World Wind Regimes - Gulf of Aden Tutorial

	NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil

World Wind Regimes - Gulf of Aden Tutorial

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Introduction

Use of SSM/I Water Vapor Data in Deducing Wind Direction

When wind flows over a hilly or mountainous barrier, vertical perturbations of the flow are induced, which affect the moisture content of the air. In a stable stratification, standing rows of wave clouds with alternating moist and dry areas are often produced downstream from the barrier. Under less stable conditions, especially over a mountain range having a gentle windward slope and a steep lee slope, vertically propagating mountain waves often result. Such waves are typified by a strong downdraft in the immediate lee of the barrier, followed by sharp upward vertical motion a little further downstream. The tendency is to produce very dry conditions at low levels in the immediate lee of the barrier. If moist conditions exist aloft, a cloud plume is produced further downstream, indicating the presence of a vertically propagating wave cloud formation.

The SSM/I water vapor algorithm produces a product (total integrated water vapor) or IWV that is most responsive to moisture content in the lower atmosphere, from the surface to 700 mb. The effect of a strong downdraft in the lee of the mountains is to reduce the measured IWV in this region through two mechanisms. First, the water vapor in the descending air spreads laterally as higher pressure is encountered reducing the thickness of the descending layer; total moisture, however, is conserved. And second, some entrainment of dryer air from aloft into the boundary layer can be expected to occur due to a number of mixing processes, including increased vertical shear in the mountain's lee, turbulent entrainment due to breaking gravity waves etc. The effect of these processes is to reduce the water vapor content per unit volume in the very layers where the SSM/I IWV signal response is maximized. The dry lee signal that results is useful not only in indicating direction of flow over the area, but also in revealing the presence of severe downdraft effects in that region.

Knowledge of such conditions is important for many military applications affecting flight operations in the local area. As an example, many unsuspecting aircraft flying at an altitude thought to enable safe passage over underlying mountainous terrain have been forced down, causing them to crash into the ground because of the severity of such downdraft conditions. The drying effect also provides the forecaster a "heads up" insight into possible anomalous radio and radar propagation effects that might occur when such unusual changes in low level moisture distribution occur.

Examples

Fig . 1. SSM/I Wind Product 28 May 2001 0507Z	Fig. 2. Topographical map of the Northern Arabian Sea region

Fig. 1 is an SSM/I (Special Sensor Microwave Imager) depiction of estimated surface-level wind speed in the region off the northeast tip of Somalia, south of the Saudi Arabian Peninsula. Immediately noticeable is the reduced wind speed indication north of the island of Socotra. The blocking effect of island barriers, leading to reduced wind speeds and reduced sea state in the island's lee is a well-known phenomenon, documented in several studies (Fett and Rabe, 1976; Fett and Burke, 1981). Also of interest is the reduced wind speed indication in the Gulf of Aden, adjacent to the northeast tip of Somalia. A range of coastal mountains exists in that region, as shown in Fig. 2. Terrain elevation ranges from a maximum of 1400 m near the northeast tip of Somalia to 2048 m near the central coastal portion of the Gulf of Aden. A similar blocking effect is probable, leading to the low wind speeds in the Gulf of Aden, suggesting that southerly flow extends continuously from the northeast portion of Somalia to Socotra. Deprived of other observations, these SSM/I wind data provide valuable indications, not only of wind speed in this region, but also, indirectly, of wind direction. Fig. 2 shows the topography of land areas associated with the Fig. 1 SSM/I image. It can be seen that a range of coastal mountains borders the northern coastline of the Somali Peninsula. These mountains extend to heights ranging from about 1400 - 2400 m.

Fig . 1. SSM/I Wind Product 28 May 2001 0507Z

Fig. 2. Topographical map of the Northern Arabian Sea region

Fig. 1 is an SSM/I (Special Sensor Microwave Imager) depiction of estimated surface-level wind speed in the region off the northeast tip of Somalia, south of the Saudi Arabian Peninsula. Immediately noticeable is the reduced wind speed indication north of the island of Socotra. The blocking effect of island barriers, leading to reduced wind speeds and reduced sea state in the island's lee is a well-known phenomenon, documented in several studies (Fett and Rabe, 1976; Fett and Burke, 1981). Also of interest is the reduced wind speed indication in the Gulf of Aden, adjacent to the northeast tip of Somalia. A range of coastal mountains exists in that region, as shown in Fig. 2. Terrain elevation ranges from a maximum of 1400 m near the northeast tip of Somalia to 2048 m near the central coastal portion of the Gulf of Aden. A similar blocking effect is probable, leading to the low wind speeds in the Gulf of Aden, suggesting that southerly flow extends continuously from the northeast portion of Somalia to Socotra. Deprived of other observations, these SSM/I wind data provide valuable indications, not only of wind speed in this region, but also, indirectly, of wind direction.

Fig. 2 shows the topography of land areas associated with the Fig. 1 SSM/I image. It can be seen that a range of coastal mountains borders the northern coastline of the Somali Peninsula. These mountains extend to heights ranging from about 1400 - 2400 m.

Fig. 3. NOGAPS Surface Analysis Valid 28 APR 2001 at 1200Z

Fig. 3 reveals that conditions of light southerly flow existed at the surface, flowing over the northeast tip of Somalia, in this depiction about 7 hours after the time of the SSM/I data.

Fig. 4 SSM/I Integrated Water Vapor 28 April 2001 0507Z	Fig. 5.

Fig. 4 is the Total Integrated Water Vapor (IWV) (kg/square m) image corresponding to the time of Fig. 1. The image reveals a decrease in IWV over the Gulf of Aden just north of the extreme northeast tip of Somalia. It will be noted that this decrease in moisture or drying effect is not entirely coincident with the low wind indication in the Gulf of Aden shown in Fig. 1. As earlier indicated, terrain elevation is lower near the tip of Somalia than further to the west. Apparently, despite the reduced height, the terrain in this region is optimally configured (gentle windward slope and sharp lee slope) to promote the strong downdraft necessary to cause the observed drying effect. In this regard note that there is no indication of drying in the lee of Socotra, even though this island has individual peaks reaching a much higher (1508 m) elevation. The figure reveals a pronounced high mountain ridge very close to the coast in this area (elevations on this chart are indicated in feet, rather than meters). Configuration appears to meet the "gentle windward slope/sharp lee slope criteria." This image reveals coastal topographical conditions conducive to promoting vertically propagating mountain waves north of the northeast tip of Somalia under conditions of southerly flow aloft.

Fig. 4 SSM/I Integrated Water Vapor 28 April 2001 0507Z

Fig. 5.

Fig. 4 is the Total Integrated Water Vapor (IWV) (kg/square m) image corresponding to the time of Fig. 1. The image reveals a decrease in IWV over the Gulf of Aden just north of the extreme northeast tip of Somalia. It will be noted that this decrease in moisture or drying effect is not entirely coincident with the low wind indication in the Gulf of Aden shown in Fig. 1. As earlier indicated, terrain elevation is lower near the tip of Somalia than further to the west. Apparently, despite the reduced height, the terrain in this region is optimally configured (gentle windward slope and sharp lee slope) to promote the strong downdraft necessary to cause the observed drying effect. In this regard note that there is no indication of drying in the lee of Socotra, even though this island has individual peaks reaching a much higher (1508 m) elevation.

The figure reveals a pronounced high mountain ridge very close to the coast in this area (elevations on this chart are indicated in feet, rather than meters). Configuration appears to meet the "gentle windward slope/sharp lee slope criteria." This image reveals coastal topographical conditions conducive to promoting vertically propagating mountain waves north of the northeast tip of Somalia under conditions of southerly flow aloft.

IMPORTANT CONCLUSIONS

1. In the absence of conventional weather charts SSM/I wind and IWV data can sometimes provide signals indicating the direction of low to mid-level flow over selected areas.

2. Perhaps more important, the IWV data can sometimes provide warning of vertically propagating mountain wave conditions that can pose a significant hazard to low flying aircraft.

3. Because of changes in low level moisture content in vertically propagating mountain wave areas, unusual effects may be noted affecting the signals of radio and radar in such regions.

REFERENCES

1. Fett, R. W. and K. M. Rabe, 1976, Island barrier effects as revealed by a numerical wave model and DMSP satellite data, Mon. Wea. Rev., Vol 6, No. 3, 324-334.

2. Fett, R. W. and S. D. Burk, 1981, Island barrier effects as observed by satellite and instrumented aircraft, and simulated by a numerical model, Mon. Wea. Rev., Vol. 109, No. 7, 1527-1541.

Author: Bob Fett
Last Updated: Mon Dec 9 10:44:09 2002
Produced by: The Composer (Ver: 1.1.2 )

NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil