Steady baroclinic annulus waves observed over a wide range of the rotation rate are simulated using a high-resolution numerical model. The performance of the model is verified by comparing the simulated and observed rates of the inward heat transport. The spectral analysis shows that all the flows obtained are composed of the dominant wave and its harmonics in addition to the mean zonal flow, and free from sidebands of the dominant mode; these wave patterns drift with the angular speed reflecting the azimuthal motion of the whole working fluid. The energy analysis reveals that the main non-linear transfer of kinetic energy is from the dominant wave to the first harmonic and from there to the series of harmonics; this process is accompanied with the passage of a significant amount of kinetic energy from the dominant wave to the mean zonal flow at low rotation rates but from the mean zonal flow to the dominant wave at medium and high rotation rates.
The validation of regional climate models is usually based on the intercomparison of the model's mean climate with the observed climatology. Albeit a prerequisite for the use of the model in a predictive mode, a successful validation of this type does not strictly test the model's ability to simulate anomalous conditions as might be associated with anthropogenic climate change. Here, we explore an alternate strategy, whereby the model's ability to reproduce the observed interannual variability is tested. The model utilized is an operational numerical weather prediction model of the German Weather Service, and it is tested for its use over East Asia and Japan in a series of 5 month-long January simulations. The model is used in a domain of 5100×5100km2, has a horizontal resolution of 56km, and 20 levels in the vertical. It is driven at its boundaries by the European Center for Medium-Range Weather Forecast (ECMWF) analysis. In validating the integrations, particular emphasis is put on the precipitation fields. For validation we use three different observational data sets: a terrestrial analysis from rain gauges, including the Automated Meteorological Data Acquisition System (AMeDAS) data of the Japan Meteorological Agency, the gridded data set of the Global Precipitation Climatology Project (GPCP), which over sea is largely based upon satellite information, and the ECMWF Re-Analysis (ERA) data set, which is produced by a model in an assimilation mode. It is demonstrated that the synoptic-scale evolution of individual low-pressure systems within the modeling domain is deterministically controlled by the lateral boundary conditions. Precipitation-spatially averaged over selected subdomains-compares remarkably well with the observations, both in terms of the monthly amounts and of the temporal evolution throughout the integration period. Using the strategy of a previous study, we analyze the year-to-year variations of the model results, both for the dynamical and precipitation fields. It is found that the modeling error is substantially smaller than the typical year-to-year fluctuations of the interannual variability. Implications of this result, concerning the model's use as a tool for down-scaling climate change, are also discussed.
The Tropical Rainfall Measuring Mission (TRMM) is a United States-Japan joint project for rain measurement from space. The first spaceborne rain radar has been installed aboard the TRMM satellite. The performance of the TRMM radar should always be validated against reference data. Use of the rainfall rate measured by ground-based instruments such as rain radar, rain gauges, or a radio link is a possible way to validate the TRMM radar. We have analyzed errors in the comparison of rainfall rates measured with the TRMM radar and with the ground-based instruments by means of a computer simulation that uses actual ground-based rain radar data. Results show that if the rainfall rate measured by the TRMM is compared with that measured by the ground-based radar at a higher correlation, the time difference between the TRMM and the ground-based radar measurements should be less than 5 minutes. Results also show that if the rainfall rate measured by the TRMM is compared with that measured by rain gauges and a radio link within a typical error on the radar measurement, nine rain gauges with 1 minute integration time or 16 rain gauges with 10 minutes integration time will be required in a pixel of the TRMM, and propagation path longer than 7km of a radio link will be required.
Properties of the quasi-stationary Rossby waves along the westerly jets are investigated with the space-time spectral analysis of 200hPa meridional wind velocity for four regions in the Northern Hemisphere summer using ECMWF data for 1980 to 1993. The observed zonal wavenumber of eastward (westward) propagating disturbances increases (decreases) as the frequency increases. The eastward propagating disturbances are stronger than the westward propagating ones. The quasi-stationary Rossby waves are seen not only in 10- to 30-day time scales but also in 30- to 90-day time scales. These observed zonal wavenumber-frequency relationships are reproduced by a β-channel model with step-like basic states. Calculated zonal wavenumber of eastward (westward) propagating solution of this model increases (decreases) as the frequency increases. The eastward propagating solutions are more strongly trapped in the westerly jet than the westward propagating ones. For these four waveguides mentioned, the results of the spectral analysis agree with the properties of the solutions deduced from the β-channel model with basic states derived from the climatological basic flow near these waveguides.
The NCEP-NCAR 40-year upper-air dataset has been used to reassess the circulation over the equatorial Pacific. The discovery from this data source of an Equatorial Mid-Tropospheric Easterly Jet (EMTEJ) is supported by radiosoundings at Galapagos. The EMTEJ is consistent with a mid-tropospheric trough along the Equator resulting from the lower-tropospheric thickness pattern controlled by the near-equatorial tongue of cold surface waters. The EMTEJ is part of a zonal circulation cell along the Equator, consisting of ascending motion and upper-tropospheric eastward outflow over the western to central Pacific, convergent inflow from the West into the upper troposphere and mid-tropospheric subsidence over the eastern Pacific, and the EMTEJ in large part fueling the upward motion to the West. The lower troposphere does not participate in this zonal circulation. During the low/warm as compared to the high/cold phase of the Southern Oscillation (SO), the EMTEJ is weaker, commensurate with reduced upper-tropospheric convergence and mid-tropospheric subsidence. The EMTEJ thus contrasts with the classical model of a Pacific Walker Cell.
The algorithm of Iwasaki (1994a) to estimate precipitable water over land was applied to 12 analysis units of a 30×30 array of pixels (about 33km×33km at the nadir point for NOAA satellites) including mountainous regions, and retrieved volume was compared with microwave radiometer data. The correlation between satellite and radiometer precipitable water was 0.81, slope and the y interception of the regression line are 1.08 and -1.68mm, respectively. Using the algorithm, the mesoscale distributions of precipitable water and its time change associated with thermally induced local circulations were visualized under the cloud-free conditions. Precipitable water on the coast increased by 5-20mm from morning to afternoon. At the same time, precipitable water increased by 0-20mm over the mountains and decreased by 0-15mm at the foot of the mountains.
Short-period disturbances in the equatorial lower stratosphere are analyzed by using radiosonde data of horizontal winds and temperature at 10 stations during the TOGA-COARE intensive observational period (IOP). The analysis is focused on disturbances whose periods are shorter than about 3 days. Time-height cross sections of both horizontal winds and temperature show the existence of disturbances with periods of about two days and vertical wavelengths of 3-5km. First, we made power and cross-spectral analyses. Dominant disturbances in northern [7°N-8°N] and southern [9°S-11°S] areas of the TOGA-COARE Large-scale Soundings Array (LSA) regions, propagate eastward and have similar structure in which vertical and horizontal wavelengths are 3-4km and several to ten thousand kilometers, respectively. The phase differences between the zonal and meridional wind components at one location, and the phase differences in wind components between the northen and southern regions, exhibit the typical structure of equatorially-trapped inertia-gravity waves. In the TOGA-COARE LSA equatorial area, dominant disturbances have vertical wavelengths of 4-5km which are a little longer than those observed in the northern and southern areas. Phase differences are clear for meridional wind components between zonally separated stations, and show that the horizontal wavelengths are three to four thousand kilometers, and that the disturbances propagate eastward. On the other hand, phase differences between tonally separated stations for zonal wind components are not significant. Next, a composite analysis was made with a reference of convective activity in the troposphere. Short period disturbances with 3-5km vertical wavelengths, which are consistent with the results of the spectral analysis, appear in the composite of lag-time-height sections in the lower stratosphere, indicating that these disturbances be strongly linked to the convective activity. Vertical phase propagation is downward in the upper troposphere and lower stratosphere, showing that these disturbances propagate energy upward there. Possible equatorially-trapped modes having the wave structures revealed by the above spectral and composite analysis are estimated using the theoretical dispersion relation. It is likely that the disturbances in the northern and southern areas correspond to n=1 eastward inertia-gravity waves. The possible modes in the equatorial regions are different for zonal and meridional wind disturbances, because odd (even) modes have zonal (meridional) wind component only. The estimated wave parameters for meridional wind disturbances are consistent with n=0 inertia-gravity waves propagating eastward. The disturbances dominant in the zonal wind component in the equatorial areas which do not show clear coherent zonal structure are probably due to a mixture of n=-1 Kelvin waves, n=1 eastward and westward inertia-gravity waves.
The moisture balance over the central part of China in May, June and July (MJJ) of 1991 is studied by utilizing the 24-hour prediction data of a global weather prediction model of JMA, in relation with the heavy rainfalls over China. The moisture balance over a domain bounded by 20-40°N and 110-125°E (Area-C, main part of China) indicates that the moisture flux convergence within this domain is mainly due to the southerly moisture inflow across the southern boundary. The precipitation in this domain increases in the early June with the abrupt increase of the southerly moisture inflow. The precipitation within this domain is highly correlated with the southerly moisture inflow crossing 20°N. To relate the southerly moisture flow into Area-C with the moisture balance in the tropical zone, the moisture balance in a domain bounded by 0-20°N and 110-125°E (Area-S, the South China Sea including the monsoon trough region) is further studied. The variation of the cross-equatorial moisture inflow in Area-S is significantly smaller than that of the moisture efflux across the northern boundary of Area-S. In Area-S, the strong correlation is found between the zonal moisture flux convergence and the southerly moisture flux across the 20°N latitude circle. This zonal moisture flux convergence is seen within the monsoon trough region between the monsoon westerly and the easterly flux in the southern rim of the Pacific subtropical anticyclone. The monsoon trough region plays the role of the channel to transport the moisture from 0-20°N zone toward north, although Area-S itself is not a moisture source region. The westeast intraseasonal oscillation of the monsoon trough, monsoon westerly and the North Pacific subtropical anticyclone have strong influence on the precipitation and westerly moisture transport over China. The features of precipitation over China in early July are characterized by a narrow intense rainfall zone over the Yangtze River Valley. The moisture flux field during this period indicates the strong moisture flux convergence within the precipitation zone.
The space-time evolution of intraseasonal convection and circulation anomalies in the 10-25-day period range over East Asia and western North-Pacific Monsoon regions are explored for four early summer seasons from 1991 to 1994. The paper primarily focuses on the linkage between tropical convection over the South China Sea, and lower-tropospheric circulation over the East Asian subtropics on this time scale. The composite results show that quasi-periodic fluctuations of convection on the 10-25-day time scale over the South China Sea, are associated with large-scale circulation in the Asian-Pacific region. The development of subtropical circulation and convection takes place through changes in large-scale circulation on this time scale. 10-25-day enhanced (suppressed) convection occurs in conjunction with well-organized cyclonic (anti-cyclonic) circulation anomalies over the South China Sea. A downstream wave train extending from the South China Sea into the North Pacific, is built up in the lower troposphere associated with this convective activity. This wave train is probably interpreted as a Rossby mode response to the anomalous heating (cooling). Following the peak of enhanced convection, the East Asian subtropical anomalous anti-cyclone maintains its strength as part of this wave train, and couples with enhanced subtropical convection along the northern flank of this anti-cyclonic cell. This development of convection in the subtropical frontal zone is due to an increase in the low-level westerlies at the northern flank of this anti-cyclonic circulation. In addition, this subtropical anomalous anti-cyclone moves southwestward from the subtropics into the South China Sea region, and appears to initiate subsequent suppressed convection. In contrast, the southwestward migration of subtropical cyclonic anomalies also occurs following the inactive convection over the South China Sea, and this behavior appears to trigger the subsequent active convection over this region. These features suggest that mutual interactions between the tropics and the subtropics on this time scale play important roles on the variability of monsoon convection and circulation over East Asia, and western North Pacific. In the upper level strongest divergent outflow (convergent inflow) occurs in the convective (non-convective) region and the significant wave train is maximally intensified in the mid-latitude at the convective peak. Upper-level circulation also appears to be forced by heating (cooling) located over the South China Sea region. These circulation features confirm that 10-25-day variations of convection over the South China Sea are one of the effective forcings for the large-scale circulation during the early summer season.
Direct assimilation using 1-dimensional variational method in the vertical (1D-VAR) was developed to incorporate vertically polarized brightness temperatures (TB's), and rain flag data (index of existence of precipitation) from the special sensor microwave imager (SSM/I), into a mesoscale numerical weather prediction (NWP) model. We used a radiative transfer model (RTM) developed by Liu (1998) to calculate TB's from NWP model variables. Observational residuals of TB's are assumed to be nonlinear functions of precipitation rates in rainy areas, and of other thermodynamic variables in rain-free areas. Quasiequilibrium assumptions on humidity and convective instability in model precipitation areas are used to assimilate TB's in rainy areas and rain flag data, which are functions of precipitation. TB's in rain-free areas are assimilated into total water content (cloud water content+humidity). Smith's method (1990) is used to calculate cloud water content and humidity from total water content. To simplify 1D-VAR, the following approximations are introduced: 1) In the tropics, variations in temperature are much smaller than variations in humidity. 2) Variations in divergence dominates the rainy areas, compared to vorticity and mass variables. Based on the above approximation, we assumed that the background error covariance of relative vorticity and the unbalanced component of mass variables is negligible, so we divided 1D-VAR into that for total water content and that for divergence. Newtonian iteration is used to solve these 1D-VAR problems. To study the assimilated variables, assimilation was applied to cases during 19-20 Dec. 1992 over the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) domain. Results indicate: 1) Precipitable water content (PWC) obtained by assimilation agreed well with PWC calculated from radiosonde data and PWC retrieved from TB's using Shibata's algorithm (1994). 2) Consistency between humidity and cloud water content profiles was attained by Smith's method (1990), although no significant improvement was seen in humidity profile accuracy by assimilation. 3) Assimilation of rain flag and TB's in rainy areas successfully produced model precipitation areas agreeing with radar observation data by Short et al. (1997). To test the impact of assimilation on NWP forecasts, experiments were conducted assimilating TB data for 19 Dec. 1992. Results indicate: 1) Assimilation modified large-scale model humidity distribution, improving large-scale humidity and precipitation forecasts for 48 hours. 2) Assimilation of rain flag and TB's in rainy areas reduced spin-up error of precipitation and positional error of mesoscale precipitation patterns. 3) The boundary between subtropical northeasterly, and equatorial weak wind areas, shifted with the change in model precipitation areas by assimilation.
Vertical propagation of stratospheric planetary waves through a zonally asymmetric basic state is investigated in a simplified numerical model for understanding co-existence effects of zonal wavenumber 1 and 2. We compare wave solutions in a zonally asymmetric basic flow with those in a pure zonal flow. The differences are considered from a view of potential vorticity (PV) distribution, to find that the zonally asymmetric basic flow has an effect of the zonal modulation of PV induction by local westerlies associated with the wave. As a result, a component forced at the bottom boundary in the zonally asymmetric flow departs from that in the zonally symmetric flow, and components unforced appear. Detailed behaviors of each wave component in the zonally asymmetric flow depend on the phase relation between the distorted basic flow and the bottom forcing.