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Faculty and Researcher Publications Faculty and Researcher Publications
2001 The California Low-Level Coastal Jet and Nearshore Stratocumulus
Kalogiros, John http://hdl.handle.net/10945/34433 1.1 THE CALIFORNIA LOW-LEVEL COASTAL JET AND NEARSHORE STRATOCUMULUS
John Kalogiros and Qing Wang* Naval Postgraduate School, Monterey, California
1. INTRODUCTION* and the warm air above land (thermal wind) and the frictional effect within the atmospheric This observational study focus on the boundary layer (Zemba and Friehe 1987, Burk and interaction between the coastal wind field and the Thompson 1996). The horizontal temperature evolution of the coastal stratocumulus clouds. The gradient follows the orientation of the coastline data were collected during an experiment in the (317° on the average in central California). Thus, summer of 1999 near the California coast with the the thermal wind has a northern and an eastern Twin Otter research aircraft operated by the component and local maximum of both Center for Interdisciplinary Remote Piloted Aircraft components of the wind is expected close to the Study (CIRPAS) at the Naval Postgraduate School top of the boundary layer. Topographical features (NPS). The wind components were measured with may intensify this wind jet at significant convex a DGPS/radome system, which provides a very bends of the coastline like Cape Mendocino, Pt accurate (0.1 ms-1) estimate of the wind (Kalogiros Arena and Pt Sur. At such changes of the and Wang 2001). The instrumentation of the coastline geometry combined with mountain aircraft included fast sensors for the measurement barriers (channeling effect) the northerly flow can of air temperature, humidity, shortwave and become supercritical. An expansion fan occurs at longwave radiation, sea surface temperature, the bend and a strong wind jet evolves liquid water and concentration and size of water downstream of the bend (Winant et al. 1988). droplets and aerosols. During the experimental period the marine The synoptic conditions during the boundary layer top was characterized by a strong experimental period were typical of a high temperature inversion of 5-10°C due to the pressure system over the central Pacific Ocean synoptic scale subsidence and its height varied with lower pressure over land and northerly winds from 400-500 m close to the shoreline to 800 m in the 2km layer close to the surface. The wind about 200 km offshore. The cloud top is just below speed was observed to increase in the boundary the temperature inversion and cloud top radiative layer and a wind jet close to the boundary layer cooling further intensified the temperature top was frequently observed some tens of inversion. The cloud depth was typically 300m with kilometers offshore. In some occasions the wind maximum liquid water content of 0.4-0.6 gkg-1 and jet was quite intense and affected significantly the effective droplet radius 8-10 m near cloud top. structure of the stratocumulus layer. The concentration of droplets varied between 150 and 200 cm-3. 2. THE MARINE BOUNDARY LAYER OF THE CALIFORNIA COAST 3. INTERACTION OF THE COASTAL JET WITH STRATOCUMULUS Two significant characteristics of the marine boudary layer of the California coast are the On the flight of 6 of July 1999 a strong wind jet stratocumulus clouds and the low-level coastal (peak wind speed of 20 ms-1) was observed at the wind jet. The cold sea surface temperatures near height of 500 m just below the temperature the coast due to upwelling and the strong inversion base (boundary layer height) about 75 temperature inversion at the top of the boundary km offshore (-122.7° longitude). Figure 1 shows layer due the synoptic scale subsidence favor the contours of the observed wind components in the development of stratocumulus cloud during the area of the wind jet. This wind jet seemed to be summer. The coastal wind jet is mainly the result connected with a zone of cloud breakup parallel to of the large horizontal temperature gradient the shore (Fig. 2). As it can be seen in the figure (baroclinity) between the cold air above the sea the north edge of the cloud free zone is at Cape Mendocino. During the day this cloud free zone * Corresponding author address: Qing Wang, Naval expanded to the south near Pt. Conception. Postgraduate School, Dept. of Meteorology, Monterey, CA 93943; e-mail: [email protected]. −1 u (ms−1) v (ms ) 900 4 4 900 −6 2.4 3.2 −6 −5 9 −7 4.8 −6 4 800 −7 800 4.8 −8 −7 −6 5.6 −8 −9 −8 5.6 8 2.4 −10 −9 700 700 −10 3.2 −11 −9 5.6 4.8 −11 −8 −12 −10 −12 −11 4 −13 600 5.6 7 600 −13 −15 −14 −12 6.4 4.8 −14 −16 −15 −13 6.4 −16 −14 5.6 −18 −17 −10 7.2 −17 500 6 500 6.4 7.2 −12 6.4 −17 8 8 7.2 −16 400 7.2 8.8 8 400 −13 −12 5 −15 8.8 Height (m) −15 Height (m) 8.8 −16 8 300 300 8.8 −15 −14 −14 −14 8.8 4 −13 −14 8 −13 −13 200 8.8 200 −12 7.2 −12 −12 8 3 −16 6.4 −11 −11 −11 100 8.8 100 −10 7.2 −10 2 −10 0 8 8 0 −18 −122.8 −122.7 −122.6 −122.5 −122.4 −122.3 −122.2 −122.8 −122.7 −122.6 −122.5 −122.4 −122.3 −122.2 Longitude (deg) Longitude (deg) Figure 1. Contours of the observed wind components (u: east, v: north) in the area of the wind jet in the time period 1730 to 1900 UTC. The white line is the track of the aircraft.
Figure 2. Visible satellite pictures in the area of the experiment. The black line at the bottom of the second picture is the horizontal projection of the approximate path of the aircraft.
Similar cloud free zones, but with less intense the west of -122.2° longitude, reaches a maximum characteristics, were observed during other at about –122.7° and then decreases again until – experimental days (19 of June, 8-9 and 16-17 of 123.3 . Thus, it is evident that there is a local th ° July). The flight track on the 6 of July reached thinning of the cloud from a 300m depth to 100m 250 km offshore keeping almost constant latitude in this area. The straight lines in Fig. 3 are linear fit (36.7°N, Monterey bay) which is just at the south by eye to the cloud top and the expected cloud edge of the cloud free zone in Fig. 2. Soundings base. This was estimated from the lifting and straight, level legs were performed. condensation level (LCL) using data from a Figure 3 shows the observed cloud base and straight level flight leg at 30m above the sea top with longitude in the time period 1700 to 1900 surface. It should be noted that during a flight leg UTC. The cloud top is coincident with the base of at a height of 425 m passing from –122.7° at about the strong temperature inversion (about 10 K 1600 UTC the cloud base at that longitude was increase in 70 m depth) capping the cloudy marine estimated to be at about 350 m assuming boundary layer. This height increases almost adiabatic liquid water. The soundings during two linearly with distance from the shore. The cloud different times at –122.2° in Fig. 4 show the rapid base on the other hand shows a rapid increase to 1000 lift of the cloud base, while the cloud top height Cloud base Cloud top changes very little. Figure 5 shows measured air Temperature inversion base temperature reduced from 30 m to sea surface 800 LCL (from 30m data) using the dry adiabatic lapse rate and sea surface temperature. The sea surface temperature shows 600 a general increase with distance from the shore but with significant variations (local minima), which are most possibly cold pools due to local Height (m) 400 upwelling. Air temperature shows a similar general + trend to increase with distance from the shore, but 200 with less variation. However, at -122.7° the air temperature shows a steep drop, which is combined with the local sea surface temperature 0 −124.5 −124 −123.5 −123 −122.5 −122 drop. Figure 5 also shows heat and humidity Longitude (deg) turbulent fluxes along the same flight leg. The local minima of heat and humidity fluxes correspond to Figure 3. Observed height of temperature the minima of the difference between sea surface inversion base and cloud top and base in the time period 1700 to 1900 UTC.
1200 1200 15:40−15:46 UTC 15:40−15:46 UTC 17:59−18:08 UTC 1000 1000 17:59−18:08 UTC
800 800
600 600 Height (m) Height (m) 400 400
200 200
0 0 280 285 290 295 300 305 0 0.1 0.2 0.3 0.4 0.5 0.6 Θ (K) q (g kg−1) v l Figure 4. Profiles of virtual potential temperature (,v) and liquid water (ql) observed near the point with latitude 36.7°N and longitude 122.2°W at the beginning and the end of the flight.
287.5 2 2 −1 T
1 286.5 0.5
286 Turbulent flux
Temperature (K) 0
285.5 −0.5
285 −1 −124.5 −124 −123.5 −123 −122.5 −122 −124.5 −124 −123.5 −123 −122.5 −122 Longitude (deg) Longitude (deg) Figure 5. Measured air temperature (T) reduced from 30 m to sea surface using the dry adiabatic lapse rate and sea surface temperature (SST) and heat and humidity turbulent fluxes
Height (m) 0.14 just below the jet core, which assists in the 200 0.14 0.1 0.24 decoupling of the cloud layer from the surface. 0.2 0.12 150 0.12 0.16 0.14 0.1 0.18 100 0.22 0.26 0.16 4. CONCLUSIONS 0.24 0.08 50 0.2 0.22 0.18 0.2 0.06 The strong low-level coastal wind jet is probably 0 −122.7 −122.6 −122.5 −122.4 −122.3 the source of the local decoupling of the cloud Longitude (deg) from the surface layer and its rapid thinning and
Height (m) 0.4 0.3 decouple the cloud layer from the surface. 200 0.4 0.5 0.7 0.2 0.4 0.6 150 0.5 0.5 0.1 0.6 0.8 100 0.7 ACKNOWLEDGEMENTS 0.6 0 50 The present work was sponsored by the National 0.7 0 −0.1 Science Foundation (NSF) grant ATM9900496. −122.7 −122.6 −122.5 −122.4 −122.3 Longitude (deg) Figure 6. The variance of the vertical wind velocity REFERENCES 2