World Wind Regimes - Central America Gap Wind Tutorial

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

World Wind Regimes - Central America Gap Wind Tutorial

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Introduction

Fig. 1. TRMM 16 Dec 00 0445Z	Fig. 2. GOES IR 16 Dec 00 1251 Z

Fig. 1 is a TRMM passive microwave radiometer image on 16 Dec 2000 at 0445Z showing evidence of the initiation of a mild Tehuantepecer wind surge with maximum wind speed of about 20 knots near the coast. If the impulse is strong, an arc cloud is commonly formed at the outward extremity of the surge. The arc cloud delineates the position where wind and sea state suddenly change from strong and high to much calmer conditions. Fig. 2 is a GOES IR view of the area about 8 hours later at 1251Z. The image reveals that an arc cloud has formed at the leading edge of the surge indicating that a further extension of the surge will follow. The arc cloud formation often reaches squall-line intensity accompanied by showers and heavy seas. Swells generated by the Tehuantepecer have been recorded reaching nearly 1000 nm to the south, to the Galapagos Islands.

Fig. 1. TRMM 16 Dec 00 0445Z

Fig. 2. GOES IR 16 Dec 00 1251 Z

Fig. 1 is a TRMM passive microwave radiometer image on 16 Dec 2000 at 0445Z showing evidence of the initiation of a mild Tehuantepecer wind surge with maximum wind speed of about 20 knots near the coast. If the impulse is strong, an arc cloud is commonly formed at the outward extremity of the surge. The arc cloud delineates the position where wind and sea state suddenly change from strong and high to much calmer conditions.

Fig. 2 is a GOES IR view of the area about 8 hours later at 1251Z. The image reveals that an arc cloud has formed at the leading edge of the surge indicating that a further extension of the surge will follow. The arc cloud formation often reaches squall-line intensity accompanied by showers and heavy seas. Swells generated by the Tehuantepecer have been recorded reaching nearly 1000 nm to the south, to the Galapagos Islands.

Background

TRMM is an acronym for "Tropical Rainfall Measuring Mission". The passive microwave radiometer on board is termed TMI (TRMM Microwave Imager). This instrument is similar to the Special Sensor Microwave/Imager or SSM/I, which has been gathering data since 1987 aboard DOD DMSP satellites. The TMI is a nine-channel system responsive to radiation at 10.65, 19.35, 21.3, 37 and 85.5 GHz. It is flown much lower than the DMSP spacecraft in a 350 km circular orbit having an inclination to the equator of 35¸. This provides for much higher resolution data retrieval than the SSM/I and varies from highest resolution footprint size of 7x5 km at 85.5 GHz to a size of 63x37 km at 10.65 GHz. The addition of the 10.5 GHz channel, which the SSM/I data do not have, enables measurement of surface winds through moderate amounts of rain that would otherwise block the DMSP system. As with SSM/I data, wind speed estimates are based upon the principle that the emissivity of water changes as wind speed increases, due to increased foaming action of water at higher wind speeds. The wind speed estimates are based upon an algorithm developed using vertically and horizontally polarized signals at 10.65, 19.35, 21.3 and 37 GHz. Reliable estimates of wind speed can be obtained through use of this algorithm up to 30-40 knots.

Examples

Fig. 3. TRMM 16 Dec 00 1315Z	Fig. 4. GOES IR 16 Dec 00 1745Z

Fig. 3 shows that the Tehuantepecer has intensified and now extends well out to sea. GOES IR data 4 hours later reveal that the trailing and apparently dissipating edge of a cold front has crossed the Gulf of Campeche (the Gulf of Mexico side opposing the Gulf of Tehuantepec). A high-pressure ridge following the cold front has begun to strengthen the pressure gradient in the area. It is therefore likely that the Tehuantepecer, based on these data, will further intensify during coming hours, with wind speeds near the coast probably increasing to well over 40 knots. The high wind speed region south of the Tehuantepecer in Fig. 3 is of special interest. It can be seen in this figure and through correlation with Fig. 4 that the region coincides with tropical disturbance cloudiness consisting of numerous convective cells. Attenuation of microwave emission by the heavy liquid water content in such areas causes faulty wind speed estimates suggested in the TRMM data. It is highly unlikely, therefore, that these wind speed indications are accurate, and more likely that relatively low wind speeds actually exist in that area.

Fig. 3. TRMM 16 Dec 00 1315Z

Fig. 4. GOES IR 16 Dec 00 1745Z

Fig. 3 shows that the Tehuantepecer has intensified and now extends well out to sea. GOES IR data 4 hours later reveal that the trailing and apparently dissipating edge of a cold front has crossed the Gulf of Campeche (the Gulf of Mexico side opposing the Gulf of Tehuantepec). A high-pressure ridge following the cold front has begun to strengthen the pressure gradient in the area. It is therefore likely that the Tehuantepecer, based on these data, will further intensify during coming hours, with wind speeds near the coast probably increasing to well over 40 knots. The high wind speed region south of the Tehuantepecer in Fig. 3 is of special interest. It can be seen in this figure and through correlation with Fig. 4 that the region coincides with tropical disturbance cloudiness consisting of numerous convective cells. Attenuation of microwave emission by the heavy liquid water content in such areas causes faulty wind speed estimates suggested in the TRMM data. It is highly unlikely, therefore, that these wind speed indications are accurate, and more likely that relatively low wind speeds actually exist in that area.

Fig. 5. COAMPS 24 hr. Forecast	Fig. 6. TRMM 20 Dec 00 1114Z	Fig. 7. TRMM 19 Dec 00 1227Z

As anticipated, winds slowly increased over the Gulf of Tehuantepec during the following days as a major cold surge spread southward over the Gulf of Mexico. Persistance of a Tehuantepecer for several days is not unusual and will occur as long as the pressure gradient through the Isthmus of Tehuantepec is maintained. The COAMPS (27 km resolution) 24 hr forecast for 20 December at 1200Z (Fig. 5) showed a 50 kt wind barb near shore in the Gulf of Tehuantepec, with 35 kt winds extending hundreds of kilometers downstream. On 20 December 2000, temperatures, even in Florida, fell below freezing, illustrative of the unusual mass of cold air that was entering the Gulf of Mexico. Fig. 6, near the time of the COAMPS forecast, shows TRMM data that reveal 40 kt winds extending almost 60 nm offshore in the Gulf of Tehuantepec. Winds closer to shore were undoubtedly much stronger. It has been reported that one ship was severely sandblasted in a Tehuantepecer having encountered a maximum sustained wind of 97 kt with gusts to 117 kt. One final point of interest. A close look at the GOES IR data (north of the TRMM data) reveals a light gray plume aligned along the axis of strongest winds. It is well known that upwelling of coastal water occurs during strong offshore wind events like the Tehuantepecer. The gray shade results from the strong divergence of surface water away from the coast. Although very subtle in this image, the gray shade may indicate this type of upwelling effect. The edge of the upwelling, again quite subtle, is more apparent in Fig. 7, acquired 24 hours earlier. Cirrus cloud contamination in the region adds some uncertainty to the interpretation. Upwelling normally widens southward due to diffusion of the cold upwelled water from the major wind speed axis, conforming also to the divergence and widening of Tehuantepecer wind effects southward from the coast. In conclusion, this study indicates that TRMM data are highly reliable in capturing a Tehuantepecer wind event, in all of its evolutions, and that the COAMPS 27 km resolution model is also highly successful in predicting the full effects and extent of Tehuantepecer winds.

Fig. 5. COAMPS 24 hr. Forecast

Fig. 6. TRMM 20 Dec 00 1114Z

Fig. 7. TRMM 19 Dec 00 1227Z

As anticipated, winds slowly increased over the Gulf of Tehuantepec during the following days as a major cold surge spread southward over the Gulf of Mexico. Persistance of a Tehuantepecer for several days is not unusual and will occur as long as the pressure gradient through the Isthmus of Tehuantepec is maintained. The COAMPS (27 km resolution) 24 hr forecast for 20 December at 1200Z (Fig. 5) showed a 50 kt wind barb near shore in the Gulf of Tehuantepec, with 35 kt winds extending hundreds of kilometers downstream. On 20 December 2000, temperatures, even in Florida, fell below freezing, illustrative of the unusual mass of cold air that was entering the Gulf of Mexico.

Fig. 6, near the time of the COAMPS forecast, shows TRMM data that reveal 40 kt winds extending almost 60 nm offshore in the Gulf of Tehuantepec. Winds closer to shore were undoubtedly much stronger. It has been reported that one ship was severely sandblasted in a Tehuantepecer having encountered a maximum sustained wind of 97 kt with gusts to 117 kt.

One final point of interest. A close look at the GOES IR data (north of the TRMM data) reveals a light gray plume aligned along the axis of strongest winds. It is well known that upwelling of coastal water occurs during strong offshore wind events like the Tehuantepecer. The gray shade results from the strong divergence of surface water away from the coast. Although very subtle in this image, the gray shade may indicate this type of upwelling effect. The edge of the upwelling, again quite subtle, is more apparent in Fig. 7, acquired 24 hours earlier. Cirrus cloud contamination in the region adds some uncertainty to the interpretation. Upwelling normally widens southward due to diffusion of the cold upwelled water from the major wind speed axis, conforming also to the divergence and widening of Tehuantepecer wind effects southward from the coast.

In conclusion, this study indicates that TRMM data are highly reliable in capturing a Tehuantepecer wind event, in all of its evolutions, and that the COAMPS 27 km resolution model is also highly successful in predicting the full effects and extent of Tehuantepecer winds.

Author: Bob Fett
Last Updated: Mon Dec 9 10:49:02 2002
Produced by: The Composer (Ver: 1.1.2 )

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