Mesoscale cyclones Okhotsk

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3 January 2004 (Fig. 2). The most remarkable features visible on the Envisat ASAR image and shown in Fig. 2c are two cyclonic mesoscale lows located to the west of Kamchatka coast. Convective vortices are also well distinguished in a field of cloudiness on NOAA AVHRR, Terra and Aqua MODIS and GOES-9

Fig. 2. Mesoscale cyclones over the Okhotsk Sea to the west of Kamchatka on 3 January 2004: (a) and (c) 89-GHz, H-polarization Aqua AMSR-E brightness temperature (Kelvin degrees) at 02:25 (a) and at 16:15 UTC (c); (b) Aqua MODIS visible image at 02:25; (d) total atmospheric water vapor content (in kg/m2) and (e) total cloud liquid water content (in kg/m2) retrieved from 23.8- and 36.5-GHz, V-polarization brightness temperatures at 02:25 UTC; (f) Envisat ASAR image at 11:13 UTC and (g) QuikSCAT-derived wind field at 08:37 UTC.

Fig. 2. Mesoscale cyclones over the Okhotsk Sea to the west of Kamchatka on 3 January 2004: (a) and (c) 89-GHz, H-polarization Aqua AMSR-E brightness temperature (Kelvin degrees) at 02:25 (a) and at 16:15 UTC (c); (b) Aqua MODIS visible image at 02:25; (d) total atmospheric water vapor content (in kg/m2) and (e) total cloud liquid water content (in kg/m2) retrieved from 23.8- and 36.5-GHz, V-polarization brightness temperatures at 02:25 UTC; (f) Envisat ASAR image at 11:13 UTC and (g) QuikSCAT-derived wind field at 08:37 UTC.

images. Aqua MODIS visible image taken at 02:25 UTC i.e. about 9 h before the Envisat data acquisition is given in Fig. 2b. Water clouds, increased total water vapor content and wind speed variations are responsible for lows' detection by Aqua AMSR-E before (at 02:25 UTC, Fig. 2a) and after (at 16:15 UTC, Fig. 2d) Envisat ASAR acquisition. Cyclonic circulation was also registered by QuikSCAT scatterometer (Fig. 2e). Maximum wind speed (12-15 m/s) was measured to the south of the large low. This is consistent with radar backscatter (brightness) variations on ASAR image. The total atmospheric water vapor content V and total cloud liquid water content Q were retrieved from TB(23.8 V) and TB(36.5 V) (Mitnik and Mitnik 2003). The maximum V and Q values located to the southeast, east and northeast from the cyclone center in the spiral bands' area reached 79 kg/m2 and 0.12-0.14 kg/m2 correspondingly. The width of the bands in Q-field was about 20-30 km. Typical values in the cyclonic eddy area were lower: V = 56 kg/m2 and Q = 0.04-0.06 kg/m2. Around the eddy, the V values decreased till 3.5-5 kg/m2. At the eddy center V = 3.7-4.5 kg/m2.

On the surface analysis map of the JMA for 3 January at 00:00 UTC the northern low the size of «300 km was outlined by 1,004-mb isobar and 12 h later it was outlined by 1,008-mb isobar. The southern low the size of «120 km was not mapped.

It is the last eddy in a chain extending from the ice edge in the central part of the Okhotsk Sea to the western Kamchatka coast. The whole chain was depicted in the cloud field on several satellite visible and IR images taken on 2-4 January. The chain is the northern boundary of the area with the mesoscale convective cells and rolls. They manifest themselves both in cloud field (Fig. 2b) and in field of the sea surface roughness (Fig. 2c). Lows are very dynamic structures and the best correspondence between the images taken by various sensors from different satellites is accomplished at small difference in data acquisition time. On ASAR image for 3 January 2004 at 11:12 UTC, the lows manifest themselves as the areas of the increased brightness (increased wind speed) spiraling around dark area (low wind speed) in their centers. The spiral structure of the eddy is detected also on MODIS and AMSR-E TB(v) images depicted in Fig. 2a, b, d. The centers of the eddy spirals look dark both on the ASAR image due to weak winds and on the NOAA and MODIS images due to low amount of clouds. The bright area south and southwest of the northern eddy center on the ASAR image results from severe winds. Wind speed decreased as the distance from the center increases. The width of this area is about 250 km. The brightest spiral bands with sharp wavelike edges imbedded in the area mark the atmospheric fronts' position near the sea surface. The width of transition zone dividing the area with low and high winds does not exceed 1-2 km. Very likely that the highest surface winds coincide with the convective cloud bands on MODIS image or somewhat shifted relative to them. The cloud bands are characterized by the increased brightness due to intense developed convection. AMSR-E-derived wind speed reached 15 m/s in a circle around the center and in the area south of it.

Fig. 3. Mesoscale cyclone in the Okhotsk Sea on 19 December 006: (a) Terra MODIS visible image acquired at 02:15 UTC, (b) surface analysis map, (c) and (d) thermobaric fields at 850 mb (c) and at 500 mb (d) of the KMA at 19 UTC and (e) reanalysis map of underlying surface temperature (isotherms are shown through 2 K) at 00 UTC. Red dot marks the center of mesocyclone.

Fig. 3. Mesoscale cyclone in the Okhotsk Sea on 19 December 006: (a) Terra MODIS visible image acquired at 02:15 UTC, (b) surface analysis map, (c) and (d) thermobaric fields at 850 mb (c) and at 500 mb (d) of the KMA at 19 UTC and (e) reanalysis map of underlying surface temperature (isotherms are shown through 2 K) at 00 UTC. Red dot marks the center of mesocyclone.

19 December 2006 (Figs. 3-5). Envisat ASAR image acquired at 00:13 UTC (Fig. 5a) shows cyclonic eddy the size of approximately 400 km, which was not revealed on the surface analysis map of the Korean Meteorological Administration (KMA) (Fig. 3b). Terra MODIS visible image (Fig. 3a) taken in 2 h after ASAR image depicts its cloud system. The MC was formed to the northeast from intense cold air outbreak over the Okhotsk Sea near stationary tropospheric trough (Fig. 3c-e). Cloud rolls typical for cold outbreak are seen to the south-west of the trough (Fig. 3a). Low-gradient thermal trough that corresponds to the high-altitude trough spreads over the whole Okhotsk Sea (Fig. 3d). The cold center at 500 hPa absolute topography map AT500 near the northeast Kamchatka coast was outlined by -45°C isotherm. Thermobaric field at AT850 map (Fig. 3c) indicates the presence of baroclinic instability of the boundary layer of the atmosphere in a northern part of the sea, which was formed as a result of the large thermal contrasts between very cold land and the relatively warm sea surface. The largest closeness of the isotherms was observed near the coast (Fig. 3e). Axis of the thermal ridge at 850 mb level was directed from the south-east to the north-west. Cold trough coinciding with narrow cloud rows on the visible image extended from continent through the Okhotsk Sea. Still one small cold tongue from continental regions adjoin the northern sea coast, wedged in warmth ridge increasing thermal and baric gradients that favored strengthening northeast winds here. QuikSCAT-derived wind speed in the mesocyclone was 12-15 m/s (Fig. 4a, b).

Fig. 4. Mesoscale cyclone over the Okhotsk Sea: (a) and (b) QuikSCAT-derived wind fields (a) on 18 December at 18:48 UTC and (b) on 19 December at 08:54 UTC; (c) 89-GHz, H-polarization Aqua AMSR-E brightness temperature on 19 December at 02:25 UTC. Dark box marks the boundaries of Envisat ASAR image shown in Fig. 5a.

Fig. 4. Mesoscale cyclone over the Okhotsk Sea: (a) and (b) QuikSCAT-derived wind fields (a) on 18 December at 18:48 UTC and (b) on 19 December at 08:54 UTC; (c) 89-GHz, H-polarization Aqua AMSR-E brightness temperature on 19 December at 02:25 UTC. Dark box marks the boundaries of Envisat ASAR image shown in Fig. 5a.

Configuration and structure of MC cloud system on NOAA-17 AVHRR infrared image and brightness (sea surface wind) field on SAR image acquired with time difference of 8 min are in a good agreement (Fig. 5). Bright area 1 in a northern part of MC was caused by strong eastern and northeastern winds. It is confirmed by structure of thermobaric field in the boundary layer of the atmosphere (Fig. 3d). Bands of alternating brightness on SAR image (Fig. 5a), as well as cloud rows on IR image (Fig. 5b) also indicate northeastern wind direction. Disturbances in the area of eddy chain distinguished on visible image are clearly seen along the boundary of wind shift 2, which coincides with the internal boundary of a bright cloud band A on IR image. The size of eddies decreases from 70 to 10 km as the distance to the MC center decreases. Small dark patches 3, typical for the eddy centers where winds are weak are clearly seen in several places along wind shift line. Their sizes are approximately 9 x 2 and 5 x 1.5 km. Small-scale cloudless zones in the centers can be revealed on IR image (Fig. 5b) not so clearly due to their small sizes (Gurvich et al. 2008).

Fig. 5. Mesoscale cyclone over the Okhotsk Sea on 19 December 2006: (a) Envisat ASAR image at 00:13 UTC; (b) NOAA-17 AVHRR infrared image at 00:21 UTC. Dark lines mark the boundaries of ASAR image. K - Koni Peninsula, Z - Zav'yalova Island.

One more bright cloud band 4, formed likely as a result of interaction of air flow with mountains higher than 1,000 m extends from Kamchatka to the west. Disturbances of sea surface wind correlated with this cloudiness are small and their contrasts against the background on SAR image are small too. Vortex chain 5 extends to the center of MC from the western Koni Peninsula. Its structure manifests itself in brightness variations on SAR image. From this follows, that the organized convection covers the whole boundary layer of the atmosphere from its upper boundary (cloudiness) till the lower boundary (surface wind). A contrast band 6, which extends from Zav'yalova Island, is merged with vortex chain 5. All cloud bands approach each other and curving anticlockwise form a circle 7 the size of 70-80 km around the almost cloudless cyclone center. These features look as eye wall and eye in typhoon. On the SAR image, a circle-like zone of strong winds fringes the central area with weak winds where imprints of convective cells 9 the size of 5-8 km can be distinguished.

Structures 5, 6 and 7 in fields of cloudiness and narrow contrast bands in field of sea surface wind on the SAR image visualize process of isolation of the warmer marine air by cold continental air which curves around it. This phenomenon is similar to warm seclusion in a synoptic-scale cyclone (Sikora et al. 2000; Young et al. 2005; Montgomery and Farrell 1992). The boundary of wind shift 10 is identified with seclusion front. Seclusion is one of the factors favorable for formation of warm core in mesoscale cyclones. Bright large rain cells 11 are seen on IR image to the south of boundary 2. Thin gray-tone very small cells 12 are observed farther south up to the boundary 2. These cells manifest themselves clearly on SAR image. Their diameters vary from 10-20 km for large cells to 1-2 km for small ones (Gurvich et al. 2008).

Sharp boundary of vortex-like structure in the field of brightness temperature 7Bh(89) separates dry air in the centre of the cyclone from the surrounding wet air mass (not shown). Large rain cells to the south of cyclone are reliably detected: 7Bh(89) increment reaches 43-48 K whereas the small cells almost do not appear due to very low Q values. A dark area 13 on SAR image (Fig. 5a) is caused by weak winds that are observed in the region where V = 3.0 kg/m2 and 7BH(89) = 160170 K. Maximum values V = 7.5-9.0 kg/m2 are marked along bright convective bands. Mesocyclone manifests itself in Q field too: average Q value equals to 0.05-0.07 kg/m2. Only individual cloud inclusions are characterized by Q = 0.11 kg/m2, suggesting that probability of precipitation is low.

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