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مباحث عمومی هواشناسی

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Amir Mohsen

متخصص بخش هواشناسی
snow_sphinx.jpg
 

Dr.ben

کاربر ويژه
دکتر از سبزه میدون و توشیبا و فلکه گاز هم عکس بزار:خجالت2:

آخه امشب اون سمت نرفتم. فردا عکس می گیرم.
یه چیزی ذهنم رو مشغول کرده. شاید این آمارهای خیلی قدیمی در مورد بارش برف های سنگین و همیشگی مثل همین آمار دادن های من در مورد بارش برف در رشت بوده باشه. :ایده:
می گن "تاریخ را فاتحان می نویسن". :99:
 

Amir Mohsen

متخصص بخش هواشناسی

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Rare December Snow Blankets Jerusalem (PHOTOS) By Jon Erdman Published: Dec 13, 2013, 2:45 PM EST

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An ultra-Orthodox Jew poses for a picture with the Dome of the Rock and the Al-Aqsa mosque's minaret in the background in the old city of Jerusalem on December 12, 2013. (AHMAD GHARABLI/AFP/Getty Images)



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The heaviest December snowstorm in at least 50 years blanketed parts of Israel, including Jerusalem into Friday, cutting power, stranding travelers, and putting the Holy City on lockdown.
Israelis were told over media and public broadcasts on Friday not to enter or leave Jerusalem and some 1,500 people were evacuated from stranded vehicles overnight, said police spokesman Micky Rosenfeld.
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Three emergency centers were set up and medics treated 350 people for cold-related symptoms, Rosenfeld said. Jerusalem Mayor Nir Barkat said he asked the military for assistance. A light snow also fell throughout the West Bank, prompting officials to close schools and government offices for the day.
The weather even featured in talks between visiting U.S. Secretary of State John Kerry and Prime Minister Benjamin Netanyahu, who said he was briefed on the emergency measures.
Kerry, a former Massachusetts senator, said the snow made him feel "at home."
"I have heard of making guests welcome and feeling at home. This is about as far as I've ever seen anything go ... giving me a New England snowstorm," Kerry said as he viewed a snow-covered Old City of Jerusalem with Netanyahu.
In the West Bank and Gaza, U.N. relief teams offered emergency services to the worst-hit communities.
In Gaza, which was experiencing its first snow in a decade, more than 500 people were evacuated from their homes, according to Hamas spokesman Ihab Ghussein.
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Upper-level wind flow pattern over the Middle East on Dec. 12, 2013. An upper-level low pressure system over Turkey and the eastern Mediterranean interacted with sufficiently cold air to manufacture snow over parts of Israel and Syria.




The cold weather was part of a storm, dubbed Alexa, which has been pounding much of Lebanon and parts of northern Syria since Wednesday, pushing temperatures below zero and dumping snow and heavy rains.
(MORE: Syrian Refugees Hit Hard By Snowstorm)
Rare snow also fell in Cairo's suburbs, the port city of Alexandria and a blanket of white covered St. Catherine's Monastery on Mt. Sinai.
(MORE: Rare Snow Blankets Cairo Area)
The culprit for this snow is a slow-moving area of low pressure aloft over Turkey and the eastern Mediterranean Sea combining with cold air to wring out snow.
This sluggish upper-level low will take its time moving east, keeping the threat of additional snow in parts of Israel, the West Bank, southwest Syria, Lebanon ?and western Jordan through Saturday.

At just under 2,700 feet above sea level, snow is not uncommon in the winter in Jerusalem. On average, at least a few flakes fall in the Holy City 7-8 days each year.



 

Amir Mohsen

متخصص بخش هواشناسی
وقتی سرچ میکردم این موضوع جالب رو هم در مورد ساز و کار هسته های گرد و خاک در مرزهای جنوب غرب و غرب کشور پیدا کردم که خوندنش خالی از لطف نیست:



[h=2]Synoptically Forced Dust Storms
[h=2]Introduction In most areas, we can classify dust storms by the broad meteorological conditions that cause them. In this section, we will examine the most common events that occur in the Middle East. These are dust storms caused by prefrontal and postfrontal winds that primarily occur in winter, and summer dust storms caused by persistent northerlies.
pre-post-frontal-montage.jpg

Note that whenever dust is a forecast consideration in your area, you should become familiar with the local atmospheric conditions that lead to strong winds under dry conditions. Each region has its own weather patterns that lead to dust storms.


Top of page
[h=2]Prefrontal Events
[h=2]Prefrontal Dust Storms Prefrontal dust storms occur across Jordan, Israel, the northern Arabian Peninsula, Iraq, and western Iran as low-pressure areas move across the region. Antecedent factors include a band of winds generated by, and ahead of, the low-pressure area that presses against a stationary high-pressure center in Saudi Arabia or the western slopes of Iran’s Zagros Mountains.
prefrontal_map.jpg

This chart depicts prefrontal winds as a low-pressure area migrates into Iraq. The southeasterly or sharqi winds that blow northward up the Tigris/Euphrates River basin are intensified as low-level flow is funneled between the Zagros Mountains to the east and the pressure gradient to the west. Toward the west, southwesterly or suhaili winds pick up dust from western Arabia and move it northeast in advance of the cold front.
sharqiPlumes.jpg

This image shows sharqi prefrontal dust plumes emanating from dry lake beds and fluvial deposits in southeastern Iraq. (Fluvial deposits are associated with rivers and streams.)
prefrontal.gif

The polar jet stream behind the cold front and the subtropical jet stream in front of it often interact dynamically to strengthen the front east of the upper-level trough. The strengthened cold front induces stronger prefrontal winds out ahead of the upper-level trough. In addition, the overlapping of these jet cores and the coupling of secondary circulations in the right rear of the polar jet and the left front of the subtropical jet enhance mid-level upward vertical velocities and increase the lifting force for blowing dust.
Under these conditions, westerly winds mobilize dust and sand across Jordan, Syria, and northwestern Saudi Arabia, transporting it east and northeastward across the Arabian Peninsula, Iraq, and the Persian Gulf countries.


Top of page
[h=2]Prefrontal Example Let’s look at an example. A prefrontal dust storm occurred on 25 March 2003. Winds in front of a powerful Mediterranean cyclone whipped up a thick dust storm that significantly impacted movement on the ground.
25mar03_wxmap.jpg

This graphic shows a 24-hour prognostic weather map valid on 25 March at 12Z as the dust storm moved across Iraq. We see the convergence of the polar front jet and the subtropical jet, indicated by the red arrows. The shaded areas indicate where dense dust storms with visibilities of less than 1 km were predicted along the associated cold front.
SeaWiFS_prefrontal_dust_25mar03_annotated.jpg

This SeaWiFS true color image taken during the morning hours shows the extent of the unfolding dust event. Areas of prefrontal dust cover northeastern and eastern Saudi Arabia and extend into Iraq where cloud cover makes it difficult to observe its full extent from satellite. In addition, postfrontal dust is forming behind the advancing cold front from Egypt and Sudan northeastward into northern Saudi Arabia and northwestern Iraq.
upperleveltough_shearline_map.jpg

The upper-level trough moved east into the northern Arabian Peninsula, resulting in fairly cloud-free skies along the front from the Red Sea. From there to the west, the building high pressure to the north resulted in convergence (a shear line) that provided lift for blowing dust and sand across Sudan and equatorial Africa.
SeaWiFS_prefrontal_dust_26mar03_MidEast_annotated.jpg

Over the Red Sea itself, satellite imagery can be used to help identify the front as the associated low-level convergence and moisture typically lead to the formation of stratiform clouds. This SeaWiFS image taken on the morning of 26 March shows cloud cover along the cold front in the southern Red Sea extending northeast along the remainder of the frontal zone to the main area of low pressure.
 

Amir Mohsen

متخصص بخش هواشناسی
[h=2]Mid/Upper Levels A mid-level trough centered over Saudi Arabia and extending northward to the eastern Mediterranean Sea is associated with the surface trough over Iraq, Kuwait, and Saudi Arabia. Strong northwesterly flow exists on the backside of the trough with 80-kt winds being reported over Egypt (in the cyan area).
postfrontal_upperair_prog_12z_26feb10_circle.jpg

Deep vertical mixing is helpful in generating dust storms. Strongdownward vertical motion, which is likely to be associated with a middle- to upper-level front within the upper-level trough shown, does two things:

  1. It helps to prevent cloud formation or evaporate preexisting cloudiness. This increases solar warming in the lower troposphere and enables strong winds to mix into the low levels from above. This requires that middle and lower tropospheric lapse rates be unstable, which is more likely to occur when strong solar heating is present. Although subsidence itself can stabilize lapse rates, even the wintertime Middle Eastern sun can often offset this stabilizing effect.
  2. Strong winds dynamically accompany the downward intrusion of upper tropospheric air into the mid-levels and near the surface. This momentum and dry air then mix to the surface, leading to clearing conditions and little precipitation on the back side of a trough.


 

Amir Mohsen

متخصص بخش هواشناسی
[h=2]Summer Shamal
[h=2]Synoptic Pattern The summer shamal is a wind that blows with persistence over Iraq and the Persian Gulf from late May to early July. Compared to winter events, summer dust storms have greater vertical motion due to high temperatures and resultant convective currents.
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The summer shamal results from a characteristic synoptic pattern in which the most prominent features are:

  • A semi-permanent high-pressure cell extending from the eastern Mediterranean to northern Saudi Arabia
  • A low-pressure cell over Afghanistan
  • Thermal low pressure associated with the monsoon trough extending into southern Saudi Arabia
As the surface pressure analysis shows, the cyclonic circulation around the region of low pressure combines with the anti-cyclonic circulation around the high-pressure cell to increase the winds over the northern Persian Gulf region. These winds are normally confined from the surface up to 5,000 feet (1500 meters). The shamal is particularly strong at ground level during daytime but weakens at the surface overnight. Shamal events can be quite long-lived, lasting several weeks. These are known as the "40-day shamal."
modisDEP_dust_sfcWinds_07aug2004.jpg

This Meteosat Second Generation (MSG) infrared image overlaid with surface pressure for 7 August 2004 shows the thermal low over southern Afghanistan and northern Pakistan. Higher surface pressure is located over the eastern Mediterranean, with lower surface pressure in the northern Persian Gulf. This produces strong northerly winds in Afghanistan and Iraq.
coamps_12hr_500hPa_heights_07aug04.jpg

This NRL MODIS DEP overlaid with surface winds confirms the presence of strong surface northerly winds, with a mesoscale dust storm taking place over western and southern Afghanistan. Dust plumes are evident in Central Iraq.
Upper-level high pressure is often found over the Saudi Peninsula during the summer shamal. The location of the center of the high can vary, as we see in the MSG infrared image and the 500-mb height composite for 7 August 2004.


 

Amir Mohsen

متخصص بخش هواشناسی
[h=2]Red Sea Dust Storm This SeaWiFS true color image shows a dust storm event that occurred around the Red Sea in July 1999.
tokar_seawifs.jpg

The large thermal contrast between the interior of Sudan and the Red Sea resulted in a strengthened pressure gradient that helped generate the dust storm. The lower terrain of the Tokar Gap provided a path for the dust to move over the Red Sea.
tokar_topo.gif

(The Tokar Gap is a low-elevation break in the mountains that flank the west side of the Red Sea.)
The dense plume of dust entering the Red Sea disperses and casts a pall over the area. The mountains to the east appear to block and turn the winds southeastward.
The Red Sea Convergence Zone also helps to keep dust trapped in the center of the Sea. The zone is formed by air flowing in from the north and south, which creates an area of convergence that traps the transported dust.
Accurately forecasting gap flows generally requires a mesoscale model with several grid cells inside the gap. Since the Tokar Gap is approximately 68 miles (110 kilometers) wide, high-resolution mesoscale models should sufficiently capture the flow.
[Note: For more information on gap winds, see COMET's Gap Winds module.]


 

Amir Mohsen

متخصص بخش هواشناسی
[h=2]Dust Devils
iraqdustdevil.jpg

Dust devils are a common wind phenomenon that occur throughout much of the world. These dust-filled vortices are created by strong surface heating and are generally smaller and less intense than tornados. Their diameters typically range from 10 to 300 feet (3 to 90 m), with an average height of approximately 500 to 1,000 feet (150 to 300 m). Dust devils typically last only a few minutes before dissipating. However, when conditions are optimal, they can persist for an hour or more. Wind speeds in larger dust devils can reach 60 mph or greater.

Click to view animation.

Dust devils form in areas of strong surface heating. This typically occurs under clear skies and light winds when the sun can warm the air near the ground to temperatures well above those just above the surface layer.
Once the ground heats up enough, a localized pocket of air will quickly rise through the cooler air above it. Hot air rushes in to replace the rising air at the bottom of the developing vortex, intensifying the spinning effect. Once formed, the dust devil is a funnel-like chimney through which hot air moves both upwardly and circularly. If a steady supply of warm, unstable air is available, the dust devil will continue to move across the ground. However, once that supply is depleted or the balance is broken in some other way, the dust devil will break down and dissipate.
large_dust_devil.jpg

Dust devils can vary greatly in size, both in diameter and vertical extent. Notice how aggressive the interaction with the surface can be.


 

Amir Mohsen

متخصص بخش هواشناسی
[h=2]Aerosol Optical Depth
modisVIS_3Mar2010.jpg

Now we'll examine a product that's based on the solar channels from MODIS midday data. The visible image is hard to interpret…
nrl_aod_middle_east_03mar2010.jpg

…but the aerosol optical depth (AOD) product shows color-coded optical depth (a strong indicator of dust) over the region.
AOD is a unitless measure of the amount of light that airborne particles, such as dust, smoke, haze, and pollution, prevent from passing through a column of atmosphere. AOD does not translate directly into surface visibility estimates because the location of the dust in the vertical is not known: it could be mostly aloft or near the surface. However, AOD serves as a first-order indicator of how dusty the atmosphere is. It is increasingly being assimilated into numerical dust forecast models and forecasters are using it as a nowcast tool to help quantify dust information near and over deserts.
In this example, we see a gathering dust storm over Syria in shades of orange, indicating an optical depth of up to about 1.5, which would likely impact visibility.
nrl_aod_middle_east_04mar2010.jpg

About 24 hours later, the dust storm has spread into the Persian Gulf. There are large values of AOD over Saudi Arabia, Iraq, and Kuwait, and lower values elsewhere.


 

Amir Mohsen

متخصص بخش هواشناسی
[h=2]Holy Land blanketed in snow after intense winter storm
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Dusan Vranic / AP
The Western Wall and the Dome of the Rock are covered in snow in Jerusalem on Dec. 13, 2013. Early snow has surprised many Israelis and Palestinians as a blustery storm, dubbed Alexa, brought gusty winds, torrential rains and heavy snowfall to parts of the Middle East.


By Stephen Kalin, Reuters
A snowstorm of rare intensity has blanketed the Jerusalem area and parts of the occupied West Bank, choking off the city and stranding hundreds in vehicles on impassable roads.
Israeli authorities said at least 20 inches of snow had fallen since Thursday and more was forecast.
"In my 54 years I don't remember a sight like this, such an amount I cannot recall," said Nir Barkat, mayor of Jerusalem.
The Israeli military helped police rescue hundreds of people stranded in vehicles on highways near the city. Read the full story.

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Darren Whiteside / Reuters
An Ultra-Orthodox Jewish man prays at the Western Wall on Dec. 13.


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Uriel Sinai / Getty Images
Ultra-Orthodox Jews enjoy the snow in the Mea Shearim neighborhood of Jerusalem's old city on Dec. 12.


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Alaa Badarneh / EPA
Palestinians enjoy a snowball fight during snowy weather in the West Bank city of Nablus on Dec. 12. The Palestinian Authority announced that schools would be closed due to adverse weather conditions.


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Ahmad Gharabli / AFP - Getty Images
Vehicles are seen stranded in snow in Jerusalem on Dec. 13. The hilltop city of Jerusalem was paralyzed by its fiercest snowstorm in years, with its mayor calling out the army to help stranded motorists.


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Abir Sultan / EPA
A young Ultra-Orthodox Jewish child plays next to a snowman in the Mea Shaarim neighborhood of Jerusalem on Dec. 12.


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Brian Snyder / AP
A woman swims in the pool at the David Citadel Hotel during a snowstorm in Jerusalem on Dec. 13.


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Alaa Badarneh / EPA
Israeli soldiers arrest two Palestinian boys near Nablus in the West Bank on Dec. 12. There were clashes after Palestinians reportedly tried to reach the al-Tour area, near the Jewish settlement of Brakha, for a snowball fight.

 
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