This study investigates the impact of anthropogenic climate change on the Indian summer monsoon, and the ENSO-monsoon teleconnection, using the transient climate change simulations of the MRI coupled model (MRI-CGCM2.2). In the present simulations atmospheric greenhouse gas (GHG) concentrations and aerosols are varied to represent observed changes during 1850-2000, and from 2001-2100, at the rate prescribed by the SRES-A2 and B2 scenarios to study the response of the monsoon climate during 2071-2100. All India annual temperature shows warming by 2.35°C (1.64°C), while all-India monsoon rainfall (JJAS) indicates an increase by 9% (6%) in SRES-A2 (B2) scenario relative to the present climate (1971-2000). Increase in the monsoon precipitation is evident over parts of south India, parts of the Bay of Bengal and northwest India. The simulations feature stronger warming over South Asia relative to the Indian Ocean, leading to enhanced land-sea temperature contrast. Enhanced moisture in a warmer scenario, coupled with enhanced monsoon circulation, leads to enhanced moisture transport over the Indian region from both the Bay of Bengal and Arabian Sea leading to increased monsoon precipitation. Decomposition of the moisture transport into divergent and non-divergent components shows that the enhanced monsoon rainfall is mainly due to intensification in the non-divergent component of the moisture transport. While strong and continued increase in monsoon rainfall suggests a change towards wetter mean state, warming of summer (JJA) Ni&ntild;o-3 SSTs suggests a change towards warmer El Ni&ntild;o like mean state in the east Pacific. The variability in the Ni&ntild;o-3 SST shows an increasing trend, as well as fluctuations about the trend. The correlation between the Indian monsoon rainfall and Ni&ntild;o-3 SST decrease particularly after 2050. This weakening of ENSO-monsoon relationship is also seen in the form of change in the impact of ENSO events on the intra-seasonal monsoon rainfall over India.
The vertical distributions of water vapor, aerosols and cloud backscattering, and three-dimensional wind were measured using a Raman lidar and a wind profiler during the passage ofa non-precipitating cold front during the night of 19-20 March 2002 over Tsukuba, Japan. The passage of the cold frontal head was identified by a sharp drop in the temperature at a height of 213 m, when the backscattering ratios sharply decreased below an altitude of approximately 2.7 km. The water vapor mixing ratios showed high values (∼2.5 g kg−1) around the frontal head. Arc clouds were observed in the watersaturated air above the frontal head in updraft regions (∼1 m s−1). The cold air mass showed the geometrical and dynamical characteristics of gravity (density) currents: nose and head identified by the lowbackscattering ratio region at the leading edge, billow-like patterns in the backscattering ratio behind the head, and feeder flows within the head (in a positive direction toward the frontal head). The observed height, and the propagation speed of the cold air mass, approximately agreed with those calculated from surface meteorological data using gravity current models, although there were uncertainties in the calculated values because of the lack of the vertical distributions of temperature and pressure data.
In order to understand cloud clusters and a meso-α-scale low, which were observed on the China Continent on 29 June 1998, numerical experiments are performed with a model which intends to resolve mesoscale organized convection, the effects of cumulus convection being incorporated as the subgrid-scale. The horizontal grid size is taken to be about 5 km for the fine grid area of the triply-nested grid model. The initial time for the numerical experiments is 00 UTC 29 June. Global analysis data (GANAL) of JMA is used as an initial condition, but two numerical experiments are performed, without and with slight reduction of low-level relative humidity of GANAL data. Results from numerical experiments indicate that in both cases, convective clouds are formed in a banded form, and a front that can be called a Meiyu front is also produced. On this front, a meso-α-scale low is also formed. The positions of the mesoscale lows predicted at 24 hours after the initial time in both cases, are almost the same. These are roughly in agreement with the position from GANAL data. Significant differences are found in the evolution stage for a period of 24 hours. In the original case, strong convective activity creates a mesoscale low at an earlier time (12 hours), and it is probably somewhat too intense compared with that observed. On the contrary, in the less humid case, three mesoscale convective systems and vortices (or closed isobars) are found at 12 hours. The easternmost vortex develops, and it becomes a mesoscale low at 18 hours. Discussions in this paper are made with an emphasis on the relation among latent instability, mesoscale convective systems, vortices, a mesoscale low and a Meiyu front. One of the significant features of the present case is that convective activity takes a banded form in a relatively large area of latent instability, and a Meiyu front is produced and intensified for the given initial conditions. As expected, latent instability becomes weaker, owing to convective activities. Eventually a latently stable area appears first around and just to the north of the convective area, and it expands northward. It is important to remark that the latently stable area to the north of the Meiyu front in the later stages is created by convective activities (not by synoptic-scale motions). In an area to the south of the front, latent instability becomes weaker gradually. However, continual rainfalls are maintained in the trailing portion of the mesoscale low till 00 UTC 30 June. This is in agreement with the observations. The relative importance of the effects of subgrid-scale cumulus convection and the grid-resolved condensation is also discussed. The importance of the latter increases with time; it contributes particularly to the development of the mesoscale low.
When a cold air outbreak occurred over the Sea of Japan, double snowbands stayed along the coastal region of the Kanazawa Plain from 15 to 16 January 2001: Snowband 1 on the seaward side, and Snowband 2 on the landward side. Their structures are studied mainly using Doppler and dual-polarization radars, and their maintenance processes are discussed. The double snowbands were maintained for about 20 hours. Snowband 1 had a stronger reflectivity and a higher echo top than Snowband 2. The cells in Snowband 2 were generated in the weak echo extending from Snowband 1 toward the land. Around the snowbands, a southeasterly land breeze, with a thickness of 400 m was present. A convergence between the land breeze and the northwesterly monsoon wind formed a strong updraft. Convective cells in Snowband 1 developed in association with the strong updraft. In Snowband 2, a weak convergence produced a weak updraft. The dual-polarization radar data showed that Snowband 1 was composed of spherical graupels, including conical ones. On the other hand, flat-shaped snow crystals were converted to snow aggregates along the trajectory of the particles within Snowband 2. Graupels were seen in the pictures taken below Snowband 1, while only snow aggregates were observed below Snowband 2. These facts indicate that the strong updraft at the land-breeze front contributed to the formation of graupels through the riming process in Snowband 1, on the other hand, snow aggregates developed through the deposition and aggregation processes in the weak updraft in Snowband 2. Some snow crystals, which contributed to the formation ofsnow aggregates in Snowband 2, were supplied from Snowband 1.
The structure and evolution of deeply developed convective cells within a long-lived multicellular cumulonimbus cloud that developed over the Huaihe River Basin in China on 13 July 1998 during the GAME/HUBEX were studied, mainly using the observational data of Doppler radars. The lifetime of the cumulonimbus cloud was longer than 3.5 hours, and its maximum echo-top height was 19 km AGL. The atmospheric condition was characterized by a large CAPE (2300 J kg−1), and weak vertical wind shear (1.6 m s−1 km−1 toward the north-northeast below 5 km AGL). In the mature stage of the cloud, a large radar-echo region stronger than 40 dBZ was observed. It was almost upright, and showed almost the same horizontal areas between 5 and 15 km AGL. Two strong updrafts existed around the upshear and downshear parts of the strong echo region, and they tilted toward the downshear and upshear sides, respectively. A downdraft developed in the eastern part of the cloud, where a cell-relative northeasterly wind was present at the low and middle levels; however, the wind was not observed in the synoptic-scale environment. The downdraft was located between the two updrafts. A descending northeasterly airflow continuously lifted the low-level air coming into the upshear-side updraft from the southwestern (upshear) side. The downdraft, and its surface outflow, did not cut off the supply of low-level air coming into the downshear-side updraft from the northern (downshear) side. The vigorous development of the cumulonimbus cloud in the mature stage was caused by the development of these adjacent convective cells. Significant processes in the formation of the mature structure are the maintenance of the upshear-side convective cell, and the change of the downshear-side updraft from downshear-tilting to upshear-tilting in association with the intensification of the downdraft in the cell-relative northeasterly wind.
Disagreement of the thermal-infrared (TIR) surface temperature and the aerodynamic surface temperature, which is an effective surface temperature in estimating surface heat fluxes using the bulk method, is a critical problem in the combination of the TIR thermometry and the bulk method. The present study investigates two methods which adjust the disagreement. One is the method using the radiometric thermal roughness length, and the other is the method using empirical parameterizations (adjustment parameter method). Recent literature has shown a priority of the adjustment parameter method, however, the present study shows the radiometric thermal roughness length is analytically identical to the adjustment parameters, and it is verified by observation data that the two methods can be equally used in estimating sensible heat flux over a rice paddy, over a wide range of vegetation density. Moreover, it is shown that the adjustment parameters quantify the effect of viewing angle of measurement on the TIR surface temperature.
Climatological monthly statistics of parameters that are widely used to characterize the environment for mesoscale convective systems in Japan is studied by using twice-daily (09 and 21 JST: Japan Standard Time) routine rawinsonde data from 18 stations of the Japan Meteorological Agency for the period between 1990 and 1999. The environmental parameters examined include CAPE (Convective Available Potential Energy), CIN (Convective Inhibition), precipitable water, SSI (Showalter Stability Index), and bulk Richardson number. The monthly medians of these parameters have the following characteristics:
1) Large values of CAPE generally occur in summer. Maximum of median CAPE is larger in the southwest than in the northeast of the Japan islands, and the month of the largest CAPE comes earlier in the southwest than it does in the northeast. At a majority of the stations, the medians of CAPE start to increase rapidly from June and remain large until September. 2) Minimum of SSI is generally found during the summer. SSI tends to be smaller for larger CAPE. The frequency that SSI becomes less than or equal to −2 K at 21 JST is significantly higher than that at 09 JST at all stations. In terms of the frequency that SSI becomes less than or equal to −2 K or 0 K, Tateno at 21 JST in August is found to be most unstable among the stations examined. 3) Most of the stations have large CIN in winter and small CIN in summer (July and August), indicating that convection may be triggered by a weaker forcing in summer. 4) Precipitable water shows a smooth seasonal variation with one peak during summer. Precipitable water at night (21 JST) is slightly higher than that at morning (09 JST) at most of the stations all the year round. 5) The largest Bulk Richardson number occurs during summer when CAPE is larger and vertical wind shear is smaller.
Turbulent diffusion from a small surface source into the boundary layer is studied by means of a wind-tunnel experiment. On the tunnel floor, a strip-shaped heater was placed perpendicularly to the mean flow. We regarded the heat as a passive scalar, and measured the air temperature in the boundary layer over the tunnel floor. The passive scalar, advected on the mean flow, diffuses in the floor-normal direction. For the diffusion of the passive scalar throughout the boundary layer, we find a characteristic length scale in the mean-flow direction. This length scale, which is always twice the boundary-layer thickness regardless of the other experimental parameters, would be usefu1 when we consider the at mospheric boundary layer over a patchy surface.
Tsuda and Hocke (2002) reduce the amount of filtering applied to raw radio occultation data in order to increase the vertical resolution of GPS based temperature retrievals in the stratosphere. The authors discuss gravity wave characteristics obtained from these optimised retrievals, including characteristics of vertical wave number spectra for vertical wavelengths below 2 km, and the meridional distribution of potential energies for such short wavelengths. The filtering of raw data is required because real measurements contain noise. In this note, we discuss the role of measurement noise for gravity wave characteristics obtained from dry temperature retrievals of GPS based radio occultation soundings. We show that for realistic noise levels, temperature fluctuations can be safely interpreted as originating from small scale atmospheric waves in the altitude region below 30 km and for wavelengths greater than 2 km, with the detailed limits depending on noise and retrieval characteristics. An optimising procedure as the one applied by Tsuda and Hocke, however, may raise noise related effects to the level of atmospheric waves for short vertical wavelengths. We further show that conclusions for vertical wavelengths less than 2 km, if at all, can only be drawn after taking the actual noise characteristics of the observations and the details of the filter procedures applied in the retrieval into account.
It has been shown by Aoki (2004, 2005) that the differential of multi-channel radiance data can be compressed into about two order smaller channels of hypothetical radiances, with loosing negligible information content. In these methods the weighting functions of the original channels are expanded with empirical orthogonal functions (EOFs), and a set of hypothetical radiances, whose weighting functions are the EOFs, are obtained by a linear combination of original channels. It was noted in these papers that to incorporate this method into the assimilation scheme of the numerical prediction, a massive look-up table is required to be applicable for various atmospheric profiles. In this table, the atmospheric profiles of temperature and water vapor, the coefficients for converting the original brightness temperatures to the hypothetical brightness temperatures, and the empirical orthogonal functions are included. A sufficiently large number of model atmospheres should be prepared for the table, so that the differential brightness temperature for an arbitrary atmospheric profile can be given by a linear correction to the brightness temperature for one of the model atmospheres in the table. The data size of such a look-up table could become as large as it impacts to the computer system. In the present paper, a preliminary study is carried out on the effect of the nonlinearity of the radiative transfer in the brightness temperature of hypothetical channels in thermal infrared regions 640-760 cm−1 and 1300-1600 cm−1. A simple method for the nonlinearity correction is proposed. It is shown that by applying the correction method proposed, the distances of the neighboring atmospheric profiles in the look-up table can be extended to more than 6 K for the temperature, and about 0.3 g/cm2 for the precipitable water.
The collection efficiency of a hydrometeor videosonde (HYVIS) for dry snow particles is re-evaluated. This is estimated from comparisons with simultaneous aircraft data taken from a warm frontal cloud. The observed cloud is relatively uniform below an altitude of 5.5 km, where the standard deviation of ice particle concentrations is less than 40%. Comparing both size spectra measured with the balloonborne HYVIS and the airborne two-dimensional optical array probe within comparable size ranges (300 to 1500 μm) reveals that the collection efliciency of the HYVIS for dry snow particles is approximately 0.5, with an uncertainty of 50%, assuming that the airborne probe provides reference concentrations in these size ranges.