A Dramatic Daytime Decrease in Water Vapor over Coastal Thailand
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Abstract:
A remarkable decrease in water vapor was observed during daytime on calm days over coastal Thailand. Precipitable water vapor (PWV) and surface specific humidity (SSH) decreased by about 5 mm and 3 g kg -1, respectively, in Bangkok from the late morning to the late afternoon on sunny days with weak low-level winds between March 1998 and December 1999. A daytime moisture decrease occurs in the lower atmosphere over coastal areas near the Gulf of Thailand, where the sea breeze often prevails. The sea breeze is relatively strong near the northern gulf. Wind profile observed over Bangkok revealed strong descending flow in the lower atmosphere during daytime, suggesting divergence of the sea-breeze circulation. This descending flow explains the decrease in daytime moisture over coastal areas.A numerical experiment assuming calm conditions successfully simulated the daytime decrease in water vapor around Bangkok. The most remarkable drying associated with descending flow was simulated around the northern edge of the Bight of Bangkok, where the coastline is sheltered by inland areas. The shape of the coastline and the topography enhanced the sea-breeze circulation around Bangkok. In another model run assuming wet soil conditions, the diurnal range of water vapor was smaller than that simulated by the control run. Clear diurnal variation was observed on calm days, especially under dry soil moisture conditions. A large diurnal range in PWV tended to appear before the monsoon onset when the low-level wind was weak and the soil moisture was dry.Keywords:
Sea breeze
Diurnal temperature variation
Precipitable water
Prevailing winds
In this paper, the seasonal changes in the diurnal variations of wind and the cloud activity at Serpong (106.7°E, 6.4°S), near Jakarta, are climatologically described. In the dry season (May-October), diurnal variation of wind accompanied with sea-land breeze circulation was prominent. In the rainy season (November-April), the diurnal variation was consistent with sea-land breeze circulation, but was not as clear as that in the dry season. The peak time of the northerly in the rainy season, similarly to that of the sea breezes at Serpong in the low level (below 1.0 km height), was earlier than that in the dry season. The maximum time in the climatological diurnal variation of the surface temperature at Serpong in the rainy season was earlier than that in the dry season. The vertical structure which was consistent with sea breeze circulation was clearer when the prevailing (daily-mean) wind was weaker in the rainy season. These results are consistent with the features of the local circulation; in other words, the local circulation depends on the diurnal variation of surface temperature and is prominent when the prevailing wind is weak. The typical diurnal variation of the wind in the rainy season was unclear when the prevailing northwesterly to westerly was strong around Serpong. Interannual variation of the diurnal variation could be detected in the transitional period from the dry to the rainy season. Cloud activity had prominent diurnal variation over West Jawa in the rainy season and was active in the early evening over land, particularly, in the mountainous area in the south of Serpong. When cloud activity was active over the mountainous area, the northerly below 1.0 km in height was prominent at Serpong, which is consistent with the feature that the development of a local cloud system is accompanied with local circulation.
Wet season
Sea breeze
Diurnal temperature variation
Dry season
Seasonality
Prevailing winds
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The diurnal variation of wind in the atmospheric boundary layer over the Kanto plain is studied in order to find the contribution of each of the land-sea breeze and the diurnal variation of turbulence as a function of the magnitude of the larger-scale pressure gradient. The land-sea breeze is found to be the main source of the diurnal variation of wind when the pressure gradient is less than 5ms-1 in geostrophic wind speed. As the pressure gradient intensifies the land-sea breeze weakens and is replaced by a turbulence-induced diurnal variation, which dominates over the land when the pressure gradient exceeds 10ms-1.
Sea breeze
Pressure gradient
Diurnal temperature variation
Pressure-gradient force
Variation (astronomy)
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Sea breeze winds are observed at various locations worldwide, but the spatially continuous mapping of sea breeze winds is rare. We have developed a scheme to map the areas of the southeastern United States (US) coast influenced by sea breeze winds using a range of surface re-analysis data to identify their occurrence. Changes in wind direction and dew point temperature are both used to detect a potential sea breeze signature, which is then confirmed by cumuliform clouds seen in satellite images or coastal fronts shown as cohesive lines in radar reflectivity images. Filters are employed to remove onshore winds not induced by the temperature difference between land and sea. From March to September 2019, this scheme identified 134 days with sea breeze occurrence somewhere in the southeastern US, a frequency of 63 percent. The number of sea breezes increased from March to July and then decreased to September. Deep inland propagation of sea breezes during this period left footprints in a band parallel to the coastline up to about 220 km inland, after which the sea breeze winds quickly diminished. Comparisons show that the findings using the scheme are consistent with site observations, theoretical estimates, and idealized and semi-idealized numerical model simulations.
Sea breeze
Prevailing winds
Dew point
Footprint
Global wind patterns
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The average wind characteristics and sea-land breeze characteristics are analyzed by using the data of wind profiler during February 2011 on Donghai island of Zhanjiang.The results show that: the prevailing wind direction is northeasterly on Donghai island during February 2011 at an altitude of 150 m and the total frequency of occurrences of three directions E,ENE,NE is 66.6%,in contrast,the total frequency of occurrences of westerly is only 1%.There is an obvious difference of occurrence frequency between westerly winds and easterly winds.The hourly changes of average monthly wind speed is restricted to small range about 1 m/s from 1:00 to 15:00;and fluctuation becomes larger during 15:00 to 20:00,the maximal speed of 2.1 m/s appears at 16:00.In February,only two days meet the conditions of sea-land breeze.In sea-land breeze day,the common duration of sea breeze is from 13:00 to 20:00,lasting 7 h,and land breeze's common duration is from 2:00 to 7:00,lasting 5 h.The mean speed of land breeze is 0.8 m/s,which are smaller than 2.1 m/s of sea breeze.The vertical height of land breeze is about 1 200 m during the sea-land breeze day,which differs slightly from sea breeze.The wind speeds of both sea breeze and land breeze have the similar trends of increasing first and then decreasing with increasing altitude.The maximal wind speed of sea breeze appears at 750 m,and that of land breeze appears at 500 m.The sea breeze circulation is much stronger than land breeze in the region of 500 to 750 m height.The wind direction basically remains unchanged above 2 km and that is the large-scale uniform systematic west wind circulation.
Sea breeze
Prevailing winds
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The study has been made from mean monthly velocities. The land and sea breezes which are the most important variations in Bombay, are found to be related with diurnal variation in pressure distribution over a wide area especially caused by the land distribution of that region. The land breeze is associated with a. high and the sea breeze with a' low '. The vertical extent of the sea breeze is found to coincide with the level where the' low' changes to ' high '.
The afternoon winds over Poona are found to be antitriptic in the lower leve'1s and they continue to be weak at higher levels due to weak pressure gradient. In some. of the non-monsoon months katabatie winds Over Poona .are found perceptible even up to 150 metres above ground level.
Sea breeze
Diurnal temperature variation
Pressure gradient
Low-pressure area
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Abstract Sea breeze winds are observed at various locations worldwide, but the spatially continuous mapping of sea breeze winds is rare. We have developed a scheme to map the areas of the southeastern United States (SEUS) coast influenced by sea breeze winds using a range of surface re‐analysis data to identify their occurrence. Changes in wind direction and dew point temperature are both used to detect a potential sea breeze signature, which is then confirmed by cumuliform clouds seen in satellite images or coastal fronts shown as cohesive lines in radar reflectivity images. Filters are employed to remove onshore winds not induced by the temperature difference between land and sea. From March to September 2019, this scheme identified 134 days with sea breeze occurrence somewhere in the SEUS, a frequency of 63 percent. The number of sea breezes increased from March to July and then decreased to September. Deep inland propagation of sea breezes during this period left footprints in a band parallel to the coastline up to about 220 km inland, after which the sea breeze winds quickly diminished. Comparisons show that the findings using the scheme are consistent with site observations, theoretical estimates, and idealized and semi‐idealized numerical model simulations.
Sea breeze
Prevailing winds
Dew point
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Sea breeze
Prevailing winds
Global wind patterns
Maximum sustained wind
MM5
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A remarkable decrease in water vapor was observed during daytime on calm days over coastal Thailand. Precipitable water vapor (PWV) and surface specific humidity (SSH) decreased by about 5 mm and 3 g kg -1, respectively, in Bangkok from the late morning to the late afternoon on sunny days with weak low-level winds between March 1998 and December 1999. A daytime moisture decrease occurs in the lower atmosphere over coastal areas near the Gulf of Thailand, where the sea breeze often prevails. The sea breeze is relatively strong near the northern gulf. Wind profile observed over Bangkok revealed strong descending flow in the lower atmosphere during daytime, suggesting divergence of the sea-breeze circulation. This descending flow explains the decrease in daytime moisture over coastal areas.A numerical experiment assuming calm conditions successfully simulated the daytime decrease in water vapor around Bangkok. The most remarkable drying associated with descending flow was simulated around the northern edge of the Bight of Bangkok, where the coastline is sheltered by inland areas. The shape of the coastline and the topography enhanced the sea-breeze circulation around Bangkok. In another model run assuming wet soil conditions, the diurnal range of water vapor was smaller than that simulated by the control run. Clear diurnal variation was observed on calm days, especially under dry soil moisture conditions. A large diurnal range in PWV tended to appear before the monsoon onset when the low-level wind was weak and the soil moisture was dry.
Sea breeze
Diurnal temperature variation
Precipitable water
Prevailing winds
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