Differences

This shows you the differences between two versions of the page.

--- stars:background [2010-03-12 12:55:14]
steinare
+++ — (current)
@@ Line 1: / Line 1: @@
-====== Background ======
-Intense meso-scale cyclones known as polar lows are frequently observed in the Arctic sector
-of the North Atlantic Ocean. During winter, cold-air outbreaks may be triggered by the large
-scale atmospheric flow exposing dry and very cold air to the relatively warm ocean surface.
-One of the ocean areas mostly favoured for polar low development is between the Norwegian
-mainland and the Svalbard island. Due to the presence of the North Atlantic Current (NAC)
-the waters in this area may be warmer than 6o C in January. The large air-sea temperature
-differences experienced during cold-air outbreaks result in the formations of atmospheric
-fronts, large heat fluxes and sometimes the development of polar lows. As polar lows are
-almost exclusively observed over open waters and are characterised by deep convective
-clouds, it is usually assumed that atmosphere-ocean heat and moist exchange are important
-contributions to the energetics of polar lows. The striking similarities to Tropical hurricanes
-displayed by the satellite images, such as clear eye and spiral bands of deep-convective
-clouds, has lead several authors to suggest the release of latent heat as one major energy
-source for at least a class of polar lows. studied a polar low south of the Bear island and
-concluded that in the final phase of its development some polar lows could act as Carnot
-engines, working between a warm (the ocean) and a cold (the tropopause) reservoir, and
-referred to these as Arctic hurricanes.
-Still, it is important to keep in mind the baroclinic nature of cold-air outbreaks. Often, polar
-lows develop from disturbances on Arctic fronts that are formed during cold-air outbreaks
-when dry Arctic air is exposed to oceanic heat fluxes. The Arctic fronts are characterised by
-strong horizontal temperature gradients in the lower troposphere and an intense low level jet. In such cases the presence of the front suggests that baroclinic instability plays a role in the
-polar low formation. Nonetheless, the deep convective clouds revealed at the same time by
-satellite images suggests that large amounts of latent heat must be released in the troposphere.
-Possibly, both baroclinic and diabatic processes are important during the early stages of polar
-low development. Pure Arctic hurricanes as described by Emanuel and Rotunno (1989) may
-represent the final stage for vertically aligned symmetric cyclones with no baroclinic potential
-left.
-Despite the fact that polar lows are almost exclusively marine phenomena, surprisingly few
-investigations of the interaction process between polar lows and the ocean. A noteworthy
-exception is PhD thesis by Linders (2009) who investigated the role of ocean surface
-temperature and air-sea fluxes on the development and intensity of polar lows. Saetra et al.
-(2008) demonstrated that during winter, the NAC sub-ducts under colder and less saline
-waters, leading to a warm subsurface core under cold upper-level waters. During strong wind
-events such warm sub-surface waters may entrain the surface by intense turbulent mixing. In
-the paper by Saetra et al. (2008), microwave satellite images revealing rapid surface warming
-during a polar low event outside the coast of Norway is presented. These finding raises some
-interesting questions. How may a rapid surface warming affect the life-cycle and intensity of
-polar lows? Also, a surface warming by entrainment of warm waters to the surface represents
-a cooling of the ocean by increased surface fluxes of latent and sensible heat. Is this a
-significant contribution to the cooling of the NAC and subsequent ocean overturning?
-The oceanic response to hurricanes has long been recognised (Price, 1983; Sanford et al.,
-; Brink, 1989). Strong turbulent mixing entrainment of cold waters from deep layers
-leads to a cooling of the sea-surface. This rapid surface cooling reduces the surface fluxes and
-inhibits further hurricane intensification. When hurricanes moves over deep cores of warm
-waters, such as the Loop Current in the Gulf of Mexico, or warm core rings this surface
-cooling is strongly reduced. The warm water will then act to insulate the entrainment of cold
-waters form even deeper layers (Hong et al., 2000; Shay et al., 2000). In such cases, strong
-hurricane intensification has been observed. In 2005, Katrina intensified into a category 5
-hurricane as it entered the warm Gulf of Mexico (Kafatos et al., 2006).
-The ocean surface warming reported by Saetra et al. (2008) has only been observed by
-microwave satellite data. During could air outbreaks the ubiquitous cumulus convection
-prevents the sea-surface to be observed by infrared sensors (IR) such as AATSR, AVHRR
-and MODIS. However, verification of such ocean response to polar lows is urgent. Here, we
-propose to use altimeter combined with SST products from both microwave and infrared
-sensors to investigate possible surface warming in connection with polar lows. As the
-altimeter measures the surface anomaly (SLA) this can be related to the ocean heat content.
-The main scientific questions to be addressed are:
-  * Can satellite IR observations in combination with altimeter be used to detect possible
-sea-surface warming caused by strong winds under polar low events?
-  * Can we identify a Polar Low Indicator based on satellite data that could be a useful
-tool for polar low forecasting?
-  * Can the forecasting of polar lows be improved by introducing coupled atmosphere
-ocean models?
-  * Does the strong turbulent mixing induced by polar lows have an anomalously strong
-impact on the cooling of the North Atlantic Current?