ON THE PHYSICAL PROCESSES THAT INFLUENCE THE DEVELOPMENT OF THE MARINE LOW-LEVEL JET

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1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 ON THE PHYSICAL PROCESSES THAT INFLUENCE THE DEVELOPMENT OF THE MARINE LOW-LEVEL JET HELMIS C. G. and SGOUROS G. Department of Environmental Physics and Meteorology, Faculty of Physics, University of Athens, University Campus, Zografou, 15784, Athens, Greece EXTENDED ABSTRACT The aim of this work is the study of the different physical processes that influence the development, the vertical structure and the characteristics of the marine Low Level Jet (LLJ). According to previous studies several mechanisms are responsible for the development and evolution of LLJs in general, including the local topography and/or the baroclinicity resulting from large scale horizontal temperature gradient in the atmospheric boundary layer, the mid-latitude fronts and synoptic gradients and the inertial oscillation due to the frictional decoupling. This study is based on data analysis from SODAR and insitu instrumentation measurements, conducted in the frame of the Coupled Boundary Layers Air-Sea Transfer Experiment in Low Winds (CBLAST-Low), during August 2003, at Nantucket Island, MA, USA. Measurements of the vertical structure of the marine atmospheric Boundary Layer (MABL) from SODAR and rawinsonde launches frequently indicated the development of a LLJ under steady upstream wind conditions (marine air), with moderate to high wind conditions at the lower 500 to 600 m above the surface. The upwind MABL was characterized by a very surface stable layer followed by slightly stable conditions at higher levels, while wind speed maxima were observed for a few hours during day or night time, above the lower temperature inversion. Most of the observed LLJs cases were characterized by a single-level jet maximum with a few exceptions with double LLJ maxima. Since various mechanisms can lead to LLJ s development and some LLJ s may be a result of multiple mechanisms, in order to reveal the influence of the different physical processes, the Hilbert-Huang Transform (HHT) was applied to the time series of the wind data from ground observations and SODAR data at different heights. The application of the HHT algorithm to the estimated v and u wind components from the SODAR, as well as the in-situ and synoptic data analysis, reveal the temporal characteristics of the variations of the wind at different levels and lead to the understanding of the characteristics of the upwind MABL and the possible causes for the development and evolution of the observed LLJs. The analysis of the wind speed data by using the HHT algorithm showed high amplitudes corresponding to contributions from the inertial motions but also from physical processes characterized with different time-scales. Keywords: CBLAST-Low, marine Atmospheric Boundary Layer, Low-Level Jet, Hilbert- Huang Transform, Turbulence 1. INTRODUCTION Of many coastal meteorological phenomena, Low-Level Jet (LLJ) over the coastal Marine Atmospheric Boundary Layer (MABL) has been the focus of several previous studies. Parish (2000) shown that the large-scale structure of the MABL and its attendant LLJ along the California coast are related with the circulation forced by the horizontal temperature contrast between land and ocean. Kallstrand (1998) found that most of the LLJ profiles measured over the Baltic Sea were the result of the inertial oscillations

2 developed due to frictional decoupling of the atmospheric boundary layer over the sea surface. Smedman et al. (1993 and 1995) found from an observational study of a marine LLJ that the mechanical shear production of TKE is large on both sides of the LLJ jet core but asymmetrical. Above the jet core TKE production decreased rather rapidly while it increased below the jet core, suggesting downward transport of TKE from the jet core region to layers near the surface. Also they found that the measured shearing stress and sensible heat flux were strongly suppressed due to the presence of the LLJ by suppressing the low-frequency fluctuations (Smedman et al. 1997). They suggested, based on the shear-sheltering mechanism, that the large eddies are suppressed since a LLJ is present above the water surface in slightly stable thermal stratifications (Smedman et al. 2004). Several mechanisms have been suggested for the development and evolution of LLJs in general. These mechanisms include the local topography and/or baroclinicity, the midlatitude fronts, the deformation frontal-genesis and the inertial oscillation due to the frictional decoupling. The last mechanism is a result of the Coriolis force when the turbulent mixing decreases due to the presence of a stable layer near the surface and leads to an inertial oscillation with super-geostrophic speeds (Blackadar, 1957). It is worth to mention that the period of the oscillation is 2πf c -1, where f c is the Coriolis parameter and at mid-latitudes the estimated inertial period is about 17 hours. The magnitude of the oscillation depends on the amount of the geostrophic departure of the wind at the start of the frictional cease (Kraus et al 1985). The aim of this work is to study the various mechanisms that can lead to LLJ s development and to reveal the influence of the different physical processes. The Hilbert- Huang Transform (HHT) which was applied to the time series of the wind data from ground observations and SODAR data at different heights, reveal the temporal characteristics of the variations of the wind at different levels and lead to understand the characteristics of the upwind MABL and the possible causes for the development and evolution of the observed LLJs. The measurements which were used for the analysis were conducted as part of the Coupled Boundary Layers and Air-Sea Transfer Low (CBLAST-Low) project, aimed at the understanding of the air-sea interaction and the coupled atmospheric and oceanic boundary layer dynamics at low wind speeds (Edson et al, 2007). 2. INSTRUMENTATION AND DATA ANALYSIS The campaign was carried out during the summer 2003 at Nantucket Island, MA, USA and the experimental site was located on the southwest coast of the island within the complex of the island Waste Water Treatment Facility, at a distance of 94 m from the waterfront. Instrumentation includes a SODAR (Remtech PA2), fast and slow response sensors on a 20 m tower at different levels and a radiosonde system. The SODAR measurements yielded mean vertical velocity (w), horizontal wind speed and direction and the standard deviations of wind direction and vertical velocity at 30 minute intervals, with vertical resolution of 40 m, lowest level 50 m and a range of 500 to 700 m depending on the acoustic noise and the atmospheric conditions. Measurements of the mean wind, temperature and relative humidity and the corresponding standard deviations at 5, 10, and 20 m height, with a sampling frequency of 1Hz and 10 minute averaged output were carried out, as well as measurements of the 3-D wind components and water vapour concentration at a sampling rate of 20 Hz at two levels (10 and 20 m) with sonic anemometers and fast hygrometers to calculate TKE, fluxes of momentum, sensible and latent heat using the eddy correlation method while radiosondes were launched at the experimental site every four to six hours.

3 Figure 1: The location of the CBLAST-low Nantucket experimental site, denoted by the red star. Predominant wind in the area is south and south-westerly. The presence of LLJ is identified using the wind measurements from SODAR and rawinsondes, following Banta et al. (2002) suggestions. The vertical profiles of the mean horizontal wind speed and direction and their East-West (u) and North-South (v) components were estimated by the SODAR at 30 min interval as well as the profiles of potential temperature (θ), relative humidity (RH), horizontal wind speed (U), direction (D), static stability (Δθ Δz -1 ) and bulk Richardson number [Ri = (g θ-1) (Δθ Δz -1 ) (ΔU Δz -1 ) -2 ] from rawinsonde launches. Since the presence of in internal boundary layer (IBL) of about 10 m at our site was revealed, the data below the 10 m height was excluded from analysis and a detailed examination of the measured momentum and heat fluxes and the stability parameter (z L -1 ) time series from the 20 m measurement level was performed, in order to ensure that we study the pure MABL (upwind of the island). This examination clarified that the wind sector from 200 to 250 degrees corresponded to pure MABL (Katsouvas et al. 2007). All data have gone through extensive data quality checking and calibrations and the dataset at 20 m was corrected for errors imposed due to the axis tilt of the sonic anemometer (Mahrt et al. 1996). The Hilbert Huang Transform (HHT) algorithm was applied to the estimated v and u components from the SODAR and the in-situ instrumentation to reveal the temporal characteristics of the variations of the wind vector at different levels. The HHT algorithm is a method which has proved to be well-suited for studies of the stable ABL (Lundquist, 2003, Sgouros and Helmis 2009) and consists of two steps; the Empirical Mode Decomposition (EMD) that breaks the original time series into a finite number of Intrinsic Mode Function (IMF) components and the application of the Hilbert transform to the time series of each IMF component (Huang et al. 1999, 2000). The frequency-time distribution of the amplitude is the complete Hilbert amplitude spectrum and represents the distribution of energy in time scales corresponding to different physical processes. Compared with the traditional Fourier analysis, the HHT method preserves the time localities of events and is well-suited for non-stationary signal analysis (Li et al. 2005). It is worth to mention that the period of the inertial oscillation for the Nantucket area (41 N, 70W) is hours or a frequency of cycles per hour (cph). A window of (+/ cph), centred on the inertial frequency, was used to estimate spectra and time-height mapping of the amplitudes with inertial motions.

4 3. RESULTS AND DISCUSSION The North America, during the summer period, is dominated by the anticyclones of the Pacific and the Bermuda-Azores in the North Atlantic (see Figure 2). The Icelandic low appears weaker than in winter, while a thermal low appears in the desert regions of south-western USA. Along the eastern coast of USA, southerly winds carrying moist air, which is lifted, cooled and condensed, produce showers and thunderstorms in the eastern coast. During the cold intrusions in the north-eastern regions of USA, there are transported either continental polar air masses, very cold, dry and stable from Canada or maritime polar, cold, moist and unstable from the northwest Atlantic. Figure 2: Synoptic circulation during the summer period over North America The synoptic conditions over Nantucket area during the first two weeks of August 2003 were characterized by the presence of a large scale trough located over the Northeastern States and a large scale anticyclone over the greater North-western Atlantic Ocean. This trough moved slowly to the east producing a relatively strong SSW flow, which was confirmed from the rawinsonde measurements above the lower 500 m. The satellite images (every 6 hours) and the 12 hour surface analyses showed no presence of a front over a large area near Nantucket. During this period measurements of the vertical structure of the MABL from SODAR and rawinsonde launches frequently indicated the development of a LLJ under steady south-westerly (SW) wind conditions (upstream marine air) with moderate to high wind conditions at the lower 500 to 600 m above the surface. The upwind MABL was characterized by a very stable surface layer at the first 100 to 150 m followed by slightly stable conditions at higher levels. The wind speed maxima and the LLJs observed during day or night time above the intense temperature inversion and lasted for a few hours. More details can be found at Helmis at al. (2005). The data from August 7 represents a well defined case of LLJ s generation and evolution.

5 Figure 3: Vertical profiles of the horizontal wind speed and direction (a), potential temperature and relative humidity (b), static stability (c) and bulk Ri number (d) from five successive rawinsonde launches at 6 and 7 August The exact launching time is indicated on the upper left part of the figure. Five rawinsonde launches were made during the period between 1846 UTC 6 August and 0004 UTC 8 August, Figure 3a gives the wind speed and direction profiles from these successive rawinsonde launches. It is seen that the wind was constantly from the SSW sector above 400 m while below 400 m the wind veered at different levels within the S to SW sectors. During the first launch wind speed increased steadily with height while a wind speed maximum (not a LLJ) developed at 230 m 5 hours later (second launch). The presence of LLJ s between 180 and 270 m above the surface are identified from the next three launches throughout August 7. Figure 3b gives the θ and RH profiles from the same soundings. A very stable layer close to the surface persisted during the entire period. Below the stable layer, a shallow (20 m) unstable IBL layer was observed during the daytime launches. The depth of the stable layer varied from 55 m (first launching) to 75 m (second launch) and finally reached the height of 120 m during the third launch. During the last two launches the very stable layer was decreased in depth while the identified LLJ developed above the top of this stable layer. Stable thermal stratification continues at higher levels with weaker gradient in all five soundings. The RH profiles show high humidity of more than 95% in the lower layer up to 100 m followed by a gradual decrease at higher levels. Figures 3c and 3d give the profiles of the potential temperature gradient (Δθ Δz -1 ) or static stability and the Richardson number (Ri) or dynamic stability, respectively, which were

6 derived from the soundings of temperature and wind. A dynamically unstable layer can be identified if the Ri exceed the critical Richardson number with a theoretical value of 0.25 (Stull 1988). The first sounding revealed a surface-based unstable layer followed by a statically very stable layer up to 70 m height. On the other hand, the flow up to 250 m was turbulent (Ri values in the range 0.05 to 0.2). At higher levels, shallow layers of large Ri values are seen in Figure 3d, suggesting the presence of local non-turbulent layers. The depth and the intensity of the very stable surface layer increased during the next two soundings, reaching its maximum depth of 120 m during the third launch. At the same time, the dynamic stability of the MABL at lower levels exhibited major changes. During the second sounding, a surface-based non-turbulent layer (Ri value 2.9) appeared, while the depth of the turbulent layer above the surface decreased to 150 m (Ri values less than 0.5) and another layer of high Ri (Ri ~ 3.16) is seen just below the wind maximum. Apparently, the high stable static stability contributes to the large Ri and hence the suppression of turbulence in the lowest layer. During the third launch the non-turbulent surface layer increased in depth to 120 m as well as in the magnitude of Ri (Ri ~ 12.9 at 90 m). This non-turbulent surface layer decoupled the upper layers from the surface and hence the layers above were free of the effects of surface drag, which is resulted in the LLJ development at 180 m (Fig. 3a), which is consistent with the frictional decoupling mechanism of LLJ development. Above the LLJ core and up to 250 m, low Ri values (around 0.01) were calculated, indicating a turbulent layer as a results of intense wind shear above the jet core in moderate stable stratification. During the fourth launch, the presence of the very stable surface layer and the persistence of the non turbulent layer between 130 and 190 m (Ri ~ 2.44 at 170 m) also supports the frictional decoupling mechanism. The depth of the stable non- turbulent surface layer decreased below 100 m during the last launch while the strong mixing due to the LLJ produced a turbulent layer between 130 and 260 m. The strong wind shear below the LLJ core generated turbulence that extends down gradually to the non-turbulent surface layer. By the time the LLJ disappeared, the surface layer becomes turbulent again (not shown). Based on the above discussions, the plausible triggering mechanism for the LLJ development is related with the very stable statically surface layer that was strengthening in depth and intensity, which suppressed the turbulence and supported the frictional decoupling mechanism. This led to the LLJ development above the dynamically stable layer while the developed LLJ and the subsequent strong mixing due to the wind shear, led to turbulent layers below and above the LLJ core. The LLJ was preserved until the destruction of the nonturbulent surface layer even though a stable surface layer still existed. The HHT algorithm was applied to the u (E-W) and v (N-S) wind components calculated from the SODAR between 1800 UTC 3 August and 1800 UTC 6 August 6, where persistent LLJ events were observed. Figures 4a and 4b give the complete Hilbert spectra (frequencies and amplitude as a function of time) for the u and v component respectively at 190 m including all the IMFs for frequencies up to 0.2 cph. In this figure frequencies are plotted as a function of time and coloured to indicate amplitude variations. The frequency window (+/ cph) centered on the inertial frequency (0.054 cph) is indicated. Figure 4a shows that IMF7 exhibits rather stationary variations with strong amplitudes during the entire time period. The IMF5 gives intense wind variations with frequencies within the frequency window during the period from 0000 UTC 5 August to 0800 UTC 6 August. The IMF6 also contributes to the wind variations with frequencies within the inertial window during the period 1100 to 2130 UTC 4 August.

7 Figure 4: The complete Hilbert spectra (frequency and amplitude as a function of time) for the u (a) and v (b) components estimated from the SODAR at 190 m height for the period 1800 UTC 3 August to 1800 UTC 6 August 2003 By combining both time periods it is possible to identify the time period during which the u component of the wind was characterized by inertial motions. From Figure 4b it is evident that only IMF4 lies within the frequency window from 1400 UTC 4 August to 1200 UTC 6 August where the v component exhibited inertial motions with moderate amplitude variations. The same method was applied to all levels of the SODAR measurements and the time periods when wind variations with inertial frequencies (within the frequency window) were identified. 4. CONCLUSIONS The analysis of measurements gave an in-depth view for the understanding of the marine ABL vertical structure and the characteristics of the marine LLJ. Very stable stratification characterized the lower part of the marine ABL while slightly stable conditions observed at higher levels. On top of the intense ground based inversion layer a LLJ was frequently developed and persisted. A plausible triggering mechanism for the LLJ development is the frictional decoupling and the subsequent inertial oscillation. Our observations revealed that prior to the LLJ development a high dynamically stable surface layer associated with increased static stability was capable to suppress the turbulence and provide a favorable environment for frictional decoupling. The increase in depth and intensity of the not turbulent stable surface layer decoupled the higher layers from the effect of surface drag leading to LLJ development. Above and below the LLJ core,

8 turbulent layers were formed indicating strong mixing due to the intense wind shear. The LLJ was preserved until the destruction of the not turbulent surface stable layer from turbulence events generated by the increased wind shear above the surface. This leads to frictional coupling even though a statically stable surface layer was present. The application of the HHT algorithm on the u and v wind components from the SODAR confirmed the inertial oscillation of the wind vector and the complete Hilbert spectra provided intense wind variations associated with frequencies (periods) close to the expected inertial oscillation over the Nantucket area. ACKNOWLEDGEMENTS This work was supported by the Special Account for Research Grants of the University of Athens (grant 10812) and the Office of Naval Research (ONR). REFERENCES 1. Banta R. M., Newsom R. K., Lundquist J. K., Pichugina Y. L., Coulter R. L. and Mahrt L., (2002), Nocturnal Low-Level Jet Characteristics over Kansas during CASES-99. Bound. Layer Meteor., 105, Blackadar, A. K., (1957), Boundary Layer Wind Maxima and Their Significance for the Growth of Nocturnal Inversions. Bull. Amer. Meteor. Soc., 38, Edson J. and co-authors, (2007), The Coupled Boundary Layers and Air-Sea Transfer Experiment in Low Winds (CBLAST-LOW). Bull. Amer. Meteor. Soc., 88, Helmis C.G., C.H. Halios, G. Sgouros, G. Katsouvas and Q. Wang, (2005), On the mean vertical structure of the marine Atmospheric Boundary Layer, WSEAS Trans. on Environment and Development, Issue 2, Vol. 1, pp Katsouvas G.D., C.G. Helmis and Q. Wang, (2007), Quadrant analysis of the Scalar and Momentum Fluxes in the stable marine Atmospheric Surface Layer, Bound.-Layer Meteor., 124, Kallstrand B., (1998), Low level jets in a marine boundary layer during spring, Contrib. Atmos. Phys., 71, Kraus, H., J. Malcher and E. Schaller, (1985), Nocturnal Low Level Jet during PUKK. Bound.- Layer Meteor., 31, Li, H., Yang, L., Huang, D., (2005), The study of the intermittency test filtering character of Hilbert-Huang transform. Mathem. Computers Simul., 70, Lundquist J. K., (2003), Intermittent and Elliptical Inertial Oscillations in the Atmospheric Boundary Layer. J. Atmos. Sci., 60, Mahrt, L., Vickers, D., Howell, J., Hojstrup, J., Wilczak, J., Edson, J. and Hare J., (1996), Sea surface drag coefficients in the Riso Air Sea Experiment, J. Geophys. Res., 101, Parish T.R., (2000), Forcing of the summertime low-level jet along the California coast, J. Appl. Meteor., 39, Sgouros G. and Helmis C.G. (2009) Low-level jet development and the interaction of different scale physical processes, Meteorol Atmos Phys 104(3), Smedman AS, Tjernstrom M., and Hogstrom U., (1993), Analysis of the turbulence structure of a marine low level jet. Bound.Layer Meteorol., 66, Smedman, A.S., Bergstrom, H. and Horstrom, U., 1995: Spectra, variances and length scales in a marine stable boundary layer dominated by a low level jet. Bound.Layer Meteorol., 76, Smedman, A.S., Horstrom, U. and Bergstrom, H., (1997), The turbulence regime of a very stable marine airflow with quasi-frictional decoupling. J. of Geoph. Res.-Oceans, 102, Smedman, A.S., Horstrom, U. and Hunt J.C.R., 2004: Effects of shear sheltering in a stable atmospheric boundary layer with strong shear, Quart. J. Roy. Meteor. Soc., 130, Stull, R. B.,1988, An introduction to Boundary Layer Meteorology, Kluwer Academic Publ., 666 pp