GENERAL ATMOSPHERIC CIRCULATION IN THE ARABIAN PENINSULA

Get Complete Project Material File(s) Now! »

GENERAL

Typically, fog at Abu Dhabi forms at night in a clear sky with a weak pressure gradient, light winds and sufficient moisture, that is, radiation fog, although a preceding advection process plays a supporting role by bringing moisture from the sea. A secondary observed form of advection is that the fog often appears to form over the southern inland part of the airport and drift, or expand, coastward on a light southerly wind off the desert.
Seasonally, the highest frequency of fog is in the autumn months of September and October and the least in summer. It tends to form earlier in the evening during the winter months and last longer after sunrise than in the summer. The 10 year study of the winter months of January and February revealed that fog occurred between 1800 to 0700 UTC and most frequently between 2200 to 0500 UTC. In summer (July and August) fog occurred between 2200 and 0400 UTC and most of it between 0000 and 0300 UTC. More often than not the fog clears within about 2 hours after sunrise. The earliest fog occurred during the study period from 1983 to 2002 was 1800 UTC and the latest was 0700 UTC.
Inspection of the fog events for the ten-year period from 1993 to 2002 revealed that there is only an 18% chance of fog on two consecutive days, while the risk of fog three days in a row is 6% and 2% and 1% for four and five days, respectively. These figures emphasise that one must not go the persistent forecasting fog. Consideration must be given to changing circumstances and conditins. A case in point is the non-occurrence of fog on the 6th October 2003.
Although inspection of the 850 hPa wet bulb potential temperature leads one to believe that there is no change of air mass during fog and no fog events, developing synoptic processes and smaller scale boundary layer influences must be taken into consideration.

PRESSURE AND PRESSURE PATTERNS

Fog forms in a weak pressure gradient across the UAE, usually 1 hPa, or less, particularly after persistent Shamal winds, or sea breezes, with attendant high humidity. Fog does not occur when a surface low pressure cell is situated to the west of the UAE, or over the western part of the UAE. It produces a dry offshore southerly flow that strengthens the early morning land breeze and delays the onset of the sea breeze until late in the afternoon and can prevent it from extending too far inland. The stronger and earlier beginning of the land breeze causes turbulent mixing of the surface air with drier air aloft, thereby prematurely curtailing th radiation cooling effect and prevents fog formation.
The presence of an anticyclone over southern Iran and the Strait of Hormuz also produces a southerly flow over the UAE and produces the same effect. When the wind remains from the north-west to north-easterly throughout the night, due to a surface low pressure cell to the south, or south-east, fog does not occur. The steady wind off the sea is also usually strong enough to maintain turbulent mixing, thereby preventing fog formation.
Air off the sea, being less influenced by nocturnal surface radiation cooling maintains warmer conditions than when it arrives from the cooler desert, with less likelihood of the air cooling to its condensation level. If the wind does swing to the south-east, due to the synoptic circulation, and increased radiation cooling over the land, it is usually too late to enable the air to cool to below the dew-point temperature.
Under Shamal conditions, such as when a surface low pressure cell is to the north-east of the UAE, fog does not form. The low level winds at night, which become light south-westerly to westerly and may even remain north-westerly, maintain warm Gulf air in circulation relative to the cooler overnight land air. Turbulent mixing of the warm and moist air from the Gulf Sea may cause a thin layer of stratocumulus, or stratus cloud, to form under a temperature inversion in the approximately 900 metres (3000 feet) atmospheric boundary layer, with sea haze moderately reducing the visibility.

TEMPERATURE

It was noted that fog forms when the air temperature falls to, or below, the maximum dew-point temperature that occurred in the previous late afternoon, or early evening. This is significant enough to be taken into consideration when determining the overnight minimum temperature. Fog does not always form immediately the air temperature has fallen to, or below, the maximum dew-point temperature earlier in the day. It can form up to a few hours later. This is not an absolutely reliable indicator, as the air temperature can fall below this level with no fog formation.

READ Nonlinear System Tdentification

HUMIDITY

As a rule the chance of fog later in the night is very good if the surface humidity exceeds the specific hourly values during the preceding evening, as indicated in figure 4.1. However, it is by no means a rigid rule, as it can be lower, particularly during the early evening. Certainly if the surface relative humidity is expected to reach, or exceed, 95%, fog must be considered.

WIND

The 10 m wind often falls to 2 knots, or less, about an hour prior to the fog forming. Allowing for surface friction effect, it can be assumed that the wind at about 2 m decreases to less than this and possibly calm at this time, which is favourable for initial fog development. Fog is most likely (87% of the time) when the wind veers during the night from north-westerly through easterly to a calm or light east-south-easterly or south-easterly wind. It is most unlikely, but not impossible, when the wind backs to south-westerly to southerly.
Fog does not occur when Shamal conditions exist and the surface wind persists from the west through north-west to north-east during the night, or most of the night before possibly becoming a temporary light south-easterly land breeze around sunrise. From the ground up to the temperature inversion the wind tends to back from south-easterly to north-north-easterly in fog conditions, but this is not always true. As the case study for the 9th to the 11 January showed, the wind can remain easterly. A southerly wind brings dry desert air and no fog. The critical factor remains the strength of the wind, a wind of around 5 knots, or less, being the most productive.

CHAPTER 1 INTRODUCTION
1.1 General
1.2 Scope of the study
1.3 Methodology
1.4 Objectives of the research
1.5 Organisation of the document
CHAPTER 2 THE UNITED ARAB EMIRATES
2.1 General
2.2 Position
2.3 Topography
2.4 Airports
CHAPTER 3 GENERAL ATMOSPHERIC CIRCULATION IN THE ARABIAN PENINSULA
3.1 Introduction
3.2 Dominating weather systems
3.3 Mean circulation charts
3.4 January circulation
3.5 April circulation
3.6 July circulation
3.7 October circulation
3.8 Wind divergence at 200 hPa
3.9 Water vapour flux
3.10 Sea surface temperature
3.11 Summary
CHAPTER 4 FOG
4.1 Introduction
4.2 Scope of the study
4.3 Method
4.4 Fog producing processes
4.4.1 Fog types
4.4.2 Radiation fog formation process
4.5 Statistics
4.6 Study of fog events
4.6.1 Introduction
4.6.2 Four consecutive fog days in summer: 19th to 22nd July 2002
4.6.2.1 NWP model data
4.6.2.2 Surface observations
4.6.2.3 Atmospheric soundings
4.6.2.4 Summary
4.6.3 Three consecutive fog days in winter: 9th to 11th January 2003
4.6.3.1 NWP model data
4.6.3.2 Surface observations
4.6.3.3 Atmospheric soundings
4.6.3.4 Fog indices
4.6.3.5 Summary
4.7 Study of non-fog producing events
4.7.1 Introduction
4.7.2 Surface low pressure cell to the west: 13th June 2003
4.7.2.1 NWP model data
4.7.2.2 Surface observations
4.7.2.3 Atmospheric soundings
4.7.2.4 Summary
4.7.3 Surface anticyclone over southern Iran: 9th November 2003
4.7.4 Surface low pressure cell to the east: 31st August 2003
4.7.4.1 NWP model data
4.7.4.2 Surface observations
4.7.4.3 Atmospheric soundings
4.7.4.4 Summary
4.7.5 Surface low pressure cell to the north-east: 23rd October 2003
4.7.5.1 NWP model dat
4.7.5.2 Surface observations
4.7.5.3 Atmospheric soundings
4.7.5.4 Summary
4.7.6 Surface low pressure cell to the north-east: 6th October 2003
4.7.6.1 NWP model data
4.7.6.2 Surface observations
4.7.6.3 Atmospheric soundings
4.7.6.4 Summary
4.8 Results
4.8.1 General
4.8.2 Pressure and pressure patterns
4.8.3 Temperature
4.8.4 Humidity
4.8.5 Wind
4.8.6 Visibility
4.8.7 Atmospheric sounding
4.8.8 Fog Indices
4.8.9 NWP model data
4.9 Forecast checklist
CHAPTER 5 SHAMAL
5.1 Introduction
5.2 Scope of the study
5.3 Method
5.4 The winter and summer Shamal
5.5 Winter Shamal: 13th to 19th November 2003
5.5.1 Introduction
5.5.2 NWP model data
5.6 Summer Shamal: 28th April to 2nd May 2003
5.6.1 Introduction
5.6.2 Sequence of events and offshore conditions
5.6.3 NWP model data
5.6.4 Surface observations
5.6.5 Atmospheric soundings
5.7 Discussion
5.8 Summary
5.9 Forecast checklist
CHAPTER 6 DUST STORMS AND DUST
CHAPTER 7 LAND AND SEA BREEZES
CHAPTER 8 RAIN BEARING TROUGHS, THUNDERSTORMS AND TROPICAL DEPRESSIONS
CHAPTER 9 CONCLUSION
REFERENCES
APPENDIX A DEFINITIONS
APPENDIX B CONVERSIONS
APPENDIX C FOG INDICES