Based on observations from a hydrometeorological network on the eastern slopes of the Annapurna Range, nearly all the annual precipitation at low elevations (< 2000 m MSL) in Nepal is in liquid form, even during the winter. However, high elevations (> 3000 m MSL) can receive up to 40% of their annual precipitation as snowfall during the winter, with the highest altitude stations (∼4000 m MSL and above) having the most total winter precipitation (which can exceed 100 cm). Significant snowstorms are associated with terrain-locked low-pressure systems that form when an upper-level disturbance passes over the notch formed by the Himalayas and Hindu Kush mountains (the so-called Western Disturbances), causing upper-level SW flow over central Nepal and orographically forced precipitation. Based on these results, a 30-year (1973-2002) climatology of these notch depressions is developed and reveals that significant interannual variability in central Himalayan winter storms exists. Weak but statistically significant correlation between notch depressions and the Polar/Eurasia teleconnection pattern was found, suggesting that the strength of the circumpolar vortex may affect the number of depressions passing through the Himalayan region. A typical snow event (11 February 2000) was the subject of an observational and modeling case study. Local precipitation (snow and rain) and other meteorological observations, as well as satellite (Meteosat-5 and TRMM) and NCEP/NCAR Reanalysis data were used, along with a cloud-resolving model with realistic topography. This study shows that significant wintertime precipitation only occurs in the central Himalayas when the large-scale flow evolves to a favorable geometry with respect to the mountains.
The US National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data are employed to study the relationship between the variation in the structure of subtropical anticyclone and the summer monsoon onset over the South China Sea (SCS). The ridge surface of the subtropical anticyclone is defined by the boundary between westerly to the north and easterly to the south [or westerly-easterly boundary (WEB) in brief]. The WEB well represents the three-dimensional structure of the subtropical anticyclone. Under the thermal wind constraint, the WEB usually tilts toward the warmer zone in the vertical. In the Asian monsoon area, the ridgelines of the subtropical anticyclone at various levels in the troposphere in winter are continuous and exhibit a relatively zonal structure. The WEB tilts southward with increasing height and the mean position of the WEB is closer to the equator. In summer, the ridgelines of the subtropical anticyclone in the middle and lower (upper) troposphere are discontinuous (continuous) and the WEB either tilts northward or is vertical, with the mean position of the WEB being located at 25-30°N. May is the period when the structure of the subtropical anticyclone varies most significantly. During the seasonal transition, the tilt of the WEB changes from southward to northward. When the WEB becomes perpendicular to the earth's surface or tilts northward, the meridional temperature gradient vanishes or becomes positive, implying the replacement of the winter monsoon by the summer monsoon. Defining the “seasonal transition axis” (STA) as the vertical ridge axis that results from the switching of the tilt of the WEB from southward to northward, it is found that the STA first establishes over the eastern Bay of Bengal and Indochina Peninsula during the first pentad of May. The onset of the Asian summer monsoon circulation is therefore closely associated with the location of the STA. When the STA reaches the northeastern SCS in the fourth pentad of May, the SCS summer monsoon occurs. The south Asian summer monsoon onset corresponds to the establishment of the STA over central India in the first two pentads of June. Based on the close relationship between the STA and monsoon onset, the area-averaged meridional mean upper tropospheric (200-500 hPa) temperature gradient in the vicinity of the WEB is suggested to be an index to define the SCS summer monsoon onset. In comparison with the onset dates as determined by the area-averaged zonal wind at 850 hPa and OLR, the dates determined by these three indices are consistent in a large number of individual years, which indicates that the establishment of the STA, or the reversal of the meridional temperature gradient, captures the essential feature associated with the SCS summer monsoon onset.
Intense convective systems were simulated for the period 19-27 December 1992, over the TOGA COARE Intensive Flux Array (IFA), making use of a cloud resolving two-dimensional model (ARPS), in order to investigate where the boundary layer air mass was detrained from convective systems over tropical oceans. Inert Euler tracers originating in the surface boundary layer were used to evaluate the detrainment amounts from the convective systems. Two peak detrainment amounts of the tracer were found around the 200 hPa, and 600 hPa levels. The 200 hPa level corresponded to the top heights of the most intense clouds. Sensitivity tests demonstrated that the detrainment peaks at the 600 hPa level were controlled by horizontal vorticity developed at the same level. The horizontal vorticity was enhanced by both the stable layers around 500 hPa, which were often observed during the period of simulation, and the melting process from ice to water.
Effects of surface topography on annular variability of the extratropical troposphere are examined by parameter sweep experiments with a simplified global circulation model. Amplitude of the sinusoidal surface topography hm of zonal wavenumber m is swept as an experimental parameter in two series of experiments; a single zonal wavenumber, two (m = 2) or one (m = 1) is assumed in the series of WN2 or WN1, respectively. In additional series of WN2-1, the ratio of superposition of these two components is swept as an experimental parameter. In each run, long time integrations for 4300 days are done under a perpetual winter condition. Characteristics of the leading mode of empirical orthogonal function (EOF) of the zonal-mean zonal wind, and the surface pressure (Ps), depend on the amplitude and zonal wavenumber of the surface topography. In the WN2 experiment, characteristics of EOF1 of zonal-mean zonal wind and Ps change dramatically around h2 = 450 m, and the annular variability is divided into two types for large and small h2. In the WN1 experiment, on the other hand, characteristics of the annular variability do not show such drastic changes as those in the WN2 experiment, and EOF1 of Ps shows annular pattern for all h1 from 0 m to 1000 m. Three typical cases are analyzed in detail; h2 = 0 m (FLAT), h2 = 1000 m (HWN2), and h1 = 1000 m (HWN1). In the HWN2 experiment, the number of the storm tracks is two, and correlation map with the index that represents the oscillatory variability at that region shows a pattern localized in longitudes around the region with high teleconnectivity, even though EOF1 of Ps shows annular pattern. On the other hand, the annular variability has a sound physical basis in the FLAT and HWN1 experiments. Importance of the number and spatial structure of storm tracks which exist at the exits of jet streams is also confirmed in the WN2-1 experiment.
Field observations were carried out to clarify the regional differences in snowflake size distributions and the causes for these differences. When snowflake size distributions can be approximately expressed by the exponential distribution ND = N0 exp(-λD), the intercept parameter N0 in different regions of Japan, becomes larger according to the distance from the coast, even under the condition of equal snowfall intensity. This information can be interpreted by the idea that supercooled cloud droplets predominate in the snow clouds initially, while the phase change from water to ice advances as the clouds move inland. The slope λ of the size distributions in the central Japan group is more gentle than that in the northern Japan group, since the adhesion strength of snow crystals is strong under the warmer condition.
We statistically investigated winter-summer climatic connectivity between the winter North Atlantic Oscillation (NAO), and the summer Okhotsk High. We found that the winter NAO persistently influences the surface air temperature, snow cover over the Eurasia continent, sea ice and SST around the Barents Sea region from winter to summer. The warm air temperature anomalies in East Siberia make a preferable condition for upper-level blocking. Warm signals around the Barents Sea region excite the Rossby wave propagating toward the Sea of Okhotsk. In addition, regional persistency of the wintertime local phenomena, and their possible influence on the summertime Okhotsk High is examined. The negative phase of the winter NAO is related to the winter and spring heavy sea ice in the Sea of Okhotsk. The heavy spring sea ice in the Sea of Okhotsk brings about the cold SST around the Sea of Okhotsk. However, since the Okhotsk High does not have correlation with the SST around the Sea of Okhotsk in the previous month, the local connection of the sea ice in the Sea of Okhotsk with the summer Okhotsk High is unlikely. Associated with an enhanced Okhotsk High, cold SST anomalies appear to the east of Japan. This cold SST anomaly is a response to the cold air advection associated with the Okhotsk High. We suggest that the winter NAO remotely regulates both the winter sea ice, and the summer Okhotsk High.
We have developed an atmospheric radar (wind profiler) for lower tropospheric observations (Lower Troposphere Radar: LTR), based on the 1357.5-MHz boundary layer radar (BLR), which we previously developed mainly for observations of the atmospheric boundary layer. System gain of this radar is improved due to newly developed large-sized active phased-array antenna, active transmitting modules with higher output power, and pulse compression technique. It has the following functions: an antenna gain of 33 dBi is obtained with a 4 m × 4 m active phased array antenna which has 96 antenna subelements, a peak output power of 2 kW is obtained by 24 active transmitting modules, and maximum S/N is improved 8 times by using a pulse compression technique which uses 8-bit optimized coding developed by Spano and Ghebrebrhan (1996c). The LTR is the first active phased-array 1.3 GHz-band wind profiler radar. It is possible to vary the beam direction by electronically steering the zenith angle within 45°. Atmospheric winds in the lower troposphere, including the atmospheric boundary layer, are obtained with high time and height resolutions in real time. Observations of atmospheric temperature are also possible using the radio acoustic sounding system (RASS) technique with speaker horns. We have confirmed LTR’s potential as a reliable tool for atmospheric observations, using simultaneous observation results with the MU (Middle and Upper atmosphere) radar, a Doppler sodar, and a radiosonde.
The dependence of longitude-dependent total ozone patterns on large-scale wave structures is analyzed by a pattern correlation of the mean fields (climatology), as well as of the decadal change (trend) for the 1979 to 1992 period. In all months, there exists a significant negative regression between total ozone and 300 hPa geopotential height patterns for the climatology (annual average: -0.18 DU/m), and for the trend (-0.14(DU/10 years)/(m/10 years)). For both, the regression coefficients exhibit an asymmetric annual variation with respect to the solstices with significantly higher values in spring, and lower values in autumn. Assuming that the ozone transport is primarily caused by the annual variation of large-scale wave transport in a background ozone field, we show that the seasonal cycles are mainly determined by the asymmetric annual variation of the gradients of the zonal mean ozone field, and not by the annual variability of the wave activity.
Late winter surface pressure anomalies over the North Pacific and North Atlantic fluctuate from year to year in a seesaw. This Aleutian-Icelandic seesaw modulates the upward propagation of planetary waves into the stratosphere, thereby causing year-to-year fluctuations in the ozone layer . We first derive the Aleutian-Icelandic seesaw index from the 40-year re-analyses (ERA-40) compiled at the European Centre for Medium-Range Weather Forecast (ECMWF). February column ozone derived from two decades of satellite ozone observations is then regressed upon the Aleutian-Icelandic index to uncover the seesaw ozone signature. The regression map obtained is contrasted with the ozone regression map associated to the Arctic Oscillation. Both the quasi-stationary and the transient eddy components of ozone are influenced by the seesaw, in a manner consistent with the seesaw imprint upon upper-tropospheric meteorological fields. The year-to-year variations in the February-mean ozone over the Aleutian and Icelandic sectors, which are anti-correlated, are shown to be dominated by the seesaw.
A comparison between two reanalysis datasets (the NCEP-NCAR and ERA-40 reanalyses) is conducted over East Eurasia. The summer sea level pressure (SLP) from these datasets are specifically compared in the two periods from 1960 to 1979, and 1980 to 1999 in order to examine the long-term homogeneity and reliability of these datasets. The SLP of the NCEP-NCAR over Mongolia and its vicinity exhibits an obvious increase between the two periods, but it is not recognized in the ERA-40. This discrepancy is mainly due to sudden increases in the SLP around Mongolia in the mid-1960s and mid-1970s in the NCEP-NCAR, which are not recognized in the ERA-40. Other observational datasets used in this study show similar variations to those appeared in the ERA-40. It is likely that the sudden increases in SLP observed in the NCEP-NCAR before the 1970s are spurious, and this result implies that the NCEPNCAR reanalysis over Mongolia and its vicinity before the 1970s does not reproduce the actual surface conditions. The ERA-40 may also have some problems such as the lower SLP than that of the observational data before the 1980s, but the difference is, in general, less than 3 hPa and this value is smaller than that recognized between the NCEP-NCAR and the observational data before the mid- 1970s. Therefore, it seems that the ERA-40 is more accurate, and is more appropriate to use than the NCEP-NCAR, for the moment, in investigations of the interdecadal climate change in the late 1970s over East Eurasia. However, care must also be taken into the above-mentioned problems.
Moistening processes in the upper troposphere are examined by relating upper tropospheric humidity (UTH) estimates from SSM/T-2 and Meteosat measurements to convective clouds over the Indian Ocean. Results are compared against NCAR/CCM3 outputs. The analysis separating the tropics into four cloud areas, i.e., deep convective, middle cloud, thin cirrus, and clear/low cloud area, indicates that the upper troposphere above the clear/low cloud area becomes drier (moister) in response to increases (decreases) in convective activity in the tropics. On the other hand, the area between deep convective cloud clusters, and clear/low cloud area, also appears to be moistened by thin cirrus originating from deep convective clouds. Although the CCM3 model reproduces the average number of cloud clusters found in the satellite observations, the model-derived UTH shows significantly different features, i.e., a drier upper troposphere within deep convective clusters, a much higher UTH over the clear/low cloud area, and a slight increase of UTH over the clear/low cloud area with respect to increases in the deep convective area, suggesting that the model needs more accurate physics for better description of the moistening/drying processes.