Vortrag in: Symposium,on Tropical Meteorology,

June 2-11, 1970, Honolulu, Hawaii.

CLIMATIC EFFECTS OF LOCAL CIRCULATIQNS IN TROPICAL AND SUBTROPICAL LATITUDES

H. Flohn, Meteorologisches Institut der Universität Bonn Deutschland

1. The best example of a simple thermally induced circulation - äs a consequence

of V. Bjerknes' well-known circulation theorem - is the diurnal system of

sea-and land-breezes. Most Islands in tropical oceans develop such a system,

. äs indicated during daytime by convective clouds above the Islands. These

are used by indigenous seafarers äs landmarks of Islands invisible from

large distances. No numerical correlation between the area of the Island

and,the size of the convective cloud system has been derived. As a conse-

quence of the small diurnal Variation of sea temperatures äs compared

with the much larger Variation of the surface temperature of the soll

caused by the radiation and heat budget, the local differences of the

heating function are nearly egual at the peaks of both fche day and night

circulations«

Above very large Islands, auch äs Borneo (750̂ QOO. km )•, giant convective

Systems grow together during the late afternoon. On the air route from

Darwin to Singapore, above the southwestern corner of this Island, Cb-

systems with a diameter of at least 400 kms at the 3OO mb-level could be

observed during two flights in opposite seasons. The occurrence of similar

giant nocturnal Systems above the adjacent seas at dawn, developed by the

convergence of land-breezes of adjacent Islands, has been described by

Braak, and confirmed by satellite infrared Images (Saha)(7).

.Therefore, it is" by "no means surprislng that the climatic effect of nocturnal

land-breezes on the diurnal Variation and on the average amounts of preci-

pitation could be demonstrated at Lake .Victoria (Flohn and Fraedrich { 3 )

Fraedrich ( 4 ) . A similar precipitation maximum at Lake Titicaca has been

described (Kessler and Monheim)( 5 )

Even along the mostly arid coasts of the Red Sea occasional nocturnal

rains are produced by a complex Interaction of the longitudinal Red Sea

Convergence Zone (RSCZ) {Pedgley ( .6 ) , a nocturnal circulation perpendicular

to the rift axis (F.\ohn) { 2 ) and rare synoptic-scale disturbances. Here

^ the steep escarpments along both sldes act togefcher with the land-sea

differences to produce combined nocturnal dowrislope and daytime up-slope

circulations. The regulär daytime interaction between the RSCZ and the up-

slope circulation produce along the escarpments, between 9OO and 2OOO m

altitude, freguent rain showers a.nd (under stable conditions) local fogs in

the ascending Inversion stratocumulus, äs first recognised by the lush green

Vegetation (C. Troll ( 9 ) .

2. The well-known local mountain vinds (valley-and mountain-breezes,slope winds)

are physically slightly different: while fche daytime ascending circulation

is produced by differential heating of the surface (varying with exposition,

slope angle, altitude, soil and Vegetation), the nocturnal down-slope

I Vi>2

l*? mootly gravitational (k/itab"J..V-; vuada) . The nighttime drainage

winda may cauoe weak convergence in the cent-3^ of the valleya» butr under

thermodynöJmlcally stähle conditiono? the effect on weather ia hardly more

than locßl fog or Inversion «tratua* If the daytinie circulation is sufficiently

atrong, äs in moet subtropical and -cropical mountains» it controls the pattern

of convective clouds and rainfall to such an extent that it may bo pictured

even in the Vegetation pattern (C. Troll <1O). Due to this effect rainfall

maasure?-cuta in volley stationa arö utterly unrepresentative. The most striking

£3Miaple of thia ia the Karakorum Mta. where r.lv£ 5 available atations in the

largo valleya yield precipltation« (P) between 8 and i5 cra/a, while runoff

data and the mass-budget of the giant glacler&i indicate that at higher altitudea

P must ^mount to at least 25O-3OO cia/a, perhaps up to 8 m/a. Satellite picturea

at midday frequently show the aimultaneoue occurrence of convective cloud

zonoa along the ridges and cloud free zonea along the valleya.

Only in exceptional casea - valleya with a width of at least 25-3O kms (Cb-scale)

in a saturated-unatable airmaaa - can the nocturnal circulatlon cauae low-level

convergence at the bottom of the valley {e.g, Cauca Rift Valley.« Colombia

(Trojer (8) } with regulär nocturnal rainfall. Thia ia replaced, at the middle

&nd higher £iopear by the usual aacending circulation with a atrong afternoon

precipitation maxiraum. In frequent caaea thla maximum ia delayed after aunaetr

until 20-22h loca.

confective cella.

until 2O-22 local t inte f due to the continued releaae of latent heat in the

In sddiclcm to auch local-acale ayatema (acale 1-1OO kms)weak orographical

circulations of a larger acale (>>1OO kma) control the air-maaa exchange

between the mountaina and thelr environment. They have been deacribed along

ths Alps {Burger and Ekhart (1), where they ahow a marked effect on the

diurnal Variation of cloudinesa and precipitation. Due to the daytime

divergence of both ayatems» a cloud-free ring ia frequently developed around

the rnountains. Many regional precipitation anomaliea apparently can be

intcrpreted aa induced by thia effect (e.g. the arid zone around Lake Rudolph

between the highlanda of Kenya and Ethiopia and the San Luia Valley in aouthern

Colorado) . One of the largeat and moat peculiar examplea of a thermally induced

circulation ia preaented by the Tlbetan highlanda durlng the whole warm aeaaon

(April-October) . Due to their altitude (45OO m) and aize (2'lO6Jon2) they form

an eievated heat aource. During July and Auguat the flux of aenaible heat into

thü air can be eatimated by compariaon with reliable data from aeverai

other central Aaiatic highlanda to be about 25O I/y/d; due to thia flux the

mid-tropospheric lapse rate (between 4 and 7 kma) in the afternoon decreaaea

from 9 C/km at the Pamir highlanda to 7-8°C/kra in the central and aouthern part,

Only above the aoutheaatern fringea ia the lapse rate equal to that of the

Standard Atmosphere.

Thua durina the warm aeason (April-Septeraber) the convective activity above

the highlanda is amazingly high. The atrong vertical transport of heat in ;,h;?

hot towers producee a quasi-stationary warm cell in the upper trcpoaphere.

Surpriaingly enough, the v2to£« «yatem ia uiaintained - Bllghtly weakened - aleo

I VI -3

during the night, and at many surface statione äs well äs at two pilot-balloon

stations (Drosh* Gangtok) the ascending valley winds are observed even in the

early morning, when in normal cases the reversed nocturnal winds reach their

peak. In the layer between 775-2O5 mbs an undiluted ascending parcel of surface

(monsoon) air is in the average 1.6 C warmer than its environment which is in turn

5.4 C warmer than the Tropical Standard Atmosphäre; the latent energy needed to

heat the air above the saturated adiabatic lapse rate is 22O Ly/d. Here the

ßtationary not tower convectiori produces the highest temperatures .in the upper

troposphere (2OO-4OO mbs) observed at any Station on the globe. Its maintenance during

the night is evidenced by surface data from several stations. Based on a combination

of all available aerological data* a model of the diurnal (daytime) circulstion could

be derived which must be completed by an even stronger "seasonal" circulation in the

same direction. The number of sirnultaneous Cb-cells has been estimated from satellite

pictures to be about 30O.

4.One of the principal effects of the local-and meso-scale thermally induced circulation?;

in tropical and subtroplcal latitudes is the spatial organization of "moist" Cb-con-

vectiori with the thermodynamical properties of not towers. A simple numerical model

of such circulations can be derived from reasonable assumptioris:. a closed stationary

clrcular system with a radius r2 = 5O kms enclosing a circular Island (r., - 2O kms) ,

where ths average radial velocity vf in the layer 8OO/1OOO mbs increases from 0 at i'

to 1OO cms at r, (div 3.3 10 i and the evaporation at the Ocean's surface

1s 0,36 cm/d (« 131 cm/a), half of which is immediately precipitated in the same area.

100

mb

8OO

1000

3.7

^̂ 100

19.4

O.9 g/kgMUMMmmm

9.45 E « 3.6, P « 1.8

w » cm s-l

The daytime circulation results in a daily precipitation of 9.45 mm <=345 cm/a) above

the Island; latent heat is transported upward in the central part of the Cb-tower above

the island ander saturated unstable conditions, and the increase of specific humidity

of the inflowing air reaches O.9 g/kg. In this case the reverse nighttime circulation

is relatively unimportant, since nocturnal evaporatiou is small^ and the averc.ge verti-

cal component 1s only a fraction of the daytime value,

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If we take into account only the larger wountainous Islands, such äs Puerto Rico,

Hawaii, Tahiti, Samoa, Fiji, New Guinea, the whole of Indonesia, and Ceylon

together with the mountains and volcanoes of Central America, the Andes of

Colombia, Venezuela, and Ecuador and the great African volcanoes (Mt. Cameroon,

Ruwenzori, Elgon, Kenya and Kilimanjaro, the Virungaß), we may estimate thafc every

day at leaet 200-3OO meso-scale Cb-systems with diameters from 25-4OO kms and a

time-scale of 6-8 hours arise, which work äs hot towers in a saturated unstable

alr-mass. In most of these cases the average annual rainfall amounts to 4OO cms

and above, up to 1O-12 m/a, on 250 days per year and more at individual stations.

Certainly a great part of the vertical heat transport with.in the Hadley cell -

perhaps äs much äs 5O percent - is due to such stationary, orographically

induced Cb-systems, which cover less than about 0.5 percent of the total area

of the tropical zone.

REFERENCES:

(!) Burger, A. and E. Ekhart 1938: über die tägliche Zirkulation der Atmosphäre

im Bereich der Alpen. Gerl. Beitr. Geophys., 49," 341-367.

(2) Flohn, H. 1965: Klimaprobleme am Roten Meer, Erdkunde, 19, 179-191.

(3) Flohn, H. and K. Fraedrich 1966: Tagesperiodische Zirkulation und Nieder-

schlagsverteilung am Victoria See {Ostafrika). Meteor. Rundschau,

19, 157-165.

(4) Fraedrich, K. 1968: Das Land-und Seewindsystem des Victoria-Sees nach

aerologischen Daten. Arch. Meteor. Geophys. Bioklim.A, 17, 186-206.

(5} Ke'ssler, A. and F. Monheim 1968: Der Wasserhaushalt des Titicacasees nach

neueren Meßergebnissen. Erdkunde, 22, 275-283.

(6) Pedgley, D.E. 1966: The Read Sea Convergence Zone. Weather, 21, 35O-358

and 394-406.'

(7) Sana, K.R. 1966: Contribution to the study of convection patterns in the

äquatorial trough zone using TIROS-IV radiation data.

Techn. Paper No. 74, Dept. of Atmos, Sei., Colorado State University,

Fort Collins.

(8) Trojer, H. 1959: Fundamentes para une Zonification Meteorologica y Clima-

tologica del Tropico y Especialmente de Columbia.

. CENICAFE, 10, 289-373.

(9) Troll, C. 1935; WUstensteppen und Nebeloasen im südnubischen KUstengebirge

Zeitsch. Ges. Erdk. Berlin, 241-281.

(10) Troll/ C. 1952: Die Lokalwinde der Tropengebirge und ihr Einfluß auf

Niederschlag und Vegetation.

Bonner Geogr. Abhandl., 9, 124-182.

[11) Flohn, H., 1968: Contributions to a Meteorology of the Tibetan Highlands.

Atmos. Sei. Paper 13O, Dept. of Atmos. Sei., Colorado State University,

Fort Collins.