Moisture budgets are analyzed using National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data over the North Pacific for a Bi-Decadal Oscillation (BDO) in precipitation that was reported by Minobe and Nakanowatari (2002). BDO in wintertime precipitation is mostly associated with moisture flux convergence, with a minor contribution from evaporation. The moisture convergence is mainly due to anomalies of wind and moisture on time scales longer than a month, except at high-latitudes where transient eddy (time scales shorter than a month) contributions are greater. When the Aleutian low strengthens (weakens), the anomalous moisture flux convergence is due to a cyclonic (anti-cyclonic) wind circulation over 30°-60°:N and anti-cyclonic (cyclonic) wind circulation over 0°-30°N. This pair of anomalous circulations is also observed on interannual time scales, but they appear independent of El Niño;/Southern Oscillation (ENSO). The anomalous circulation over 0°-30°N is associated with sea-level pressure anomalies in the tropics (20°S-20°N) and this result is confirmed with the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and the NCEP real-time marine data. This low-latitude anomalous circulation plays a dominant role in precipitation variability in Hawaii on the bi-decadal and interannual time scales.
Such cloud radiative propenies as reflectance, transmittance, and absorptance of mixed-phase clouds have been simulated for the stratocumulus cloud observed on 30 January 1999 within the Japanese Cloud and Climate Study (JACCS) program, in which simultaneous observations of the cloud microphysical and radiative properties were conducted. The stratocumulus cloud was in mixed-phased condition, and highly heterogeneous vertically and horizontally with different mixing ratios of liquid water droplets and ice particles. A vertically-homogeneous, plane-parallel mixed-phase cloud model could reproduce the observed visible (VIS; wavelength < 0.7 μm) band radiative properties, but it could not reproduce the near-infrared (NIR; > 0.7 μm) band radiative properties. A multi-layered, plane-parallel mixed-phase cloud model could consistently reproduce the observed VIS-band and NIR-band radiative properties within the measurement accuracy. It is found that the venical profiles of water droplets and ice particles are an important factor to determine the radiative properties of mixed-phase clouds. The simulated results suggested that the NIR-band reflectance and absorptance could vary by more than 0.1 due to different vertical distributions of the cloud microphysical properties even the model clouds had constant liquid-water-path and ice-water-path. The visible and near-infrared solar reflection can be also affected by the vertical profiles of cloud microphysical properties; it is suggested that the conventional passive remote sensing under the assumption of vertical-homogeneity, may bring large errors in estimation of the microphysical propenies of mixed-phase clouds.
In order to resolve the contradiction between the theories proposed by Rodwell and Hoskins (1996) and Chen et al. (2001), a linear response of global/hemispherical atmosphere to a subtropical heat source is studied, using two different numerical models. The first model treats the response as a linear combination of wave components (Hough functions), which are eigenmodes of the homogeneous equations. The second model is based on quasi-geostrophic approximation. First, the response to zonally uniform heat source is investigated. In the case of the heat source centered at the equator, geopotential and zonal wind perturbation fields are expressed by Rossby and Kelvin modes, while vertical and meridional flows are represented by gravity modes. On the other hand, the case of the heat source centered off the equator reveals that the cell reaching winter hemisphere is dominant due to mixed Rossby-gravity mode. Second, the response to zonally localized heat source centered at 25°N is investigated. The validity of quasi-geostrophic approximation in the subtropics is verified. It is found that the geopotential and horizontal wind perturbation fields can be expressed only by Rossby modes, while the contribution of gravity modes is stronger for the vertical flow. On the basis of these results we calculate the response in the observed basic wind, but the wavetrain shown in Chen et al. (2001) cannot be reproduced. On the other hand, the descent west or northwest of the heat source, which is examined in Rodwell Hoskins (1996), appears in the resting basic field as well as in the basic zonal flow. The mechanism producing this descent is discussed in detail.
This study investigates major characteristics of summer (June-August) climate variability in Taiwan during the period of 1950-2000 and associated regulating processes imposed by large-scale background variations, including the western North Pacific summer monsoon (WNPSM), the Pacific interdecadal climate change (PICC) related to abrupt climate change in the late 1970s, and the western North Pacific (WNP) tropical cyclone (TC) activity. In Taiwan, summer rainfall and temperature anomalies tend to be opposite in sign: anomalous cold summer is often wet, and vice versa. Based upon characteristics of temperature and rainfall anomalies in Taiwan, low-level circulation pattern overlying Taiwan, and the phase of PICC, we categorize Taiwan’s summer climate variability into six major types. The major regulatory mechanism for four of these types is anomalous vertical motion over an east-west elongated low-level circulation anomaly across Taiwan, a feature associated with the WNPSM variability. For the other two types, the primary regulating mechanism is anomalous moisture transport by a southwest-northeast oriented low-level circulation anomaly over the WNP, which is a salient feature of the PICC. Influence of the WNP TC activity on Taiwan’s summer climate is examined in terms of TC rainfall (rainfall in Taiwan induced by TC passages) variability. For the types of summer climate variability with a low-level anomalous high over the WNP, the circulation enhances vertical wind shear to suppress TC genesis and the subsequent TC activity over the WNP, leading to decreased TC rainfall in Taiwan. On the other hand, increased TC rainfall does not necessarily occur in Taiwan for summer climate variability types with a low-level anomalous low over the WNP. A favorable condition for TC rainfall in Taiwan is to have more TCs forming in a region southeast of Taiwan, which are then steered by the WNP anomalous low to recurve northward toward Taiwan.
In this study, we have investigated the dominant horizontal patterns of the barotropic component of the atmosphere, the external forcing to it, and the nonlinear scale interactions, using the EOF and SVD analyses for 50 years of DJF mean NCEP/NCAR reanalysis. According to the EOF analysis, the horizontal pattern of the EOF-1 of the atmosphere represents the Arctic Oscillation (AO), and the EOF-2 represents the PNA teleconnection pattern. The SVD-1 for the barotropic component and the external forcing represents the structure of AO with the external forcing characterized by topographic forcing. The energy balance for the SVD patterns, however, shows that the external forcing of the SVD-1 damps the AO-like anomaly of the barotropic component. The SVD-2 represents the PNA, and the time series of the coeflicient indicates a link with El Niño/La Niña. The external forcing of the SVD-2 appears also to damp the PNA-like anomaly. Furthermore, the SVD-1 for the barotropic component and the nonlinear scale interactions represents the AO-like anomaly with the large nonlinear scale interactions over half of the Northern Hemisphere. The energy balance for this SVD pattern shows that the nonlinear scale interactions excite the AO-like anomaly. Similarly, the SVD-2 shows the energy increase by the nonlinear scale interactions the same as SVD-1. The nonlinear scale interactions are further decomposed in linear interactions with the climate basic state and the reminder. We find by the energetics analysis that the AO-like anomaly is mostly excited by the interactions with the basic state. It is concluded from the SVD analyses that the extreme events associated with low-frequency variabilities are not induced by the low-frequency forcing, but mostly by the energy supply from the climatebasic state.
The purpose of this article is to document genesis, development and dissipation of a subsynoptic-scale typhoon-like cyclone, that caused severe weather on 13 October 2003, over the eastern part of the Kanto area. In this documentation, practically all available data obtained operationally by the Japan Meteorological Agency are used. The subtropical low of interest formed initially as a mesoscale vortex at 0000UTC 8 October over the ocean near Okinawa Island (∼26°N, 128°E), where the western end of a quasi-stationary polar front was located. The genesis region is characterized by a weak baroclinicity, and nearly saturated low-level atmosphere, with a negative Showalter Stability Index. Several mesoscale convective systems developed prior to the formation of the subtropical low in the genesis region. It is inferred that diabatic heating, by the release of latent heat in deep convection helped spin up pre-existing relative vorticity associated with the quasi-stationary polar front. In addition, an upper-level short wave trough spawned a cut-off low over the region, helping the vortex to develop into the subtropical low. When matured ∼50 h later, the subtropical low exhibited remarkable axisymmetry in the wind and thermal structures, with the diameter of l00∼200 km. The cloud pattern also displayed a cloud-free eye-like structure. After it traveled slowly northwards for four days, the subtropical low was overtaken by a synoptic scale trough, and underwent the processes known as the extratropical transition of a typhoon, while it moved towards the east. Finally, briefly discussed are similarities and dissimilarities between the present subtropical low, and hurricane-like subtropical cyclones that develop in other regions of the globe, such as the Mediterranean Sea and the central/eastern Pacific Ocean.
On 16 January 2001 during the field experiment named WMO-01, a meso-β-scale snowband extending eastward from Toyama Bay was observed in the coastal area of eastern Hokuriku district, middle part of Japan. This snowband stagnated for about half a day and brought a snowfall of about 50 cm in this region. Numerical simulations of this snowband are performed using a nonhydrostatic cloud resolving model (NHM), with a horizontal grid size of 1 km. The NHM well reproduces many characteristics of the observed snowband. The snowband forms over the convergence zone between a cold southerly land breeze and a northwesterly winter monsoon, to which heat and moisture is supplied from relatively warm sea surface. Convective snow cells with the horizontal scale of a few km successively form at the northern part of the snowband and propagate east-southeastward. Meso-γ-scale convective snow systems are organized, consisting of developed snow cells in the snowband. A cold pool forms under the snowband. The diabatic cooling due to the sublimation of snow is responsible for the formation of the cold pool. After the land breeze became weak, the cold pool contributes to the maintenance of the horizontal convergence with the northwesterly winter monsoon at the northern edge of the snowband by compensating for the weakened land breeze. Therefore, the formation of the cold pool is significant for the maintenance of thesnowband.
Seasonal variations of heat and water balances have been estimated by using a multi-layer soil model for the Tibetan Plateau in 1997-1998. Input data was from routine meteorological data, and calculation was carried out for 14 stations in 1997 and 17 stations in 1998. Compared with 1997, annual precipitation was smaller in northwestern and larger in southeastern Tibet in 1998. The variations of latent heat flux from the ground surface, connected with the changes of soil water content that are controlled by change of precipitation. Thus, the latent heat flux of 1998 was larger than that of 1997 in southeast Tibet, but in northwest Tibet, the situation was reversed. The calculation result for Wetness Index (WI), showed that from 1997 to 1998, the climate in northwest Tibet become more dry, and southeast Tibet wetter. The climatic difference became more severe between those two regions. In northeast Tibet, soil water contents were smaller and sensible heat flux was larger in 1998, than in 1997, because the precipitation was relatively small in the springtime of 1998. In southeast Tibet, the amount of the precipitation were almost the same in the springtime of 1997 and 1998, but the rainfall amount became larger, and concentrated in the latter monsoon season in 1998, this led to the increase in soil water contents, and the amount of evaporation. Those increases were not as sharp as the changes of precipitation, and most of the precipitation contributed to sharply increase the run off, and flowed into the rivers. After monsoon seasons, the differences between the amount of precipitation in 1997 and 1998 became very small, and the soil water contents were almost the same in the end of those two years. Soil water contents changed with the precipitation.
Over the tropical land regions scatter plots of the rain rate (RPR), deduced from the TRMM Precipitation Radar (PR) versus the observed 85 GHz brightness temperature (T85v) made by the TRMM Microwave Imager (TMI) radiometer, for a period of a season over a given geographic region of 3° × 5° (lat × lon), indicate that there are two maxima in rain rate. One strong maximum occurs when T85v has a value of about 220 K, and the other weaker one when T85v is much colder ∼150 K. AIso these two maxima are vividly revealed in plots of RPR vs. (T19v-T37v). The strong maximum occurs when (T19v-T37v) is ∼20 K and the weaker maximum when it is greater than ∼40 K. Together with the help of a) earlier investigations based on airborne Doppler Radar observations and b) radiative transfer theoretical simulations, we infer the strong maximum is a result of relatively weak scattering due to super cooled rain drops and water coated ice hydrometeors associated with a developing thunderstorm (Cb) that has a strong updraft. The other maximum is associated with strong scattering due to ice particles that are formed when the updraft collapses and the rain from the Cb is transitioning from convective type to stratiform type. Incorporating these ideas with a view to improve the estimation of rain rate from existing operational method applicable to the tropical land areas, we have developed a rain retrieval model. This model utilizes two parameters, that have a horizontal scale of ∼ 20 km, deduced from the TMI measurements at 10, 19, 21 and 37 GHz (T10v, T19v, T21v, T37v). The third parameter in the model, namely the mean horizontal gradient <dT85v/dr> (K km−1) within the 20 km scale, is deduced from TMI measurements at 85 GHz that have the scale of ∼5 km. Utilizing these parameters our retrieval model is formulated to yield instantaneous rain rate on a scale of 20 km. This retrieval model is initially tuned with the help of a limited amount of PR rain rate. After initial tuning, the model is applied to widely different tropical land areas, and for different seasons. Our estimates of instantaneous rain rate, on a scale of 20 km, and seasonal averages on a scale of 3° × 5° (lat × lon) agree better with PR than that given by the operational TMI rain retrievals.
In this study, eigenmodes and singular modes are analyzed for a dynamical system of the atmosphere, linearized about a winter basic state in order to understand the dynamics of the Arctic Oscillation (AO). Since the fluctuations of the sea-level pressure are dynamically linked with the barotropic component of the atmosphere, the AO is investigated in the framework of a barotropic model. As a result of the analysis, we find multiple eigenmodes which are similar to the AO, with a negative pole in the Arctic, and positive poles in the Pacific and Atlantic sectors. Since some of the eigenmodes are unstable, a linear drag is introduced to shift the eigenvalues in order to pick up different eigenmodes as a singular mode with resonant behavior. It is demonstrated that the singular eigenmode of the dynamical system emerges resonantly as the AO, in response to the arbitrary forcing. The resonant growth is allowed for multiple eigenmodes, including the unstable modes, and therefore the AO described by the neutral mode under the strong friction is recognized as the least damping mode excited by the tail of the resonant response curve of the singular eigenmode. In reference to the result of the nonlinear simulation of the AO, using the same barotropic model, we may conclude that the AO is a physical mode ofa dynamical system for the global atmosphere.
Long-term trends in summer precipitation totals, the number of rainy days, and precipitation intensity were investigated with daily rainfall datasets for China from 1961 to 2000. Total precipitation significantly increased in summer in the Yangtze River basin and northwestern China, while total precipitation decreased in other regions. The number of rainy days increased in the Yangtze River basin, and over northwestern China. In contrast, the number of rainy days decreased over Tibet and over northern and northeastern China. Seasonal mean precipitation intensity became large at most of the stations in China. To investigate trends in heavy rainfall, daily rainfall totals during the whole investigation period were grouped into ten classes, with class width equal to l0% of total number of rainy days from 1961 to 2000, and the number of rainy days, and summer mean rainfall amount for each class, were obtained for each summer. A simple linear fit was made to determine the linear trends in the 10-class precipitation time series. The upper 20 percentile of daily precipitation totals showed a statistically significant increase over the Yangtze River basin, and over northwestern China during the study period. Class average precipitation decreased in almost 10 classes over Tibet and north and northeastern China.
Aerosol number concentrations were observed in conjunction with in-situ meteorological parameters aboard the patrol vessel SOYA in February 2003 over the ice-covered Okhotsk Sea. The concentrations of particles with radii larger than 0.35 μm were close to the background level, while concentrations of particles less than 0.35 μm were close to the remote continental level. During the observation period, relatively higher and lower aerosol number concentrations (HA and LA) were observed, depending on synoptic weather conditions. Backward trajectory analyses revealed that for the HA period, the air mass in the study region traced back to the Sea of Japan; for the LA period, the air mass originated from the northern ice-covered area. This suggests that sea ice influences the aerosol number concentration by suppressing the amount of oceanic sea salt supplied to air masses. Using the observational data and an empirical formula, differences in surface solar radiation between the HA and LA periods were alsoexamined.
A convective cloud varies considerably in physical structure from development to dissipation. This variation results in changes in the Raindrop Size Distribution (RSD) in the precipitation from a convective cloud. The characteristics of RSD in the life stages of a convective cloud are compared, based on the data measured by an optical raindrop spectrometer during the field projects; TREX (1996), X-BAIU-98 (1998), and X-BAIU-99 (1999), performed on the southern and western coasts of Kyushu Island in the Baiu season. The convective clouds that passed over the observational site were classified as being in the mature, or the dissipation stage. The relationship between the shape of the RSD, and the convective activity of the convective clouds, varied according to the stage of development. When clouds are in the mature stage, the raindrop number concentration in every size range increases as the convective activity increases. On the other hand, when clouds are in the dissipation stage, the raindrop number concentration with larger sizes tends to increase as the convective activity in the mature stage increases. On the basis of these new findings obtained from observations, the major precipitation mechanism is expected to change in the life stages of convective cloud.