Identification and formation mechanism analysis of spatial pattern snowfall in central plain of guilan (delta snow) by using weather and research forecast(WRF) model

To monitoring the spatial snow cover, by using MODIS images (Aqua and Terra satellites), during the cold season (October to March) in the years 2005 to 2012 (9 years). These images, with daily intervals and spatial resolution of 250 meter were studied for 9-year. Snow surface zones in the plains of Guilan observed special triangular pattern that approximately matches the Sefidrud River Delta. To identify snow-covered areas, the snow area index (NDSI) was applied. Due to low reflectance of snow in the infrared bands and high reflectance in the visible bands, this indicator can be very useful in detecting snow cover from other phenomena. Using GIS software and algorithms to detect snow Guilan plain snow zones were identified. Results were extracted from the digital map. Then, this layer over layed on digital elevation map(DEM) of the area; the spatial pattern of snow area was prepared.
Data and To investigate the mechanism of pattern formation in Delta Snow, 6 events which lead the snowfall in delta form were selected, in the plains of Guilan during 8 year, was selected. Then, Daily and 6-hour maps of pressure, temperature, geopotential height, vertical velocity, zonal and meridional wind components fields from NCEP/NCAR data over a region consist of Iran with 2.5º×2.5 º horizontal resolutions were analyzed. To avoid prolongation of paper, system in March 2012 and the results obtained are presented in detail.
To evaluate reason of this process and simulate more details on a smaller scale ,weather and research forecast(WRF) model was used. model on Guillan, with three horizontal resolutions of 27, 9 and 3 km and the three-hour time step, was run. 10-meter wind, 2-meter temperature, relative humidity and cross section of dynamic parameters such as relative vorticity and vertical velocity were investigated. Study of the synoptic structure of this systems leads to the formation of this phenomenon, show the origin of this system is the anticyclone (high) with a central pressure greater than 1035 that is formed on west of Europe and East of Atlantic ocean (Azores high pressure) and With the move to the East and are expanded over the Middle Caspian. Anticyclone has spread to the southern shores of the Caspian and clockwise circulation in the lower troposphere cause flows north and northwest, from the Caspian Sea towards the coast. Surface cyclone in the center of Iran and anticyclone in north of Iran, increasing pressure gradient and wind speed on the southwest coast of the Caspian Sea. In the early hours of arrival system to Iran, in 850 hPa, ridge on the southern latitude and trough on the northern latitude (Parts of northern Russia) and anti-clockwise circulation due to the trough on the northwest of Iran, South and southwest cold flow, moves from the Iranian plateau to higher latitudes.
Based on WRF model output, 2-meter temperature and 10-meter wind pattern, shows forming a strong north and northwest flow in the southwestern margin of the Caspian Sea that correspond to the surface maps. South of the Alborz mountain range in height from sea level to adapt to the higher level of 900 hPa, therfore trough and south and southwestern currents is overcome, but Manjil gap is only natural passage and cross region between the northern and southern currents of Guilan plains and southern Alborz. wind Cross section shows that above latitude 37º, in the lower levels of the troposphere, according to surface anticyclone dominance and anti-clockwise circulation of high-pressure, as north and northwest currents are observed mostly. But, in the middle and upper troposphere, the western wave and trough, wind direction along the Manjil meridian is southwest. In other words, the vertical wind shear and instability in the layer between 700 and 800 hPa levels occur. Furthermore, Manjil gap(latitude 36/7),is place that horizontal wind shear between the two sides of the Alborz Mountains were seen. 2-meter temperature pattern and cross sections of temperature, temperature gradient (temperature Changes from 5 to 8 degrees Celsius in the aerial distance less than 20 km) between the southern high lands of Alborz (lower temperature) and plains of Guilan (higher temperature) clearly shows. Before the establishment of the high-pressure core over the Caspian Sea, tropospheric cooling of the lower layers is not enough for snowfall in the plains of Guilan. Cold air, due to crossing thermal trough of middle levels troposphere from the Iranian plateau and Alborz Mountain (have greater height and lower surface temperature compared with Delta Sefidrud) makes the cold currents of the Iranian plateau in the White River Valley prevail and happen snowfall in the plains of Guilan. Based on the model output at 3 km spatial scale, cross section and horizontal wind shear and substantially horizontal temperature gradient shows formation of local front according to Manjil gap. Regardless of the physical and dynamics mechanisms of snowfall, the topography of the region and Manjil gap in the Alborz Mountains is the only way to Influence of the cold plateau air to Guilan plain that corresponded with Delta pattern formation. Following the establishment of a cold anticyclone over central Caspian and strengthen cooling the lower of tropospheric layers over central Caspian, temperatures on the plains of Guilan and the temperature contrast between the Guilan plain and the southern Alborz, (Qazvin plain) decreases. Furthermore, located southwestern margin of the Caspian Sea coast in the Western branch 850 hPa trough, that accompanied with strong northern currents along the western and central Alborz, lead to dominance the northern winds to the south of the Alborz and spread to 700 hPa. Cross section of wind has also confirmed that under these conditions increases instability and vertical wind shear.