Severe Bora on the mid - Adriatic

The Adriatic severe bora can partly be explained by the two-dimensional hydraulic theory, as one of the possible mechanisms of a strong surface downslope wind. A comparison of the application of the hydraulic theory shows a similar severe bora mechanism on the northern and mid-Adriatic, with the exception of the Senj area where special bora effects have been observed. As the theory could not completely explain Adriatic bora, this required a study ofa three-dimensional bora structure. The results have been achieved through analyses of the interaction of the upstream bora structure (sounding measurement atZagreb and Belgrade) and the downstream bora structure at Split. The only available aerological data at Split date back to the period 1959-1963 and can be used for a comparison of estimated hydraulic parameters on the lee side. Key word index: Severe bora, hydraulic theory, three-dimensional analysis. Saietak U radu je pokazano da dvodimenzionalna hidraulidka teorija, kao jedan od moguiih mehanizama jakog zavjetrinskog vjetra pri tlu, moZe djelomidno objasniti pojavu olujne bure na Jadranu. Neposredna usporedba primjene hidraulidke teorije ukazuje na jednak mehanizam olujne bure na sjevemom i srednjem Jadranu s izuzetkom na podrudju Senja gdje dolaze do izralaja lokalni efekti. Poito ova teorija nije u potpunosti mogla objasniti olujnu buru na srednjem Jadranu, to je.zahtijevalo istraZivanje trodimenzionalne strukture bure. Ovi rezultati dobiveni su na osnovi analize interakcija navjetrinske strukture atmosfere, iz radiosondainih podatakaZagrebai Beograda u terminima 00 i l2 UTC kao i zavjetrinske strukture u 6 situacija s olujnom burom ujedinom raspoloiivom periodu visinskih mjerenja u Splitu (1959-1963). Aerolo5ki podaci Splita posluZili su zausporedbusprocijenjenimparametrimauzavjetrini. Kljudne rijeii: Olujna bura, hidraulidka teorij4 trodimenzionalna analiza questions arose: Are the sounding measurements at Zagreb end Belgrade rcpresentative of upstream bora layer in the mid-Adriatic? Is there any essential difference in the vertical atmospheric structure between Zagreb and Belgrade ? The answers to these questions are the first attempt to confirm the basic characteristic flow during the occurrence of the mid -Adriatic bora. HYDRAULIC THEORY APPLICATION A short review ofthe hydraulic theory The basic characteristics ofthe hydraulic theory are briefly illustrated in cases of continuous atmospheric stratification, one and two-layer models. One-layer model Long (1953) was the first to consider theoretically and experimentally the application of the hydraulic theory to a uniform flow across the mountain with a discontinuity in density. It assumes a twodimensional hydrostatic steady flow with two homogenic fluids of different density. The lower neutral layer is covered with a very thin temperature inversion. In that case the model is reduced to a one-layer model and the motions and continuity equations for shallow water are valid. Only the disturbed lower layer falls over the obstacle and there are no imposed pressure gradients of the upper layer. As this rarely occurs in the real atmosphere, the hydraulic theory can not be often applied. The vertjcal wind profile during a bora event is oftcn similar to the assumptions of the theory, as a wind above the bora layer is light or reversed. Therefore, there is no pressure gradient on the cold lower layer. According to the hydraulic theory for single layer model, a certain relationship must exist among the upstream wind Uo, the stability Nrt (N, the BruntVdisiilii frequency), the mountain height /r, the depth of the upstream layer Ho and the depth of the temperature inversion d: {=,*!ri -lP^'n (l) H6-" 2'o 2'o 4=+, g'=N:d , N2=gAP Jg'Ho ' p)z Fo is the Froude number representing the ratio of the wind speed and the velocity of the long gravity waves. From the equations of momentum and mass conservation Bainse (1987) obtained the relations of dependson FoandHo: Hrvatski meteorolo5ki dasopis, 28, 1993 The first curve is presented on Figure lb by AF and the second curve by BC. To the left of the curves AB and AF the flow is either supercritical (f',> I ) or subcritical ( Fo< I ). To the right of BC the obstacle height is sufficiently large to completely block the flow. In a partially blocked case, flow over the obstacle crest is controlled by the local condition F..: l. A hydraulic jump may be attached on the upstream ( region ABCD ) or downstream ( region ADE ). In the region EAF the flow may be either partially blocked or supercritical, depending on the initial conditions. C o nli nuou s slratiJi c ati o n The analyses of aircraft measurements during bora occasions on the northem Adriatic in the ALPEX period 1982 helped Smith to construct a new hydraulic model using Long's equation for the continuous stratification N, and uniform upstream flow Uo ( Smith, 1985). It is presumed that the critical streamline at the level Ho splits over the mountain with the lower branch descending rapidly, by6", towards the lee side. Above H, a weak flow is present, at least in comparison with the strong perturbance below. Between the split streamlines the air is well mixed so that the density in that region is constant. The descending of the critical streamline is obtained by: g" = 6"/ h= hl (3) ,=l+-ll+#)" (4) * =#lt. ;(;'.0)"'1"' The Scorer pararneter / is the maximum horizontal wave number with which steady linear gravity waves can propagate in the vertical. If the Scorer parameter decreases with height, there will be a range of wave number over which gravity waves can have a periodic vertical structure only near the ground. f is ttre effective mountain height. The vcrtical profile of velocity perturbations u7$,,2) and the height of ff, (the descending layer on the lee side) is obtained from the relation: (n\ ur(x,,2) = U ol I 6", cos lz I \/ fr, =, ;,,=+, fr, t=lt(rf * t)'t' nnn Ht =Htl, Ho =Hol, 6"r=6", / *llor;)-' -i-1, e) (5)