1. Introduction
Polar lows (PLs) are intense maritime cyclones with horizontal scales between 200 and 1000 km that develop poleward of the main baroclinic zone (Rasmussen and Turner 2003). Since satellite observation started to operate in the late twentieth century, there have been a number of studies on PL formation over high-latitude oceans including the Barents Sea (Rasmussen 1985), the Norwegian Sea (Wilhelmsen 1985), the Labrador Sea (Mailhot et al. 1996), the Gulf of Alaska (Businger 1987), the Bering Sea (Businger and Baik 1991), and the Southern Ocean (Carleton and Carpenter 1990). Because some PLs resemble tropical cyclones with a cloud-free eye and spiral cloud bands (Businger and Baik 1991; Nordeng and Rasmussen 1992), several studies proposed that PLs develop through a cooperative interaction between the vortex and diabatic processes such as condensational heating and surface heat fluxes (Rasmussen 1979; Emanuel and Rotunno 1989). On the other hand, because some PLs look like miniature extratropical cyclones (ECs) with a comma-shaped cloud pattern (Reed and Duncan 1987), several other studies proposed that PLs develop through baroclinic instability (Mansfield 1974; Duncan 1977). Rasmussen and Turner (2003) pointed out that both condensational heating and baroclinic instability are important for PL development, and that there are a variety of hybrid systems with both baroclinic and convective processes involved. The variability and hybrid characteristics of PLs are also confirmed by idealized numerical experiments (Yanase and Niino 2005, 2007; Terpstra et al. 2015). In addition, upper-tropospheric disturbances can play an important role in PL development (Grønås and Kvamstø 1995).
The Sea of Japan is located at relatively low latitudes (35°–45°N) compared to the regions where PLs typically develop [e.g., the Norwegian Sea (60°–75°N) and the Barents Sea (70°–80°N)]. The Sea of Japan is a marginal sea of the North Pacific Ocean, and is surrounded by the Asian continent and the Japanese islands including Honshu and Hokkaido (Fig. 1). During the winter, northwesterly flow dominates the Sea of Japan under the influence of the East Asian winter monsoon, which is particularly intense on the western side of an EC. The northwesterly flow is associated with cold air outbreaks from the cold Asian continent, where the flow gains sensible and latent heat from the relatively warm ocean (Ninomiya 1989). These processes create a shallow baroclinic and convective layer, which seems to be favorable for PL development. The PLs over the Sea of Japan are accompanied by strong winds and heavy snowfall and can cause significant disruption to human activities, damage, and loss of life. For example, a train fell from the Amarube Bridge in western Honshu due to strong winds on 28 December 1986 causing six deaths and injuring six others, and a Soviet vessel of 6000 tons foundered off Hokkaido on 26 February 1981 (Ogura 1991). These characteristics and disasters have motivated many previous studies of PLs over the Sea of Japan (Table 1). These studies demonstrate that PLs are diverse in geographical distribution and in synoptic-scale conditions. For example, some PLs move southward off the western coast of Hokkaido and northern Honshu (Ninomiya 1991; Fu et al. 2004; Wu and Petty 2010; Shimada et al. 2014), while others move southeastward or eastward over the southern part of the Sea of Japan off the coast of Honshu (Ninomiya et al. 1990; Shimada et al. 2014).
Climatological SST (°C) for the cold season (average from October 1979 to March 2015). Areas with (without) shading indicate sea (land) areas resolved by the TL319 resolution of the JRA-55 reanalysis. The Sea of Japan is defined by the sea area within the black frame for the present study.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
The PLs reported in previous studies and their scores in the PL tracking using the JRA-55 reanalysis. TIME, LON, and LAT indicate time, longitude, and latitude, respectively, of the observed PLs at the mature stage estimated from satellite images or surface weather charts are given. PRS indicates that the SLP field of JRA-55 shows a negative anomaly sufficient for the tracking (rank A), negative anomaly insufficient for the tracking (rank B), or negligible anomaly (rank C). TRACK indicates that the SLP minima are connected to construct a track (rank A) or not (rank B). VOR indicates that the vorticity field at 850 hPa exceeds 10 × 10−5 s−1 (rank A) or not (rank B). WIND indicates that the near-surface wind speed exceeds 15 m s−1 (rank A) or not (rank B). TEMP indicates that the difference between SST and temperature at 500 hPa is larger than 43°C (rank A), is between 39° and 43°C (rank B), or is less than 39°C (rank C). VOR, WIND, and TEMP are assessed at the time of PL’s maximum intensity only for the rank A cases in TRACK, or are not assessed (identified by em dash) for the rank B cases in TRACK.
Climatological studies of PLs are necessary for a comprehensive understanding of the tracks and synoptic-scale conditions in which PLs occur. The climatology of PLs in several different regions has previously been studied using satellite imagery (Carleton 1985; Yarnal and Henderson 1989a,b; Blechschmidt et al. 2009). A database of PLs over the Nordic seas based on subjective analysis of satellite observations has been compiled by trained forecasters at the Norwegian Meteorological Institute (Noer et al. 2011). The database is used for the climatology of PLs including geographical distribution, interannual variability, environmental fields, and validation of reanalysis data (Mallet et al. 2013; Laffineur et al. 2014; Terpstra 2014; Rojo et al. 2015). In addition, several recent studies have employed objective tracking of PLs using high-resolution (horizontal grid spacing of several tens of kilometers) model datasets that cover global or large regions (Zahn and von Storch 2008; Xia et al. 2012; Chen and von Storch 2013; Zappa et al. 2014). Tracking algorithms are useful for climatological studies of PLs, because they can automatically and objectively detect a large number of PLs from a model dataset based on specified criteria. However, most of these climatological studies have focused on the North Atlantic Ocean and its marginal seas, and few climatological studies have examined PLs over the Sea of Japan. Although Yarnal and Henderson (1989a,b) and Chen and von Storch (2013) analyzed the climatology of PLs over the entire North Pacific Ocean, they did not describe the local distribution of PLs over the Sea of Japan in greater detail. While Ninomiya (1989) analyzed several PLs over the Sea of Japan, the period was limited to three months from December 1986 to February 1987. In 2013, the Japan Meteorological Agency (JMA) released a new global reanalysis dataset, the Japanese 55-year Reanalysis Project (JRA-55; Kobayashi et al. 2015). JRA-55 is a promising dataset for studying the climatology of PLs over the Sea of Japan, because its horizontal resolution of about 60 km potentially resolves the low pressure and large vorticity associated with PLs.
The purposes of the present study are 1) to compile a dataset of PLs over the Sea of Japan by applying the tracking algorithm based on Zappa et al. (2014) to the JRA-55 reanalysis, 2) to determine the climatology of their geographical distribution, seasonal transition, and direction of motion, and 3) to assess the synoptic-scale conditions responsible for the development and motion of PLs using composite analyses.
The paper is organized as follows: section 2 describes the methodology of the PL tracking using the JRA-55 reanalysis. Section 3 presents the climatology of the PLs over the Sea of Japan for 36 cold seasons (October–March 1979–2015). Section 4 examines synoptic-scale conditions during PL development using composite analyses. Section 5 discusses the climatology of PLs over the Sea of Japan, and section 6 summarizes the results.
2. Data and methodology
a. The JRA-55 reanalysis
We use the global atmospheric reanalysis, JRA-55 (Kobayashi et al. 2015), for the tracking analysis of PLs and composite analysis of synoptic-scale conditions. JRA-55 uses the four-dimensional variational data assimilation (4DVar) system of the JMA. As a lower boundary condition, it uses sea surface temperature (SST) from the Centennial In Situ Observation-Based Estimates of the Variability of SSTs and Marine Meteorological Variables (COBE; Ishii et al. 2005). The global model used for the analysis has a TL319 spectral horizontal resolution (~60 km) and 60 vertical levels in a η hybrid coordinate. The horizontal resolution of this state-of-the-art model is expected to resolve low pressure and large vorticity associated with meso-α-scale (200–2000 km; Orlanski 1975) cyclones including PLs, although it still has difficulties in resolving meso-β-scale (20–200 km) structures within cyclones including cloud bands and cloud-free eyes. The dataset is available on a Gaussian grid for the TL319 spectral resolution.
We mainly use 6-hourly analysis fields at 0000, 0600, 1200, and 1800 UTC for sea level pressure (SLP), geopotential height, temperature, and horizontal winds. In addition to these analysis fields, we also utilize some forecast fields provided by JRA-55. The forecast fields of SLP at 0300, 0900, 1500, and 2100 UTC are used in between the 6-hourly analysis fields to construct pseudo-3-hourly SLP fields, which are used for calculating PL tracks as accurately as possible. The forecast fields of the surface heat fluxes are also available as averages from the beginning of forecasts up to 3 h (0000–0300, 0600–0900, 1200–1500, and 1800–2100 UTC), which are diagnosed with a bulk formula following Monin–Obukhov similarity theory (JMA 2013).
We analyze the cold season from October to March, because most PLs are observed during this season. While JRA-55 covers the period from January 1958 to the present, most satellite observations have only been assimilated since the late 1970s. This potentially introduces inhomogeneities into the data. Therefore, we only analyze the period from October 1979 to March 2015 (36 cold seasons).
b. Tracking and classification of PLs
Tracks of PLs are objectively determined by applying the PL tracking algorithm of Zappa et al. (2014) to the JRA-55 reanalysis with minor modifications: while Zappa et al. (2014) used 3-hourly vorticity maxima at 850 hPa of ERA-Interim and the ECMWF operational analysis for their PL tracking, we use 3-hourly SLP fields as mentioned above because the vorticity fields are only available at 6-hourly intervals in the JRA-55 dataset. First, the SLP data are spectrally filtered to retain horizontal scales of typical PLs using a bandpass filter of T40–T100 spectral resolution (200–500 km in half wavelength). Minima with negative anomalies exceeding −1 hPa in the filtered SLP fields are detected at every 3-h time step. Next, the SLP minima are linked between contiguous time steps to form tracks by using the minimization of a cost function that depends on the track smoothness (Hodges 1994, 1995) and is subject to adaptive constraints on displacement and smoothness (Hodges 1999). Hereafter, we only use the track information at 0000, 0600, 1200, and 1800 UTC, because the analysis fields including vorticity and temperature are only available at 6-hourly intervals. Based on Zappa et al. (2014), we only use the periods of the tracks when the filtered vorticity (T40–T100 in spherical harmonics) within 0.5° radius from the SLP minimum exceeds 2 × 10−5 s−1 at 850 hPa. Only tracks with a lifetime of at least 6 h (2 time steps) are retained.
The cyclones detected by the tracking method include not only PLs, but also ECs and false disturbances that are not observed in satellite imagery. To obtain a set of robust tracks of PLs, further criteria are employed based on previous studies. The criteria are assessed at the time when a PL reaches its maximum intensity in the vorticity at 850 hPa. At first, we examine all the detected cyclones that satisfy the following weak thresholds at least once in their life span: the vorticity at 850 hPa exceeds 6 × 10−5 s−1 within the radius of 1° from the cyclone center, and the sea surface wind speed (wind speed at 10 m above the sea level) exceeds 12 m s−1 within the radius of 2.5° from the cyclone center. These cyclones are PL candidates that are subject to further classification.
While the T40–T100 filter used in the tracking algorithm removes most of the synoptic-scale structures of ECs, it still leaves some meso-α-scale cores of ECs. Therefore, an additional criterion is necessary for the discrimination between PLs and ECs. Because PLs are convectively unstable systems, previous studies consider the temperature difference between the sea surface and atmosphere at 500 hPa as a criterion for PLs: the thresholds used in the previous studies are the temperature difference (SST minus temperature at 500 hPa) of 43°C (Zahn and von Storch 2008; Xia et al. 2012; Zappa et al. 2014) or 39°C (Chen and von Storch 2013). Figure 2 shows a scatterplot for the SST and temperature at 500 hPa averaged within 1° radius of the cyclone center. The gray dots indicate all the detected cyclones that are found over the western North Pacific Ocean (20°–70°N, 120°E–180°) at the time of their maximum intensity. For reference, several detected cyclones are subjectively linked to PLs or ECs based on satellite imagery, surface weather charts and previous studies: the purple dots are linked to observed PLs (see section 2c), whereas the green dots are linked to meso-α-scale cores of ECs that have warm and cold frontal systems in JMA’s surface weather charts. The observed PLs occur frequently in the part of the plot where the difference between SST and temperature at 500 hPa exceeds 43°C. On the other hand, the observed ECs become more dominant as the temperature difference becomes lower. Thus, the criterion of the temperature difference effectively distinguishes PLs from ECs, although the boundary is not sharp. There are two extreme exceptions that are associated with relatively high temperatures at 500 hPa of around −18° and −21°C in Fig. 2. These are shallow meso-α-scale cloud systems within weak monsoon flows in March. It should be noted that subjective classifications based on satellite imagery and surface weather charts also have difficulty in distinguishing between PLs and ECs for some cases. In the present study, we employ the threshold of 43°C at the time of their maximum intensity to obtain robust cases of PLs. As this criterion uses the value of SST, only the cyclones over the ocean are analyzed. This is justified because PLs are known to be primarily maritime phenomena that need the energy supply through surface heat fluxes over the ocean. Hereafter, we analyze only those PLs that reach their maximum intensity in the vorticity at 850 hPa over the Sea of Japan (Fig. 1).
SST and temperature at 500 hPa for all the cyclones over the western North Pacific Ocean that are detected by the tracking algorithm. Purple and green dots indicate PLs and ECs, which are subjectively classified using satellite imagery and surface weather charts. The dashed lines indicate the differences of 39° and 43°C between SST and temperature at 500 hPa.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
The reanalysis PL dataset may contain some false PLs that are not observed in satellite images, or may miss some observed PLs. The number of false and missed detections depends on the thresholds of PL intensity. Previous studies used different thresholds for intensity: Zappa et al. (2014) employed a threshold of 6 × 10−5 s−1 for vorticity at 850 hPa and a threshold of 15 m s−1 for surface wind speed, whereas Zahn and von Storch (2008) employed a threshold of −2 hPa for filtered SLP and a threshold of 13.9 m s−1 for surface wind speed. Therefore, we examine two thresholds for vorticity at 850 hPa within 1° radius from the SLP minimum (6 and 10 × 10−5 s−1), and two thresholds for wind speed near the sea surface within 2.5° radius from the SLP minimum (12 and 15 m s−1). Figure 3 shows a scatterplot for vorticity at 850 hPa and surface wind speed. When we use the threshold of 6 × 10−5 s−1, we obtain more samples of PLs, but find many false disturbances that are not identified in satellite images (not shown). Therefore, we have decided to employ the threshold of 10 × 10−5 s−1 for the vorticity at 850 hPa, although the number of samples is reduced. The number of PLs is a little different between the thresholds of 12 and 15 m s−1 for the surface wind speed (272 and 244, respectively), when the vorticity exceeds 10 × 10−5 s−1. Based on Zappa et al. (2014), we employ the threshold of 15 m s−1 for the surface wind speed, which is also used in one of the previous definitions of PLs (Rasmussen and Turner 2003). Hereafter, this combination of the thresholds for vorticity and wind speed is referred to as the strong threshold, and is the main criteria used for our analysis.
Vorticity at 850 hPa (10−5 s−1) and surface wind speed (m s−1) for PLs over the Sea of Japan. Dashed lines indicate the vorticity thresholds of 6 and 10 × 10−5 s−1 and wind speed thresholds of 12 and 15 m s−1. Values indicate the numbers of the PLs in the four areas that are separated by the threshold lines.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
In summary, PLs in our classification need to satisfy the following criteria at the time of their maximum intensity in the T40–T100-filtered vorticity at 850 hPa:
The maximum of T40–T100-filtered vorticity at 850 hPa within a 1.0° radius exceeds 10 × 10−5 s−1.
The maximum of surface wind speed within a 2.5° radius exceeds 15 m s−1.
The average of the difference between SST and temperature at 500 hPa in a 1.0° radius exceeds 43°C.
c. Validation of the detected PLs
The PLs detected from the JRA-55 reanalysis have to be validated based on independent datasets including satellite images and surface weather charts. Figure 4 shows two examples of observed PLs that are accompanied by well-developed cloud signatures in satellite imagery. In the surface weather charts, both PLs over the Sea of Japan are located to the west of synoptic-scale extratropical cyclones (Figs. 4a,b). The PL on 21 January 1997 moved southward (Figs. 4c,e), whereas the PL on 6 January 1997 moved eastward (Figs. 4d,f). Both cases are accompanied by active cumulus convection. The tracking algorithm successfully represents the tracks of the southward- and eastward-moving PLs (Figs. 4g and 4h, respectively). While the tracking algorithm also detects the tracks of meso-α-scale cores of the ECs over the western North Pacific Ocean, the criterion of the temperature difference distinguishes the PLs from the ECs (white and black filled circles, respectively).
PLs obtained by the tracking algorithm on (left) 1800 UTC 21 Jan 1997 and (right) 1200 UTC 06 Jan 1997: (a),(b) JMA's surface weather charts, (c),(d) satellite imagery 6 h prior to the mature stage, (e),(f) satellite imagery at the mature stage, and (g),(h) PLs detected by the tracking algorithm. The large open circles are cyclones classified as PLs, whereas the large black filled circles are the other cyclones including ECs. Black curves indicate tracks of cyclones prior to the specified times where small circles indicate 6-hourly locations. The contours indicate SLP (contour interval is 4 hPa). The shadings indicate the vorticity at 850 hPa exceeding 1.0 × 10−4 s−1.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
Zappa et al. (2014) validated their PL tracking over the North Atlantic Ocean by using a subjective dataset based on satellite imagery compiled by trained forecasters at the Norwegian Meteorological Institute (Noer et al. 2011). Unfortunately, there are no equivalent datasets for PLs over the Sea of Japan. Therefore, we utilize several alternative sources of information on observed PLs.
The times and locations of several remarkable PLs can be obtained from previous studies (Table 1). The locations of observed PLs are estimated from satellite imagery and surface weather charts with an accuracy of 1°–2° in the longitudinal and latitudinal directions at the nearest 6-hourly time (0000, 0600, 1200, or 1800 UTC). The table also describes whether the observed PLs are reproduced in the JRA-55 reanalysis and are detected by the tracking methodology. The observed PLs are linked to the JRA-55 tracks, if the tracks are found within 2.5° radius from the observed PLs. All the 19 observed cases are detected as disturbances with negative SLP anomaly exceeding −1 hPa (rank A in the PRS column). Thus, the JRA-55 reanalysis is very capable of representing the PLs in the previous studies. Then, the tracking algorithm successfully constructs tracks for 16 cases among the 19 (rank A in the TRACK column). Among the 16 tracks, 13 tracks satisfy all the criteria (rank A in the VOR, WIND, and TEMP columns), 1 track does not satisfy the strong thresholds for intensity (rank B in the VOR or WIND columns), and 2 tracks do not satisfy the threshold for temperature difference of 43°C (rank B or C in the TEMP column). Note that PLs over the Sea of Japan are sometimes accompanied by multiple vortices. For example, Tsuboki and Asai (2004) observed two meso-β-scale vortices embedded in a meso-α-scale PL on 23 January 1990. The present tracking algorithm detects only a center of the meso-α-scale cyclone rather than centers of the two meso-β-scale vortices for this case, while the raw TL319 field of the JRA-55 reanalysis marginally represents two meso-β-scale vorticity maxima.
There are two sources that report formation of mesoscale disturbances over the Sea of Japan based on satellite imagery for several years. Although they do not distinguish PLs from other weak or small disturbances, we examine whether these disturbances are represented by the PL tracking with the JRA-55 reanalysis. To reject weak or small disturbances in a simple manner, we only select disturbances that are accompanied by at least one closed contour in the SLP field of the JMA’s weather charts (the contour interval is 4 hPa). First, Gurvich (2013) and its succeeding work have archived a database of mesoscale disturbances over the Sea of Japan based on Moderate Resolution Imaging Spectroradiometer (MODIS) images. Because the MODIS observations are only available for irregular time intervals, we determine the locations of disturbances at the nearest 6-hourly time (0000, 0600, 1200, or 1800 UTC) using JMA’s surface weather charts. Using the archive from January 2011 to December 2013, 36 disturbances are obtained. Among them, 29 cases are detected as disturbances with negative SLP anomaly exceeding −1 hPa in the JRA-55 reanalysis, 24 cases are tracked by the algorithm, and 14 cases satisfy all the PL criteria of vorticity, wind speed, and temperature difference. Second, the JMA analyzed cloud characteristics (nephanalysis charts) including the distribution of several cloud types, cloud streets, and locations of meso-α- and meso-β-scale disturbances based on pattern and movement of clouds observed in geostationary satellite images until May 2003. Using the nephanalysis charts, we have compiled 6-hourly locations of 35 meso-α-scale disturbances for 6 winter seasons (December–February) from December 1997 to February 2003. Among the 35 disturbances, 27 cases are detected as disturbances with negative SLP anomaly, 19 cases are tracked, and 8 cases satisfy all the PL criteria. While JRA-55 represents SLP anomalies accompanied by these disturbances, the ratios of successful classification of PLs for these sources are lower than that for the cases in Table 1. This is partly because these sources still contain weak or small disturbances, and partly because the criteria may be too strong. An observed database of PLs over the Sea of Japan that considers their intensity would be useful for examining the thresholds of the PL tracking algorithm more strictly.
In summary, the tracking of PLs using the JRA-55 reanalysis reproduces the intense PLs reported in the previous studies, whereas it still has some difficulty in representing some PLs observed in satellite imagery. Although the lower thresholds can reduce the miss detection rate, it increases the false detection rate. Among the 122 tracks with the strong threshold since January 1997, for which we have geostationary satellite imagery, 11 tracks are not accompanied by organized cloud systems. Because most of these false detections occur near the coast of the Asian continent, the algorithm may detect weak disturbances without convection just downstream of the topography. If we use the tracks with the weak threshold, the ratio of false detection is increased. Therefore, to obtain a robust result, the present study mainly analyzes the PLs that satisfy the strong thresholds, and focuses on the mature stage of the PLs when the vortices are intense.
3. Tracks and frequencies of the PLs
From the tracking analysis, the climatology of mature PLs can be computed, including the geographical distribution, seasonal frequency, and direction of motion. Figure 5a shows the tracks and locations at maximum intensity in vorticity at 850 hPa (black dots) for PLs using the strong thresholds. The locations of the maximum intensity are concentrated off the coasts of the Japanese islands (Hokkaido and Honshu; see also Fig. 1) rather than off the coast of the Asian continent [i.e., on the southeastern side of the midline between the Asian continent and the Japanese islands (downstream of the northwesterly monsoon flow and the westerly jet stream)]. Some PLs move eastward over the Sea of Japan, whereas others, particularly those to the west of Hokkaido, move southward. An average duration between genesis and maximum intensity is about 15.6 h. The genesis of PLs (gray dots) occurs to the west of Hokkaido, to the east of the part of the Korea Peninsula that is nearest to the Asian continent, and centrally over the Sea of Japan. The first and second of these genesis regions are also found in the study of Chen and von Storch (2013), whereas the last one is not. This discrepancy seems to be partly due to these authors only analyzing PL tracks with a southward component, and to the tracking algorithm with JRA-55 not representing initial stages of PLs with weak vortices. Thus, the present study focuses on the mature stage of PLs with intense vortices.
Tracks of PLs over the Sea of Japan for (a) strong and (b) weak thresholds. Gray dots indicate the genesis locations of PLs, and black dots indicate the locations where PLs reach their maximum intensity in vorticity at 850 hPa. Red and blue curves indicate that the meridional component of PL motion is northward and southward, respectively.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
Figure 6a shows the monthly frequency of the PLs for the strong thresholds. The PLs form most frequently during midwinter (from December to February). The mean number of PLs per cold season is 6.8 events with a year-to-year standard deviation of 3.6 events (not shown). Thus, the interannual variability of PL formation is large with a minimum of 1 event and a maximum of 15 events per season. It would be interesting, in a future study, to examine whether this variability is associated with the occurrence of cold air outbreaks and of the East Asian winter monsoon.
Average monthly frequency of PLs over the Sea of Japan during the 36 cold seasons for (a) strong and (b) weak thresholds.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
The geographical distribution of PL tracks for the weak thresholds is similar to that for the strong thresholds, although the number of tracks is larger (Figs. 5b and 6b). Some PLs with the weak thresholds develop to the east of the part of the Korea Peninsula that is nearest to the Asian continent (around 40°N, 130°E). Previous studies reported formation of meso-β-scale vortices within the Japan sea polar airmass convergence zone (JPCZ; Asai 1988), which is a convergence or confluence zone of polar airstreams forming on the lee side of the high mountain area in the Asian continent (Rasmussen and Turner 2003). Although weak or small vortices are also interesting disturbances over the Sea of Japan in winter, we analyze only the PLs with the strong thresholds to reduce false detection as mentioned in section 2c.
The detected PLs have a tendency to move eastward or southward (Fig. 5a) depending on the region. Figure 7 shows the latitudinal distribution of directions of motion of PLs averaged for 6 h prior to the time of maximum intensity, where PLs that move less than 0.2° for 6 h are discarded. The PLs at lower latitudes in the Sea of Japan (<~39°N) frequently move eastward, whereas the PLs at higher latitudes (>~39°N) move in a broader range of directions between southward and eastward. The relationship between the directions of motion of PLs and synoptic-scale conditions are examined in the next section.
Latitudinal distribution of directions of motion of the PLs with the strong threshold averaged for 6 h prior to their maximum intensity.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
4. Composites of atmospheric fields during the PL development
a. PL groups used for the composite analysis
Composite analyses are conducted for the PLs with the strong threshold in each of the three areas: the northern, middle, and southern parts of the Sea of Japan (referred to as NJ, MJ, and SJ, respectively) shown in Fig. 8. These areas are 4° × 4° boxes where the PLs frequently reach their maximum intensity. As the directions of the PL motion depend on the regions over the Sea of Japan (Figs. 5a and 7), the frequencies of directions of motion are examined for the three composite areas (Fig. 9). The PLs in the NJ area show two dominant directions of motion: southward and eastward (Fig. 9a). Therefore, the composite analysis for the NJ area is conducted for two groups: the southward-moving group (NJs), which covers 90° in the direction of motion between southwest and southeast, and the eastward-moving group (NJe), which covers the direction of motion between southeast and northeast. In the MJ and SJ areas, PLs most frequently move eastward. Therefore, the composite analysis is conducted for the eastward-moving group in the MJ and SJ areas (referred to as MJe and SJe, respectively), which covers the direction of motion between southeast and northeast. In summary, the composite analysis is conducted for the four groups—NJs, NJe, MJe, and SJe—to depict planetary- and synoptic-scale conditions for PLs in different areas and for different directions of motion.
Number of the PLs within 1° × 1° boxes at the times of their maximum intensity. The rectangles labeled NJ, MJ, and SJ indicate the areas used for composite analysis.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
Frequencies of eight directions of motion for PLs averaged for 6 h prior to their maximum intensity in composite areas (a) NJ, (b) MJ, and (c) SJ.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
The key time (0 h) is defined as when a PL reaches its maximum intensity in the vorticity at 850 hPa (the black dots in Fig. 5a). Lag composite analysis at +x h (−x h) is conducted using the atmospheric fields x h prior to (after) the key time. We separate raw atmospheric fields into two time scales: mean and anomaly. The mean is obtained by applying a 15-day running mean to the raw data, whereas the anomaly is the difference of the raw data from the mean. Thus, the raw dataset (referred to as the total field) is the sum of the mean and the anomaly. The mean represents the time scales of seasonal transition and longer phenomena, whereas the anomaly contains the time scales of synoptic-scale and mesoscale disturbances including ECs and PLs.
b. Planetary- and synoptic-scale conditions
The SLP field gives basic atmospheric information that can be compared with surface weather charts. Figure 10 shows the total fields of SLP and horizontal winds at 850 hPa at 0 h for each PL group. The total fields are not dominantly characterized by signals of the PLs in the composite areas, because they contain planetary- and synoptic-scale high and low pressure systems. On a planetary scale (several thousand kilometers), a high pressure system is evident over the cold Asian continent (the Siberian high), whereas a low pressure system forms over the relatively warm North Pacific Ocean (the Aleutian low). This pressure pattern causes northwesterly flow associated with the East Asian winter monsoon over the Sea of Japan for all the composites. On synoptic and meso-α scales (from several hundred to a few thousand kilometers), SLP is relatively low around and to the east of the composite areas.
Composite of total fields for SLP (shading; hPa) and horizontal winds at 850 hPa (vector) at 0 h: (a) NJs, (b) NJe, (c) MJe, and (d) SJe. Black rectangles indicate the composite areas.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
To focus on synoptic- and meso-α-scale characteristics, Fig. 11 shows the anomaly fields of SLP and horizontal winds at 850 hPa for −24, 0, and 24 h. For all the composite groups at 0 h (the middle panels), significantly negative SLP anomalies extend southeastward from the composite area on synoptic scales. These negative anomalies are associated not only with the low pressure of meso-α-scale PLs in the composite areas (black rectangles), but also with that of synoptic-scale ECs to the east or southeast of the PLs. For NJs (Fig. 11b), the minimum of the SLP anomaly occurs to the east of the composite area, which implies that the ECs are more intense than the PLs. On the other hand, for NJe, MJe, and SJe (Figs. 11e,h,k), the SLP minima appear within the composite areas, which implies that PLs are intense. Note that the average vorticity at 850 hPa at the PL’s maximum intensity (not shown) for NJs (12.7 × 10−5 s−1) is also smaller than those for NJe, MJe, and SJe (15.0, 16.1, and 14.0 × 10−5 s−1, respectively). From −24 to 24 h, negative SLP anomalies shift eastward in all composites, which corresponds to an eastward motion of the ECs, although it is difficult to distinguish between PLs and ECs in the composite fields. For individual cases (not shown), PLs are rarely identified at −24 and 24 h. Therefore, the eastward shifts of negative SLP anomalies seem to represent the eastward motion of synoptic-scale ECs.
Composite of anomaly fields for SLP (shading; hPa) and horizontal winds at 850 hPa (vector): (a)–(c) NJs, (d)–(f) NJe, (g)–(i) MJe, and (j)–(l) SJe. Dots indicate the significance level of 0.05 for the SLP anomaly. Lag composite at (left) −24 h, (middle) 0 h, and (right) 24 h. Black rectangles indicate the composite areas.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
The horizontal winds at 850 hPa within the composite areas are associated with multiscale pressure systems around the Sea of Japan as mentioned above: the planetary-scale northwesterly flow associated with the East Asian winter monsoon, the synoptic-scale northerly flow on the western side of the EC, and the cyclonic circulation accompanied by the meso-α-scale PL. For the NJs, northerly wind dominates over the composite area (Fig. 10a), because the extratropical cyclone to the east is strong (Fig. 11b). On the other hand, the horizontal winds for NJe, MJe, and SJe are not northerly in the eastern part of the composite area (Figs. 10b–d), because the cyclonic circulation of PLs is relatively intense (Figs. 11e,h,k). As discussed later, these differences in horizontal wind directions seem to affect the PL motion.
The midtropospheric conditions can also affect the dynamics of the PLs. Figure 12 shows the total fields of geopotential height and horizontal winds at 500 hPa. For all the composite groups, a westerly jet stream is apparent in the midlatitudes, and is deflected around the Sea of Japan on planetary or synoptic scales. Therefore, the composite areas for MJe and SJe are dominated by the strong westerly flow. Although the NJ area is located to the north of the strong westerly jet stream, the horizontal wind is still westerly in the southern half of the composite area for both NJs and NJe. The horizontal wind in the northern half of the area for NJs is northeasterly, whereas that for NJe is southwesterly.
Composite of total fields for geopotential height (shading; m) and horizontal winds at 500 hPa (vector) at 0 h: (a) NJs, (b) NJe, (c) MJe, and (d) SJe. Black rectangles indicate the composite areas.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
The midtropospheric conditions can be associated with synoptic-scale disturbances. Figure 13 shows the anomaly fields of geopotential height and temperature at 500 hPa at 0 h. All the composite areas are dominated by synoptic-scale upper-level cold troughs with significantly negative height anomalies and negative temperature anomalies. As in the anomaly fields of SLP, the upper-level cold troughs also move eastward for all the composite groups (not shown). Previous case studies also observed upper-level cold troughs above PLs over the Sea of Japan (e.g., Ninomiya 1989; Fu et al. 2004; Shimada et al. 2014). The composite analysis in Mallet et al. (2013) also shows upper-level cold troughs during PL formation over the Nordic seas and over the Labrador Sea, although the troughs over the Nordic seas move southward. The upper-level cold trough for NJs is located just above the composite area, whereas the upper-level cold trough for NJe, MJe, and SJe occurs to the west of the composite areas. Note that the difference in the locations of the upper-level cold troughs between NJs and NJe is also related to the difference in the total horizontal winds within the composite areas as discussed above.
Composite of anomaly fields for geopotential height (shading; m) and temperature at 500 hPa (contour interval is 1°C) at 0 h: (a) NJs, (b) NJe, (c) MJe, and (d) SJe. Dots indicate the significance level of 0.05 for the geopotential height anomaly. Black rectangles indicate the composite areas.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
As the sensible and latent heat fluxes from the sea surface are considered to be important for convective processes and PL development, we analyze the forecast fields of the surface heat fluxes. Figure 14 shows the total fields of the sum of the sensible and latent heat fluxes and the ratio of sensible heat flux to latent heat flux (the Bowen ratio) over the ocean at 0 h. The sum of the sensible and latent heat fluxes is large over the Sea of Japan, particularly on the western side of the composite areas. It exceeds 400 W m−2 in the southwestern part of the composite area for all the composite groups. The Bowen ratio is near or above unity in most of the composite area for NJs and NJe, whereas it is less than unity for MJe and SJe. Therefore, the sensible (latent) heat flux becomes more dominant at higher (lower) latitudes over the Sea of Japan.
Composite of total fields of heat fluxes from the ocean surface at 0 h. The shading indicates the sum of sensible and latent heat fluxes (W m−2). The contours indicate the ratio of sensible heat flux to latent heat flux (contour interval is 0.2); the black contours indicate the ratio of 1.0 or larger, whereas the gray contours indicate the ratio less than 1.0 (only positive value is shown): (a) NJs, (b) NJe, (c) MJe, and (d) SJe. Black rectangles indicate the composite areas.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
Figure 15 shows the anomaly fields of the sum of the sensible and latent heat fluxes for NJs. The surface heat fluxes over the Sea of Japan at 0 h are larger than those at −24 and 24 h. Thus, the PLs form during the period of significant increase in surface heat fluxes, which is also the case for NJe, MJe, and SJe (not shown). The increase is caused by strong northerly winds and low temperatures in the lower troposphere associated with the EC to the east of the PLs. The low atmospheric temperature over the Sea of Japan mainly originates from the cold Asian continent, and partly from the Okhotsk Sea (see also Fig. 10). At the mesoscale, the northerly flow is partly enhanced on the western side of the PL due to the circulation of the PL itself, whereas the flow is weakened on the eastern side of the PL (see also Fig. 11). Thus, the surface heat flux is particularly large on the western side of the composite areas. This characteristic is similar to the results of numerical simulations of a PL over the Sea of Japan (Yanase et al. 2002) and of a PL over the Barents Sea (Føre et al. 2012).
Composite of anomaly fields for the sum of sensible and latent heat fluxes from the ocean surface (shading; W m−2) for NJs at (a) −24 h, (b) 0 h, and (c) 24 h. Dots indicate the significance level of 0.05.
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
5. Southward-moving and eastward-moving PLs
The PLs over the Sea of Japan are classified into two types according to their directions of motion (Figs. 5, 7, and 9): the southward- and eastward-moving PLs. The synoptic-scale conditions are also different between the two types as shown in section 4b. This section discusses differences between the two types of PLs in more detail.
To examine the environmental flow in the lower and middle troposphere, Fig. 16 shows hodographs of environmental horizontal winds at 900, 800, 700, 600 (filled circles), and 500 hPa (triangles) for all the composite groups. The horizontal scale of environmental flow should be large enough to cover the area of strong wind accompanied by a PL, but small enough to represent synoptic-scale wind accompanied by an EC. Because it is difficult to determine a strict horizontal scale of the environmental flow, we examine two different sizes of circular areas to calculate averages: one has a radius of 3° (Fig. 16a) and the other a radius of 5° (Fig. 16b) around the PL centers. Note that the axisymmetric component of the PL circulation is completely canceled by the area average. The hodographs of the total fields for the 3° average (solid lines in Fig. 16a) are consistent with the direction of PL motion: the southward-moving group (NJs; a black line) is characterized by southward–southeastward wind between 900 and 500 hPa; whereas the eastward-moving groups (NJe, MJe, and SJe; red, green, and blue lines, respectively) are characterized by southeastward–northeastward wind. It should be also noted that the average propagation speed of PLs (not shown) for NJs (4.1 m s−1) is smaller than those for NJe, MJe, and SJe (6.0, 9.0, and 7.9 m s−1, respectively), which is consistent with the difference in environmental wind speed between the southward- and eastward-moving groups. The hodographs of the total fields for the 5° radius average (solid lines in Fig. 16b) are similar to those for the 3° radius average, except that the southeastward component for NJs becomes more dominant. Thus, the horizontal winds of the 3° radius average seem to represent the PL motion better than that of the 5° radius average.
Composites of hodograph for the total wind fields (solid lines) and 15-day-mean wind fields (dashed lines) for NJs (black), NJe (red), MJe (green), and SJe (blue). The horizontal winds are averaged within a radius of (a) 3° and (b) 5° from the PL center. The winds are assessed at 900, 800, 700, 600 (filled circles), and 500 hPa (triangles).
Citation: Journal of Climate 29, 2; 10.1175/JCLI-D-15-0291.1
The hodographs for the 15-day mean fields (dashed lines in Figs. 16a,b) represent the time scales of seasonal transition and longer phenomena. The mean fields are responsible for the eastward flow, particularly at upper levels, which is associated with a strong jet stream during winter. The eastward flow is more intense at lower latitudes, which is consistent with the frequent occurrence of eastward-moving PLs at lower latitudes (Figs. 7 and 9). On the other hand, the southward flow for NJs is not represented by the mean field, which implies that synoptic-scale disturbances are responsible for the southward motion of the NJs group.
The southward-moving PLs are located just below upper-level cold troughs during their mature stage (Fig. 13a). The cold air in the midtroposphere causes active convection, which is favorable for PL development through condensational heating. The southward motion and active convection are apparently different from characteristics of synoptic-scale ECs. On the other hand, the eastward-moving PLs are located slightly to the east of upper-level cold troughs (Figs. 13b–d). This location seems to be favorable for PL development through baroclinic interaction including dynamically induced ascending motion ahead of an upper-level mobile trough. The eastward motion and baroclinic interaction are somewhat similar to characteristics of ECs. Thus, the eastward-moving PLs are sometimes difficult to distinguish from ECs, although typical PLs are smaller than synoptic-scale ECs, and are accompanied by active cumulus convection near the cyclone centers (Figs. 4b,f). Because there are cyclones that have intermediate characteristics between typical PLs and ECs, a definite distinction between the two types of cyclones may be intrinsically difficult. Some tracking analyses in previous studies rejected northward-moving PLs to avoid detecting some ECs (Zahn and von Storch 2008; Chen and von Storch 2013), which are different from our definition of PLs that allow a northward component of PL motion.
Different types of PL motion are also found for PLs over the Nordic seas (Terpstra 2014). Terpstra (2014) classified the atmospheric conditions associated with PL genesis based on the mean flow and vertical shear (thermal wind) of horizontal winds between 925 and 700 hPa: if the vertical shear and mean flow are in the same (opposite) direction, the environment is classified as forward (reverse) shear. Terpstra (2014) showed that the forward shear PLs move eastward more frequently and feature similarity with ECs, whereas the reverse shear PLs move southward more frequently. Figure 16a shows that the environmental flow of the southward-moving PLs is characterized by a decrease in the southward component with altitude (reverse shear), whereas the environmental flow of the eastward-moving PLs is characterized by an increase in the eastward component with altitude (forward shear). Thus, the forward and reverse shear PLs over the northeast Atlantic Ocean in Terpstra (2014) resemble the eastward- and southward-moving PLs over the Sea of Japan in the present study, respectively. Note that the southward-moving PL on 21 January 1997 (left panels in Fig. 4), which is also reported by Yanase et al. (2002, 2004) and Fu et al. (2004), is classified as a reverse shear PL by Kolstad (2006).
6. Summary
To understand the climatology of PLs over the Sea of Japan, the tracking algorithm based on Zappa et al. (2014) has been applied to the JRA-55 reanalysis. This first attempt successfully detects most of the PLs reported in previous studies, although there are a few caveats: the reanalysis occasionally causes miss or false detection, and the algorithm has some difficulty in discriminating between eastward-moving PLs and ECs. The PLs frequently reach their maximum intensity on the southeastern side of the midline between the Asian continent and the Japanese islands. The frequency of the PL development is high during the midwinter from December to February.
The composite analyses based on the area and dominant direction of PL motion depict the synoptic-scale conditions associated with the PL development. Synoptic-scale ECs to the east of the PLs enhance the cold northerly flow over the Sea of Japan, and increase heat fluxes from the sea surface around the PLs. The PLs develop under the influence of upper-level cold troughs. These results are consistent with previous case studies. In addition, the sea surface heat fluxes during the PL formation contain a higher portion of sensible (latent) heat at the higher (lower) latitudes.
Furthermore, our study has revealed two dominant directions of PL motion: southward and eastward. The southward-moving PLs occur off Hokkaido and northern Honshu. They are steered by the strong northerly flow in the lower troposphere, which is caused by relatively intense ECs to the east of PLs and by northwesterly winter monsoon flow. Upper-level cold lows just above the PLs are favorable for PL development through condensational heating of cumulus convection. On the other hand, eastward-moving PLs are widely observed off the coasts of Honshu and Hokkaido. They seem to be steered by the planetary-scale westerly flow in the midlatitudes during winter. Upper-level cold troughs slightly to the west of the PLs seem to be favorable for PL development through baroclinic processes. The results of the composite analyses demonstrate average synoptic-scale conditions associated with the PL formation, although the conditions for the individual PLs vary from case to case.
This study has demonstrated that a PL tracking algorithm and high-resolution model output are valid tools for understanding the climatology of PLs, although they may not be completely consistent with observations. If more sophisticated reanalyses with higher resolution become available in the future, it may be possible to more accurately identify and track weaker PLs, determine the genesis and lysis stages more accurately, and allow for a more detailed study of the structure and dynamics of PLs. We will also continue to improve the tracking algorithm by examining several atmospheric variables and spectral filter bands among other parameters to construct a more accurate and more comprehensive climatology of PLs (Bracegirdle and Gray 2008; Xia et al. 2012). Along with the objective tracking analysis, accumulation of case studies is indispensable for understanding structures and mechanisms of various types of PLs. In particular, a satellite-based PL database for the Sea of Japan that considers intensities and sizes of PLs will be useful for more accurate validation of the tracking algorithm. Longer-term datasets will provide us a larger number of PL samples and higher statistical significance in composite analysis. It would also be interesting to apply the tracking approach to the climatology of PLs over the other marginal seas of the North Pacific Ocean including the Okhotsk Sea and the Bering Sea, and to the PLs in the global simulations for paleo and future climates.
Acknowledgments
We acknowledge helpful comments by Drs. Kozo Ninomiya, Kent Moore, Leonid Mitnik, Udai Shimada, Yasutaka Hirockawa, Moriaki Hinokio, Taro Shinoda, Hisashi Nakamura, Tomonori Sato, Masaya Kato, Qoosaku Moteki, and three anonymous reviewers. The atmosphere and SST datasets made use of JRA-55. Wataru Yanase was supported by JSPS KAKENHI Grants 23740349 and 25106703. Hiroshi Niino was supported by JSPS KAKENHI Grant 24244074. Matthias Zahn was supported through the Cluster of Excellence “CliSAP” (EXC177), Universität Hamburg, funded through the German Science Foundation (DFG). Irina Gurvich’s research was partially supported by Grant FEB RAS 15-I-1-009o. Thomas Spengler was supported by the Norwegian Research Council as part of the project High Impact Weather in the Arctic (Project 207875).
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