Satellites and sensors

The potential of satellite retrievals for PL studies was demonstrated in many papers (Claud et al. 1993, 2004; Carleton et al. 1995; Heinemann 1995; Heinemann and Claud 1997; McMurdie et al. 1997; Mitnik et al. 1996, 2004, 2007; Moore and Vonder Haar 2003; Rasmussen and Turner 2003). Registration of warm core in polar lows from the measurements of brightness temperature at frequency 53.6 GHz by the Advanced Microwave Sounding Unit on a board of NOAA-15 (Moore and Vonder Haar 2003) and obtaining detailed distribution of sea surface wind from the X-band Real Aperture Radar (RAR) images acquired from Okean series satellites (Kalmykov 1996) were especially important to understand better PL structure and development. Strong correlation of the surface wind and cloudiness fields was revealed by joint analysis of RAR and infrared images of the mesoscale cyclones and cold air outbreaks with resolution of 1-2 km (Mitnik 1990; Mitnik et al. 1996).

Appearance of new passive and active microwave and optical sensors installed on recently launched satellites such as QuikSCAT, Terra, Envisat, Aqua and ADEOS-II stimulated their application for detailed study of structure and evolution of MCs (Chunchuzov et al. 2000; Gurvich et al. 2008; Mitnik et al. 2004; Sikora et al. 2000; Vachon et al. 2000; Young et al. 2005). The West Bering Sea and the Okhotsk Sea, for which information on mesoscale marine weather systems is very limited, were chosen for the case studies of MCs taking into account the future climatological compilation. In this study, a combination of the following three microwave sensors is used: the Advanced Microwave Scanning Radiometer (AMSR-E) of the Aqua satellite, the Advanced Synthetic Aperture Radar (ASAR) of Envisat satellite and the SeaWinds scatterometer of QuikSCAT. 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 a (200-2,000 km) and meso p (20-200 km) scales (Orlanski 1975). Visible and infrared (IR) images were obtained from the Terra and Aqua spectroradiometer MODIS. All these sensors possess by the higher spatial resolution and the presence of additional spectral channels compare to similar instruments launched earlier such as SSM/I, ERS scatterometer, AVHRR. Due to the small-scale characteristics of polar lows, the reduction in spatial truncation errors provided by increased resolution is of primary importance. Optical images give an insight into condition on the upper boundary of the marine boundary layer of the atmosphere whereas the radar images - on its lower boundary. Characteristics of the whole layer can provide passive microwave measurements. This acquires distinctive importance in relation to the progress in high resolution simulation of the structure of mesoscale atmospheric vortices restoring fields of total water vapor content, snow content, liquid water content (Fu et al. 2004b).

The Aqua AMSR-E measures the brightness temperatures TBs at six frequencies v = 6.925, 10.65, 18.7, 23.8, 36.5 and 89.0 GHz with vertical (V) and horizontal (H) polarization. Instant field of view (IFOV) increases with the decrease of frequency from approximately 3.5 x 6 km (cross track x along track) at 89.0 GHz to approximately 43 x 75 km at 6.925 GHz. AMSR-E antenna beam scans within swath width of 1,450 km, incidence angle is 55°. Sensitivity of microwave channels is -0.34 K at 6.925 GHz, -1.2-1.4 K at v = 89.0 GHz and 0.6-0.7 K at other frequencies (Kawanishi et al. 2003).

AMSR-E measurements allow to visualize the structure of cloud liquid water content, precipitation, and surface winds, to reveal ice clouds, to retrieve fields of the total water vapor content V, total cloud liquid water content Q and sea surface wind W, to assist with the detection of developing mesoscale cyclones circulation centers, etc. Retrieval algorithms used for MC study were developed using simulated brightness temperatures (see below).

Envisat ASAR operates at C-band (wavelength is 5.6 cm). Wide swath mode regime (swath width is 405 km) is optimal for MC study. Spatial resolution of ASAR data is 75 x 75 m (Envisat Special Issue 2001). ASAR images map accurately the location of atmospheric fronts, the cyclone centers, reveal the fine-scale and mesoscale features of sea surface wind field in various weather systems, in the areas of organized roll and cell convection including regions where this convection does not expressed in cloud field.

QuikSCAT scatterometer provides indirect measurements of winds at the sea surface and is a powerful tool to assist in both the detection and intensity of polar low circulations. QuikSCAT is capable of measuring wind speeds between 0 and 20 m/s with a 2 m/s accuracy, and has a directional accuracy of about 20°. One thousand and eight hundred kilometer swath during each orbit provides «90% coverage of Earth's oceans every day. QuikSCAT-derived wind fields can be downloaded with a resolution of 25 x 25 or 12.5 x 12.5 km from http://manati. orbit.nesdis.noaa.gov/quikscat/. Polar areas are covered completely.

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