"Cutoff low" (COL) is a low-pressure system that develops between the middle (~5000 m altitude) and the upper troposphere (~10000 m altitude). It propagates holding cold air derived from polar air inside. The cold, heavy air moving over the upper troposphere destabilizes the atmosphere below, and often accompanies localized weather disasters such as tornadoes and quasi-linear convective systems.

In order to monitor past and future (about 3 days) COL's activity, this site objectively extracts COLs from JMA GSM forecasts using a method based on Kasuga et al. (2021), and automatically visualizes the obtained variables (center coordinates, intensity, size, background gradient), which we call "COL map".

The figures shows weather charts for upper troposphere (200 hPa) in the left and the mid troposphere (500 hPa) in the right. The solid black lines are geopotential height contours. They are equivalent to the isobars on the surface weather map, and COLs corresponds to the area where the geopotential height contours are closed and depressed. The areas where the contours are not closed but still lower altitude than the surrounding area are called "troughs" and are also monitored as the early stage of COLs.

In addition, not only low pressure systems (COLs and troughs) but also high pressure systems (highs and ridges) can be extracted, and blocking highs (high pressure systems in the upper troposphere that stays in the same location and causes abnormal weathers) can be also monitored.

The types of low and high pressure systems are distinguished by colors of the circles.

Upper figures

• Blue: COLs
• Green: Troughs

Lower figures

• Blue: Highs
• Green: Ridges

These are collectively referred to as (upper-level) disturbances.

Each extraction point is indicated by a "dot", "circle" and "arrow" which visualize the "intensity", "size" and "background gradient" respectively.

• The "dots" indicate the central coordinates and their colors indicates intensities of the disturbances. The intensity is expressed as a slope of the geopotential height, in units of [m (100 km)^-1] (dimensionless), and is called optimal slope (S_o). This value become negative for anticyclones and ridges. To establish an absolute intensity criterion is an issue for the future, though we currently regard So below 10 m (100 km)^-1 is relatively weak, and above 20 m (100 km)^-1 is relatively strong. Some weak disturbances are removed as noise.
• The "circles" indicates an estimate of the horizontal size of the depression and is adjustively selected from 100, 200, 300, ..., 2600 km. This radius is called optial radius (r_o). Note that the disturbance for r_o = 100 km may be noise and is removed.
• The "arrows" indicate the background slope of the geopotential height field around the depression as a gradient vector, and their colors indicate the magunitude of gradient. The unit is [m (100 km)^-1], and is called background slope (S_BG). Basically, the background gradient tends to point to the south because tropical regions have higher geopotential height than that of polar regions because tropical regions have thick troposphere proportional to its mean temperature. Due to the geostrophic wind balance, the disturbances are expected to move toward perpendicular in clockwise to the background gradient.

The blue and red triangles indicate the geopotential height minima and maxima, respectively, of COLs and highs.

The gray shadings indicate the two-dimensional distribution of concavity, in [m (100 km)^-1]. They are denoted as AS⁺ (AS plus) and AS^- (AS minus), respectively. These extrema correspond to the center locations and intensities (S_o) of the disturbances.

The value on the right shoulder of each point is the ratio of the background gradient (S_BG) to the intensity of the disturbance (S_o), called slope ratio SR (=S_BG/S_o). Large SR means disturbances are weaker than backgrounds and exhibits less curved contours, so we remove disturbances larger than SR=3 as noise.

• Mid-latitude ocean-atmosphere interaction hotspots under the changing climate | Terms | Cutoff lows (in Japanese)
• JAMSTEC | APL Column | Atmospheric Blocking (in Japanese)
• Kasuga, S., M. Honda, J. Ukita, S. Yamane, H. Kawase, A. Yamazaki, 2021:
  Seamless Detection of Cutoff Lows and Preexisting Troughs. Mon. Wea. Rev., 149, 3119-3134, https://doi.org/10.1175/MWR-D-20-0255.1.
  (Press release, in Japanese)

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