to the SST anomaly.
Double-Fourier-Series (DFS) spectral method is applied to a large-size problem of barotropic instability of double-shear flow on the sphere. The computing source is the NEC SX-5 parallel vector processors, with the maximum vector length of 512. It is demonstrated that the DFS spectral model is robust and stable even for such a large-sized intensively nonlinear problem, and can simulate well the multiple scale phenomenon without losing accuracy. In addition to the efficiency on serial computing, represented with O(N² log2 N) operations as opposed to O(N³) for the spherical harmonics spectral method, with N the truncation, the DFS spectral model also preserves the efficiency on parallel computing on vector architecture, due to its nature of two dimensional transform. The parallel performance increased slightly with the resolution, and nearly 33.5 percent (26.8 GFLOPS) of the theoretical peak performance (80 GFLOPS) was achieved in the highest-resolution experiment. The zonal-mean absolute vorticity, of which initial condition is characterized as two peaks in both hemispheres, evolves with time into a nearly constant value over the hemisphere. On the other hand, the meridional gradient of the absolute vorticity increases around the equator. The kinetic energy per unit mass is calculated for each total wavenumber, where a disturbance field of a single total wavenumber is separated by an 8th-order spherical harmonics filter. Kinetic energy spectrum shows two distinct subranges, each with a constant slope. The subrange, other than the viscous subrange, shows a slightly increasing slope with time and approaches l-3 (l is the total wavenumber) in the matured stage, when a single large vortex is formed. As the resolution increases, the subrange other than the viscous subrange extends to the higher wavenumber domain, due to low viscosity. Numerical convergence of the solution with a fixed viscosity is discussed in terms of time averaged zonal-mean statistics of the zonal-flow.
This study investigated the dynamic motion of atmospheric water advection by an analytic method called colored moisture analysis (CMA), that allows for the estimation and visualization of atmospheric moisture advection from specific source regions. The CMA water transport model includes balance equations with the upstream scheme and, uses external meteorological forcings. The forcings were obtained from the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiments (GAME) reanalysis. A numerical simulation with 79 global sections was run for April to October 1998. The results clearly showed seasonal variations in advection associated with large-scale circulation fields, particularly a difference between rainy and dry seasons associated with the Asian monsoon. The paper also proposes a new definition of southwest Asian monsoon onset and decay, based on the amount of water originating from the Indian Ocean. Earliest onset occurs over southeastern Indochina around 16- 25 May. Subsequent onset occurs in India one month later. These results agree with previous studies on the Asian monsoon onset/end. The CMA provides a clearer, more integrated view of temporal and spatial changes in atmospheric circulation fields, particularly Asian monsoon activities, than previous studies that focused only on one or two distinct circulation features, such as precipitation or wind speed. Furthermore, monsoon transition in a specific year, 1998, first became analyzable, whereas the previous studies used climatologies.
Numerical experiments are performed to get a better understanding of the three-dimensional structure of a squall line system under an environmental wind that was observed on 22 February 1993, and used for the GCSS (GEWEX Cloud System Studies) model intercomparison (Redelsperger et al. 2000). The primary objective of this study is to clarify the effects of the observed environmental wind with directional shear on the squall line. More specifically, it is shown that the main features of the structure of the squall line under the observed wind can be explained more clearly by interpreting the wind as a superposition of a jet-type wind in the east-southeastward direction, and a wind with a low-level shear in the north-northeastward direction. For this purpose, numerical experiments are made not only with the use of the observed wind but also with a simplified wind mentioned above. In the latter case, two numerical experiments with, and without, the low-level north-northeastward shear are performed. These numerical experiments indicate that the low-level shear plays an important role on the observed location of enhanced convection at a leading edge, and that a stratiform cloud and pressure field are primarily explained by the jet-type wind. The properties of the squall lines obtained from the present model are also compared with those from the GCSS model intercomparison and other studies. A sensitivity of the properties of convective systems to different orientations of initial, roll-shaped buoyancy is also examined, with a focus on the role of the environmental wind as well as the selforganization mechanism of convection.
A field experiment of snowfall systems, WMO-01, was conducted over the Japan Sea in the winter of 2001. During the WMO-01 period, cold-air outbreaks and passages of synoptic-scale cyclones were observed several times. By utilizing global analysis (GANAL) data, with a horizontal resolution of 1.25 degrees, the apparent heat source Q1, apparent moisture sink Q2, differential advection of moist static energy Dh, potential vorticity PV, sensible heat flux S and latent heat flux LvE were estimated to investigate characteristic features of weather over the Japan Sea. It is shown that the cold-air outbreak during 12 to 17 January was remarkable, where Q1, upper-level PV;S;LvE, and -Dh were the largest during the WMO-01 period. The cold-air was accompanied with the upper-level high PV anomaly, and the air-mass transformation took place strongly over the Japan Sea. The values of S + LvE reached approximately 600 Wm-2, and -Dh was large, indicating that the convective activity was strong. The heat and moisture budgets were compared with previous studies for the Japan Sea in winter. Generally speaking, it is shown that the budgets were similar to previous studies. Next, the 14 January case was examined in detail as a typical example of the cold-air outbreak. The convergence band (= JPCZ) was found extending southeastward from the east of joint of the Korean Peninsula to the San-in and Hokuriku areas. The L modes of convective clouds (parallel to the prevailing northwesterly wind) were seen on both sides of the JPCZ, and the T modes (normal to the NWly wind) were found on the northeastern side. A synoptic-scale cold trough was seen in eastern Asia at 500 hPa, and the polar jet was located on the southern side of the Japan Sea. Additional 1000 km-size PV anomalies slowly propagated eastward repeatedly along the northern side of the jet. This case was studied by using a non-hydrostatic cloud-resolving model with 5 km horizontal resolution (5 km-NHM). Characteristic features of the JPCZ were well reproduced. The 5 km-NHM outputs were also used to examine the heat and moisture budgets. Q*1 and Q*2 are defined similarly to Q1 and Q2, respectively, but the ice phase is included. Large differences from the GANAL analysis were found near the surface and above the height of 4 km. Near the surface, the evaporation of snow or rain may occur for the 5 km-NHM, while it may not for the analyzed Q1 and Q2 due to the vertically coarse data. The sensitivity experiments of cases with or without ice phase were also examined. It is found that the general features are similar in both cases, indicating that the ice phase is not very important for the diabatic process of clouds in this study.
Sensitivity studies are performed on the assimilation of TRMM (Tropical Rainfall Measuring Mission) Microwave Imager (TMI) derived rainfall data into a mesoscale model, using a four-dimensional variational data assimilation (4DVAR) technique. A series of numerical experiments is conducted to evaluate the impact of TMI rainfall data on the numerical simulation of Hurricane Bonnie (1998). The results indicate that rainfall data assimilation is sensitive to the error characteristics of the data, and the inclusion of physics in the adjoint model. In addition, assimilating the rainfall data alone is helpful for producing a more realistic eye and rain bands in the hurricane, but does not ensure improvements in hurricane intensity forecasts. Further study indicated that it is necessary to incorporate TMI rainfall data together with other types of data, such as wind data into the model, in which case the inclusion of the rainfall data further improves the intensity forecast of the hurricane.
The climatological meridional atmospheric temperature structure in the Okhotsk region in summer is characterized by a poleward increase of the surface air temperature. Under this anomalous temperature gradient, the stationary anticyclone, referred to as the Okhotsk high, occasionally occurs. The relationship of the interannual variation of the meridional temperature gradient anomaly is statistically investigated, with those of the Okhotsk high, the sea surface temperature, and the global atmosphere mainly using NCEP reanalysis data. The correlation between the anomaly and the Okhotsk high is quite high. That is, in years when the meridional temperature gradient is positive, the Okhotsk high appears more than normal. The anomaly is composed of two independent factors, the warmness of eastern Siberia and the coldness of the northwestern North Pacific. The vertical structures of the anticyclone, related to the Siberian warmness, are different from those related to the Pacific coldness. The former anticyclone has deep structure, whereas the latter has shallow structure. Therefore, there are two types of the Okhotsk High. The warmness of Siberia is connected to the Rossby wave propagating along the northern coast of the Eurasian continent. The coldness of the North Pacific is, on the other hand, influenced by the variation of the tropical Pacific. Consequently, the interannual variation of the Okhotsk high is influenced by completely different remote sources; one is the tropical Pacific, and the other is the highlatitude areas facing the Arctic Ocean. To understand the interannual variation of the Asian summer monsoon, which is closely related to the occurrence of the Okhotsk high, both the Arctic and the tropics should be considered.
The optical and chemical properties of atmospheric aerosols were measured over the Pacific Ocean during two R/V Mirai cruises. One was the MR02-K02 cruise (winter cruise), crossing the western Pacific from Japan to 5°S between February 21 and March 31, 2002. The second was the MR02-K04 cruise (summer cruise), that covered the western Pacific Ocean, the Indian Ocean, and the seas around Indonesia from Japan to 10°S between June 25 and August 22, 2002. The surveys mainly encountered three kinds of air masses, maritime air masses (marine aerosol), air masses influenced by fossil fuel combustion (FF aerosol), and those influenced by biomass burning (BB aerosol). From 10°N to 20°N, marine aerosols were observed both in the summer and winter cruises with the value of the single scattering albedo (ω) above 0.98 in the summer and, 0.92 in the winter. The concentrations of anthropogenic components such as elemental carbon, vanadium, and arsenic were somewhat low. Sea salt particles play a dominant role in the variation of the scattering coefficients, and the optical thickness in this region. Between 30°N and 40°N, FF aerosols were observed on both cruises. With the absorptive aerosols, the mean ω in the winter cruise was 0.74, while with the transparent aerosols in the summer cruise, the mean ω was 0.92. This seasonal difference comes mainly from differences in the concentrations of elemental carbon. There were BB aerosols around Indonesia in the summer, with a mean ω of 0.72. This air mass contained the most absorptive aerosol measured on these cruises. The chemical compositions of BB particles were different from the other aerosols, with carbonous components and non-sea salt potassium in high concentrations and metal components, such as vanadium and arsenic, in low concentrations.
A polar mesoscale cyclone (PMC) developed over the east coast of Asia in an atmospheric general circulation model (AGCM) that used seasonally varying climatological sea surface temperatures (SSTs). The AGCM had 52 layers and triangular spectral truncation at wavenumber 106 (T106L52). This study compares the model PMC, which formed in February, 8 years after model spin-up, with observational studies. The simulated PMC formed over the western Sea of Japan under the influence of a short-wave trough propagating around a cold upper low. PMC genesis occurred as a surface trough over the Sea of Japan. The surface trough extended northward from a major cyclone that developed over the western North Pacific south of Japan. Large sensible heat fluxes from the sea surface facilitated PMC genesis by decreasing the vertical stability and maintaining a thermal gradient in the lower troposphere. Signifi- cant weakening of the PMC as it passed over Japan suggests that energy supplied from the surface played an important role in the evolution of the PMC. The PMC subsequently developed rapidly over the western North Pacific east of Japan, under the influence of the upper cold low and a short-wave trough. Precipitation, diabatic heating, and energy from the sea surface all increased as the PMC developed. The evolution of the simulated PMC matched the evolution of observed PMCs. Realistic formation of synoptic-scale circulation systems, such as the upper cold low, short-wave troughs, and a major cyclone in the model, are crucial if PMCs are to be simulated realistically in the AGCM.
In this study, local energetics analysis is conducted for the blocking formation in the North Pacific, in order to investigate the condition of a transient ridge to become a blocking. Among the total number of 452 ridges, 88 are identified as blocking during 51 winters from 1950 to 2001. Kinetic energy budget is then performed in the framework of the vertical mean and sheared flows, and the energetics terms, including the barotropic-baroclinic interactions, C(Ks,Km), are analyzed. As a result, we find that a blocking becomes Ω type for large C(Ks,Km), and it becomes dipole type for small C(Ks,Km). It is also shown for the large CðKs;KmÞ, that a ridge develops to a blocking when the flux convergence of mechanical energy of the mean flow, B(Km + φm), is positive around the ridge. On the contrary, a ridge flows away downstream when B(Km + φm) is negative there. The positive B(Km + φm) around the blocking is associated with the enhanced negative BðKm þ fmÞ at the upstream jet, due to the intensified mechanical energy flux from the upstream jet. Therefore, it is found that the sign of the flux convergence of mechanical energy around the transient ridge is the condition for the ridge to become a blocking.
Seasonal and decadal-long variations of aerosols in the Asia Pacific region, over the seas east off the Asian continent, have been analyzed using AVHRR data from NOAA-11 and NOAA-14 satellites. The aerosol optical thickness at wavelength 0.5 mm and A° ngstro¨m exponent were retrieved by means of the so-called two-channel algorithm for the period of about 12 years, between November 1988 and January 2001. Evident seasonal variation was found in the geographical distributions of aerosols over the region. Excepting the interval of strong influence by the Pinatubo volcanic eruptions, the aerosol optical thickness has gradually increased almost all over the region during the analyzed period. The present results verified the fact that the Asian Pacific region is under significant influence of natural and anthropogenic aerosols from the Asian continent.
This study attempts to identify differences in mechanisms responsible for heavy rainfall occurring over geographically different regions, focusing on dynamical and thermodynamical aspects of the heavy rainfall, in both Korea and the central US. To this end, model simulations and climatology of the summertime synoptic-scale features are analyzed. Modeling studies with different precipitation physics revealed that the removal of the convective instability by the cumulus parameterization scheme is an essential process for heavy rainfall event over the US, whereas it plays an insignificant role in reproducing heavy rainfall over Korea. From the comparison of climatological characteristics of dynamic and thermodynamical features, it is evident that summertime climatology over Korea is characterized by stronger baroclinicity. Climatologically, the Korean peninsula is characterized as thermodynamically neutral in contrast to large convective available potential energy (CAPE) over the US.