17:15 〜 19:15
[AAS10-P08] Vertical structure and regional characteristics of upper-tropospheric cold lows
★Invited Papers
キーワード：寒冷低気圧
A cold low (CL) is a type of cyclone that forms when a trough deepens in the upper troposphere, with cold air concentrated at its center. The atmosphere beneath a CL often becomes unstable, leading to mesoscale disturbances and extreme weather events such as torrential rain. Recently, a new method was developed to objectively identify CLs from instantaneous height fields (Kasuga et al., 2021). Since this method detects CLs based on the geometric features of the height field, it remains unclear how deep and intense the associated cold temperatures and vertical motions extend toward the surface. In this study, we investigate the vertical structure of temperature and vertical velocity associated with CLs detected by this method, as well as their temporal evolution and regional characteristics.
We use originally distributed CL indices, including position, radius, and intensity, derived from six-hourly geopotential height fields in the JRA-55 atmospheric reanalysis dataset for the period 2012–2021. Additionally, we applied the method of Kasuga et al. to temperature fields to obtain indices characterizing temperature depressions. To analyze the relationship between low-pressure intensity and cold air intensity, we examined the bivariate frequency distribution in the North Pacific basin. Specifically, we compared the intensity of individual CLs at the 500-hPa level with the maximum cold air intensity—defined as the highest grid-wise cold air intensity within the CL’s radius—at the 500-, 700-, 850-, and 925-hPa levels. The results indicate that as the low-pressure intensity increases, the cold temperature anomaly also intensifies, with this relationship holding strongest from the 500-hPa to 700-hPa levels. However, at 850 hPa and below, the positive correlation between low pressure and cold air intensity weakens.
Next, we classified CLs at 500 hPa into three categories based on the intensity of cold air (categories 1, 2, and 3, from weakest to strongest) and examined the wintertime frequency distribution of detected CLs and the horizontal distribution of the 5th percentile value of upward velocity for each category. From these distribution maps, we focused on two regions: Region 1 located near the Kamchatka Peninsula, where intense upward motion is observed, and Region 2 off the coast of California in the eastern Pacific, where a high-frequency region appears only in category 3. We then performed composite analyses of CLs detected within these regions for each category. The results show that in all categories, the CL in Region 1 exhibits a westward-tilted structure, whereas the CL in Region 2 is nearly vertical. Additionally, in both regions, as the cold air intensifies, the height depression and the upward motion strengthen. In particular, in Region 1, the vertical motion in category 1 shows strong upward motion 0º-10º east of the low-pressure center, whereas in category 3, the region with strong upward motions extends to 0º-20º. Moreover, the two maxima observed around 10º and 15º in category 3 were found to be about 1.5 times larger compared to those in category 1.
Reference: Kasuga et al.,2021, MWR 3127,35
Figure Caption
(a)Frequency distribution of wintertime CLs for category 3 of cold air intensity. (b) the 5th percentile value of upward velocity for category 3 detected within the radius of the CLs. (c) Longitude-pressure section of composite mean temperature anomalies (color shading) and geopotential height anomalies (white contours) with respect to the individual cyclone center for category 1 (top) and category 3 (bottom) for Area 1. (d) As in (c) but upward velocity (shading) (e-f) As in (c-d) but for Area 2.
We use originally distributed CL indices, including position, radius, and intensity, derived from six-hourly geopotential height fields in the JRA-55 atmospheric reanalysis dataset for the period 2012–2021. Additionally, we applied the method of Kasuga et al. to temperature fields to obtain indices characterizing temperature depressions. To analyze the relationship between low-pressure intensity and cold air intensity, we examined the bivariate frequency distribution in the North Pacific basin. Specifically, we compared the intensity of individual CLs at the 500-hPa level with the maximum cold air intensity—defined as the highest grid-wise cold air intensity within the CL’s radius—at the 500-, 700-, 850-, and 925-hPa levels. The results indicate that as the low-pressure intensity increases, the cold temperature anomaly also intensifies, with this relationship holding strongest from the 500-hPa to 700-hPa levels. However, at 850 hPa and below, the positive correlation between low pressure and cold air intensity weakens.
Next, we classified CLs at 500 hPa into three categories based on the intensity of cold air (categories 1, 2, and 3, from weakest to strongest) and examined the wintertime frequency distribution of detected CLs and the horizontal distribution of the 5th percentile value of upward velocity for each category. From these distribution maps, we focused on two regions: Region 1 located near the Kamchatka Peninsula, where intense upward motion is observed, and Region 2 off the coast of California in the eastern Pacific, where a high-frequency region appears only in category 3. We then performed composite analyses of CLs detected within these regions for each category. The results show that in all categories, the CL in Region 1 exhibits a westward-tilted structure, whereas the CL in Region 2 is nearly vertical. Additionally, in both regions, as the cold air intensifies, the height depression and the upward motion strengthen. In particular, in Region 1, the vertical motion in category 1 shows strong upward motion 0º-10º east of the low-pressure center, whereas in category 3, the region with strong upward motions extends to 0º-20º. Moreover, the two maxima observed around 10º and 15º in category 3 were found to be about 1.5 times larger compared to those in category 1.
Reference: Kasuga et al.,2021, MWR 3127,35
Figure Caption
(a)Frequency distribution of wintertime CLs for category 3 of cold air intensity. (b) the 5th percentile value of upward velocity for category 3 detected within the radius of the CLs. (c) Longitude-pressure section of composite mean temperature anomalies (color shading) and geopotential height anomalies (white contours) with respect to the individual cyclone center for category 1 (top) and category 3 (bottom) for Area 1. (d) As in (c) but upward velocity (shading) (e-f) As in (c-d) but for Area 2.