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Temperature data in the mountain forest regions are often extrapolated from temperature data recorded at base stations at lower elevation. Such extrapolation is often based on elevation differences between target regions and base stations at low elevation assuming a constant temperature lapse rate throughout the year. However, this assumption might be problematic where slope circulation is active and decoupled from the regional circulation. To model the seasonal change in the lapse rate, the authors compared daily maximum ( Tmax) and minimum temperatures ( Tmin) observed at a mountain forest site (Kog–Ma; 1300-m altitude) with those observed at the bottom of the basin (Chiang–Mai; 314-m altitude) in northern Thailand, where slope circulation is active and decoupled from the regional circulation. The difference in Tmax between Kog–Ma and Chiang–Mai (Δ Tmax; Kog–Ma minus Chiang–Mai) was relatively unchanged throughout the year. However, the difference in Tmin between Kog–Ma and Chiang–Mai (Δ Tmin) changed seasonally. Thus, assuming a constant lapse rate throughout the year could cause large errors in extrapolating Tmin data in mountainous areas in northern Thailand. The difference Δ Tmin was related to nighttime net radiation (Rn), suggesting that nocturnal drainage flow affects the determination of Δ Tmin. This relationship would be useful in formulating seasonal changes in the lapse rate for Tmin. As Rn data are generally unavailable for meteorological stations, an index that relates to the lapse rate for Tmin and is calculated from Tmax and Tmin data is proposed. This index might be useful for accurately estimating Tmin values in mountainous regions in northern Thailand. [ABSTRACT FROM AUTHOR]

Abstract: Tropical seasonal forests play an important role in global cycling of carbon, water and energy in terms of inter-annual variation in the growing season. We investigated year-to-year variations in the canopy duration and transpiration period as measures of growing season length using time series data of radiative transmittance and sap flux in a teak plantation in Thailand over a 3-year period. In 2001, leaf-out and tree transpiration commenced earlier than in 2002 and 2003 following exceptionally heavy rainfall in the late dry season, strongly suggesting that enhanced soil moisture advanced the timing of leaf unfolding and commencement of tree transpiration. The commencement of leaf-fall and decline in tree transpiration at the beginning of 2003 were late in coming in correlation with a prolonged rainy season. Declines in tree transpiration were directly controlled by soil moisture conditions at the beginning of the dry season, with time lags between the timing of leaf-fall and decline in tree transpiration. These results revealed that soil moisture conditions are a major determinant of large inter-annual variation in the growing season at this site. The variation in canopy duration and transpiration period in this study, from 2001 to 2004, spanned about 40 and 60 days, respectively. This was much larger than the inter-annual variations previously reported in temperate deciduous forests, implying a profound potential impact on surface energy balance and canopy-atmosphere water and carbon exchange on an annual time scale. [Copyright &y& Elsevier]

Using data for one year, we examined the vertical wind speed profiles on a mountain slope covered with forest in northern Thailand under a tropical monsoon climate. We defined two profile patterns: higher wind speeds at greater heights (Pattern 1) and lower wind speeds at greater heights (Pattern 2). We classified 9.4% of the data as Pattern 2; this pattern tended to occur during the night, at low wind speeds, and with high outgoing longwave radiation. In addition, stable stratification and decoupling between the canopy surface air and the overlying layers were observed when Pattern 2 occurred frequently. These facts suggested that Pattern 2 was caused by a nocturnal drainage flow. The occurrence of Pattern 2 showed a clear seasonal trend, indicating that there is a seasonal trend in the occurrence of nocturnal drainage flows. Pattern 2 was observed more frequently between August and February and less frequently between March and July. This corresponded to the seasonal trend in wind speed, but did not correspond to the seasonal trend in the outgoing longwave radiation. [ABSTRACT FROM AUTHOR]