UC San Diego UC San Diego Previously Published Works Title When land breezes collide: Converging diurnal winds over small bodies of water Permalink https://escholarship.org/uc/item/9dt4w30b Journal Quarterly Journal of the Royal Meteorological Society, 140(685) ISSN 0035-9009 Authors Gille, ST Llewellyn Smith, SG Publication Date 2014 DOI 10.1002/qj.2322 Peer reviewed eScholarship.org Powered by the California Digital Library University of California When land breezes collide: Converging diurnal winds over small bodies of water Sarah T. Gille,a∗ and Stefan G. Llewellyn Smithb,a aScripps Institution of Oceanography, University of California San Diego bDepartment of Mechanical and Aerospace Engineering, University of California San Diego ∗Correspondence to: Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0230. E-mail: [email protected] Over enclosed and semi-enclosed bodies of waters, the land-breeze/sea-breeze circulation is expected to be modified by the presence of opposing coastlines. These effects are studied using satellite scatterometer surface wind observations from the QuikSCAT and ADEOS-2 tandem mission from April to October 2003. Winds are studied for six bodies of water: the Red Sea, the Gulf of California, the Mediterranean, the Adriatic Sea, the Black Sea, and the Caspian Sea. These bodies of water are large enough for the geographic orientation of the diurnal winds relative to the coastline to match the expected orientation for a straight coastline. Land breezes from opposite coastlines converge in the middle of these bodies of water, and in some cases the convergence line is shifted substantially away from the mid-point between opposite coastlines. Displacements in the convergence line appear likely to be explained by differences in the strength of the diurnal winds emanating from opposite coastlines, associated with differential heating or with island/peninsula effects, and by geographic displacements associated with large-scale mean wind patterns. Copyright c 2013 Royal Meteorological Society Key Words: Diurnal winds; scatterometer winds; enclosed and semi-enclosed seas Received . Citation: . 1. Introduction and at distances up to 500 km from shore, particularly in summer (Gille et al. 2003). The far reach of the land Daytime heating on land causes warm air to rise over breeze implies that over bodies of water with diameters the land surface, and results in onshore flows of cool air, up to 1000 km, land breezes from opposite coasts might while nighttime cooling over land reverses the process (e.g. be expected to converge and therefore to interact. The Simpson 1994; Miller et al. 2003). For centuries this wind objective of this study is to make use of data from the pattern has been known as the sea breeze when it flows from 2003 tandem QuikScat/ADEOS-2 scatterometer mission the ocean to land during the day (e.g. Dampier 1699) and as to evaluate locations where land breezes from opposite the land breeze when it flows from land to sea at night. (As coastlines would be expected to converge. a shorthand, in this study, we will use the term land breeze We examine six large enclosed or semi-enclosed bodies to refer to both the landward and seaward components of of water, as listed in Table 1. These are selected to cover the diurnal winds over water.) Satellite scatterometer winds a range of Northern Hemisphere latitudes and a range of have demonstrated themselves to be a valuable tool for sizes, all sufficiently large to be observable from microwave assessing the ubiquitous character of diurnal winds along scatterometry with a 12.5 km footprint. We also examined coastlines spanning a wide range of latitudes (Gille et al. Lake Superior, but it proved to be too far north and/or too 2003, 2005; Wood et al. 2009). small to exhibit a consistent pattern of diurnal variability. The QuikSCAT scatterometer demonstrated that over the Large enclosed or semi-enclosed bodies of water in the ocean land-breeze effects are detectable at all latitudes, Southern Hemisphere are few in number and were not Copyright c 2013 Royal Meteorological Society Prepared using qjrms4.cls [Version: 2011/05/18 v1.02] Table 1. Enclosed or semi-enclosed bodies of water considered in this influence wind patterns. The Earth’s rotation deflects the study, ordered by latitude. Angular deflections of the major axis are measured in degrees to the right of a line perpendicular to the coast. land breeze so that it is not strictly orthogonal to the Average angles are computed using only points in which the direction shoreline (e.g. Alpert et al. 1984). One might hypothesize of flow is offshore in the morning and onshore in the evening. Here, to that interactions of converging land-breeze fronts could be consistent with the figures, the angular deflection for Mediterranean weaken the land breeze or rotate the orientation of wind represents only the Eastern Mediterranean, encompassing the region to vectors away from the circulation predicted for semi-infinite the East of Sicily and excluding the Adriatic. The angular deflection for coastlines. A more fundamental issue is to assess the the full Mediterranean, excluding the Adriatic, is 41 ± 1o. processes that influence the position of the convergence line, 2 to determine when the sea-breeze convergence is essentially Body of water area (km ) mean major axis at the center of the body of water, and when and why it can latitude deflection be displaced from the center. Black Sea 4.4 105 44◦ N 54 2o In this paper, in section 2, we introduce the scatterometer Adriatic Sea 1.4 × 105 43◦ N 49 ± 6o data used for this analysis. Section 3 evaluates whether the Caspian Sea 3.7 × 105 41◦ N 49 ± 2o orientation of the land breeze or other basic characteristics Mediterranean 2.5 × 106 35◦ N 37 ± 2o differ for small bodies of water compared to the full Gulf of California 1.6 × 105 28◦ N 26 ± 3o ocean. Section 4 evaluates conditions in which one coastline Red Sea 4.4 × 105 22◦ N 16 ± 3o dominates another coastline. Finally results are summarized × ± in section 5. 2. Data considered, because the land breeze is predominantly a summertime phenomenon, and the QuikSCAT/ADEOS-2 Scatterometers are normally flown in sun-synchronous tandem mission took place during Northern Hemisphere orbits, meaning that they cross the equator at the summer. same local time everywhere (see e.g. Gille et al. 2003, The land-breeze circulation over a semi-enclosed sea 2005; Wood et al. 2009). For example, the QuikSCAT can be thought of as a simple reversal of the sea- scatterometer measured at about 6:00 and 18:00 on its breeze circulation over an island or peninsula. Island sea ascending (northward) and descending (southward) satellite breezes have been the subject of considerable modeling passes, and the SeaWinds scatterometer aboard ADEOS- and theoretical work (e.g. Malkus and Stern 1953a,b; II measured at 22:30 on its ascending pass and 10:30 Stern 1954; Olfe and Lee 1971; Neumann and Mahrer on its descending pass. The diurnal wind ellipse (e.g. 1974; Mahrer and Segal 1985; Niino 1987; Jiang 2012a; Haurwitz 1947; Schmidt 1947) cannot be characterized by Robinson et al. 2011). On islands and peninusulas, the one satellite alone, as the sun synchronous orbit samples at convergence lines where sea breezes from opposite coasts the Nyquist frequency of the diurnal cycle. For six months, intersect are associated with vertical convection, enhanced from April to October 2003, the QuikSCAT and SeaWinds cloud cover, high precipitation, and thunderstorm activity flew as a tandem mission, and these measurements have (e.g. Frank et al. 1967; Pielke 1974; Simpson 1994; provided a means to tease out details of the diurnal Carbone et al. 2000; Jury and Chiao 2013). Sea-breeze variability of the wind on a global scale (e.g. Gille et al. ‘collisions’ are thought to contribute to high precipitation 2005; Wood et al. 2009). rates over some tropical islands (Carbone et al. 2000) Here we use the high-resolution (12.5 km) QuikSCAT and to trigger formation of the Morning Glory bore in and ADEOS-2 wind products to repeat the land-breeze north Queensland, Australia (Clarke 1984). One of the analysis carried out by Gille et al. (2005) for the six bodies complications in studying converging sea breezes over land of water listed in Table 1. Figure 1 shows the time-averaged is that the processes are dependent on orography and land winds for these six bodies of water. surface (e.g. Carbone et al. 2000). Following the procedure discussed by Gille et al. (2005), Enclosed or semi-enclosed bodies of water offer a we average swath wind vectors measured by the two simpler framework for assessing converging diurnal wind scatterometers onto a 0.125◦ latitude by 0.125◦ longitude fronts, since the water surface is relatively homogeneous. grid for four time bins corresponding to the four satellite In the vicinity of the convergence line, the flow is not equatorial crossing times: 6:00, 10:30, 18:00, and 22:30. subject to orographic effects or land-surface variations. Averaged measurements from these four times are used to Neumann and Mahrer (1975) reviewed historic land-breeze project winds onto an ellipse and to identify the orientation observations associated with lakes and formulated an of the wind ellipse, the amplitude of the diurnal variability, idealized axisymmetric theory for a small lake with and the time of maximum offshore wind. Figure 2 shows the no background wind field. A number of more recent length of the semi-major axis of the diurnal wind ellipses (in investigations have brought newer modeling approaches colors) and sample ellipses (black ellipses).