Preliminary Results from the Air Mass Transformation Experiment (AMTEX) .... the AMTEX region with successive days of MCC visible on 21, 22, and ..... Page 9 ...
4 C T S A R P
i fj j T D P l c S ^ s f r 7 No. 44
November 1976
An occasional series reporting on U.S. and international GARP scientific, technical, and planning activities, developments, and programs, presented as a public service to the meteorological community by the American Meteorological Society through arrangements with the U.S. Committee on the Global Atmospheric Research Program of the National Academy of Sciences-National Research Council. Opinions expressed in GARP TOPICS do not necessarily reflect the point of view of the U.S. Committee. Preliminary Results from the Air Mass Transformation Experiment (AMTEX) D. H. Lenschow
National Center for Atmospheric Research* Boulder, Colo. 80303 E. M. Agee
Department of Geosciences, Purdue University, West Lafayette, Ind. 47907 Abstract
The field phases of AMTEX, a GARP subprogram on air-sea interaction implemented by Japan, were conducted over the East China Sea in the environs, of Okinawa, Japan, during the last two weeks of February in 1974 and 1975. Investigators from Australia, Canada, and the United States also participated in this experiment. The weather was generally very favorable for this study of air mass transformation processes in 1975 because of an extensive cold air outbreak during most of the experimental period. A basic synoptic data set was obtained from 6 h soundings from an array of aerological stations centered at Okinawa. In addition, satellite, hourly surface and surface marine, oceanographic, boundary layer, radiation, radar, cloud physics, and aircraft data were obtained and have been or will be available in published data reports or on magnetic tape. Preliminary results from 1974 and 1975 reported at the Fourth AMTEX Study Conference and joint United StatesJapan Cooperative Science Program Seminar, "Air Mass Transformation Processes over the Kuroshio in Winter," held in Tokyo, 26-30 September 1975, are presented and discussed. 1. Introduction
The Air Mass Transformation Experiment (AMTEX) was organized and implemented by the Japanese as a GARP subprogram on air-surface interaction at a time i The National Center for Atmospheric Research is sponsored by the National Science Foundation. 1346
and place of large transfer of energy from the sea to the atmosphere. The planning for this experiment, which was first proposed to the Joint Organizing Committee in 1970, and summaries of the first three study conferences and Steering Committee meetings are presented by Lenschow (1972) and Lenschow and Agee (1974). The field phase was conducted over the East China Sea within a 300 km hexagonal array of observation stations centered at Okinawa during the last two weeks of February in both 1974 and 1975. Australia, Canada, and the United States also participated in this international experiment that is attempting to clarify the processes by which heat, moisture, and momentum are transferred from the ocean surface through the boundary layer and into the overlying free atmosphere in a region of large temperature contrast between the warm, northward flowing Kuroshio (Japan Current) and relatively cold, southward-flowing continental air. The resulting transfer of energy from the ocean fuels the development of atmospheric disturbances on several interacting scales: individual cumulus clouds (1-10 km), clusters of clouds organized into mesoscale patterns (10-100 km), mediumscale cyclonic disturbances (—1000 km), and large planetary-scale disturbances (>3000 km). The approximate location of the Kuroshio as well as the location of the AMTEX station array is shown in Fig. 1. Lenschow and Agee (1974) presented a more deVol. 57, No. 11, November 1976
Bulletin American Meteorological Society
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FIG. 1. DMSP satellite photograph at 1240 JST on 19 February 1975 showing the approximate location of the Kuroshio, the AMTEX island (solid circles) and ship (open circles) stations, and the part of the Electra flight path for which measurements are shown in Fig. 6.
tailed map showing the locations of the AMTEX station array and a survey of the recorded observations, synoptic weather features, and preliminary results from the 1974 field program. Since then, the 1975 field program was successfully executed with basically the same set of observations and the Fourth AMTEX Study Conference and a joint United States-Japan Cooperative Science Program Seminar, titled "Air Mass Transformation Processes over the Kuroshio in Winter," was held in Tokyo, 26-30 September 1975. At this meeting, more detailed results from 1974 and preliminary results from the 1975 field program were presented, and future activities of AMTEX were discussed. In this report we summarize the results of this conference, discuss the synoptic weather situation during the 1975 field program, present some further preliminary results, and discuss the data availability and future plans. 2. Synoptic weather features during AMTEX 75
There were two important, distinct differences between the synoptic events of AMTEX 74 and AMTEX 75. First, AMTEX 74 began with the end of a cold pe-
riod and a return to warmer weather that lasted for several days (see Lenschow and Agee, 1974). Such a weather cycle allowed for only one cold air outbreak during the first field period. In 1975, however, the field program began with a major outbreak of cold air, which allowed sufficient time for subsequent cold air outbreaks to occur during the field period. Second, during AMTEX 75 a Taiwanese depression developed that matured into an extratropical cyclone within the hexagonal network, a synoptic feature absent during AMTEX 74. This event was highly desirable from the standpoint of studying the effects of air-sea interaction on synoptic-scale motions. As indicated in the chronology of significant weather events during AMTEX 75 given in Table 1, the AMTEX 75 field program began on 14 February with 2
2 A cold air outbreak during AMTEX is defined as polar air over the AMTEX station array capable of producing a convective boundary layer. The outbreak may be evidenced by cloud patterns of mesoscale cellular convection and is usually represented by sea surface water more than 5°C warmer than the air.
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conditions favorable for cyclogenesis within the experimental measurement network. A depression of 1016 mb developed just east of Taiwan on the morning of 13 February, which appeared as an organizing synopticscale cloud system on the Defense Meteorological Satellite Program (DMSP) satellite imagery. At 0900 JST, 14 February, the center of low pressure (1004 mb) was located 300 km north-northwest of Naha, Okinawa. With warm, moist air feeding the storm system at low levels and cold, dry advection aloft, a vigorous frontal squall line developed and moved through the AMTEX network. Thunderstorms with gusts to 25 m s" moved across the Kadena Air Force Base at 1300 JST, with over 30 m s" gusts recorded nearby at the Naha Meteorological Observatory. With the passage of this deepening and expanding storm system, cold air was drawn southward from Asia over the East China Sea and the AMTEX site. By 0900 JST, 15 February, the cyclone had deepened to 982 mb and was located about 900 km east of Japan, with a trailing cold front some 800 km southeast of Okinawa. The storm moved northeast at 20 m s" , and eventually reached a pressure of 956 mb as it moved farther out to sea. As cold air streamed southward over the AMTEX network, the DMSP satellite image at 0912 JST, 15 February, showed well-organized patterns of mesoscale cellular convection (MCC) over the East China Sea. Strong northerly flow with persistent patterns 1
1
1
TABLE 1.
of MCC prevailed over the AMTEX region for the next 2 days. A representative photograph of the cloud structure during this 3-day cold period is shown in Fig. 2, which is the DMSP image at 1155 JST, 16 February. The 18-19 February was a transition period during which a weak trough of low pressure moved eastward across the Yellow Sea. A DMSP photograph at 1240 JST, 19 February 1975, is shown in Fig. 1. By 0900 JST, 20 February, a disorganized region of low pressure was beginning to consolidate into a cyclone center east of Japan. The 500 mb analysis at this time showed strong northerly flow over the Yellow and East China seas. A jet stream was positioned over this region with strong, cold air advection and 500 mb level winds in excess of 50 m s" . Positive vorticity advection (PVA) in the Yellow Sea further enhanced the northerly flow, which 24 h later added support to cyclone development east of Japan. Again a strong, cold air outbreak enveloped the AMTEX region with successive days of MCC visible on 21, 22, and 23 February. Associated with this outbreak was a record-breaking 20 cm snowfall at Tokyo. A "surge" line associated with the advancement of this cold air can be seen in the DMSP photograph in Fig. 3, at 1221 JST, 20 February. The MCC patterns that followed are shown in Fig. 4, a DMSP image taken at 1203 JST, 21 February 1975. The next several days can be described as a period of
Significant weather events and Electra research flights during AMTEX 1975.* Major Synoptic Features
Period
Feb. 1975
Warm
13 14
Developing Taiwanese depression. Taiwanese low NW Okinawa (1004 mb); cold front passage at Naha.
Cold
15 16
Taiwanese low east of Japan (982 mb); cold air outbreak (MCC). Cold air outbreak continues (MCC).
17
Cold air outbreak weakens (MCC).
18
Cool
Cold
Warm
1
Electra Flight Times (JST)
Location of Flight Patterns
1110-1706
Near Keifu and buoy array.
1111-1659
Near Keifu and buoy array; also south of Keifu to 26°N.
Trough low pressure over Yellow Sea.
1056-1519
19
Trough of low pressure east of Korea; cold air over Yellow Sea.
1031-1739
Near Miyako, then north to 30°N, 124°30 E. NW over Ryofu and on to 30°N, 126°30'E.
20 21
Cyclone east of Japan with PVA south of Korea; surge line. Cold air outbreak (MCC).
1153-1800
22
Cold air outbreak continues (MCC).
1159-1652
23
Anticyclone over East China Sea; cold air outbreak weakens (MCC).
Miyako and Tarama; then NW to 28°N, 128°E. Miyako; then north to 26 N, 125°30 E.
24 25 26 27
Anticyclone over Japan. Cyclone over Sea of Japan NE of Korea. East-west cold front over Korea. East-west cold front over AMTEX region; frontal wave NW of Okinawa. Weak cold air outbreak (MCC).
1056-1510
West to 26°30'N, 126°E.
1034-1520
West to 27°N, 125°E.
1105-1758
SE to 21°30 N, 133°40'E.
28
/
P
/
* The presence of mesoscale cellular convection (MCC) as observed from satellite photographs is indicated
/
Bulletin American Meteorological Society transition as high pressure drifted eastward. By the morning of 24 February the AMTEX region and eastern China were under southerly flow as the high-pressure center moved to the southeast of Japan. The concluding days of the experimental period saw a new, but weaker, frontal system move through the network. As the field program ended, the front was located to the southeast of the region and there was a poorly organized pattern of MCC visible.
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The average daily temperature at Naha shown in Fig. 5 reflects quite well the observed synoptic changes during the field period and the occurrence of air mass modification within the AMTEX region. The circled dots in the Naha temperature trace imply that MCC was visible on satellite photographs within the hexagonal network. Also shown in Fig. 5 is the average daily temperature trace at Kagoshima, located approximately 200 km to the north of the field array. Systems affecting
FIG. 2. DMSP satellite image at 1155 JST on 16 February 1975 showing mesoscale cellular convection (MCC) over the East China Sea and the AMTEX hexagonal network. Closed cells exist within the network, while open cells are present to the west toward the China mainland. The vortex street downwind of Cheju Island, which has a maximum elevation of 1950 m, is a commonly observed feature during cold air outbreaks. In addition to the AMTEX stations the location of an Oregon State buoy is also identified.
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FIG. 3. DMSP satellite image at 1221 JST on 20 February 1975 showing a comma cloud associated with a secondary vorticity maximum. Development of a "surge line" marks the onset of a new cold air outbreak and increased northerly flow.
Bulletin American Meteorological Society
FIG. 4. DMSP satellite image at 1203 JST on 21 February 1975 showing the pattern of MCC associated with the second cold air outbreak during AMTEX 75. This pattern contains mostly open cells, whereas that shown in Fig. 2 is mostly closed cells.
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3. DMSP satellite information on imagery received at Kadena Air Force Base, Okinawa, during AMTEX 75.*
TABLE
Satellite No.
Pass No. 1, JST
Pass No. 2, JST
7529 8531
~0900 ~1200
~2100 ~2400
Picture Quality Poor Good
VHR, HR, MI VHR, HR, WHR
WHR MI
* Daytime imagery is VHR, HR, MI, and nighttime imagery is WHR, MI, HR.
FIG. 5. A plot of average daily temperature at Naha and Kagoshima, Japan, for the period 10 February 1975 to 2 March 1975. Daily temperature was obtained by averaging hourly reports. Circled values for the Naha trace imply that MCC was visible from satellite photographs over the AMTEX hexagonal network, which is seen to be in response to colder air.
Kagoshima were sufficiently intense to bring about similar changes in the weather at Naha, as evident in the comparison of temperature traces for both stations. 3. Observations
a.
Synoptic
The vicinity of Okinawa and the East China Sea is an excellent location for the study of air mass modification because of the large air-sea temperature difference that commonly occurs during winter. The need for synoptic data made this location even more suitable due to the network of island stations whose observations are routinely collected by the Japan Meteorological Agency (JMA). During AMTEX, three special ships and three island stations were added to the network, w hich formed an irregularly shaped hexagon, with Naha as the center and headquarters of the field program (see Fig. 2). Supporting observations were obtained from the JMA stations at Miyako Island and Kagoshima. AddiT
TABLE
2. Location and height of the six AMTEX aerological island stations and three observation ships. BlockStation
Latitude, N
Longitude, E
Height, m
Kagoshima Naze Naha Miyako Jima Ishigaki Jima Minamidaito Jima
47-827 47-909 47-936 47-927 47-918 49-945
130°36 129°33' 127°40' 125°17' 124°10'
Keifu Maru Ryofu Maru Nojima
JBOA JGZK JAZP
31°38' 28°23' 26°12' 24°47 24°20' 25°50' 28°00'
282.6 295.0 29.1 39.2 7.2 15.7 8.0 5.7 5.3
Stations
/
29°30' 23°30'
/
i25°oo' 127°00' i28°oo'
tional synoptic observations were available from Cheju Island (south of Korea) and commercial ships traversing the East China Sea. Table 2 gives the location and elevation of the nine official aerological stations of AMTEX operated by the JMA. Aerological soundings were made every 6 h, except every 12 h from the Nojima, during the second week of the experiment. The data, which have been collected and published in Volume 2 of the AMTEX 75 Data Report, include upper air soundings of pressure, temperature, humidity, and winds, except that winds aloft were not measured from the observation ships Ryofu Maru and Nojima. Hourly surface data, which were obtained from 25 island stations and 5 research ships in the AMTEX area, include observations of pressure, temperature, humidity, wind speed and direction, precipitation, weather and amount, form and height of clouds, and, from the ships, sea surface temperature and the heights and periods of the wind waves and swell. Weather radar observations from the 5.7 cm wavelength radar network covered the area. b. Satellite
observations
Several polar-orbiting meteorological satellites operated by the Soviet Union and the United States photographed the AMTEX area during AMTEX 75, but the principal source of satellite products was the DMSP of the U.S. Air Force. The location of a DMSP receiving station at Kadena Air Force Base, Okinawa, provided useful coverage of satellite cloud photography in real time as well as for postanalysis. DMSP satellites 7529 and 8531 were operational during the entire field period, providing two daily passes each over the field network. Information on the amount and type of data collected is given in Table 3. The terminology used in describing the imagery resolution is 1) HR—high resolution (2 nm) in the visible scan (0.4-1.2 ^m), 2) VHR—very high resolution (1/3 nm) in the visible, 3) MI—high resolution in the infrared (8-13 /*m), and 4) WHR—very high resolution in the infrared. Photographs provided by Detachment 8 of the Air Weather Service at Kadena were made available on a daily basis to interested scientists and to the Naha Meteorological Observatory. Routine products were prepared in 16 shades over a temperature range from near 210 K to 310 K. Selected thermal cuts were also prepared at the request of Texas A8cM and Purdue universities.
Bulletin American Meteorological Society c. Special
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observations
The observational network and the observations collected during 1975 were very similar to those during 1974. Since these have already been described in some detail by Lenschow and Agee (1974), the emphasis here will be on differences in the special observations between the two periods. 1) B O U N D A R Y LAYER
A wide variety of boundary layer observations by groups from Japan, Australia, Canada, and the United States was obtained during the experiment. Direct measurements of turbulent fluxes at fixed points were made from an island-based tethered balloon, two ship stations, two offshore towers, and four onshore towers. Spectral density techniques for estimating turbulent fluxes were also used. The sensors included cup, propeller, hot-wire, and sonic anemometers for wind fluctuations and infrared and Lyman-alpha hygrometers and fine-wire thermocouple psychrometers for humidity fluctuations. Indirect estimates of turbulent fluxes were obtained by the bulk transfer coefficient method. Observations of boundary layer structure at higher levels were obtained from lowlevel sondes, a tethered balloon and an acoustic sounder on Miyako Island, and a kite on Tarama Island. A mesometeorological array of 12 spar buoys from Oregon State University capable of measuring and recording wind speed and direction, wet and dry bulb temperature, sea temperature (at the surface and at 5 and 15 m depth), and solar radiation was deployed on the edge of the continental shelf northeast of the Keifu Maru. However, only one buoy located at 28°26.6 N, 125°25.3'E operated successfully and only for about the first 5 days of the experiment. The NCAR Electra aircraft flew nine research flights originating from Naha, Okinawa, for a total of 50 h mostly at or below 300 m altitude during the experiment on the days and at the locations shown in Table 1. Scientists from six United States institutions participated in the Electra flight program. Measurements included mean horizontal wind, humidity, upward and downward infrared and short-wave radiation, air and surface radiation temperature, and fluctuations in air velocity, temperature, and humidity. In addition, the size distribution of cloud particles was measured during some cloud penetrations, and high-frequency measurements of temperature and velocity were compared (by the University of California at San Diego) with concurrent tower measurements on Miyako Island on 21 and 22 February. A frontal investigation was conducted on the 28 February flight southeast of the AMTEX station array. Most of the rest of the flights were devoted to boundary layer investigations of air mass modification and to observations of boundary layer structure in regions observed by satellite photographs to have MCC. An example of the measurements taken from the Electra on 19 February 1975 at 100 m height along the track plotted in Fig. 1 from 26°51.8 N, 127°29.6'E to 28°50.2 N, 127°29.7 E is shown in Fig. 6. The satellite /
/
/
/
FIG. 6. Plots of 30 s averages of temperature, humidity, and wind speed and direction at 100 m altitude, as well as surface temperature and surface saturated mixing ratio measured along the Electra flight track from 26°51.8'N, 127°29.6'E to 28°50.2'N, 127°29.7'E on 19 February 1975 from 0145 to 0225 GMT (1045-1125 JST). The measurements were taken about h after the satellite photograph in Fig. 1.
picture was taken at 1240 JST, and the airplane measurements were taken from 1045 to 1125 JST. A small Japanese research aircraft also made boundary layer observations near Amami Island. 2)
CLOUD
Besides the satellite pictures, photographs were taken from commercial airlines flying between Tokyo and Manila. Stereoscopic cloud photographs were taken from the ground at Miyako Island. Radar observations were taken on Miyako Island with 8.6 mm and 3.2 cm wavelength, vertically pointed radars and with Doppler radars at the same wavelength. Raindrop size distribution at the ground and airborne measurements of the size and liquid water content of raindrops and cloud droplets were made also around Miyako Island. 3) RADIATION
The total downward and the total short-wave (solar) downward radiation flux at the surface were measured at four island stations and on four research ships. Above the surface, radiative flux was measured from aircraft and with radiation sondes at Naha, Okinawa. 4) OCEANOGRAPHIC
Oceanographic observations of temperature, salinity, and current with depth were obtained from the three
fixed ships ( R y o f u Maru, Keifu Maru, and Nojima),
as
well as four roving research vessels that cruised through the northwestern half of the AMTEX area. In addition to the Oregon State University buoys mentioned previously, an ocean data buoy, located at 28°20'N, 126°05'E, measured wind speed and direction, wet and dry bulb temperature, pressure, temperature at 3, 10, and 25 m depth, solar radiation, current speed and direction, orientation and inclination of the buoy, wave height and period, and electric conductivity.
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Preliminary results from both 1974 and 1975 were discussed at the study conference held in Tokyo in September 1975 and are included in the Scientific Report of the Fourth AMTEX Study Conference (AMTEX Report No. 8). Large-scale studies of air mass modification utilizing the special AMTEX aerological network and available satellite data were presented by several investigators. Estimates of the heat transfer averaged over the AMTEX region based on either the budget method using the 6 h rawinsonde data (Ninomiya and Nitta) or the bulk aerodynamic method using 3 h surface layer observations of wind, temperature, and humidity (Kondo) were reported. Good agreement was obtained between the two independent methods. In addition to the budget and the bulk aerodynamic methods for estimating heat fluxes, the fluxes were measured directly by the eddy correlation technique. Direct measurements from towers, ships, and aircraft were reported. A reasonably good comparison was obtained between direct flux measurements and bulk aerodynamic estimates measured from the NCAR Electra aircraft. Indirect methods of estimating fluxes based on spectral density techniques were also discussed. Further work needs to be done, but preliminary indications are that the methods may be quite useful when direct measurements are not possible, or feasible. During a period of cold air outbreak, the total amount of latent and sensible heat transfer was estimated to be >700 W m~ , whereas during the warm period the average heat transfer was
