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Polar Lows , Firestorms, and Duststorms.docx

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Environmental Science
Tanzina Mohsin

POLAR LOWS What are polar lows? – Cyclones forming over the open sea in polar regions WHERE DO POLAR LOWS FORM? Over the open sea in polar regions – Several hundred kilometers in diameter – Last a day or two – Sometimes called “Arctic hurricanes” due to well defined “eye” – Winds must be gale force (>50km/h) - Poleward of the polar front, the division between cold, polar air and warm, tropical air, storms can form. These are referred to as polar lows. Sometimes, due to some POLAR LOWS ARE REFFERED TO : the division between , warm tropical air and cold polar air (mT and cP) similarities to hurricanes, they have been called polar hurricanes or Arctic hurricanes. HOW DO THEY FORM? They form when cold Arctic air forms over sea or ice or continental land surface, moves over to much warmer ocean water - This change is surface properties is reffered to as WHAT? The ARCTIC FRONT - There is a strong temperature gradient between the air mass , and the ocean current beneath (around 20 to 40 degrees) , and this difference is greater than what is seen in a midlatitude cyclone - SO, the air mass receives substantial heating from the ocean current below, and this is the MAJOR DRIVER or influence for the storm to occur This, in turn, induces vertical motion by convection, followed by surface convergence and a counterclockwise vortex. The ocean also provides moisture - to the air mass and towering cumulus clouds form as the air rises. Continental cold arctic air (cP or cA) moves over open sea water (relatively warm) Heat source (sensible and latent) of the open sea fuels the storm – In many ways similar to hurricanes • Sea surface temperatures (SSTs) as energy source • Eye formation • Warm core • Dissipate over land POLAR LOWS FORMATION AND DURATION - How long do polar lows tend to last? Polar lows form quickly reaching maximum intensity within 12 to 24 hours - Dissipate quickly Most last less than 12 hours The contrast in temperature between the air mass and the ocean surface beneath is quite large. By tropical standards, the polar ocean is quite cold, perhaps only a few degrees above freezing. Nevertheless, the ocean waters can be 20 to 40C warmer than the Arctic air mass, a stronger temperature difference than experienced by tropical cyclones. Thus the air mass receives substantial sensible heating from the ocean surface below and this is the major driver of the storm. This, in turn, induces vertical motion by convection, followed by surface convergence and a counterclockwise vortex. The ocean also provides moisture to the air mass and towering cumulus clouds form as the air rises. As the air diverges at the upper level, cirrus clouds form giving the storm its characteristic hurricane-like appearance. This provides a second source of energy to the storm, latent heat. Most polar lows extend to the tropopause, roughly 8 km above the surface in polar regions. Polar lows can achieve central low pressures of 980 mb, typical of hurricanes and strong midlatitude cyclones. Winds can be gale force, which are winds that exceed 62 km/h. These storms produce heavy frozen precipitation, hail and snow. The polar low is often a tight circular storm resembling a tropical cyclone. It can be similar to a tropical cyclone in other ways as well. It often has a clear central core, an eye. The core is warm relative to its surroundings; like a hurricane but not like a midlatitude cyclone. Unlike a tropical cyclone, polar lows fully develop within 24 hours and dissipate within a couple of days. They move quickly compared to other storms, often exceeding 50 km/h. When polar lows move over land they quickly dissipate as they are cutoff from their main energy source, the sensible heat flux from the ocean. Where do polar lows form? Where do they form? Polar regions over open sea – WHY ? relative warmth is energy source • Northern Hemisphere » Atlantic Ocean » Pacific Ocean » Canadian Seas • Limited by sea ice conditions , WHAT KIND OF CONDITIONS ARE NECESSARY? open water a necessary condition, without it , polar lows will not form This type of storm is smaller than both midlatitude cyclones and tropical storms, ranging in size from 200 to 1000 km in diameter. They typically form from November to March in the North Pacific, North Atlantic, especially east of Greenland. They occasionally form east of Greenland and in the Beaufort Sea of the Arctic Ocean. They have been observed on the Southern Hemisphere, particularly in the Tasman Sea near New Zealand. WHERE DO POLAR LOWS OCCUR IN CANADA? Most polar lows occur in the Labrador Sea • Hudson Bay and Baffin Bay are limited by annual sea ice cycle –polar lows occur in the fall until the sea ice forms Firestorms - What is a firestorm? A firestorm is a self-sustaining convective cell that forms from an intense fire - Can form pyrocumulus cloud - How are firestorms caused? These storms are caused naturally in forest fires as well as by human actions such as a targeted explosion. How are firestorms formed? How do they form? – Initial trigger is a localized fire • Lightning/forest fire • Bomb • Chemical explosion • Nuclear explosion – Updraft forms due to heating of surface air (chimney effect) What draws in more oxygen for combustion and detritus for burning? SURFACE CONVERGENCE – Surface convergence (winds) which draws in more oxygen for combustion and more detritus for burning – Winds can be gale or hurricane strength – Latent heat is NOT a major fuel source for these storms – Radiant heat from fire can spontaneously cause other fires in the vicinity – The orangish “cloudlike” appearance is not a cloud but formed from ash – Clouds however can form (pyrocumulus) above the fire storm – Formed due to condensation of moisture in the updraft – Firestorm ash provides cloud condensation nuclei for cloud droplets to form on The firestorm begins by an initial combustion, started by lightning or a targeted explosion such as a bomb. This initial fire quickly releases heat energy. The air is warmed and rises in what is called a chimney effect. Localized warming is followed by rapidly rising air, an intense thermal. At the surface, the rising induces a vortex of convergence of surface air. This converging air can be intense, often with gale and hurricane strength winds. This surface air fuels, and thus sustains, the firestorm in two ways. First, the incoming air provides a source of oxygen. The initial burn depletes the oxygen which would normally act as a brake on the storm. The converging air provides a new source of oxygen and enables combustion to continue. Second, the strong converging winds draws into the fire any loose detritus which is often flammable material, thus providing burnable material for the firestorm. Firestorms ca HOW TO PYROCUMULUS CLOUDS FORM? 1. The intense heating of the storm causes air to rise. (results in a updraft) 2. WHERE DOES THE CONDENSATION NUCLIE COME FROM ? ash from the firestorm 3. The ash from the firestorms provides an abundance of cloud condensation nuclei. These nuclei are needed as surfaces for condensation to take place. 4. Well above the surface combustion, the rising air cools and if there is sufficient moisture in the air, reaches saturation and a pyrocumulus cloud forms. (SATURATION, and PYROCUMULUS CLOUD FORMATION OCCURS) 5. This cloud is not to be confused with the firestorm itself – which does have a cumulus cloud-like shape, although orange in color. This appearance is a result of the ash produced by firestorm, essentially smoke. The pyrocumulus cloud appears white or grey in color above the firestorm. It frequently appears above forest fires which often contain one or more firestorms. It also been observed following volcanic erup
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