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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| The Cua Viet River forms a valley between mountains in Vietnam through which southwesterly winds frequently funnel. These winds, often called the "Winds of Laos", are accelerated in venturi fashion - at times reaching gale intensity (Brand and Lester, 1969). The winds cause heavy seas in a narrow strip as they move out over the South China Sea. In an opposing manner, mountain barriers of coastal Vietnam restrict the movement of such flow. Seas in the lee of the mountain barriers are relatively calm for a certain distance, depending mainly upon the strength of the flow, vertical and horizontal wind shear, and stability of the lower atmosphere. Quite frequently DMSP VHR data delineates the exact areas of rough and calm seas through sunglint effects. |
| Fig 1. Location Map for Cua Viet River |
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| In Fig. 1, it can be seen that a gap exists between mountain ranges southwest of the towns of Dong Ha, Quang Tri, and Cua Viet. The Qua Viet River runs through this gap. During the southwest monsoon, winds can funnel through this gap and accelerate through venturi action to cause gale force winds a short distance offshore. |
| Fig. 2. | Fig. 3. |
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| In Fig. 2, note that the Primary Specular Point (PSP) is inland, only a
short distance from the South China Sea. The PSP is defined as that point
on the earth's surface where the sun's rays are reflected directly to the
satellite along a great circular arc perpendicular to the satellite
subpoint track (Fett and Bohan, 1977). Calm areas of water reasonably
close to the PSP reflect brilliantly into the spacecraft sensor. Rough
areas, on the other hand, act to reflect much of the light away from the
spacecraft sensor and are therefore perceived as darker toned areas. The
cone-shaped bright areas along the coast are therefore interpreted to be
calm. Such areas typically converge, while darker areas, indicative of
rougher seas, typically diverge away from the coast. is precisely defined. On this date, however, winds were not particularly strong, or sunglint would have been further disrupted due to high seas, and as a result, reflectance would have been considerably more reduced. | |
| Fig 4. DMSP VHR 22 August 1974, 0213 GMT |
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| Another example of the "Winds of Laos" blowing through the Cua Viet River area is shown in Fig. 4. Note the darker gray shade in the lee of the valley on this date, indicative of rougher seas diffusing the sunglint to a greater extent. The protective areas have become narrower and more elongated, indicating a stable stratification of flow with little lateral mixing (Deardoff, 1976). The elongated pattern suggests weaker winds at the surface increasing rapidly with height, i.e., strong vertical shear. |
| Fig. 5. QSCAT Surface Winds. 20 Aug 2001, 1522Z |
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| Fig. 5 shows QuikScat surface winds over the South China Sea to the east
of Vietnam. Ordinarily the lone 30 kt wind offshore of Vietnam,
surrounded by much lighter wind speeds, would be considered suspect. This
is especially true since the wind barbs are coded black, indicative of
possible rain contamination. However since this measurement is in the lee
of the Cua Viet valley near 17¸N, the region where especially strong gap
winds are known to occur during the period of the Southwest Monsoon, it is
likely that the measurement is correct and that that the "Winds of Laos"
are active at this time. Note also that surrounding winds turn
anticyclonically over the region implying a high-pressure ridge axis where
rain contamination would be unlikely to occur. Further verification that
rain is not causing a problem can be seen in Fig. 6. Ships transiting parallel to the coast past this region could experience a very abrupt transition from light wind speeds and slight seas to very strong winds and much rougher sea state conditions. |
| Fig. 6. GMS-5 Infrared Image. 20 Aug2001, 1531Z |
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| The GMS-5 image (Fig. 6) provides verification of clear skies off the coast of Vietnam at the time of the QuikSCAT image (Fig. 5), precluding a rain contamination problem with the data. The example is especially compelling since synoptically, as shown by the satellite data, strong winds offshore of the Qua Viet Valley would not otherwise be suspected. |
Author: Bob Fett Last Updated: Mon Dec 9 10:41:39 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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