Ann. Geophysicae 18, 235±246 (2000) Ó EGS ± Springer-Verlag 2000

Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts

S. Federico1, G. A. Dalu2, C. Bellecci3, M. Colacino2 1 CRATI S.c.r.l c/o University of Calabria, I-87036 Arcavata di Rende (CS) Italy, e-mail: federico@fermi.®s.unical.it 2 IFA-CNR Tor Vergata, via Fosso del Cavaliere, n. 100, I-00133 Rome, Italy 3 S.T.F.E. Department, Engineering Faculty. University of Tor Vergata. Via di Tor Vergata, I-00133 Rome, Italy

Received: 19 March 1999 / Revised: 29 September 1999 / Accepted: 15 October 1999

Abstract. We study the relative importance of sea-land show that the ¯ow is highly ageostrophic, and that the and mountain-valley thermal contrasts in determining ¯ow intensity increases from sunrise to reach its max- the development of thermally forced mesoscale circula- imum in the afternoon but before sunset, which suggests tions (TFMCs) over a mountainous peninsula. We ®rst that, in the late part of the day, the conversion of analyse the energetics of the problem, and using this potential energy into kinetic energy is balanced by the theory, we interprete the numerical simulations over dissipation. Calabria, a mountainous peninsula in southern Italy. The CSU 3-D nonlinear numerical model is utilised to simulate the dynamics and the thermodynamics of the Key words: Meteorology and atmospheric dynamics atmospheric ®elds over Calabria. Results show the (climatology; mesoscale meteorology) importance of orography in determining the pattern of the ¯ow and the local climate in a region as complex as Calabria. Analysis of the results shows that the ener- getics due to the sea-land interactions are more ecient when the peninsula is ¯at. The importance of the energy due to the sea-land decreases as the mountain height of 1 Introduction the peninsula increases. The energy stored over the mountain gains in importance, untill it is released by the In the early hours of the morning the sun warms the readjustment of the warm mountain air as it prevails east-facing slopes ®rst, then during the mid part of the over the energy released by the inland penetration of the day the south-facing slopes and the west-facing slopes in sea breeze front. For instance, our results show that over the afternoon. The local thermally driven winds follow a peninsula 100 km wide the energy over the mountain the development of the convective boundary layer and the energy in the sea-land contrast are of the same (CBL), with some delay. Over a mountainous peninsula order when the height of the mountain is about 700 m, the convective boundary layer has di€erent depths and for a 1500 m convective boundary layer (CBL) depth. altitudes; the resulting horizontal thermal gradients Over the Calabrian peninsula, the energy released by the drive mesososcale ¯ows. Over the slopes, the diabatic hot air in the CBL of the mountain prevails over the ¯ux in the CBL drives a mountain-valley breeze, and, energy released by the inland penetration of the sea air. in addition, since a peninsula is bounded by the sea, the Calabria is about 1500 m high and about 50 km wide, sea-land thermal contrast at the coast drives a sea-land and the CBL is of the order of 1500 m. The energy over breeze. the mountain is about four time larger than the energy The scope of this study is to quantify the relative contained in the sea-land contrast. Furthermore, the importance of the sea breeze and of the valley breeze over energetics increase with the patch width of the peninsu- a peninsula. We evaluate analytically the mesoscale la, and when its half width is much less than the Rossby available potential energy (MAPE) due to the sea-land radius, the MAPE of the sea breeze is negligible. When contrast, the mountain-valley contrast, and the moun- its half width is much larger than the Rossby radius, the tain-sea contrast. We then use these results to examine breezes from the two opposing coastlines do not the role of the sea-land and the mountain-valley breezes interact. Over Calabria peninsula, numerical simulations in the thermal convergence over Calabria. Calabria is a peninsula located in southern Italy, which ranges between 38120 and 40, and between 16300 and 17150E. On the west Calabria is bounded by the Correspondence to: S. Federico Tirrenian Sea, and on the south and on the east by the 236 S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts

Ionian Sea. The topography (Fig. 1) is characterised by e€ects of sea-land and mountain-valley contrast because the ridges of the Apennines. From north to south, there of their relevance in determining the local climate. are four main ridges: Catena Costiera, La Sila, Le Serre, Three simulations are presented and compared with and Aspromonte. There are four main peaks: Montalto considerations on the energetics: in the Aspromonte, Pecoraro in the Serre, Botte Donato 1. The sea-land breeze over a ¯at peninsula in the La Sila and Serra Dolce Dorme the north side of 2. The mountain-valley breeze the Catena Costiera (coastal ridge). These mountains are 3. The combination of the sea-breeze with the mountain- steep with an altitude of 1500±2000 m or 1.5 to 2.0 km valley breeze and a half width of few tens of kilometres. The pass of Marcellinara, a gap between La Sila and Le Serre, is In the ®rst case we consider no orography and a sea- located in the narrowest part of the Calabrian peninsula land contrast as if the Calabria peninsula were ¯at, in in the W±E direction, its width is 30 km from coast to this case the forcing is only due to the sea-land contrast. coast. In Calabria there are three main valleys close to In the second case the real topography of Calabria is the sea: Sibari to the east on the Ionian Sea, and Gioia introduced but we ignore the sea-land contrast (the sea Tauro and Lamezia to the west on the Tyrrhenian Sea. is substituted by ¯at land), in this case the forcing is due Previous studies suggest that the local climate in to the mountain valley contrast. In the third case the real Calabria is controlled by the sea breeze and by the sea-land contrast and the real topography of Calabria upslope ¯ow, which occur frequently under calm or are both present, in this case the forcing is due to nearly calm conditions and clear skies, typical of the the combined sea-land contrast and mountain-valley warm season in Calabria (Baldi et al., 1997; Baldi et al., contrast. 1998). Local ¯ows are strongly conditioned by the chief of the region, which plays a fundamental role in determining the location and the strength of the 2 Energetics due to the sea-land contrast cumulus convection. Fair weather cloud distribution is over a ¯at peninsula strongly related to the speci®c morphology of the peninsula, which induces upslope ¯ows and sea breezes, In order to evaluate the role of sea-land contrast in accompanied by vigorous convergence and upwelling determining the local ¯ow intensity, we evaluate the over the crests of the ridges in the interior of the energetics over a ¯at peninsula. The mesoscale available peninsula. Therefore it is interesting to examine the potential energy, MAPE, is the energy di€erence

Fig. 1. Topography of Calabria S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts 237 between the initial and ®nal con®guration (Green and Dalu, 1980):

MAPE ˆ PEi PEf ZR‡L Zh  gh gh ˆ dx i f z dz : 1† H H R‡L† 0 Here 2L is the width of the peninsula, h is the depth of the mesoscale ¯ow, and R is e-folding distance of the ¯ow intensity from the coastline (Dalu and Pielke, 1989). hi is the initial diabatic temperature perturbation, and hf is the ®nal temperature perturbation. In the ®nal state the potential energy is lower, the di€erence is converted into kinetic energy of the mesoscale ¯ow or dissipated. The sea-breeze stream-function, computed using linear theory is:  z jxj=R0 w x; z†ˆw0H e ; h0   Fig. 2. Stream-function of two interacting breeze cells as a function of z 1 z z2 h2 z ‡ h peninsula width. The stream-function of the peninsula is normalized H ˆ ln 0 ‡ ln 0 2† 2 to the stream-function of a single sea breeze cell h0 2p h0 z z h0 N h2 N w ˆ 0 0 ; R ˆ h 0 : 0 2 0 0 f ZR‡L Zhi Hz MAPE ˆ PEi PEf ˆ 2 dx g hi z†z dz Here N0 is the Brunt-Vaisala frequency and f is the H L 0 Coriolis parameter where h0 is the depth of the CBL R‡L hf over the peninsula and R0 is the Rossby radius. The sea- Z Z Hz breeze intensity has an e-folding distance from the 2 dx g hf z†z dz coastline equal to a Rossby radius (Dalu and Pielke, H 1993). 0 0 RN 2h3 R ‡ L†N 02h3 The stream-function over two adjacent lands with ˆ 0 i 0 f : 5† CBL of di€erent depths, h1 and h2 respectively, is: 3 3   0 N h2 z N h2 z Here N0; N0 are the initial and the ®nal Brunt-Vaisala 0 1 jxj=R1 0 2 jxj=R2 0 w x; z†ˆ H e H e 3† frequencies, respectively, and Hz and Hz are the initial 2 h1 2 h2 and ®nal vertical potential temperature gradients, re- Over a peninsula with the coastlines at x ˆ L and at spectively. The ®nal con®guration and the ®nal Brunt- x ˆL, the stream-function is: Vaisala frequency are determined by mass conservation and by thermodynamics: wpeninsula x; z†ˆw x ‡ L†‡w x L† ( 02 2 hi  N0 ˆ N0 z  hf 6† jx‡Lj=R0 jxLj=R0 hi ˆ w0H e e ; hf ˆ R R‡L h0  4† The mean sea-breeze intensity U is the square root of 2MAPE divided by the volume, 2 R ‡ L†hi, involved in The ¯ow intensity, Eq. (4), plotted as a function of the the adiabatic redistribution of air masses: width of the peninsula (Fig. 2) shows that, when L < R0,  R N 2h3 1=2 the two sea-breeze cells in Eq. (3) interfere. Further- U ˆ 1 0 i 7† more, when L  R0, the breeze intensity becomes R ‡ L 2 R ‡ L† negligible, and, when L  R0, the two breeze cells are independent, therefore we study the mesoscale energet- Figure 4a, b shows the mesoscale available potential energy and the mean wind intensity as computed by Eqs. ics over a peninsula where 0 < L < R0. Then R in Eq. (1), using Eqs. (2, 4), is given by: (7) and (8). The CBL heights are 1.0, 1.5 and 2.0 km,  respectively, L ranges from 0 to 200 km. The sea-breeze w z intensity grows as the boundary layer height increases w x ˆ R ‡ L†; z†ˆw x ˆ R ; z†ˆ 0 H peninsula 0 e h and as the width of the peninsula increases, to reach its 0 maximum when L ˆ R, the ¯ow intensity ranges from Figure 3a, b shows the initial and ®nal con®guration of 2.5 to 5.5 ms1 when the boundary layer ranges from 1 air masses for a ¯at peninsula. The MAPE is: to 2 km. 238 S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts a

b

a

Fig. 3. a Air masses in a heated boundary layer over a ¯at peninsula. hi is the initial boundary layer depth, 2L is the patch width and R is the Rossby radius. b As in a but after the adiabatic redistribution of air masses. hf is de®ned by mass conservation. R and L as in a

Where L  R, the two sea-breezes are disconnected and independent. The sea-breeze MAPE in a semi- in®nite land bounded by a semi-in®nite sea is:

RN 2h3 MAPE ˆ 0 i 8† 3

3 Energetics of the mountain-valley b and the sea-mountain ¯ow Fig. 4. a Mesoscale available potential energy for the sea-land contrast Figure 5a, b shows the initial and ®nal distributions of shown in Fig. 3a, b. The three curves refer to hi ˆ 1000 m, air masses over a land with a low mountain. Figure 5c, d hi ˆ 1500 m, hi ˆ 2000 m. L as in Fig. 3a. b Mean wind velocity for shows the initial and ®nal con®gurations over a land the sea-land contrast shown in Fig. 3a, b. The three curves refer to h ˆ 1000 m, h ˆ 1500 m, h ˆ 2000 m. L as in Fig. 3a with a high mountain. When the land is bounded by the i i i sea, i.e. the peninsula case, the CBL depth over the sea is zero. When the ridge is bounded by ¯at land, there is a CBL over the plain as well. In general the CBL depth at z ˆ h1. The initial potential energy, due to the sea- over the mountain is di€erent than the CBL over the land contrast, PEisea, is: plain. ZR‡L Zh1 dh PE ˆ 2 dx g i z dz; d# ˆ H h z† 9† isea H i z 1 3.1 Energetics due to the sea-land contrast, L 0 where there is a mountain where h1 is the CBL depth and Hz is the initial potential In this section, in the evaluation of the potential energy, temperature vertical gradient. The ®nal potential energy, we take as reference the temperature of the air initially PEf sea, is: S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts 239

Fig. 5. a Air masses in two heated boundary layer over mountainous adiabatic redistribution so that hm < hf 1. hf 1 and hf 2 represent the terrain, for a small mountain. hm is the mountain height. R and L as in ®nal heights of the boundary layer h1 and h2, respectively. R and L as Fig. 3a. h2 is the boundary layer depth over the mountain and h1 is in Fig. 3a. c As in a but for a high mountain. d As in b but for the boundary layer depth over the ¯at terrain. b As in a but after an hm > hf 1, for a high mountain

ZR‡L Zhf 1 ZR‡L Zhf 1 dh H0 PE ˆ 2 dx g f z dz; d# ˆ H0 h z† PE ˆ 2 dx g z h z†z dz f sea H f z f 1 f sea H f 1 0 h x† 0 0 h Lx 10† L m †L Z Z 0 Hz 2 dx g hf 1 z†z dz 0 h1 where hf 1 is the ®nal depth of the sea air, H ˆ Hz, H z hf 1 and h x† is the mountain pro®le, and it is zero over 0 0  the sea. N 02 h ˆ 0 h3 R ‡ L†h2 Lh m 12† We ®rst consider the energetics of a low mountain, 3 f 1 m f 1 2 hm < hf 1, and later we study the case of a high The mesoscale available potential energy is MAPE ˆ mountain, hm > hf 1. Referring to Fig. 5a, b, i.e. to a PE PE . The ®nal geometry is determined by low mountain, the initial potential energy, PEisea, is: isea f sea mass conservation, while the ®nal Brunt-Vaisala fre- R‡L h quency is determined by the thermodynamics (Fig. 5b): Z Z 1 2 3 Hz N0 h1R ( PEisea ˆ 2 dx g h1 z†z dz ˆ ; N 02 ˆ N 2 h1 H 3 0 0 h 11† f 1 13† L 0 L h1 hf 1 ˆ hm ‡ R gH 2 R‡L† R‡L N 2 ˆ z 0 H In the case of a high mountain (Fig. 5c, d; hf 1 < hm) the initial potential energy is given by Eq. (11) and the ®nal And the ®nal potential energy PEf sea is: PEf sea is: 240 S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts

ZR‡L Zhf 1 The mesoscale available potential energy is H0 PE ˆ 2 dx g z h z†z dz MAPE ˆ PEimountain PEf mountain. The ®nal geometry f sea H f 1 is determined by mass conservation, while the ®nal L 0 Brunt-Vaisala frequency is determined by the thermo- Lx L hm †L dynamics (Fig. 5b): Z Z 0 Hz 8 ‡ 2 dx g hf 1 z†z dz > 002 2 h2‡hmh1 H > N0 ˆ N0 Á < hf 2hf 1 hmh 0 h L f 1 h ˆ h L ‡ R 1 hm f 1 m 2 R‡L† R‡L 19† "# > 4 : R h2‡hm† Lhm 02 Lh hf 2 ˆ ‡ N0 3 f 1 R‡L 2 R‡L† ˆ hf 1R ‡ 14† 3 2hm For a high mountain (Fig. 5c, d; hf 1 < hm) the initial Mass conservation and thermodynamics determine the potential energy is given by Eq. (17) and the ®nal ®nal con®guration and the Brunt-Vaisala frequency PEf mountain is: (Fig. 5d): ZR‡L Zhf 2 8 H00 < 02 2 h1 z N0 ˆ N0 PEf mountain ˆ 2 dx g hf 2 z†z dz hf 1 H 1 15† R2h2 ‡2Lh h R†2Rh 0 h : h ˆ m 1 m m f 1 f 1 L Á hmh L f 1 h †Lx Z hm mZ L H00 2 dx g z h z†z dz 3.2 Energetics due to the CBL over the mountain H f 2 0 hf 1 In this section, in the evaluation of the potential energy, "# h3 we take as reference the temperature of the air initially 002 f 2 2 2 3 ˆ N0 R ‡ L† hf 2hf 1 ‡ hf 1 at z ˆ h2 ‡ hm. 3 3 The initial potential energy, due to the hot air in the " 2h3 h h † h h2 h h † CBL over the mountain, is (Fig. 5a, c): 002 f 1 m f 1 f 2 f 1 m f 1 N0 ZR‡L hZ1‡hm 3hm hm dh PE ˆ 2 dx g i z dz; ! !# imountain H h h3 h3 1 h4 h4 ‡ f 2 m f 1 m f 1 L h1 3 hm 6 hm d#i ˆ Hz h2 ‡ hm z† : 16† 20† Here h1 is the CBL depth, Hz is the temperature gradient, hm is the mountain height, and h2 is the CBL Mass conservation and thermodynamics determine depth over the mountain. the ®nal con®guration (Fig. 5d) and the ®nal Brunt- The initial potential energy, PEimountain Eq. (16), is: Vaisala frequency: 8 ZR‡L hZ2‡hm 002 2 h2‡hmh1 > N0 ˆ N0 Hz <> hf 2hf 1 PEimountain ˆ 2 dx g h2 ‡ hm z†z dz 1 2 2 2 H †R hm‡2Lh1hmR Rhm 21† hf 1 ˆ L h1 > L "#: R h ‡h † h ˆ 2 m ‡ Lhm h ‡ h †3 2 f 2 R‡L 2 R‡L† ˆ N 2R 2 m h ‡ h †h2 ‡ h3 : 0 3 2 m 1 3 1

17† 3.3 Discussion on the energetics

PEimountain is the same for a high or a small mountain. For the evaluation of potential energy in the ®nal In the previous two sections we have shown the con®guration we ®rst consider the energetics of a small energetics of a sea-land and a mountain-valley contrasts mountain, hm < hf 1, and later the case of a high where a mountain is located. Where there is a mountain mountain, hm > hf 1. bounded by a valley, i. e. a mountain-valley contrast, the For a small mountain the ®nal potential energy total MAPE is given just by the second contribution PEf sea is: (Sect. 3.2) because the isentropic air above the valley does not change its potential energy between the initial ZR‡L Zhf 2 and ®nal state. However, this air participates in the H00 z motion taking its kinetic energy from the potential PEf mountain ˆ 2 dx g hf 2 z†z dz H energy of the warm air above the mountain. In the case 0 h f 1 "#of a mountainous peninsula, i. e. the sea-mountain h3 2 contrast, the total MAPE is given by the sum of both ˆ N 002 R ‡ L† f 2 h h2 ‡ h3 18† 0 3 f 2 f 1 3 f 1 contributions due to the redistribution of the air above the sea and the hot air above the mountain. S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts 241

In the following discussion the CBL depth over the The mean velocity increases as the boundary layer sea is assumed to be zero. However when the ridge is increases. In addition it is almost constant as a function bounded by a plain the depth of the CBL over the plain of the peninsula width due to the increase of both the is assumed to be equal to the CBL depth over the MAPE and the volume, V . In the speci®c case of mountain. Calabria (h1  1500 m, L  50 km, hm ˆ 1500 m) the Figure 6a, c and e shows the MAPE released by the analytical computed mean wind is about 5.5 ms1. sea breeze for a CBL depth of 1000, 1500, and 2000 m. The development of the CBL over a ¯at terrain is: The di€erent curves within the same frame refer to Zt0 di€erent heights of the mountain 0, 500, 1000, 1500, 2Q h t†ˆ 0 q t0†dt0 ˆ h 1 cos xt†† 2000 m. 2 0 N0 Figure 6a, c and e shows that the MAPE released by 0 the sea breeze increases as the depth of the CBL where Q the diabatic bouyancy source: increases. However, the sea breeze MAPE decreases as the height of the mountain increases, since the presence Q ˆ Q0 sin xt†He h z†r x†ˆQ0q t†He h z†r x† of a mountain acts as an obstacle to the inland The CBL over the mountain sides is a function of the penetration of the sea breeze. The barrier e€ect exerted slope and of its exposition. When the ridge runs north- by the mountain on the sea breeze increases as the south, there is a time lag dt between the west and the east mountain height increases. side and a depth di€erence dh: Figure 6b, d and f shows the MAPE released by the 2h 2h air which was initially in the CBL over the mountain. dt ˆ m ; dh ˆ m h sin xt†; Di€erent frames refer to di€erent CBL depths 1000, xL L 0 23† 1500, and 2000 m, di€erent curves in the same frame 2p h h x ˆ ; sin m  m refer to di€erent heights of the mountain 500, 1000, day L L 1500, 2000 m. The CBL depth over the ridge is assumed to be equal to the CBL depth over the ¯at land. Figure In Calabria, dh is of the order of 100 m. In addition, dt is 2 2 1=2 6b, d and f shows that the MAPE released by the warm less than one hour and adds up to T ˆ f ‡ k † , air over the mountain increases as the depth of the CBL which is the delay of the mesoscale ¯ow response to the and as the height of the mountain increases. This CBL growth, T is of the order of 1.5 h (Dalu et al., MAPE is zero when the mountain height is zero. 1991). Therefore, the mesoscale ¯ow is stronger on the Therefore, there is for each width of a peninsula a east side in the morning hours, while in the afternoon it height of the mountain at which the sea breeze MAPE is stronger on the west side. When the ridge runs west- equals the MAPE released by the mountain. With a east, the CBL is deeper on the south side, the di€erence mountain peninsula bounded by the sea, the total grows in time and reaches its maximum at sunset: MAPE is the sum of the sea breeze MAPE with the 2h dh ˆ m h 1 cos xt†† MAPE released by the mountain air, as is shown in L 0 Fig. 7. The MAPE released over a peninsula is computed This e€ect becomes relevant when dh is a sizeble fraction by adding the sea breeze MAPE contribution to the of h1. MAPE contribution due to the hot air over the mountain, as is shown in Fig. 7a, c and e. The 4 The simulations di€erent frames refer to di€erent CBL depths 1000, 1500, and 2000 m, while the di€erent curves within the Calabria is a narrow peninsula with high mountains, same frame refer to di€erent heights of the mountain and, since its width is less than one Rossby radius in the 0, 500, 1000, 1500, 2000 m. The dashed curves refer to east-west direction and two Rossby radii in the north- the ¯at peninsula case, Fig. 4a. In this case, the south direction, we expect the induced atmospheric MAPE, released over a peninsula, increases as the perturbation to be ageostrophic. boundary layer increases and as the peninsula width During the warm season, the local ¯ow is dominated increases. by the sea breeze and by the mountain valley breeze. In the particular case of Calabria (h1  1500 m Observations show that the sea and valley breezes merge L  50 km hm  1500 m) the energy released by the in the early afternoon, producing a strong updrought. In inland penetration of the sea is about a quarter of the order to examine the relative importance of the sea-land energy released by the redistribution of the hot air over breeze and of the mountain-valley breeze over Calabria the mountain. The mean wind intensity is the square we simulate the dynamics and the thermodynamics root of two times the kinetic energy density, KE. KE is using a 3-D nonlinear, hydrostatic, incompressible form the ratio between the MAPE and the volume, V , of the of the Colorado State Mesoscale Model CSUMM. This air mass involved in the mesoscale ¯ow: model is described in Pielke (1974, 1984), Mahrer and Pielke (1977a, b) and McNider and Pielke (1981). The V ˆ 2 R ‡ L† h ‡ h †Lh 22† 2 m m entire domain (Fig. 1) extends for 320 km in the N-S Figure 7b, d and f shows the mean velocity ®eld for the direction and 320 km in the E-W direction. The hori- mountainous peninsula. The dashed curves show the zontal resolution is Dx ˆ Dy ˆ 3:33 km with 16 uneven- case of a ¯at peninsula bounded by the sea, Fig. 4b. ly spaced vertical levels, which extend from ground to 242 S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts 243 b of local ¯ows. Indeed the di€erence between the Fig. 6a±f. Mesoscale available potential energy for sea-land and simulated mean ®eld intensity and the one shown in mountain-valley contrasts in the mountainous terrain shown in Fig. 10 (i.e. sea-mountain contrast) is remarkably high. Fig. 5a±d. a, c and e refer to the sea-land contrast and correspond to The mean ¯ow simulated is less intense than that di€erent heights of the CBL h1 ˆ 1000; h1 ˆ 1500; h1 ˆ 2000 m. b, d and f brefer to the mountain-valley contrast and correspond to computed using the energetics. This di€erence is due to di€erent heights of the CBLs h1 ˆ h2 ˆ 1000; h1 ˆ h2 ˆ 1500; h1 ˆ the fact that in the numerical model both physical and h2 ˆ 2000 m. Di€erent curves in the same frame represent di€erent numerical dissipations are present. mountain heights, hm ˆ 500; hm ˆ 1000; hm ˆ 1500; hm ˆ 2000 m. Dashed lines refer to the ¯at terrain case, Fig. 4a, and are shown for completeness 4.2 Simulations of the mountain-valley and of the sea-mountain ¯ows 13 000 m, with a dissipative layer above 7000 m. The time step is 15 s and the integration time is 24 h, starting First we discuss the results concerning a run in which the from sunrise. A pre-run enables balancing of the real topography of the Calabria peninsula is introduced, meteorological ®elds with the lower boundary drag and the sea-land contrast is not considered. Then we due to the topography and to the surface roughness. An present the results of a run in which the real topography initial weak environmental ¯ow is assumed to jumpstart and the real sea-land contrast of the Calabria peninsula the surface ¯uxes, which allows us to initialise the are both introduced. The meteorological and soil friction velocity uÃ, #Ã, and qÃ. parameters are as in the previous case. Figure 9 shows As reported in previous studies (Baldi et al., 1997), the wind vector at 16:00 LST and at 75 m above the and, the (Baldi et al., 1997, 1998) climatic conditions for ground in the case of mountain-valley contrast. The diabatic driven local ¯ows are more favourable in simulated maximum wind intensity is about 7 ms1, summer, therefore in the next sections we report results showing less intense kinetic energy when locally com- for the 16, July, under calm large-scale winds and clear pared to the case in which both forcings are present. sky conditions, as often observed in Calabria in summer These results are in agreement with our energetics and sometime' in winter as well (Colacino, 1992), and considerations and show that the orographic forcing is, (Colacino et al., 1997). We simulate the atmospheric in the case of the Calabria peninsula, larger than the ¯ow in three di€erent con®gurations. forcing due to sea-land contrast in determining local mesoscale ¯ows. 1. We consider a ¯at peninsula and a sea-land contrast In the case of sea-mountain contrast (Fig. 10), i.e. as in a Calabria with no topography. both forcings are considered, there is a strong conver- 2. We introduce the real topography of the Calabria gence in the centre of the region. The simulated ¯ow peninsula but neglect the sea-land contrast, the sea is intensity maximum is larger than 9 ms1 and there are substituted by ¯at land. four main convergence peaks, associated with the four 3. The real land-sea contrast and the real topography of main mountains of the peninsula. There is also evidence Calabria are both present. of two climate axes associated in the north and south part of the peninsula with the ranges Catena Costiera and Appennino Calabrese, respectively. 4.1 Sea breezes simulations

In this section we discuss the results for a run in which 5 Conclusions the sea-land contrast is introduced as if the Calabria peninsula had a uniform topography of 10 m a.s.l. (¯at The present study investigates the relative role of the terrain). sea-land contrast and of the mountain-valley contrast in We show results when the large-scale ¯ow is weak determining the local thermal convergence pattern over 1 1 ug ˆ 0:0ms , vg ˆ 0:5ms , the soil is rather dry and a mountainous peninsula with particular reference to the static atmospheric stability is weak, b=2 Kkm1. Calabria. We use an analytical and numerical approach The extent of Calabria in the W-E direction is less to study the problem. Our results show that the than one Rossby radius, therefore the Ionian and topography plays a fundamental role in determining Tirrenian Sea breeze fronts meet, in the early afternoon, the mesoscale ¯ows. Indeed the presence of mountains in the centre of the region. They de®ne two climate axes highly enhances the wind intensity. In particular, for the in the north and south part of the region. Calabria peninsula, the importance of the mountain- Figure 8 shows the wind vector at 16:00 LST at 75 m valley contrast is larger than the sea-land contrast, above the ground. A strong convergence of air masses in therefore the local climate is due to the presence of the the centre of the region is evident. The maximum wind sea but also to its high topography. Our energetic intensity is about 5 ms1, roughly half of the maximum analysis is in good agreement with the order of wind intensity of the case in which the real orography of magnitude of the simulated ®elds. The simulated mean Calabria is present. wind is less intense than that predicted using MAPE. This result is in good agreement with our consider- This is related to the particular morphology of Calabria ations on the energetics, which show the important peninsula, which is very rugged and less than one contribution of a mountain in determining the strength Rossby radius wide. Therefore, energy losses due to 244 S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts 245

Fig. 10. As in Fig. 8 but for the sea-mountain ¯ow Fig. 8. Wind vector at 75 m above the ground simulated on July 16 at 16:00 LST for the sea-breeze case. u ˆ 0:0 m ; v ˆ 0:5 m ; b ˆ 2K G s G s km friction and dissipation are important, furthermore, since the circulation is highly ageostrophic, the breezes do not reach maturity and not all the potential energy is released. In fact, our MAPE evaluation of the mean wind is made considering an inviscid ¯uid, capable of releasing all the available potential energy. However, the relative contributions, i.e. the sea breeze, mountain- valley breeze and the combination of the sea breeze with the mountain-valley breeze in the simulated case and in the theoretical case are well reproduced. Despite the limitations of the numerical model in which there is dissipation due to the physics and also dissipation due to the numerical scheme, and considering also the limita- tions of the energetics theory which is non dissipative and highly ecient in releasing the potential energy, both approaches are useful in gaining knowledge of mesoscale ¯ows.

Acknowledgements. S. Federico acknowledges the support of INEA, project POM-MISURA2, contract A05. G.A. Dalu ac- knowledges the support of ASI. We are in debt to the reviewers for their suggestions and for their contributions in improving this work. We are grateful to P. Aversa, and J. D. Dalu in their help for writing the paper and draughting the ®gures. Topical Editor D. J. Webb thanks B. W. Atkinson and another referee for their help in evaluating this paper. Fig. 9. As in Fig. 8 but for the mountain-valley ¯ow References

Baldi, M., M. Colacino, G. A. Dalu, E. Piervitali, and Z. Ye, b Simulazioni a mesoscala su regioni a topogra®a complessa: Fig. 7a±f. Mesoscale available potential energy and mean wind when applicazioni alla Calabria, IFA CNR, Roma R.I. 97±1, 1997. both contrasts, i.e. sea-land and mountain-valley, are present Baldi, M., M. Colacino, G. A. Dalu, E. Piervitali, and Z. Ye, (Fig. 5a±d). a, c and e refer to the MAPE and correspond to CBL Thermally forced atmospheric ¯ow over complex terrain in heights of h2 ˆ 1000; h2 ˆ 1500; h2 ˆ 2000 m. b, d and f refer to the Southern Italy, Il Nuovo Cimento, 21C (4), 417±437, 1998. mean velocities for h2 ˆ 1000; h2 ˆ 1500; h2 ˆ 2000 m. Di€erent Colacino, M., Mediterranean meteorology, In winds and currents curves in the same frame represent di€erent mountain heights of the Mediterranean basin, Proc. NATO-ASI, Ed H. Char- hm ˆ 500; hm ˆ 1000; hm ˆ 1500; hm ˆ 2000 m. Dashed lines refer to nock. Reports in Meteorology and Oceanography 40, 1±38. the ¯at terrain case, Fig. 4a, and are shown for completeness Harvard University Press, USA, 1992. 246 S. Federico et al.: Mesoscale energetics and ¯ows induced by sea-land and mountain-valley contrasts

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