We performed a 20-year numerical experiment over the period 1985 to 2004 using a high-resolution North Pacific Ocean General Circulation Model (NPOGCM) and a 20 km-resolution regional climate model (RCM20) to clarify the impact of the Kuroshio large meander (LM) on the climate around Japan. The NPOGCM reproduced the two primary quasi-stationary states, straight path (SP), and large meander (LM), although the periods during which each state prevailed differed from those indicated in the observational data. The NPOGCM result also showed that the Kuroshio LM causes a cold sea surface temperature anomaly to the south of the Pacific coast of the central Japan. Using the result as a lower boundary condition, a continuous numerical integration was performed by the RCM20. An 8-year composite analysis of the atmospheric circulations of the RCM20 simulation for the Kuroshio LM and SP showed that, in both winter and summer, substantial decreases in the upward surface turbulent heat flux, the frequency of precipitation, and the frequency of steep horizontal gradients in equivalent potential temperature over the ocean are caused by the cold sea surface temperature anomaly. Similar effects are evident over the land area of Japan, although they are less intense, at most 20-50 % of magnitude over the cold sea surface temperature anomaly area, and limited to the coastal region on the Pacific Ocean side in the central part of the country.
A global nonhydrostatic model was used to evaluate the reproduction skill of cold and warm rain over the ocean at low latitudes and investigate their responses to global warming. In response to global warming, surface precipitation at low latitudes (30°S-30°N) in the simulations using mesh sizes of 7 and 14 km (R7 and R14, respectively) increased by 1.9 % and 2.6 %, respectively. It was found that the increase in precipitation in the higher horizontal resolution model R7 was caused by the increase in cold and warm rain and that in R14 was due to the increase in cold rain. In R7, the net increase in cold rain occurred due to the increase in stronger precipitation (> 40 mm hr-1), most of which compensated for the decrease in weaker precipitation (< 40 mm hr-1). In contrast, warm rain increased in almost all ranges of precipitation intensity. The fractional coverage of warm (cold) rain increased (decreased) robustly for both mesh sizes in the simulations. Analysis of the contribution of dynamic and thermodynamic environmental changes to the changes in cold and warm rain revealed a strong dependency on dynamic regimes in their effects. The lifespans of cold and warm clouds at low latitudes (defined by the ratio of the sum of cloud water and cloud ice paths to the precipitation flux) and possible changes related to global warming were also evaluated. On an average, in all precipitation intensities, there was no significant change in the longevity of cold clouds in response to global warming. In contrast, the lifespan of warm clouds was reduced in most of the sea surface temperature anomaly regimes.
Simple methods are formulated using an ensemble forecast to identify the sensitive initial perturbations that grow in a specified region at the verification time. These methods do not require the tangent-linear or adjoint models, but use an ensemble forecast to obtain approximated solutions. Input to the sensitivity calculation can be any ensemble forecast integrated from initial conditions perturbed with the bred vector, singular vector, or ensemble Kalman filter methods. Two formulations are presented here to approximate the adjoint and singular vector methods using an ensemble forecast. The ensemble singular vector sensitivity, which has already been applied in previous studies, is obtained with a single eigenvector calculation. The ensemble adjoint sensitivity only requires an even simpler matrix-vector multiplication. To validate the formulations, ensemble-based sensitivity analysis has been conducted in a few cases. First, the two methods were applied to identify the sensitive initial perturbations that grow in the verification region over Japan in January and August 2003. The first singular vector mode indeed achieves the largest amplitude at the verification time, but that is not necessarily true after the verification time. Both methods can identify the sensitive regions more specifically than the regions with large ensemble spread in cases with a mid-latitude cyclone and with a tropical cyclone. The monthly-mean sensitivity in January 2003 indicates the effect of Rossby waves and synoptic disturbances in upstream sensitive regions over Siberia, Tibet, and a downstream sensitive region in the north-western Pacific; the sensitivity in August 2003 suggests the influence of the Asian summer monsoon. Next, for an August 2002 storm case in Europe, global 20-km resolution simulations were conducted from the initial conditions perturbed by the ensemble singular vector method to compare with the unperturbed simulation. In the perturbed simulation, the cyclone is deeper by a few hPa in its north-east sector with more precipitation north of the Alps more consistently with observations. These results indicate that reasonable sensitive regions can be identified with our methods.
This paper analyzes the behavior of the precipitation cores (PCOs) in three cumulonimbus clouds that caused localized heavy rainfall on 18 August 2011 in the Kanto region, Japan, and their relationship with temporal variations in surface rainfall intensity. The 3D structure of the cumulonimbus clouds was observed at 2-min intervals using a research X-band dual-polarization radar. A PCO was defined as a 3D contiguous region that contained one local maximum of horizontal radar reflectivity Zh, and the PCOs were automatically detected using adaptive thresholds. Subjective tracking procedures using the PCO dataset, which was based on observations made every 2 min, identified 15 PCOs during the total lifespan of the three cumulonimbus clouds. The PCOs generally descended toward the ground after their appearance aloft. The average appearance height of the PCOs of 5.25 km above sea level (ASL) was slightly above the ambient 0°C level (5.1 km ASL). The duration of each PCO was roughly proportional to its appearance height. Of the 12 temporal peaks at maximum surface rainfall intensity (> 10 mm h-1) recorded from the three cumulonimbus clouds, 10 were associated with the descent of PCOs. In each cumulonimbus cloud, the first PCO was detected 10-12 min before rainfall heavier than 10 mm h-1 was recorded. These results indicate that the behavior of PCOs is closely related to the onset of strong surface rainfall and subsequent fluctuations in surface rainfall intensity.
We investigate the formation mechanism of the summer Okhotsk High (OH) in terms of the land-atmosphere coupling in Siberia. A reanalysis data indicates that the formation mechanism of the OH clearly differs between early and late summer because it changes from a nearly barotropic to a baroclinic structure with seasonal changes. Then, we assess the influence of springtime snow cover on the formation of the late summer OH with the baroclinicity using a regional climate model. The model performs well in reproducing the land-atmosphere coupling in eastern Siberia and the OH in August 2008, when abnormal weather prevails in Japan, in conjunction with the intensively developed OH. The August OH develops with a distinct baroclinic structure owing to increased surface heating, which is related to land-atmosphere coupling in response to reduced spring snow cover in eastern Siberia. The land-atmosphere coupling can help to reinforce and maintain the baroclinic structure through surface heating, forming strong surface anticyclone to the southeast over the Sea of Okhotsk. Our results suggest that the late summer OH is a regional climate system that involves coupling among the atmosphere, the cool ocean, and the warm land surface.
During summer, disturbed weather brings frequent bouts of heavy rain to Japan in two types of cloud systems, one with high lightning activity and the other with low lightning frequency. To find the basis for this difference, data from three Baiu and two non-Baiu Japanese heavy rain events in 2008-2009 were compared with videosonde data from various cloud systems across East and Southeast Asia. This analysis suggests that heavy rain is produced by two different precipitation mechanisms: by graupel growth in high-electrical-activity clouds and by frozen drop growth in low-electrical-activity clouds. High electrification is achieved only in graupel-growing clouds, where numerous ice crystals also grow and the riming electrification process is active.
Interannual variations of regional mean summer precipitation, especially its extremes have high impacts on the agriculture, water resources, and ecosystem and environment in Mainland China. Using observational rainfall data at 596 meteorological stations in China from 1961 to 2008, we investigated the features of regional mean summer rainfall variations based on rotated empirical orthogonal function (REOF) decompositions. Moreover, we found that there are 21 regions where variations of summer seasonal mean rainfall are very regional. These 21 regions cover most of the territory of China except for some parts in west and southwest China. The time series of coefficients of the leading REOF mode for a particular region explains a large part of variance of the summer rainfall in that region, ranging from 40 % to 90 %. The periodicities and extreme years of precipitation in the 21 regions are notably different, thus suggesting a statistically independent feature of the summer mean rainfall variations among different regions. A survey has been conducted for correlations of regional mean June-August rainfall with indices including El Nino and the Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and several atmospheric teleconnections. Outcomes of this survey suggest that the rainfall variations in 21 regions in mainland China are possibly influenced by signals from both tropical and mid-latitudes. The results of the present work provide a base for further investigation on the possible mechanisms of regional summer rainfall variations; moreover, the present work is helpful for policy making in meteorological disaster prevention and mitigations in China.
Laboratory experiments were conducted for characterizing the performance of two commercially available instruments employed for the measurement of light absorption and scattering coefficients of aerosols at λ = 405, 532, and 781 nm (using three-wavelength photoacoustic soot spectrometer; PASS-3) as well as at 375 nm (using photoacoustic extinctiometer; PAX) based on photoacoustic spectroscopy and reciprocal nephelometry, respectively. The calibration factors (conversion factors from the readout to real values) associated with scattering measurements, estimated using gaseous molecules, mono-disperse polystyrene latex and ammonium sulfate particles, and/or poly-disperse ammonium sulfate particles, are in good agreement with one another, typically within 5 %, 5 %, and 10 % at 375, 405, and 781 nm, respectively. In contrast, a significant particle size dependency was observed for the calibration factors at 532 nm, which is possibly because of a combination of differences in the polarization states of the lasers relative to the scattering planes and large truncation angle. Considering the estimated effective truncation angle, the typical uncertainties in calibration factors for scattering when measuring non- or weakly light-absorbing particles, with volume-based geometrical diameters of less than 700 nm, were estimated to be 12 %, 7 %, 34 %, and 17 %, at 375, 405, 532, and 781 nm, respectively. The typical uncertainties in the calibration factors for absorption measurements, which were determined using poly-disperse propane soot particles, were estimated to be 6 %, 4 %, 8 %, and 11 %, at 375, 405, 532, and 781 nm, respectively. The calibration factors for absorption determined by the poly-disperse soot particles at 375 and 405 nm were 48 % and 36 % smaller than those by light absorption of NO2 molecules possibly because of NO2 photolysis, although good agreement was observed at 532 nm. These results suggest that the photolysis effect should be taken into account when light absorption by NO2 is used for calibration at 375 and 405 nm.
This paper presents regime diagrams illustrating the parametric dependence of dynamical balance in a superrotating atmosphere produced in a quasi-axisymmetric idealized system with strong horizontal diffusion studied previously by the present authors. In this system, the superrotation is maintained by the Gierasch mechanism, which possibly explains the four-day circulation in the atmosphere of Venus. Our previous paper developed a theoretical model of this system to estimate the superrotation strength and showed that the parametric dependence of the superrotation strength can be consolidated into three non-dimensional external parameters. The present study analyzes the theoretical model to determine boundaries of the regimes based on the dynamical balance and plots theoretical regime diagrams, which are important to understand the non-linear dynamical system and are useful to clearly describe the parametric dependence. Further, a parametric limit of the theoretical model is also estimated and included in the diagrams. The parametric limit shows both a lower limit for the horizontal diffusion and an upper limit of the superrotation strength in the Gierasch mechanism. The regime diagram demonstrates that the superrotation in the cyclostrophic balance is realized when the horizontal Ekman number is in a certain range whose width is mainly controlled by the vertical Ekman number. Numerical solutions covering a vast region in the parameter space are obtained by time-integrations of the primitive equations, and the dynamical regimes in the numerical solutions are compared with the theoretical regime diagrams. The theoretical regime diagrams agree well with the numerical results in most regions, confirming the validity of the theoretical model. Multiple equilibrium solutions are obtained when the horizontal Ekman number is lower than the theoretical limit. Moreover, they show that the Gierasch mechanism can maintain the superrotation even with the horizontal diffusion weaker than the predicted lower limit, but cannot generate superrotation from a motionless state.