Abstract:On the basis of the pentad rainfall data averaged from 1961 to 2000, the seasonal march of rainfall in the Philippines is analyzed in this study. The relation to the atmospheric circulation at the 850 hPa level is also discussed.To investigate the temporal and spatial features of rainfall, the Empirical orthogonal function (EOF) analysis was applied to rainfall data. The result showed two dominant modes in the seasonal march of rainfall. The first mode reveals the increase of rainfall amount in the entire Philippines during summer monsoon while the second mode represents the contrast between the west and east coasts in the seasonal march of rainfall. The rainy season starts simultaneously over the entire west coast in the middle of May and withdraws gradually from northern stations around November. And on the east coast, the rainfall amount increases in autumn and winter rather than in summer. These regional differences between west and east coasts are considered to correspond to the seasonal change of Asian summer monsoon and orographic effect.The seasonal march of rainfall in the Philippines is characterized by the sudden change of atmospheric circulation around the Philippines. Particularly, the onset and peak of rainy season on the west coast are influenced by the eastward shift of the subtropical high and the evolution of the monsoon trough with southwesterly, respectively. The increase of rainfall on the east coast is related with the weakened monsoon trough around early September.
This study investigated interannual variations in seasonal march of rainfall in the Philippines by revealing onset and withdrawal pentads of rainy seasons from 1961 to 2000. For defining the onset and withdrawal of rainy season, the empirical orthogonal function (EOF) analysis was applied. As a result, the onset of summer rainy season, when started in mid-May on the average, was frequently delayed and fluctuated more greatly after the latter half of the 1970s. Such characteristics were not found in the onset of autumn rainy season, which corresponds to the increase in rainfall amount on the east coast. To clarify causes of the long-term change in the onset timing of the summer rainy season, we classified transition patterns of atmospheric circulation related to the onset of the summer rainy season by applying the EOF analysis to spatial anomalies of geopotential height at 850 hPa level. The first two dominant EOF modes showed three important triggers of the onset of the summer rainy season in atmospheric circulation: (1) the northeastwards shift in the subtropical high over the western North Pacific, (2) the evolution of the monsoon trough over the northern South China Sea and (3) the great approach of the easterly wave. Additionally, interannual variations in the time coefficients of EOF1 have a positive tendency on the boundary of the latter half of the 1970s and are significantly correlated with those in the onset of the summer rainy season. That is, it was suggested that the change of the onset timing in the summer rainy season after the latter half of the 1970s was related to a long-term change in transition patterns of atmospheric circulation connected with the onset of the summer rainy season.
This study investigates the seasonal march patterns of rainfall in the Philippines from 1951 to 2012 and their long-term variability. In order to clarify the dominant patterns in the seasonal march of rainfall, an empirical orthogonal function (EOF) analysis was applied to pentad rainfall data of 30 stations. For the first EOF mode (EOF1), we obtained a pattern related to the summer rainy season. We then applied cluster analysis to the time coefficients of EOF1 in each year to classify the seasonal patterns of the summer rainy season. As a result, the patterns were classified into six clusters. We found a long-term change in the pattern appearances with three anomalous patterns frequently observed since the 1990s: (1) a pattern that has an indistinct dry season and a prolonged peak rainfall, (2) a pattern that has a distinct dry season and an earlier withdrawal of the summer rainy season, resulting in a shortened rainy season, and (3) a pattern with a distinct dry season as well as delayed onset and withdrawal of the summer rainy season. This study also shows the relations between these three patterns and the lower atmospheric circulation at the 850 hPa level around the Philippines. Consequently, large positive and negative anomalies in geopotential height were observed around the Philippines for the distinct and indistinct dry seasons, respectively. The duration and condition of the dry season were greatly affected by the strength and location of the subtropical high especially for February-March. It is also noteworthy that the timing of the onset (withdrawal) of the summer rainy season is clearly related to that of the onset of the westerly (northerly) wind in the zonal (meridional) component around the Philippines. Further, the duration and amount of peak rainfall were directly influenced by the strength of the westerly winds in the zonal component. These three anomalous patterns tended to appear in the years when the warm or cold event of the El Niño-Southern Oscillation (ENSO) occurred. This study suggests that the long-term variability in the seasonal march of rainfall is considerably influenced by the variability in ENSO.
To investigate the spatial and temporal variability of the urban heat island, a high-spatial density meteorological observation system was set up in the Tokyo ward area by Tokyo Metropolitan Research Institute for Environmental Protection (TMRIEP) and Tokyo Metropolitan University from July 2002 to March 2005. The observation system was named Meteorological Environmental Temperature and Rainfall Observation System (METROS) and consisted of two observation networks named METROS20 and METROS100; METROS20 was made to observe meteorological factors (wind direction and speed, pressure, rainfall etc.) on the roofs of 20 buildings; METROS100 was made to observe temperature and humidity in instruments screens of 106 elementary schools. Since April 2005, observations of temperature and humidity were continued with the instruments screens of elementary schools by TMRIEP. This observation network was maintained until March 2010. Based on their observations, temporal and spatial characteristics of thermal environment of Tokyo have been investigated such as temperature range, especially in summer. For example, warmer areas differ between daytime and nighttime as shown by spatial patterns in rate of time exceeding 30 degrees Celsius and number of sultry nights: the warmer area is located from central part to northern part of the Tokyo ward area during daytime and from central part to coastal area during nighttime.Key words: high density spatial meteorological observation system, Tokyo, METROS100, MET-ROS20, temperature : 高密度気象観測システム 東京 METROS100 METROS20 気温 309 - -
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