A prognostic cloud prediction scheme, designed for large-scale models, has been incorporated into a Single Column Model (SCM) and used to simulate the cloud cluster properties observed during the 19-26 December, 1992 Tropical Oceans Global Atmosphere (TOGA) - Coupled Ocean Atmosphere Response Experiment (COARE). Results from the SCM simulations have been compared with simulated profiles obtained from the Goddard Cumulus Ensemble Model (GCEM). Observed large-scale advective temperature, water vapor, and surface fluxes have been used as forcings to run the SCM and the GCEM. Results indicate that the SCM produces mixed profiles of cooling/warming and drying/moistening in the vertical, which are highly sensitive to the prescribed surface fluxes. Errors in the temperature and moisture profiles simulated by the SCM are about ±3 K and ±3 g kg-1, while those from the GCEM are approximately -2 K and 1 g kg-1 at most levels. The SCM produced -10% errors in the relative humidity above 700 mb and -30% at the surface, while in the GCEM, the errors were about 10 to 15% at most levels. The distributions of precipitation rates are fairly well simulated, but the high cloud fractions are slightly underestimated by the SCM as compared to the GCEM. The cloud liquid water is underestimated, but the ice contents are slightly overestimated by the SCM. Results show that the SCM has been able to simulate the distributions of temperature, moisture, and precipitation rates fairly well as compared to the GCEM and other GEWEX Cloud System Study (GCSS) model intercomparison products. Sensitivity studies have been carried out to investigate the implications of different physical processes in the SCM. Results indicate that the interactions between various physical processes are nonlinear, and a mere substitution of the heating and moistening profiles from the GCEM may not be able to reproduce the observed temperature and moisture values by the SCM. The SCM has also been used to simulate the distributions of large-scale cloud cluster properties and their diurnal variation during the disturbed and suppressed periods of convection. Results show that the diurnal variations of simulated values are in agreement with many observational studies conducted by different authors over the TOGA-COARE.
A series of numerical experiments was made to search for the basic features of diurnal wind variation in the lee of a mountain range using a two-dimensional dry version of the MRI non-hydrostatic model. The features of diurnal wind variation were classified into four regimes according to prevailing wind speed with respect to the characteristic behavior of the lee convergence zone (LCZ). Under moderate prevailing wind, the LCZ propagates downwind far into the leeside plain during the daytime accompanied by a surge of strong wind. This is in qualitative agreement with the daytime advance of downslope wind observed in the Canterbury Plains in New Zealand, and the Kanto plain in Japan.
Multi-scale summer (Jun-Aug) rainfall variability over China and its long-term link to global sea surface temperature (SST) variability are studied for the period of 1955-1997. First, the dominant spatial and temporal patterns of the observed rainfall anomaly are studied by empirical orthogonal function (EOF) analysis. By a wavelet transform, interannual and decadal-interdecadal variabilities as well as a trend are found, with different dominance, in the first two EOF modes. EOF1 shows a sudden shift in rainfall anomaly over China in the late 1970s, representing overall wetter conditions in central China and drier conditions in northern and southern China in the 1980s than the conditions in the 1960s. This sudden shift is associated with a quasi-in-phase reinforcement between bidecadal and quadridecadal variabilities. EOF2 represents an increasing trend in the rainfall anomaly in broad central and southern China, especially in the Yangtze River valley, without an apparent shift in the late 1970s. The lack of such a shift is associated with an out-of-phase partial cancellation between a bidecadal cycle and the trend around that time. Second, to understand the long-term rainfall variability that is linked to global SST variability, the singular value decomposition (SVD) analysis for the two fields is carried out. SVD1 links drought conditions in northern China and flood conditions in central China to an El Nino-like SST anomaly distribution. This mode shows both an apparent trend and a regime shift in the late 1970s, which do not coexist in the rainfall EOF modes. SVD2 links the rainfall anomaly in the area between the Yangtze River and the Yellow River and the opposite anomaly in southern China to a wave-like SST anomaly distribution in the eastern Pacific from tropics to extratropics. SVD3 links the rainfall anomaly in the Yangtze River valley to the SST anomaly in the western Pacific centered near 20°N 140°E. The rainfall variability in different areas of China that can be attributed to SST effects results from the interplay of the SVD modes. The most significant links found from SVD analysis are verified by cross-correlation functions. A scenario for a long-term link on the trend scale between the rainfall over China and global SST variabilities, through the associated large-scale circulation, is presented.
Multi-spectral, dual-polarization measurements made by the satellite-borne Special Sensor Microwave Imager (SSM/I) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) radiometers cannot discriminate satisfactorily strong, convective rain from weak, stratiform rain. This degrades the quality of the rain information deduced from these radiometers, particularly over land. With the objective to improve the quality of this rain information on land, in this study we have developed a mesoscale area-average rain retrieval method. This method is derived from ATI concepts outlined by Doneaud et al. (1984) and Lopez et al. (1989), and it requires tuning based on calibrated radar, and/or surface rain gauges. As a building block of this method, the fractional rain area, fR, is first determined in a mesoscale grid box of 2°× 3° with the help of a threshold on the 85 GHz scattering depression. Using SSM/I data, fR is shown to correlate well with the corresponding ground-truth area-average rain rate, Rg, deduced from 15-minute rain gauge observations. Based on fR, a method is then developed to retrieve area-average rain rate, RfR. In order to demonstrate the usefulness of this method, nine-months of RfR are retrieved from SSM/I data over three grid boxes in the Northeast United States. In the three grid boxes, RfR can explain about 64% of the variance contained in the corresponding Rg. When average 85 GHz scattering depression in the rain area is also included as a parameter in this method, the variance explained drops to 55%. Thus, we find statistically that the algorithm based solely on fractional rain area gives a better result. With the aid of this tunable method, because of the wider TMI swath, one can use this procedure to fill gaps in space and time present in the rain measurements of the Precipitation Radar (PR) in the TRMM mission.
The relationship between the soil moisture content and the leaf transfer coefficient for water vapor ce was examined. The correlation between these variables are represented by the soil moisture content at the wilting point, and a numerical parameter that changes with vegetation type and soil type. The present ce model that considers these soil parameters was examined, with a simulation of the heat balance over a sorghum field conducted using the canopy model and routine meteorological data. The calculated results agree well with observed values. The relationship between the spectral reflectance and ce was also examined. The value of ce decreases with decreasing the reflectance ratio (r1220 nm/rl550 nm) and vegetation vigor index (VVI). The VVI is a useful index to obtain information concerning the vegetation vigor when the soil moisture content is not known.
The life cycles of SEAM (Southeast Asian Monsoon) and WNPM (Western North Pacific Monsoon) are monitored by the climatological pentad mean OLR and 850 hPa [u, υ], as well as utilizing the vertically integrated moisture flux vector [Qλ, Qψ] and associated precipitation minus evaporation (P - E). SEAM reaches its peak in late July in conjunction with full establishment of a gigantic heat low over the Asian Continent due to surface heating. The height of WNPM occurs two to three weeks later due to the delayed intensification of the monsoon trough not directly by in situ surface warming, but indirectly by remote influence of the Asian heat low. SEAM owes its existence to the westerly moisture influx from the Arabian Sea and further upstream. On the other hand, the easterly moisture influx from the Eastern Pacific is the major contributor to WNPM, relegating the role of moisture import from the SEAM domain to that of secondary importance. The climatological pentad mean data are then partitioned into symmetric and asymmetric components with reference to the equator. SEAM is characterized by dominance of the asymmetric mode, an implication that it is essentially an asymmetric monsoon system regulated by north-south heat contrast. The contribution of the asymmetric mode to the summer mean moisture budget of SEAM is as high as 82%, while the corresponding contribution due to the symmetric mode is only 18%. In comparison, the asymmetric and symmetric modes equally contribute to WNPM and as such, it is an asymmetric and symmetric mixed, hybrid monsoon system. The summer mean moisture budget of WNPM is accomplished by almost equally significant contributions of the symmetric and asymmetric modes, i.e., 58% from the former, while 42% from the latter mode. The symmetric mode exerts a strong influence on the maintenance of the climatological equatorial Walker circulation. It is the asymmetric Hadley circulation that furnishes moisture to equatorial convections over the updraft portion of the Walker circulation in the eastern Indian ocean. Activated convections then enhance the symmetric zonal flow along the equator, which is westerly over the Indian Ocean while easterly over the western Pacific.
To understand the mechanism of hierarchical organization of the tropical super cloud cluster and its eastward propagation, we performed numerical experiments using a 2D cumulus-scale resolving model (Yamasaki, 1984). In the experiments, synoptic-scale convection similar to super cloud cluster (SCC) developed with a reasonable eastward phase velocity of 3-6 ms-1, and a realistic hierarchy of convection. The hierarchy includes large-scale cloud cluster (LCC) with a scale of 1, 000-1, 500 km, cloud cluster (CC) with a scale of O(100 km), mesoscale convection (MC) with a scale of a few to several tens of kilometer, and individual cumulus convection. The eastward propagation of SCC occurs favorably under the condition that the environmental surface flow is easterly. The surface easterly, with its superposition with convectively induced surface flows, enhances evaporation from the sea surface and moistens the lower-level atmosphere more effectively in the eastern side of the convection than the western side, which assures successive formation of new clouds to the east. This result supports the mechanism of WISHE suggested by Emanuel (1987), and Neelin et al. (1987). However, we also found that WISHE operates only as a preconditioning of the cloud formation. The formation is in practice contributed by mesoscale (O(10)-O(100) km) gravity waves that are induced by inner convection of SCC (that is, IVIC, CC and LCC) and propagate at phase velocities of 10-20 ms-1. The gravity waves play the role as follows. Under the condition of WISHE (the existence of the environmental surface flow), the waves propagate eastward from the existing convections and their upward motions initiate low-level clouds in some distance to the east where the air is moist enough. The clouds grow into taller convections aided by the subsequent passages of upward motions of gravity waves. The growing convections in turn induce gravity waves one after another to the east, and likewise contribute to generate the convections to the east. In this manner each of MC, CC, and LCC organizes itself into CC, LCC, and SCC, respectively, with the help of the mesoscale gravity waves. Additionally, equally important in maintaining the hierarchy is the interaction between the evaporatively driven cold pool and the environmental low-level warm flows, whose importance is now familiar in various mesoscale convective systems. The interaction is found to be essential to the maintenance of MC and CC, in particular at their formative and developing stages. It is also shown that MC, which seems to have received little attention in the previous studies of SCC, is one of significant classes of hierarchy in the organization of SCC. Furthermore, discussion is given concerning whether or not the SCC and the induced larger-scale disturbance can interact cooperatively. The development and maintenance of the eastward-propagating large-scale disturbance interacted with SCC can be explained by gravity (Kelvin)-wave CISK.
A quasi-stationary precipitation band, which persisted for more than 10 hours, was observed along a mesoscale front over the Kanto area, when Typhoon 9426 (Orchid) approached the Japan Islands. The band was analyzed using data of special upper sounding at Tsukuba, two Doppler radars, and operational meteorological observations. The band had the carrot-shaped form and characteristic features of the back-building type. In the precipitation band, which aligned from south to north, a multi-cellular structure was observed. New cells were repeatedly generated at the southern edge of the band, and moved northward, expanding westward. Numerical simulations with the horizontal resolution of 2 km were performed using the Meteorological Research Institute mesoscale non-hydrostatic anelastic model. The characteristic features of a quasistationary precipitation band was well simulated, although a mesoscale front was generated about 100 km southeast of the observed position. This lasted for more than 3 hours, and reproduced features of the back-building type. In the simulated band, the carrot-shaped form is produced by the northwestward movements of the cells, and the enhancement of the cells on the eastern flank of the band. By changing environmental fields and cloud microphysical processes, sensitivity experiments were conducted to investigate the formation mechanisms of the precipitation band. It was shown that the supply of high equivalent potential temperature air, from the upwind side of the middle level wind, and the vertical wind shear are essential
The world's greatest monsoons and deserts co-exist at the same latitudes on the subtropical Afro-Eurasian Continent. To investigate the mechanisms for these diverse subtropical climates, we conduct atmospheric general circulation model experiments under an idealized land-sea distribution featuring an Asian continent with a straight south coast at 17°N. Sea surface temperature and land surface parameters are all set zonally uniform to avoid a priori longitudinal preference for rainfall. The model with flat land surface produces a realistic zonal distribution of precipitation, with arid (wet) conditions over the western (eastern) part of subtropical Afro-Eurasia in association with a gigantic high pressure system over the ocean surface. The key to maintaining the western deserts is the slow precipitation-soil moisture interaction, which keeps the northward migration of the monsoon rainbelt to lag behind the sun. This is demonstrated by conducting a perpetual summer integration, where the Sahara and Arabia are soaked in heavy rain. The model monsoon starts abruptly in late June, three months after the spring equinox when a northward temperature gradient is established near the ground on the south coast. The onset is associated with explosive growth of a westward-traveling moist baroclinic wave. Further analysis suggests a Geostrophic Monsoon model: The geostrophy resists and delays the formation of a thermally direct circulation, until baroclinic instability triggers the rapid onset.