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Cold pool
In atmospheric science, a cold pool (CP) is a cold pocket of dense air that forms when rain evaporates during intense precipitation e.g. underneath a thunderstorm cloud or a precipitating shallow cloud. Typically, cold pools spread at 10 m/s and last 2–3 hours. Cold pools are ubiquitous both over land and ocean.
The characteristics and impact of cold pools vary depending on the properties of the parent convection, namely its rain rates, and the large-scale environment in which they originate. Cold pools can have a strong impact on cloud cover and organization, by triggering new convection at the gust front and suppressing clouds in its interior.
Cold pools can be detected and studied using observations, high resolution numerical simulations, and simple conceptual models.
Cold pools spread radially away from the rain event along the surface as a moving gust front. When the gust front passes, cold pools cause an increase in wind speed and a sudden drop in specific humidity and in air temperature. In large-eddy simulations, they reach 10 km in radius, whereas, in reality, they can become as large as 50–100 km in radius and last 2-3 h on average.
Cold pools consist of a large-scale mass of cold air surrounded by warmer air (according to the American Meteorological Society). Over the ocean, these masses of cold and dense surface air are mostly caused by cooling through evaporation of precipitation from shallow and thunderstorm clouds in unsaturated air. Evaporation of precipitation requires energy, which is used in the form of latent heat, making the air inside a cold pool denser than the environmental air. In addition, the falling rain drags the air around it. These effects accelerate the air mass towards the surface, leading to a rapid decrease in surface air temperature and generating a divergent flow that moves radially away from the location of the precipitation. As the density current spreads horizontally outward, dry and cold air is injected into the boundary layer due to the penetrative downdrafts. This dries the central area of the cold pool, sometimes referred to as the cold pool wake, more than the edges. Cold pools spread radially away from the rain event along the surface as a moving gust front. When the gust front passes, cold pools cause an increase in wind speed and a sudden drop in specific humidity and in air temperature. The gust front is associated with wind enhancement and mechanical lifting of moist air. The cold pool gust front can therefore usually be identified as a mesoscale cloud arc.
Cold pools end when their features can no longer be distinguished from the large-scale flow, i.e., when their signature in meteorological variables is within the background variability of the environment. The temperature in the interior of a cold pool usually recovers faster than the temperature at the edge, since air from above the boundary layer is entrained into the cold pool wake. Additionally, the moisture recovers much more quickly than the temperature.
Cold pools are ubiquitous both over land and ocean. Nearly all shallow clouds in the trade-wind region that produce precipitation rates larger than 1 mm/h are associated with cold pools.
Cold pool characteristics differ depending on the depth of the parent convection (deep or shallow). The properties of cold pools formed in the trades (characterized by shallow convection) were observed to vary significantly from properties of cold pools formed from tropical deep convection. For example, they are associated with temperature drops that are on average 2 K weaker, and they experience less drying and smaller wind speed enhancement. The trade wind region is mainly drier and is characterized by subsiding motion that caps the growth of convection and maintains clouds shallow.
Cold pool
In atmospheric science, a cold pool (CP) is a cold pocket of dense air that forms when rain evaporates during intense precipitation e.g. underneath a thunderstorm cloud or a precipitating shallow cloud. Typically, cold pools spread at 10 m/s and last 2–3 hours. Cold pools are ubiquitous both over land and ocean.
The characteristics and impact of cold pools vary depending on the properties of the parent convection, namely its rain rates, and the large-scale environment in which they originate. Cold pools can have a strong impact on cloud cover and organization, by triggering new convection at the gust front and suppressing clouds in its interior.
Cold pools can be detected and studied using observations, high resolution numerical simulations, and simple conceptual models.
Cold pools spread radially away from the rain event along the surface as a moving gust front. When the gust front passes, cold pools cause an increase in wind speed and a sudden drop in specific humidity and in air temperature. In large-eddy simulations, they reach 10 km in radius, whereas, in reality, they can become as large as 50–100 km in radius and last 2-3 h on average.
Cold pools consist of a large-scale mass of cold air surrounded by warmer air (according to the American Meteorological Society). Over the ocean, these masses of cold and dense surface air are mostly caused by cooling through evaporation of precipitation from shallow and thunderstorm clouds in unsaturated air. Evaporation of precipitation requires energy, which is used in the form of latent heat, making the air inside a cold pool denser than the environmental air. In addition, the falling rain drags the air around it. These effects accelerate the air mass towards the surface, leading to a rapid decrease in surface air temperature and generating a divergent flow that moves radially away from the location of the precipitation. As the density current spreads horizontally outward, dry and cold air is injected into the boundary layer due to the penetrative downdrafts. This dries the central area of the cold pool, sometimes referred to as the cold pool wake, more than the edges. Cold pools spread radially away from the rain event along the surface as a moving gust front. When the gust front passes, cold pools cause an increase in wind speed and a sudden drop in specific humidity and in air temperature. The gust front is associated with wind enhancement and mechanical lifting of moist air. The cold pool gust front can therefore usually be identified as a mesoscale cloud arc.
Cold pools end when their features can no longer be distinguished from the large-scale flow, i.e., when their signature in meteorological variables is within the background variability of the environment. The temperature in the interior of a cold pool usually recovers faster than the temperature at the edge, since air from above the boundary layer is entrained into the cold pool wake. Additionally, the moisture recovers much more quickly than the temperature.
Cold pools are ubiquitous both over land and ocean. Nearly all shallow clouds in the trade-wind region that produce precipitation rates larger than 1 mm/h are associated with cold pools.
Cold pool characteristics differ depending on the depth of the parent convection (deep or shallow). The properties of cold pools formed in the trades (characterized by shallow convection) were observed to vary significantly from properties of cold pools formed from tropical deep convection. For example, they are associated with temperature drops that are on average 2 K weaker, and they experience less drying and smaller wind speed enhancement. The trade wind region is mainly drier and is characterized by subsiding motion that caps the growth of convection and maintains clouds shallow.
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