USING ENVISAT ASAR FOR THE STUDY OF WINTER MESOSCALE CYCLONES IN THE ASIAN MARGINAL SEAS

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1 USING ENVISAT ASAR FOR THE STUDY OF WINTER MESOSCALE CYCLONES IN THE ASIAN MARGINAL SEAS M. L. Mitnik, L. M. Mitnik, and I.A. Gurvich V.I. IL ichev Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences 4 Baltiyskaya St., 69004, Vladivostok, Russia, E-maisl: mitnik@poi.dvo.ru, maia@poi.dvo.ru, gurvich@poi.dvo.ru ABSTRACT Winter mesoscale cyclones (MСs) are frequently formed over the northern Asian Marginal Seas. They are often associated with precipitation and severe winds causing ice drift and serious disturbance in fishery and transport operation at the sea. The MCs were detected by screening Envisat ASAR archive images acquired over the Northwest Pacific in Highresolution ASAR images of selected MCs were compared to satellite visible and infrared imagery, QuikSCAT-derived wind fields, Aqua AMSR-E-derived fields of total atmospheric water vapour content V, total cloud liquid water content Q and wind speed W as well as with surface analysis and upper-air analysis maps.. INTRODUCTION Any cyclone to the north of the polar front which divides cold air to the north and warm air to the south, and the size of which less than 2000 km is defined as a mesocyclone []. Taking into account an Envisat ASAR swath width 405 km the main attention was given to MCs the size less than 500 km. It corresponds to meso α ( km) and meso β ( km) Orlansky scales [2]. Mesocyclones form over the sea during winter months when cold air mass moves across relatively warmer water. Polar lows, the most intensive MCs, are considered as a subtype: they are characterized by wind speed exceeding 5 m/s. Their size is usually less than 800 km but some may span as little as 00 km. Spiral polar lows have considerable similarity to tropical cyclones including the presence of a clear eye at the centre of the cloud vortex and a warm core. MCs are not often noted on the weather charts issued by local weather bureaus due to their small scales, short life time (typically between 2 and 6 hrs) and development in remote data sparse regions. The main sources of quantitative spatial information to examine MCs are satellite data and fields of geophysical parameters retrieved from measurements of various satellite sensors. However the satellite visible and infrared observations can capture only the structure of the storm tops that is serious limitation. The internal MC structure is often masked by the upper cloud layer. A satellite wide swath Real Aperture Radar [] and a SAR such as Envisat ASAR, RADARSAT SAR and ALOS PALSAR are important tools for the study of MCs over the ocean [4]. SAR images provide deeper insight into internal properties of MCs since they can contribute the high-resolution surface wind field, to mark the accurate location of the atmospheric fronts and MC centres at the sea surface as well as the presence of the small-scale organized variations of surface wind with scales from several hundreds to several tenths of kilometres corresponding to the meso γ (2 20 km) and micro α ( m) Orlansky scales []. Promising sources of regularly available remotely sensed data (as opposite to occasional ASAR images) are the Aqua and ADEOS-II Advanced Microwave Scanning Radiometers (AMSR-E and AMSR) [], the QuikSCAT Seawinds scatterometer and Terra and Aqua MODIS spectroradiometer. All these sensors are characterized by a wide swath and possess by improved spatial resolution and/or have additional spectral channels compare to such sensors as the SSM/I, AMSU, AVHRR, etc. Availability of high- and medium-resolution satellite data such as provided by SAR and MODIS in combination with the present models [5, 6] can improve the understanding and the predictability of MCs. The purpose of this paper is to apply multi-spectral satellite data to the study of MCs and their surrounding environment from active and passive microwave (Envisat ASAR, Aqua AMSR-E and QuikSCAT SeaWinds) and visible/infrared (MODIS) satellite data. 2. DATA The Envisat ASAR images were downloaded from an ESA web site. They represent the so-called quick look (QL) images and have the strongly reduced spatial and radiometric resolution. In spite of this they are valuable source of information on the features of spatial organization of the cyclones, the location of their centres, the areas of high surface winds, wind direction, the presence of sea ice. The Envisat ASAR precision processed images (PRI) have a spatial resolution of 50 m x 50 m in wide swath mode that allowed studying the fine details of radar signatures. The ASAR images were acquired in and cover the Japan, Okhotsk and Bering seas and the northwest Pacific Ocean between N and 0 80 E. More than 2000 QL and 280 PRI images are now in our database. SAR images of the MCs in different stage of development were selected for the study. Proc. Envisat Symposium 2007, Montreux, Switzerland 2 27 April 2007 (ESA SP-66, July 2007)

2 The SAR images were compared with the microwave brightness temperatures measured by Aqua AMSR-E and ADEOS-II AMSR provided by EORC JAXA, QuikSCAT-derived wind speed fields downloaded from website as well as with the Aqua and Terra MODIS visible and thermal infrared (IR) images downloaded from website and the NOAA AVHRR images available from several web sites. Satellite images were compared also with the surface analysis maps and ice condition maps of the Japan Meteorological Agency. Detailed quantitative analysis of structure and parameters of MCs was carried out for several cases.. MESOSCALE CYCLONES.. Bering Sea 5 January Small mesoscale cyclone in the southwestern Bering Sea, which was formed in a cold dry air in the rear of another deep cyclone, was detected by Aqua MODIS, Envisat ASAR and QuikSCAT SeaWinds on 5 January 2005 (Fig. ). AS opposite to the deeper cyclone, it was not revealed in pressure field on the surface analysis map of the Japan Meteorological Agency (JMA) (not shown). Cyclone is clearly visible as the spiral-like structure just off the coast. Two Commander Islands are seen at the bottom of the images. A cloudless area in the center of the cyclone is its eye (Fig. a) where weak winds are observed (Fig. b). Open and closed mesoscale convective cells (MCC) typical for cold air outbreaks are clearly seen to the east of bright convective band both on the daytime (Fig. a) and nighttime (Fig. c) MODIS images taken with the time difference of.5 hrs. Circular and crescent imprints of the MCC in the sea surface roughness [6] are visible in the lower right of the ASAR image (Fig. b). Several curvilinear features that spiral in towards the eye as well as the small-scale MCC are distinguished to its southwest and south on all images (Fig. ). The cloudless center, convective spiral band, which extends far to the south, the mesoscale linear and cellular features to the east of the band are reliably detected on the AMSR-E 89-GHz image at horizontal polarization having a spatial resolution of 5 km x 5 km (Fig. d). Variations of the brightness temperature T B (89H) in the cyclone s area exceed 60 K. From a comparison of MODIS infrared and AMSR-E T B (89H) images acquired simultaneously it follows that the maximum T B (89H) values (> 20 K) are observed in the convective bands near the cyclone s center. The lowest T B (89H) values (<70 K) were measured over cloudless area to the south of the center. The low is also detected as cyclonic structure in AMSR- E retrievals showing a distinct mesoscale signal in the fields of total cloud liquid water content Q and total water vapor content V (Fig. 2). Fields of Q and V were derived by application of algorithms [7] to the measured brightness temperatures at 2.8 and 6.5 GHz at vertical polarization. Spiral structure is better expressed at 00:55 UTC (Fig. 2a,b) than at 4:25 UTC (Fig. 2c,d) that very likely reflects dissipation of the cyclone. Analysis of the brightness temperatures at all AMSR-E channels and the results of modeling allows to conclude that probability of precipitation is high in the spiral bands near the center where Q > 0.5 kg/m 2. Mesoscale organized convection manifests itself in mesoscale variations of Q and V (Fig. 2c,d). (a) (b) (c) (d) Figure. Mesoscale cyclone in the southwestern Bering Sea on 5 January 2005: Aqua MODIS images: (a) at 00:55 UTC and (c) at 4:25 UTC; (b) Envisat ASAR image at 0:2 UTC, and (d) 89-GHz, H-pol Aqua AMSR-E brightness temperature.

3 (a) (b) (c) (d) Figure 2. Fields of total cloud liquid water content (a), (c) and total water vapor content (b), (d) derived from Aqua AMSR-E measurements taken on 5 January 2005 at 00:55 UTC (a) and (b) and at 4:55 UTC (c) and (d). 2 February Cold air advection on the warmer sea surface in combination with influence of orography were responsible for intensive convective activity to the east of Kamchatka Peninsula on 2-22 February that was registered by Envisat ASAR and other satellite sensors (Fig. ). Envisat ASAR image acquired on 2 February at 0:4 UTC depicts the well-developed mesoscale cyclone in the southwestern Bering Sea (Fig. a). Kamchatka Peninsula, Karaginsky Island and Commander Islands 2 are visible both on ASAR and on Aqua MODIS visible images. In spite of the time difference of 9 hrs, a good agreement is observed between cloud field and the ASAR signatures, which are determined mainly by the sea surface wind field. It may be concluded that synoptic situation changed slowly during this period. It follows also from the surface analysis maps of the JMA for 00:00 and 2 UTC on 2 February. Kamchatka (a) (b) Kamchatka (c) Kamchatka Figure. Mesoscale cyclone in the southwestern Bering Sea on 2 February 2004: (a) Envisat ASAR image at 0:2 UTC; (b) Aqua MODIS visible image at 0:5 UTC and (c) QuikSCAT- derived wind field at 09:09 UTC.

4 The most striking features of the ASAR image are associated with the polar low and with the sharp frontal boundaries 4 and 5 of the second mesoscale cyclone. Its center 6 was located to the south from the first one (Fig. b) and to the west of ASAR swath. The second cyclone with the minimum central pressure 990 mb was shown on the weather maps of the JMA as opposite to the first one. The ASAR image illustrates, in detail, the spiral-form structure of the surface wind field around the eye of the low, wave-like features of the frontal boundaries, the zones with the increased wind speed and other features, which were revealed partially earlier [, 4]. The surface wind convergence zones are characterized by sharp wind field gradients across the frontal boundary that converges toward the eye. QuikSCAT-derived wind field was obtained at 09:09 UTC (Fig. c). The centers of both cyclones and the sharp frontal boundaries are clearly distinguished in Fig. c and correspond to the brightness field of ASAR image..2. Okhotsk Sea January The most remarkable features visible on the Envisat ASAR image and shown in Fig. 4c 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 images. Aqua MODIS visible image taken at 02:25 UTC i.e. about 9 hrs before the Envisat data acquisition is given in Fig. 4b. 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. 4a) and after (at 6:5 UTC, Fig. 4d) Envisat ASAR acquisition. Cyclonic circulation was also registered by QuikSCAT scatterometer (Fig. 4e). Maximum wind speed (2-5 m/s) was measured to the south of the large low. This is consistent with radar backscatter (brightness) variations on ASAR image. (a) (b) (c) (d) On the surface analysis map of the JMA for January at 00:00 UTC the northern vortex the size of 00 km was outlined by 004-mb isobar and 2 h later it was outlined by 008-mb isobar. The southern eddy the size of 20 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 MCC and rolls. They manifest themselves both in cloud field (Fig. 4b) and in field of the sea surface roughness (Fig. 4c). Eddies 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 January 2004 at :2 UTC, eddies 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 T B (ν) images depicted in Figs. a,b,d. The centers of the eddy spirals looks 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 -2 km. Very likely that the highest surface winds coincide with the convective cloud bands on MERIS 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 5 m/s in a circle around the center and in the area south of it. (e) Figure 4. Convective vortices and MCC over the Okhotsk Sea to the west of Kamchatka on January GHz, H-pol Aqua AMSR-E brightness temperature at 02:25 (a) and at 6:5 UTC (d); Aqua MODIS visible image at 02:25 (b), Envisat ASAR image at : UTC (c) and QuikSCAT-derived wind field at 08:7 UTC (e).

5 February Mesoscale cyclone is clearly visible as the comma like structure just off the marginal ice zone (MIZ) in the northern Okhotsk Sea (Fig. 5). Head of the comma is characterized by the increased brightness (wind speed). A region of low radar backscatter that covers the center of circulation and the surrounding zone is due to weak wind speeds. The wave-like features with the wavelength of approximately 20 km are observed along the whole frontal boundaries dividing the areas with the strong and weal winds. Linear structures with the alternating brightness cross the area of strong winds from the frontal boundary till the MIZ. The wave-like features are detected on Aqua MODIS infrared image acquired at 6:55 UTC as opposite to the linear bands, which are visible only partially (Fig. 5b). It is due to insufficient air humidity that is confirmed by analysis of Q and V fields (Fig. 5c,d) retrieved from AMSR-E data. The maximum Q-values reach 0. kg/m 2 and the typical V- values near the MIZ and to the south of the frontal boundary are 4-5 kg/m 2. (a) (b) (c) (d) Figure 5. Mesocyclone over the Okhotsk Sea on February 2007: (a) Envisat ASAR image at :40 UTC, (b) Aqua MODIS infrared image at 6:55 UTC, and Aqua AMSR-E-derived fields of total cloud liquid water content in kg/m 2 (c) and total atmospheric water vapor content in kg/m 2 (d) at 6:55 UTC. 4. DISCUSSION AND CONCLUSIONS The clearly surface manifestations and accurate location of the mesoscale cyclones, frontal boundaries, wavelike disturbances on the fronts, mesoscale convective cells and rolls, etc. were found on many Envisat ASAR images of the northwest Pacific Ocean. Surface imprints of the atmospheric features can indicate wind direction needed for wind speed retrieval with C-band SCAT model CMOD4 or CMOD5. AMSR-E retrievals showed the distinct features in the fields of total cloud liquid water content and total atmospheric water vapour content correlated with radar signatures. The MC areas were characterized by appearance of water clouds with low and moderate Q values kg/m 2 and by the higher V values against the surrounding areas. The frontal structure of the most intense MCs was depicted by position of maximum Q values near frontal boundaries and high V gradients. Remarkable degree of similarity between ASAR, visible/infrared and 89-GHz brightness temperatures was found. Individual ASAR images and time series of visible/infrared images, microwave radiometer and scatterometer measurements allowed to study the life circle of the most intense MCs. Incorporating data from multiple satellite platforms improves the temporal resolution through employment of a greater number of satellites and increases the amount and kind of information collected. However, the detection of severe or violent MCs depends mainly on radar images and their quantitative estimates. There are many unresolved problems associated with MCs in spite of intensive study, special experiments both at northern and southern latitudes and development of cloud resolving models simulating 'real world' MCs. The detailed mesoscale and submesoscale variations of oceanic and atmospheric parameters in the area of MCs are the subject to further careful and rigorous investigation. 5. ACKNOWLEDGMENTS This study has been carried out within ESA ERS project AO-40, Envisat project AO-ID-9 and ESA-IAF

6 Bear project AO This work is partly supported by RFBR Projects: а and р_vostok_а, and Russian State project: Investigation of ocean-atmosphere system with passive and active microwave sensing from new generation of satellites. Aqua AMSR-E measurements were provided the Japan Aerospace Exploration Agency (JAXA) within the cooperation between the JAXA and the POI FEB RAS (project AD2M-RA-UF002). 6. REFERENCES. The European Polar Low Working Group GHeineman/ eplweg/eplwgop.htp 2. Orlanski, I. (975). A rational subdivision of scales for atmospheric processes. Bull. Amer. Meteor. Soc., 56, (5), Mitnik, L.M., Hsu M.-K. & Mitnik M.L. (996). Sharp gradients and mesoscale organized structures in sea surface wind field in the regions of polar low formation. The Global Atmosphere and Ocean System, 4, (4), Chunchuzov, I., Vachon, P.W., & Ramsay, B. (2000). Detection and characterization of polar mesoscale cyclones in RADARSAT Synthetic Aperture Radar images of the Labrador Sea, Canadian J. Remote Sensing, 26, Kawanishi, T., Sezai, T, Ito, Y. et al., (200).The Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E), NASDA s contribution to the EOS for Global Energy and Water Cycle Studies. IEEE Trans. Geoscience and Remote Sensing. 4, (), Ninomiya, K., Nishimura, T., Suzuki, T., & Matsumura, S. (200). Polar Low Genesis over the East Coast of the Asian Continent Simulated in an AGCM. J. Meteorol. Soc. Japan. 8 (4), Fu, G., Guo, J. & Zhang, M. (2004). High-Resolution simulation and analysis of the mature structure of a polar low over the Sea of Japan on 2 January 997. Adv. Atmos. Sciences, 2, (4), Mitnik, L.M. (992). Mesoscale coherent structures in the surface wind field during cold air outbreaks over the Far Eastern seas from the satellite side looking radar. La mer, 0, Vachon, P.W., Adlakha, P., Edel, H., Henschel, M., Ramsay B., Flett, D., Rey, M., Staples, G., & Thomas, S. (2000). Canadian progress toward marine and coastal applications of Synthetic Aperture Radar. Johns Hopkins APL Technical Digest, 2, (), Mitnik, L.M. & Mitnik, M.L. (200). Retrieval of atmospheric and ocean surface parameters from ADEOS-II Advanced Microwave Scanning Radiometer (AMSR) data: Comparison of errors of global and regional algorithms. Radio Sciences, 8, (4), 8065, doi: 0.029/2002RS