Air Circulation in Forested Areas - ACS Symposium Series (ACS

13 Air Circulation in Forested Areas Effect on Aerial Application of Materials LEO J. FRITSCHEN

Chemical and Biological Controls in Forestry Downloaded from pubs.acs.org by UNIV LAVAL on 05/04/16. For personal use only.

College of Forest Resources, AR 10, University of Washington, Seattle, WA 98195

During summer, the forested areas of the Northwest and Southwest United States are dominated by high pressure systems which are characterized by subsiding a i r . The combination of subsiding air and marine or local inversions yield very stable conditions. Circulation in valleys capped by inversions is characterized by turbulent upslope, upvalley winds during heating periods and laminar downslope, downvalley winds during cooling periods. Interception of radiant energy by vegetative canopies produces an inversion at the crown closure level while radiant cooling raises the inversion above the canopy at night. Materials released below these inversions tend to drift below the inversion until a thermal chimney is encountered. The success o f any a e r i a l a p p l i c a t i o n depends i n p a r t upon the meteorology and the micrometeorology (both w i l l be r e f e r r e d t o as meteorology) o f the area t o be t r e a t e d . The meteorology i s i n t u r n r e l a t e d t o the g e n e r a l l o c a t i o n w i t h i n the c o n t i n e n t , topography, time o f year and time o f day. The combination o f these f e a t u r e s y i e l d s p e c i f i c environmental c o n d i t i o n s which can be p r e d i c t e d s u c c e s s f u l l y i n some areas a t c e r t a i n times o f the year. The purpose o f t h i s paper i s t o d e s c r i b e the combination of f e a t u r e s and the r e s u l t i n g c l i m a t i c c o n d i t i o n s i n the P a c i f i c Northwest d u r i n g summer p e r i o d s as they may r e l a t e t o a e r i a l app l i c a t i o n of materials. General C i r c u l a t i o n The excess h e a t i n g i n the t r o p i c a l regions o f the e a r t h , r e l a t i v e to other r e g i o n s , causes r i s i n g a i r over the t r o p i c s . T h i s warm a i r moves both n o r t h and south. Due t o the r o t a t i n g e a r t h , the northward moving a i r i s d e f l e c t e d t o the r i g h t and becomes a w e s t e r l y f l o w , thus the northward flow i s slowed and a i r p i l e s up

0097-6156/ 84/ 0238-0175S06.00/ 0 © 1984 American Chemical Society


176

C H E M I C A L AND BIOLOGICAL CONTROLS IN FORESTRY

at about 30 degrees N. Because o f the p i l e up o f a i r and the heat l o s s by r a d i a t i o n , some o f the a i r s t a r t s t o descend forming a h i g h pressure zone. A i r that descends flows r a d i a l l y outward. Again the northward f l o w i n g a i r i s d e f l e c t e d to the r i g h t and becomes the p r e v a i l i n g w e s t e r l i e s i n the middle l a t i t u d e s w h i l e the a i r that flows to the south i s d e f l e c t e d t o the west and becomes the n o r t h e a s t e r l y trades of the low l a t i t u d e s . At the s u r f a c e , a i r that flows outward from a h i g h pressure zone i s replaced by s i n k i n g a i r o r i g i n a t i n g h i g h i n the t r o p o sphere. This s i n k i n g i s r e f e r r e d t o as subsidence and g i v e s r i s e to the upper a i r s t a b i l i t y that dominates the P a c i f i c Northwest d u r i n g the summer. The s i n k i n g a i r warms a t the dry a d i a b a t i c lapse r a t e and, without the a d d i t i o n o f moisture, has a very low r e l a t i v e humidity. Because o f the warming and d r y i n g o f the a i r , s u b s i d i n g a i r i s c h a r a c t e r i s t i c a l l y very c l e a r and c l o u d l e s s . Subsidence may occur i n stages g i v i n g r i s e t o two o r more i n v e r s i o n s . The s u b s i d i n g a i r has a w e s t e r l y o r n o r t h w e s t e r l y t r a j e c t o r y i n the P a c i f i c Northwest. The number and i n t e n s i t y of i n v e r s i o n s i n c r e a s e a t S e a t t l e w h i l e t h e i r height decreases from s p r i n g t o f a l l as the P a c i f i c High i n t e n s i f i e s (Figure 1) ( 1 ) . Along the west coast a t lower l e v e l s , warm moist a i r over the P a c i f i c Ocean i s advected over the c o l d u p w e l l i n g c o a s t a l c u r r e n t s g i v i n g r i s e to c o o l moist a i r and frequent f o g . This l a y e r , 300 to 600 m t h i c k , i s o v e r l a i n w i t h the warm dry s u b s i d i n g a i r r e s u l t i n g i n extremely s t a b l e c o n d i t i o n s . The c o o l moist a i r f r e q u e n t l y invades the lower c o a s t a l v a l l e y s and the Puget Sound lowland w h i l e the higher topography i s exposed t o the warm dry a i r . Strong surface h e a t i n g may f i n a l l y wipe out the lower or marine i n v e r s i o n l e a v i n g the higher subsidence i n v e r s i o n s . When the c o o l marine a i r i s dammed by the C o a s t a l and Cascade mountains (Figure 2) and i f the pressure i s g r e a t e r on the west s i d e than on the east s i d e , a foehn wind may r e s u l t on the east side (2-3). The foehn winds are warm dry descending winds which r e s u l t i n c l e a r sky c o n d i t i o n s and may induce i n v e r s i o n s . Topography Topography a l s o i n f l u e n c e s s t a b i l i t y a t the lower atmospheric l e v e l s . Night time r a d i a t i o n a l c o o l i n g o f the surface produces low i n v e r s i o n s which grow deeper d u r i n g the n i g h t . Strong surface h e a t i n g during the day u s u a l l y e l i m i n a t e s the r a d i a t i o n a l i n v e r s i o n s . A d d i t i o n a l l y , a i r i n mountain v a l l e y s and i n basins heats f a s t e r during the day and c o o l s more r a p i d l y a t n i g h t than a i r over the p l a i n s . The amount o f h e a t i n g o r c o o l i n g depends upon the steepness and o r i e n t a t i o n o f the s l o p i n g s u r f a c e s , and the degree o f v e g e t a t i v e cover. East f a c i n g slopes heat e a r l i e r i n the day than do west f a c i n g s l o p e s , however s o u t h e r l y slopes reach g r e a t e r temperatures and produce g r e a t e r i n s t a b i l i t y than n o r t h e r l y s l o p e s , Figure 3 ( 3 ) . C o o l i n g o f these s u r f a c e s i s a l s o dependent upon s l o p e , o r i e n t a t i o n and v e g e t a t i v e cover.

Air Circulation in Forested Areas


FRITSCHEN

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I

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I

I

111

I

1

Aug

Sep

L

T e m p e r a t u r e d i f f e r e n c e (K)

May

Jun

Jul

Oct

Figure 1 . Afternoon inversions at Seattle during May to October 1 9 5 7 - 6 1 . Mean inversion heights ("base and top): magnitude (potential temperature difference); percentage frequency of inversion occurrence; wind speed and direction at the inversion base for the principal (solid lines) and secondary inversions (dashed l i n e s ) . (Adapted from Réf. 1 . )


CHEMICAL A N D BIOLOGICAL CONTROLS IN FORESTRY

Figure 2. A n t i c y l o n i c foehn w i t h damming o f c o l d a i r : T, a i r temperature; p, a i r p r e s s u r e ; and e, vapor p r e s s u r e ; s u f f i x o, values a t the ground l e v e l ; and 1, values on the leeside slope. (Reproduced w i t h permission from Ref. 3. Copyright 1975» U n i v e r s i t y o f Tokyo Press.)

Figure 3. Temperature on a c o n i c a l shaped mountain Values are ranked from T l (warmest) t o T5 ( c o l d e s t ) (Reproduced w i t h permission from Ref. h. Copyright 1967, Harvard U n i v e r s i t y Press.)

13.

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179


Slope and v a l l e y winds The r e s u l t of s o l a r h e a t i n g on upper slopes as contrasted to lower slopes produces l e s s dense a i r which r i s e s up the s i d e s l o p e , Figure 4, b ( 4 ) . The steeper s u n l i t slopes act as n a t u r a l chimneys e s p e c i a l l y i f they are barren and a s s o c i a t e d w i t h draws or r a v i n e s . S i m i l a r l y about midmorning, the head of v a l l e y s are heated w i t h respect to the base which promotes a i r flow up the major a x i s of the v a l l e y (Figure 4, c ) . U s u a l l y t h i s flow s t a r t s a f t e r the slope f l o w s . However, both depend upon the o r i e n t a t i o n of the v a l l e y and the v e g e t a t i o n cover. The upslope or u p v a l l e y flows are u s u a l l y t u r b u l e n t . L a t e r i n the day, as the s o l a r h e a t i n g becomes l e s s intense on the side s l o p e s , upper p o r t i o n s c o o l q u i c k l y by r a d i a t i o n a l l o s s to the c l e a r c o l d sky. Vegetated surfaces c o o l more q u i c k l y than bare rock s u r f a c e s . The a i r a s s o c i a t e d w i t h the c o o l s u r faces becomes dense and s t a r t s to d r a i n down the slope t a k i n g the path of l e a s t r e s i s t a n c e (Figure 4, e ) . This process occurs on e a s t e r l y slopes e a r l y i n the afternoon. These downslope or drainage winds are laminar i n nature and tend to flow l i k e water, u s u a l l y through the stem space i f t r e e s are present. They can be dammed momentarily by any o b s t r u c t i o n l i k e v e g e t a t i o n , road f i l l s , fences or narrowing of v a l l e y s . L a t e r i n the evening, the head of the v a l l e y c o o l s w i t h respect to the base and the c o l d dense a i r begins to flow down the c e n t r a l p o r t i o n of the v a l l e y u s u a l l y above the v e g e t a t i o n (Figure 4, g ) . The downvalley winds tends to be stronger than the downslope winds having a maximum speed some d i s t a n c e above the v e g e t a t i o n . Note that the drainage winds p e r s i s t during the hours of darkness and u n t i l enough s o l a r h e a t i n g causes a r e v e r s a l . These up- and down- s l o p e , and v a l l e y winds are c y c l i c i n nature, s p e c i f i c f o r a given drainage, and can be very p r e d i c t a b l e i n the absence of f r o n t a l systems. Winds were s t u d i e d i n the Carbon R i v e r V a l l e y near Mount R a i n i e r ( 5 ) . The l o n g i t u d i n a l s e c t i o n a l winds are shown i n F i g ure 5 a and b. The down-valley or mountain wind p e r s i s t e d u n t i l midafternoon on 9 and 10 August. The u p - v a l l e y or v a l l e y winds s t a r t e d at the base of the v a l l e y at 1200 on 10 August and i n creased i n t h i c k n e s s during the afternoon. Above these v a l l e y winds were a n t i - v a l l e y and anti-mountain winds which v a r i e d i n d i r e c t i o n from the lower winds by 90 to 180 degrees. Another example of up- and down-valley winds along the Rio Grand R i v e r near Los Alamos, N.M. i s shown i n Figure 6 ( 6 ) . Duri n g June, the drainage winds s t a r t about 1900 (WNW to NNW through the n i g h t ) and give way to the downvalley winds about 0530. The upslope winds (SE) s t a r t around 0900. Note the sudden change i n wind d i r e c t i o n that occurs s h o r t l y a f t e r s u n r i s e w i t h i n c r e a s i n g a i r temperatures. In north-south v a l l e y s , the e a s t - f a c i n g slopes are s u n l i t e a r l y i n the morning w h i l e the w e s t - f a c i n g slopes are s u n l i t l a t -


CHEMICAL AND BIOLOGICAL CONTROLS IN FORESTRY

Figure k. Schematic i l l u s t r a t i o n of the slope and valley winds where: A, i s about sunrise; B, about midmorning; C, about noon; D, afternoon; E, early evening; F, early night; G, midnight; and H, dawn. (Reproduced with per mission from Ref. k. Copyright 19&Ί > Harvard University Press.)


FRITSCHEN


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Figure 5 . Mountain and valley winds i n the Carbon River Valley near v i c i n i t y of Mount Rainier, Washington. Local down valley d i r e c t i o n — ; AV, antivalley wind; AM, antimountain wind; M, mountain wind; V, valley wind. (Reproduced with permission from Ref. 5 . Copyright 1 9 6 6 , Springer Verlag.)



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Figure 6. Slope and valley winds for Ik selected days i n June 1 9 8 Ο . (Reproduced with permission from Ref. 6. Copy right 1981, Brent M. Bowen.)


13.

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183

e r i n the day. This may cause up slope winds on the e a s t e r l y slopes e a r l y i n the day and downslope winds on the w e s t e r l y s l o p e s . The t h i c k n e s s o f the warmed a i r l a y e r i n c r e a s e s upslope reaching maximum t h i c k n e s s near the top o f the s l o p e s . The gene r a l i z e d slope and v a l l e y winds are dependent upon the shape and o r i e n t a t i o n o f the v a l l e y and the v e g e t a t i v e cover. Therefore, winds i n mountain v a l l e y s can be extremely complex being i n f l u enced by r e g i o n a l and l o c a l c o n d i t i o n s . The general c i r c u l a t i o n may r e i n f o r c e o r oppose these l o c a l convective winds. Their r e l a t i o n s h i p may change suddenly and over short d i s t a n c e s — sometimes winds d i f f e r i n g by 90 degrees are separated by the t r e e crowns. Drainage wind systems f o r s p e c i f i c v a l l e y s have been presented by (7-13). The wind f i e l d o f a l a r g e v a l l e y i n France was discussed by ( 1 4 ) . Valley inversions As the c o o l dense a i r a s s o c i a t e d w i t h slope and v a l l e y winds a c cumulates i n the bottoms o f the v a l l e y s , warmer a i r i s pushed up i n the center o f the v a l l e y c r e a t i n g an i n v e r s i o n which i n c r e a s e s i n depth and s t r e n g t h during the n i g h t . This i n v e r s i o n u s u a l l y occurs a t 2/3 t o 3/4 o f the height o f the v a l l e y and gives r i s e to the thermal b e l t . The s t r e n g t h o f t h i s i n v e r s i o n i s dependent upon the c o n f i g u r a t i o n , o r i e n t a t i o n and v e g e t a t i v e cover o f the v a l l e y , Figure 7 (15, 4 ) . Temperature i n v e r s i o n s i n other v a l l e y s have been s t u d i e d by (16-18). Canopy i n v e r s i o n s During d a y l i g h t p e r i o d s , strong r a d i a n t h e a t i n g produces a warmed zone o f a i r near the height o f crown c l o s u r e and another i n v e r s i o n (0948 through 1608, Figure 8) (19). At n i g h t , r a d i a n t c o o l ing o f the v e g e t a t i o n c o o l s the l a y e r o f a i r a s s o c i a t e d w i t h the v e g e t a t i o n which moves the i n v e r s i o n above the p l a n t canopy (2028 through 0556, Figure 8 ) . These canopy i n v e r s i o n s produce a d i s t i n c t i v e microclimate e i t h e r w i t h i n the stem space during the day or w i t h i n the canopy d u r i n g the n i g h t . During n i g h t time hours the canopy i n v e r s i o n may be strengthened by c o l d a i r d r a i n i n g down the slopes w i t h i n the stem space. The s t r e n g t h o f the canopy i n v e r s i o n depends upon the d e n s i t y of the stand. They tend to be more pronounced i n dense stands than i n sparse stands. Likewise the windspeed w i t h i n dense stands i s u s u a l l y l e s s than i n more open stands (Figure 9) ( 1 9 ) . The low l e v e l j e t i n the stem space i s stronger i f an understory i s absent. Furthermore, when the wind i s blowing i n t o a f o r e s t from a c l e a r i n g , the windspeed i s reduced to a low constant speed i n two t o three h e i g h t s (Figure 10) (19). The standard d e v i a t i o n of the reduced canopy windspeed i s low i n d i c a t i n g a more o r l e s s constant windspeed r e g a r d l e s s o f the e x t e r n a l wind. Thus the


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Figure 7 · Schematic temperature profile (right hand) showing the position of the thermal "belt on the slope i n relation to the shape of the valley cross section. (Reproduced with permission from Ref. 3 . Copyright 1 9 7 5 , University of Tokyo Press.)

\.0666 ... 1400

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mm

F i g u r e 8. Average a i r temperature p r o f i l e s i n a 27 m Douglas f i r f o r e s t . Shaded area represents v e r t i c a l vege tation density.


FRITSCHEN


F i g u r e 9. Comparison of normalized wind p r o f i l e s of v a r i o u s v e g e t a t i v e canopies where Ζ i s the height above the ground, Η i s the height of the top of the canopy and U i s wind speed. 1, dense cotton ( 2 1 ) ; 2, Douglas f i r f o r est (19); 3, dense c o n i f e r w i t h understory (22); 4, moder a t e l y dense c o n i f e r stand w i t h no understory ( 2 0 ) ; 5, dense hardwood j u n g l e w i t h understory (23); and 6, i s o l a t e d c o n i f e r stand ( 2 4 ) .


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13.

FRITSCHEN

A ir Circulation in Forested Areas

187

combination of the canopy i n v e r s i o n and low wind speed creates a d i f f e r e n t m i c r o c l i m a t e i n the stem space. In a d d i t i o n to the canopy i n v e r s i o n s , h e a t i n g of the f o r e s t f l o o r creates an unstable zone (Figure 8) which tends to i n h i b i t d e p o s i t i o n of the l e s s dense m a t e r i a l s . Again, t h i s e f f e c t i s more pronounced i n more open f o r e s t s .


I m p l i c a t i o n to a e r i a l spraying I n v e r s i o n s tend to i n h i b i t mixing of a i r below the i n v e r s i o n w i t h that above the i n v e r s i o n . The combination of the P a c i f i c h i g h pressure, topograpy and g e n e r a l l y c l e a r sky c o n d i t i o n s found i n the P a c i f i c Northwest during the summer months r e s u l t s i n m u l t i l e v e l i n v e r s i o n s (e.g. subsidence, marine a i r , v a l l e y and canopy i n v e r s i o n s ) . A l l of these could a f f e c t a p a r t i c u l a r s i t e . How ever, the v a l l e y and canopy i n v e r s i o n s are the most common. These, i n combination w i t h slope and v a l l e y winds, g r e a t l y a f f e c t the d i s t r i b u t i o n of p a r t i c l e and gaseous d i s p e r s o i d s . Fluorescent p a r t i c l e s (3 ym i n diameter, d e n s i t y of 4 and s e t t l i n g v e l o c i t y of 1.3 mm/sec) r e l e a s e d w i t h i n the canopy, be low canopy i n v e r s i o n , tended to remain below the i n v e r s i o n regard l e s s of the r e l e a s e height and d r i f t w i t h the flow u n t i l they reach some s o r t of a thermal chimney, Figure 11 (25). Openings i n the canopy, streams and l a k e s could act l i k e thermal chimneys. Furthermore, low d e n s i t y m a t e r i a l r e l e a s e d above these i n v e r s i o n s tends to remain above the i n v e r s i o n s and d r i f t w i t h the flow (25). Spread of m a t e r i a l s r e l e a s e d w i t h i n the canopy was more r a p i d than m a t e r i a l s r e l e a s e d above the canopy. They tended to spread l a t e r i a l l y f a s t e r and f i l l the space below the i n v e r s i o n . The v e r t i c a l extent of plume mixing was determined by s t a b i l i t y and v e g e t a t i v e d e n s i t y . In a study where 5.1 μπι diameter f l u o r e s c e n t p a r t i c l e s (de p o s i t i o n v e l o c i t y , 54 mm/sec) were r e l e a s e d at 26 m over a sage brush and grass s i t e , 93 percent (an average of 2.1 percent per u n i t height up to 45 m) of the m a t e r i a l remained a i r b o r n e at 842 m from the r e l e a s e p o i n t (26). The temperature d i f f e r e n t i a l be tween 10 and 6 m was 0.5 C and the windspeeds were 2.0 and 6.7 m/s at 2 and 30 m, r e s p e c t i v e l y . Based upon the above statements, i t appears that the best time of day f o r a e r i a l a p p l i c a t i o n i n v a l l e y s i t u a t i o n s i s when the laminar drainage winds are present e i t h e r i n the e a r l y morn ing ( d a y l i g h t p l u s 2 hours on e a s t e r l y slopes) or l a t e afternoon. Drainage wind p e r s i s t s longer on w e s t e r l y s l o p e s . Late afternoon may be u n d e s i r a b l e because of the p o s s i b i l i t y of e i t h e r up-slope or v a l l e y winds being present. To be e f f e c t i v e , m a t e r i a l s ap p l i e d should have a l a r g e enough t e r m i n a l g r a v i t a t i o n a l s e t t l i n g v e l o c i t y to penetrate the canopy i n v e r s i o n ; otherwise they may d r i f t f o r long d i s t a n c e s above the i n v e r s i o n . The m a t e r i a l should be of a nature that when i t impacts upon v e g e t a t i o n i t does not break up i n s m a l l e r p a r t i c l e s or d r o p l e t s which can be



c

30 m

1

m

/

8

1 2

1

F i g u r e 11. Dosage i n t e n s i t i e s 1 0 ~ (min l i t e r " " ) i n a h o r i z o n t a l plane (at 1 m) and i n v e r t i c a l planes along the plume center l i n e s of f l u o r e s c e n t p a r t i c l e s r e l e a s e d i n a 27 m Douglas f i r f o r e s t . I s o l i n e s a r e i n powers o f 10. Release p o i n t s , and plume center l i n e s are shown · and - - -, r e s p e c t i v e l y . (a) r e l e a s e a t 1 m; (b) r e l e a s e a t 10 m; and ( c ) r e l e a s e a t 20 m. Average temperature p r o f i l e s and wind d i r e c t i o n and speeds are shown f o r s e l e c t e d towers and h e i g h t s . (From 20).

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c a r r i e d on the drainage winds. I f drops are used, an evaporation r e t a r d a n t should be used t o m a i n t a i n drop s i z e and thus reduce drift.

Literature Cited 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

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