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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| COLD SURGE EFFECTS OF THE ASIAN WINTER ANTICYCLONE With the onset of the Northern Hemisphere winter, an intense surface high- pressure system develops over the East Asia continental region, centered south of Lake Baikal. The dominating characteristics of this anticyclone can be seen by noting the huge geographical area over which it lies. The anticyclone is sustained and intensified by strong radiational cooling over the frozen land mass and by consistent cold air advection from Arctic latitudes. The occurrences of winter season cold air outbreaks cause high pressure to extend eastward over coastal waters off the China mainland. Strong winds turning anticyclonically around this eastern extension of high pressure and the associated weather is referred to as a Northeast Monsoonal Surge (NEMS). These surges produce strong, steady, northerly to northeasterly monsoon winds along the East Asian coast near Sakhalin Island, across the Sea of Japan, the Yellow Sea, the East China Sea, the Philippine Sea and into the South China Sea. A cold surge or NEMS is generally progressive, starting first in the far north and then moving southward with time. A major surge takes about 6-7 days before its influence is apparent at near-equatorial locations. Blocking action, as detected at the 500-mb level near 160ùE, appears important in anchoring low pressure in the region north of Japan, and thereby providing a stationary channel for cold northerly wind surges. The influence of the winds, as indicated above, extends well to the south through the area of the maritime continent, which consists of the Indonesian and Malaysian Islands. |
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| This image shows an example of the initiation of a NEMS and the development of cloud lines (streets) north of Korea in a classic example from the Navy Tactical Applications Guide (NTAG) series (Fett and Bohan, 1986). Isotachs are superimposed on this image. Note, in typical fashion, that the cold surge follows passage of a frontal band and developing low- pressure center east of Japan. Also of interest is the notable wind speed increase from land to water. |
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| The major synoptic features of a Northeast Monsoon Surge (NEMS) are shown in this figure. The Sea Level Pressure (SLP) pattern shows an intense 1044 mb high centered near 45N 110E with a ridge of high pressure extending southeastward over the South and East China Seas. This low- pressure center, with a frontal cloud band extending southwestward, defines precursor circulation and cloud patterns to the onset of a NEMS. Cloud lines in westerly to northwesterly flow are apparent in the Sea of Japan and in the East China Sea, north of Taiwan. Bulging of high pressure southward is characteristic of a cold surge as cold air of the high pressure drains toward lower latitudes. |
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| This image of GMS-5 IR data, within an hour of the previous image, has NOGAPS wind data superposed. The cloud lines, noted in the previous image north of Korea, have formed in response to northwesterly flow sweeping into the Sea of Japan. Cloud line formation occurs in the marine boundary layer only under conditions of strong vertical wind shear. The convective activity brought about by cold air flowing over warmer water results in mixing, bringing stronger winds from aloft down to lower levels. In this respect, note that winds are much stronger over the open water areas than over land where they are also slowed due to effects of surface friction. The cold surge at this time has already reached past Taiwan. However, the winds in this region, indicated by the NOGAPS model, are quite light, ranging from 5 to 15 kt. It appears, and succeeding slides will show that the NOGAPS spectral global model that has an approximate equivalent 80nm grid size, cannot resolve the mesoscale wind pattern through the 100nm wide Taiwan Strait. In fact, in this depiction of the NOGAPS wind overlay there are no winds greater than 15 kt shown south of Central Japan. |
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| The 13/1200Z COAMPS surface wind analysis, on the other hand, indicates northerly winds of 30 kt in the Taiwan Strait, and generally 20 to 25 kt winds extending northeastward to Southern Japan. These winds are much stronger than those of the NOGAPS analysis shown in the previous slide. Although the COAMPS analysis is based on data 12 hours later than the NOGAPS analysis, these wind speed differences are more likely related to the finer grid size and therefore higher resolution of the mesoscale model, rather than to a general, sudden increase in wind speed over the region. This is particularly true within the Taiwan Strait channel where additional grid points in the mesoscale model can capture funneling effects due to venturi action as winds enter and exit that region. The capture of venturi action is aided by improved, higher resolution topography in the COAMPS model. |
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| This is an image of the F-14 Special Sensor Microwave Imager (SSM/I) 13/1233Z surface wind field. SSM/I data are gathered from the DMSP satellite system, flown at an altitude of 850 km. This instrument senses wind speed over water because of changes in emissivity of the water due to increased capillary wave action and foam development with increased wind speed. Resolution of the SSM/I sensor is adequate (38x30 km at 37 GHZ) to detect speed acceleration due to funneling through the Taiwan Strait. To the north of the Strait, winds generally less than 15 kt are indicated, increasing to over 25 kt in the Strait and then decreasing south of the Strait. Note that higher winds are observed over the ocean areas to the northeast and southeast of Taiwan, while a leeside light wind area is shown immediately south of the Taiwan landmass, which provides a sheltering effect. Also note that the wind data revealed in this figure closely reflect the COAMPS wind speed depiction shown previously (note especially high wind speeds in the Yellow Sea west-southwest of Korea) “ verification that the COAMPS model output is realistic. |
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| This is a QuikSCAT depiction of the area about 10 hours later than the previous SSM/I image. QuikSCAT refers to the active microwave radar aboard NASAËs Ocean Wind satellite that gathers wind speed and direction information over the ocean from backscattering off of capillary waves. QuikSCAT has a resolution of about 25 km, which is small enough to capture the gap wind effects we have been describing. This QuikSCAT image showing three passes over the Western Pacific region captures not only mesoscale wind features like the Taiwan Strait wind funneling and leeside sheltering, but also the general synoptic scale pattern and extent of the NEMS. Note the directional shear associated with the frontal cloud band north of about 25N, and also the speed shear zone that extends southward to below 15N across the Philippines. The strong northwesterly flow across the East China Sea is also clearly shown. |
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| This slide shows the COAMPS 12 hr surface wind forecast with verifying time (VT) at 14/0000Z. This VT is one hour after the previously shown QuikSCAT image. The synoptic scale frontal band direction shear zone as well as the speed shear zone marking the leading portion of the NEMS over the Philippine area are clearly shown in the COAMPS wind forecast. The mesoscale wind funneling through the Taiwan Strait and Taiwan leeside wind shelter are also evident in the forecast winds. The wind speed pattern of the East China Sea shown in the QuikSCAT image, maximum near Southern Japan decreasing toward Taiwan, is correctly forecast. All of these features are of a size that can be captured and forecast by the 27 km grid size COAMPS run. It is noted that the forecast winds are not in exact agreement with the QSCAT wind speeds. COAMPS appears to have advected the front eastward a little faster than the QuikSCAT wind indicates, while being a little slow on the southward movement of the NEMS leading edge. By visual inspection the COAMPS forecast wind speeds appear to generally be a little lower than the QuikSCAT speeds. In an operational forecast situation, once such movement and intensity characteristics can be determined for a given forecast run, appropriate modifications can be made to specific forecasts. Over the several day sequence of a given NEMS event, or over several separate NEMS, close monitoring of NWP model forecast skills and biases may be established and appropriately applied to specific forecasts. |
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| This is a GMS IR image almost 12 hours later than the previous image, with cloud track winds superimposed. The lowest level (800-900 mb) winds are encoded green; mid-level (600-799 mb) winds are encoded yellow; and higher- level (400-599 mb) winds are encoded blue. A tremendous surge of northwesterly winds is shown extending across the Yellow Sea and into the East China Sea. While the green vectors indicate low-level speeds from 800-900 mb, the instability off cold air rushing out over warmer water creates instability resulting in the transfer of high winds from aloft down to the sea surface. Given the long fetch of sustained 30 kt winds with no change in direction, it is guaranteed that any ships unfortunate enough to be in that region would be experiencing very high and dangerous sea conditions. This is one of the aspects of cold surge events that often catches the uninitiated off-balance. The normal mid-latitude expectation is for improving conditions following frontal passage, and, with the nearest low hundreds of miles away, it would be difficult for the layman to explain extremely heightened sea state in that location. Cold surges of this type in the past have been responsible for many disasters at sea. |
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| This is a higher resolution visible GMS-5 view with TRMM microwave imager
(TMI) data superimposed. TMI retrievals appear over water, while
corresponding GMS visible data appear outside the TMI pass and over land
where its orbit intersects with the GMS data. The TMI instrument senses
wind speed over water because of changes in emissivity of the water due to
increased capillary wave action and foam development with increased wind
speed, similar to data obtained by the Special Sensor Microwave Imager
(SSM/I) of the Defense Meteorological Satellite Program (DMSP). However,
the TRMM satellite is flown at a much lower altitude than DMSP (350 km vs.
830 km); the lower altitude permits increased resolution using an
algorithm that is similar to that used for SSM/I. The increased
resolution offered by TMI is evident in a comparison of 37 GHz footprints
for TMI (footprint 16x9 km) as opposed to DMSP (footprint 38x30 km).
These data, acquired on 14 Jan 2001, over 7 hours after the preceding
image, reveal that cold surge effects of the NE Monsoon are still alive
and well. Cloud lines indicating strong vertical wind shear are evident
in the northern portion of the image. Cloudiness on the east side of
Taiwan provides proof of northeasterly winds impinging on that area
against the mountains in that region. The barrier, or sheltering effect
of the island, producing very weak winds off the southwest tip of Taiwan
as revealed in the TMI data, is a characteristic NE Monsoonal effect, as
are the stronger winds in the strait between Taiwan and China. The
venturi effect in this area makes this strait one to be avoided by ships
during cold surge events. Be aware that rain affects the response of the
TMI. It is therefore likely that the yellow patches, normally indicative
of higher wind speeds in the region south of Taiwan, are really indicative
of shower activity in that area. Therefore, the wind speed indications in
that region are unreliable. IMPORTANT CONCLUSIONS 1. A frontal cloud band typically forms along the leading edge of the NEMS. 2. During a NEMS, enhanced northerly winds will occur through the Taiwan Strait due to topographic funneling. 3. The synoptic scale features of a NEMS are generally well forecast by global models, but mesoscale features, such as the enhanced northerly winds through the Taiwan Strait, are below global model resolution. 4. Mesoscale models run at various nested grid sizes (54, 27 and 9 km) provide increasing atmospheric feature resolution as grid spacing decreases. The higher resolution permits accurate depiction of wind speed strength exiting over the open water areas from mountain gap regions, such as that into the Yellow Sea and through narrow straits as in the Taiwan example. REFERENCES Fett, R.W. and W. A. Bohan et al., 1986, Navy Tactical Applications Guide, Vol. 6, Tropics, Weather Analysis and Forecast Applications, Naval Research Laboratory, Monterey, CA, 93940, pp 182. |
Author: Bob Fett Last Updated: Mon Dec 9 10:38:45 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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