Tropospheric air samples were collected aboard an aircraft up to an altitude of 7km. Vertical and horizontal distributions of tropospheric methane (CH4) over Japan were measured using a gas-chromatograph equipped with a flame ionization detector. CH4 mixing ratios in vertical and latitudinal profiles varied widely. The CH4 mixing ratio was high when the air from the mid-to-high latitudes of the Asian continent source region of CH4 was sampled. The CH4 mixing ratio was highest in air from Siberia and rather low in air from China. When this air mixed with air from the stratosphere—very clear from the high O3 mixing ratio—the effect on the CH4 mixing ratio was not clear because CH4 is photochemically stable and does not apparently decrease from the upper troposphere to the lower stratosphere. When the aircraft went directly across tropopause folding, however, very high O3 and low CH4 mixing ratios were observed. The distance of transport from the intrusion region would be an important factor for both O3 and CH4 mixing ratios. Low CH4 mixing ratio was observed when the air was sampled in maritime air mass because they were not affected by Asian continental sources. Between February 1986 and February 1992, mean CH4 mixing ratios over Japan (25-44°N, 124-144°E) steadily increased. Using data between November and February, we obtained an average annual increase of 12.0±7.0 (confidence interval: 70%) ppbv/year using a linear least square fit. From February 1992 to December 1993, the mean volume mixing ratio decreased slightly. The overall annual increase from February 1986 to February 1996 was 9.6±4.9 ppbv/year. These results indicate that the increase was larger in the mid-1980s than in the 1990s and has been declining.
To determine the magnitude of explosion earthquakes and B-type earthquakes at Sakurajima Volcano, where the long-period component often dominates, we installed a long-period seismograph at Kagoshima Local Meteorological Observatory Station E. We derived the magnitude formula for the vertical component of the long-period seismograph, MELV=logAV+1.37logΔ+0.69, from data on techtonic earthquakes whose focal parameters were determined by the Fukuoka District Meteorological Observatory (FDMO). This formula has no systematic discrepancy from the magnitude determined by FDMO, and is applicable to volcanic earthquakes up to M4.0. Applying this formula, we found that MELV of 171 explosion earthquakes in 1996 were 1.3-3.1 and those of 120 B-type earthquakes in January 1996 were 0.7-1.8. We similarly derived a magnitude formula for the vertical component of a short-period seismograph at Station E and calculated explosion earthquake and B-type earthquake magnitude, and compared them with MELV. We found that explosion earthquake MELV averages about 0.21 grater than the short-period magnitude, and about 0.14 grater for B-type earthquakes. This is because the short-period seismograph cannot record original explosion earthquake and B-type earthquake amplitudes where the long-period component dominates.
The Asian summer monsoon was studied viewed from the energy budget by analyzing the radiative flux budget at the top of the atmosphere and the atmospheric heat budget. (1) The contrast of heat sources in northern summer between land and sea and between the upper and lower atmosphere over land, and (2) atmospheric heat advection moderating these contrasts are summarized as follows;
(1) The net radiative flux at the top of the atmosphere (NET) over land dominates that over the ocean in mid and high latitudes during northern summer. The NET land-ocean contrast is created by stratus clouds over the ocean in high latitudes after snow cover mostly disappears over land. Seen from the vertically integrated atmospheric energy budget, seasonal change similar to that of NET is found in sensible heating over land in high latitudes. In mid-latitudes, sensible heating is dominant from spring to early summer, and evaporation is active in late summer. Energy estimated exceeding NET is transported from the atmosphere to the ocean in high latitudes besides NET in July-August. Before the onset of the summer monsoon circulation (in April), sensible heating near the surface contributes to increase the low-level temperature over land, especially under the downward motion of Hadley circulation and extratropical stationary waves (Iran Plateau and northern China).
(2) After the onset of summer monsoon circulation (in June), the heat source due to precipitation in Southeast Asia dominates sensible heating over the Eurasian Continent. Upward motion balances this condensational heat source. Strengthened downward motion warms upper-level air over central Asia and sensible heating near the surface is canceled by northerly cold advection. Strengthened warm southerly advection contributes to troposphere warming over the Tibetan Plateau and the northwestern Pacific in the mid-latitudes. Condensational heating in both of the above regions appears to contribute to the strengthening of the lower-level southerly flow.