Incorporating Effects of Forest Litter in a Snow Process Model
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- Della Lawson
- 5 years ago
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1 57th EASTERN SNOW CONFERENCE Syracuse, New York, USA, 2000 Incorporating Effects of Forest Litter in a Snow Process Model ABSTRACT J. P. H A R D Y, 1 R. MELLOH, 1 P. ROBINSON, 1 AND R. JORDAN 1 N e t s o l a r r a d i a t i o n o f t e n d o m i n a t e s t h e s n o w s u r f a c e e n e r g y e x c h a n g e d u r i n g a b l a t i o n i n m a n y c o n i f e r f o r e s t s. R e f l e c t i o n o f s o l a r r a d i a t i o n f r o m t h e s n o w s u r f a c e d e p e n d s n o t o n l y o n s n o w p r o p e r t i e s, b u t a l s o o n f o r e s t l i t t e r l y i n g o n a n d w i t h i n t h e s n o w p a c k. W e k n o w o f n o v a l i d a t e d m o d e l r e p o r t e d i n t h e l i t e r a t u r e t h a t a c c o u n t s f o r t h e i n f l u e n c e o f f o r e s t l i t t e r o n s n o w s u r f a c e e n e r g y e x c h a n g e s. T h e p u r p o s e o f t h i s w o r k i s t o t e s t a n e x i s t i n g a l g o r i t h m s a b i l i t y t o a c c u m u l a t e f o r e s t l i t t e r i n s n o w l a y e r s a n d t o p r e d i c t t h e s u b s e q u e n t e f f e c t o f l i t t e r o n t h e s n o w s u r f a c e a l b e d o. F i e l d s t u d i e s i n a c o n i f e r s t a n d o f r e d s p r u c e -b a l s a m f i r i n n o r t h e r n V e r m o n t, U S A, p r o v i d e d k e y d a t a f o r v a l i d a t i o n, i n c l u d i n g s u b c a n o p y r a d i a t i o n, m e t e o r o l o g y, s n o w d e p t h, a n d i m a g e s o f l i t t e r a c c u m u l a t i o n. W e r a n t h e l i t t e r a l g o r i t h m c o u p l e d w i t h t h e s n o w m o d e l S N T H E R M f o r t h e a b l a t i o n s e a s o n, a n d p r e d i c t io n s c o m p a r e d w e l l w i t h m e a s u r e m e n t s o f s n o w d e p t h, s n o w s u r f a c e l i t t e r c o v e r a g e, a n d s n o w s u r f a c e a l b e d o b e n e a t h t h e c o n i f e r c a n o p y. M o d e l r e s u l t s s u g g e s t t h a t f o r t h i s f o r e s t a n d a b l a t i o n s e a s o n, t h e c u r r e n t l i t t e r a l g o r i t h m r e a l i s t i c a l l y d i s t r i b u t e s l i t te r i n t h e s n o w p a c k t h r o u g h t i m e w i t h v a l i d a t e d e f f e c t s o n s n o w s u r f a c e l i t t e r c o n c e n t r a t i o n a n d a l b e d o. T h e p o o r r e l a t i o n s h i p b e t w e e n d a i l y t o t a l w i n d s p e e d a n d c h a n g e i n l i t t e r c o v e r a g e o n t h e s n o w s u r f a c e s u g g e s t t h a t, f o r t h i s f o r e s t a n d a b l a t i o n s e a s o n, i n c o r p o r a t i n g w i n d e v e n t s i n t o t h e a l g o r i t h m w i l l n o t i m p r o v e t h e r e s u l t s. K e y W o r d s : A l b e d o, F o r e s t l i t t e r, S n o w m o d e l l i n g 1 U.S. Army Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, New Hampshire , USA jhardy@crrel.usace.army.mil
2 INTRODUCTION F o r e s t s a r e w e l l k n o w n t o a l t e r t h e a c c u m u l a t i o n o f s n o w o n t h e g r o u n d a n d t o m o d i f y n e a r - s u r f a c e m i c r o m e t e o r o l og y, w h i c h c o n t r o l s s n o w m e l t r a t e s. N e t s o l a r r a d i a t i o n ( i n c o m i n g m i n u s r e f l e c t e d ) o f t e n d o m i n a t e s t h e s n o w s u r f a c e e n e r g y e x c h a n g e d u r i n g a b l a t i o n i n m a n y c o n i f e r f o r e s t s ( P r i c e a n d D u n n e , M e t c a l f e a n d B u t t l e , H a r d i n g a n d P o m e r o y , D a v i s e t a l , H a r d y e t a l ). T h e s o l a r r a d i a t i o n i n c i d e n t o n t h e s n o w s u r f a c e d e p e n d s o n f o r e s t s t r u c t u r e a n d v a r i e s w i t h t r e e s p e c i e s, s k y c o n d i t i o n s, a n d s o l a r a n g l e. R e f l e c t i o n o f s o l a r r a d i a t i o n f r o m t h e s n o w s u r f a c e d e p e n d s n o t o n l y o n s n o w p r o p e r t i e s ( g r a i n s i z e, p a r t i c l e d e n s i t y, a n d l i q u i d w a t e r c o n t e n t ), b u t o n f o r e s t l i t t e r ( l e a v e s, t w i g s, n e e d l e s, b a r k, a n d b r a n c h e s ) l y i n g o n a n d w i t h i n t h e s n o w p a c k a n d p r o t r u d i n g v e g e t a t i o n. L i t t e r d e p o s i t e d o n t h e s n o w s u r f a c e a c c u m u l a t e s i n l i t t e r h o r i z o n s a s t h e s n o w p a c k d e v e l o p s. T h e s e l i t t e r h o r i z o n s, u s u a l l y s e p a r a t e d b y s u b s e q u e n t s n o w f a l l s, a r e r e -e x p o s e d d u r i n g m e l t. N e t s o l a r r a d i a t i o n i n a f o r e s t e n v i r o n m e n t t h u s d e p e n d s o n f o r e s t s t r u c t u r e a n d t i m e o f y e a r, a s w e l l a s t h e a c c u m u l a t i o n a n d r e-e x p o s u r e o f f o r e s t d e b r i s. M a n y i n v e s t i g a t o r s c o n s i d e r a l b e d o o f t h e f o r e s t c a n o p y ( e. g., O t t e r m a n e t a l , L i s t o n , B e t t s a n d B a l l , N a k a i e t a l , N i a n d W o o d c o c k i n p r e s s ) o r t h e e f f e c t s o f d u s t a n d s o o t o n s n o w s u r f a c e a l b e d o ( e. g., W i s c o m b e a n d W a r r e n , G e r l a n d e t a l ). O t h e r s q u a l i t a t i v e l y d e s c r i b e t h e e f f e c t t h a t p r o t r u d i n g v e g e t a t i o n h a s o n s n o w a l b e d o ( P o m e r o y e t a l ) o r e v e n m a k e m u l t i p l e m e a s u r e m e n t s o f n e t r a d i a t i o n b e n e a t h a f o r e s t ( P r i c e a n d D u n n e , N a d e a u a n d G r a n b e r g ). M e a s u r e m e n t s o f a l b e d o b e n e a t h a f o r e s t c a n o p y a r e r a r e, a n d w h e n t h e y d o e x i s t a r e l i m i t e d ( H a r d i n g a n d P o m e r o y , P o m e r o y a n d D i o n ). M o s t s n o w p r o c e s s m o d e l s u s e t h e w e l l-k n o w n f a c t t h a t s n o w a l b e d o d e c r e a s e s a s s n o w m e t a m o r p h o s e s t o l a r ge r g r a i n s, w h i l e i g n o r i n g l i t t e r e f f e c t s, w h i c h c a n b e s u b s t a n t i a l ( J o r d a n , D i n g m a n , A l b e r t a n d K r a j e s k i , P o m e r o y e t a l ). P r o g r e s s i n t h i s a r e a h a s b e e n s e v e r e l y l i m i t e d d u e t o t h e l a c k o f w i n t e r d a t a o n t h r o u g h -c a n o p y r a d i a t i o n, s n o w a l b e d o, a n d l i t t e r f a l l i n f o r e s t e d e n v i r o n m e n t s. A p r e v i o u s s n o w p a c k e n e r g y b a l a n c e s t u d y i n t h e b o r e a l f o r e s t i n t r o d u c e d a p r o v i s i o n a r y t e c h n i q u e t o e s t i m a t e l i t t e r f a l l a n d i t s i n c o r p o r a t i o n i n t h e s n o w p a c k ( H a r d y e t a l ). T h e t e c h n i q u e i n c o r p o r a t e s l i t t e r w i t h i n a c c u m u l a t i n g s n o w l a y e r s w i t h c o n t i n u e d d e p o s i t i o n d u r i n g s n o w a b l a t i o n, a n d r e e m e r g e n c e o f l i t t e r a s t h e s n o w p a c k m e l t s. W h e n c o u p l e d w i t h t h e s n o w m o d e l S N T H E R M ( J o r d a n ), t h e l i t t e r m o d e l p r o v i d e d t h e n e c e s s a r y c o r r e c t i o n t o o b t a i n a g r e e m e n t b e t w e e n m o d e l a n d a c t u a l s n o w a b l a t i o n a t t h e b o r e a l f o r e s t s i t e, b u t t h e p r e v i o u s s t u d y l a c k e d q u a n t i t a t i v e l i t t e r a n d a l b e d o d a t a t o v a l i d a t e t h e m o d e l. T h e p u r p o s e o f t h i s r e s e a r c h i s t o p r o v i d e f u r t h e r v a l i d a t i o n o f t h e H a r d y e t a l. ( ) l i t t e r m o d e l, u s i n g a c o m p r e h e n s i v e f o r e s t l i t t e r a n d a l b e d o d a t a s e t c o l l e c t e d i n a t e m p e r a t e f o r e s t i n n o r t h e r n V e r m o n t, U S A ( M e l l o h e t a l. i n p r e p ). S e c o n d a r i l y, o u r o b j e c t i v e i s t o a s s e s s o p p o r t u n i t i e s t o i m p r o v e t h e e q u a t i o n t h a t c o n t r o l s l i t t e r i n p u t r a t e s t o t h e m o d e l, t o a c c o u n t f o r i m p o r t a n t m e t e o r o l o g i c a l e v e n t s, e s p e c i a l l y h i g h w i n d s. STUDY AREA T h e s t u d y a r e a i s l o c a t e d w i t h i n a s m a l l c o n i f e r f o r e s t i n t h e W -9 s u b -w a t e r s h e d ( k m 2, ?2 6?N, ?2 8?W ) o f t h e S l e e p e r s R i v e r R e s e a r c h W a t e r s h e d ( k m 2 ) i n n o r t h e r n V e r m o n t, U S A. R o b i n s o n e t a l. ( i n p r e p ) d e t e r m i n e d t h e f o r e s t c h a r a c t e r i s t i c s o f t h e p l o t. T h e c o n i f e r p l o t m e a s u r e s 2 2 m? 2 2 m a n d i s d o m i n a t e d b y r e d s p r u c e b a l s a m f i r (Picea rubens Abies balsamea ) (Fig. 1). The average tree h e i g h t i s m a n d t h e a v e r a g e c r o w n h e i g h t i s 8. 0 m. T o t a l D B H ( D i a m e t e r a t B r e a s t H e i g h t o r 1. 5 m f r o m t h e g r o u n d ) p e r h e c t a r e o f a l l s t a n d i n g t r e e s, l i v i n g a n d d e a d, i s m. T h e p e r c e n t c o v e r o f t h e b a l s a m f i r c a l c u l a t e d a s B a s a l A r e a ( B A ) o f l i v e t r e e s i s a p p r o x i m a t e l y 6 1 % c o m p a r e d t o 3 9 % f o r t h e r e d s p r u c e. T h e y o u n g r e d s p r u c e a p p e a r t o b e d y i n g u n d e r t h e d e e p s h a d e o f t h e m a t u r e b a l s a m f i r, m a n y n o t l i v i n g b e y o n d 0. 1-m DBH. T h e s h r u b l a y e r i s v o i d o f a n y w o o d y v e g e t a t i o n b e t w e e n 0. 5 m a n d 2. 0 m. T h e g r o u n d c o v e r w a s n o t s u r v e y e d, b u t a n a b u n d a n c e o f f a l l e n t r e e s a n d b a l s a m f i r s e e d l i n g s w a s o b s e r v e d.
3 Figure 1. Photo taken on 9 April 1999 (day 99) in the conifer forest. Note the litter on the snow surface and the abundance of fallen trees on the ground. A p p r o x i m a t e l y 0. 8 k m f r o m t h e c o n i f e r s i t e i s t h e l o c a t i o n o f a m e t e o r o l o g i c a l s t a t i o n s i t u a t e d i n a l a r g e o p e n a r e a a n d i n o p e r a t i o n s i n c e A t t h e o p e n s i t e, t h e m e a n a n n u a l t e m p e r a t u r e i s a b o u t 4 C a n d t h e m e a n a n n u a l p r e c i p i t a t i o n i s a b o u t m m. S n o w f a l l a v e r a g e s m m w i t h a m a x i m u m s n o w w a t e r e q u i v a l e n t ( S W E ) n o r m a l l y t o m m ( P a n g b u r n e t a l ). METHODS Forest characterization: Meteorological and snow measurements P r i o r t o t h e / s n o w s e a s o n, t h e f o r e s t c o m m u n i ty s u r r o u n d i n g t h e c o n i f e r s i t e w a s q u a n t i f i e d u s i n g s a m p l i n g m e t h o d s d e s c r i b e d i n D a u b e n m i r e ( ) a n d B r o w e r e t a l. ( ). P a r a m e t e r s a n a l y z e d i n c l u d e d s p e c i e s c o m p o s i t i o n, a b u n d a n c e, r e l a t i v e s p e c i e s d e n s i t y, a n d p e r c e n t c o v e r i n s h r u b a n d t r e e l a y e r s. R o b i n s o n e t a l. ( i n p r e p ) p r o v i d e a d e t a i l e d e x p l a n a t i o n o f t h e s e s a m p l i n g m e t h o d s. D u r i n g t h e s n o w a b l a t i o n s e a s o n ( l a t e F e b r u a r y t h r o u g h A p r i l ) M e l l o h e t a l. ( i n p r e p ) c o n t i n u o u s l y m e a s u r e d i n c o m i n g s o l a r a n d l o n g w a v e r a d i a t i o n t h r o u g h t h e c a n o p y a n d r e f l e c t e d s o l a r r a d i a t i o n f r o m t h e s n o w p a c k u s i n g a r r a y s o f f o u r u p - a n d f o u r d o w n l o o k i n g r a d i o m e t e r s ( E p p l e y p y r a n o m e t e r s, P S P ) i n t h e c o n i f e r s i t e. T w o u p l o o k i n g E p p l e y p y r g e o m e t e r s ( P I R ; 8 - t o 5 0 -? m w a v e l e n g t h ) m e a s u r e d i n c o m i n g l o n g w a v e r a d i a t i o n b e n e a t h t h e c o n i f e r c a n o p y. T w o - m e t e r, s u b c a n o p y a i r t e m p e r a t u r e, r e l a t i v e h u m i d i t y, a n d w i n d s p e e d w e r e a l s o m e a s u r e d i n t h e c o n i f e r s i t e. M e a s u r e m e n t a n d d e p l o y m e n t d e t a i l s a r e o u t l i n e d i n M e l l o h e t a l. ( i n p r e p ). T h e m e t e o r o l o g i c a l p a r a m e t e r s m e a s u r e d a t t h e o p e n s i t e c o n s i s t o f a i r t e m p e r a t u r e a n d r e l a t i v e h u m i d i t y, w i n d s p e e d a n d d i r e c t i o n ( 2 -m a n d 6-m h e i g h t ), c o n t i n u o u s s n o w d e p t h, i n c o m i n g a n d r e f l e c t e d s o l a r r a d i a t i o n, i n c o m i n g a n d o u t g o i n g l o n g w a v e r a d i a t i o n, a n d p r e c i p i t a t i o n. T h e s n o w p a c k p r o p e r t i e s w e r e m e a s u r e d a n d m o n i t o r e d t w o w a y s. F i r s t, a n u l t r a s o n i c d e p t h s e n s o r m e a s u r e d s n o w d e p t h f r o m a s i n g l e p o i n t i n t h e f o r e s t. T h i s d e p t h g a u g e m e a s u r e d t h e d i s t a n c e f r o m t h e s e n s o r t o t h e s n o w s u r f a c e a n d p r o v i d e d a c o n t i n u o u s r e c o r d o f s n o w d e p t h f o r v a l i d a t i o n o f o u r m o d e l r u n s. S e c o n d, s n o w d e p t h a n d i t s v a r i a b i l i t y w e r e r e g u l a r l y m e a s u r e d b y m a n u a l s u r v e y s, a s w e r e o c c a s i o n a l m e a s u r e m e n t s o f s n o w d e n s i t y a t t h e s t u d y s i t e d u r i n g t h e a b l a t i o n s e a s o n. A t p e a k a c c u m u l a t i o n a n d t o i n i t i a l i z e t h e s n o w m od e l, t h e s n o w d e n s i t y p r o f i l e a t m v e r t i c a l r e s o l u t i o n u s i n g a cm 3 d e n s i t y c u t t e r w a s m e a s u r e d a l o n g w i t h s n o w t e m p e r a t u r e s u s i n g d i a l s t e m t h e r m o m e t e r s.
4 Snow surface and in-pack litter contamination A n o v e l, r a d i o -l i n k e d c a m e r a s y s t e m c o l l e c t e d a n d t r a n s m i t t e d r e a l -t i m e d i g i t a l i m a g e s o f t h e s n o w s u r f a c e c o n d i t i o n. M e l l o h e t a l. ( i n p r e p ) p r o v i d e s p e c i f i c s o f t h e d e s i g n a n d d e p l o y m e n t o f t h e c a m e r a s y s t e m. T h i s t e c h n i q u e d o c u m e n t e d s n o w s u r f a c e l i t t e r c h r o n o l o g y f r o m F e b r u a r y t h r o u g h A p r i l i t s a c c u m u l a t i o n a n d r e -e x p o s u r e d u r i n g m e l t. T h e d o w n l o a d e d a n d p r o c e s s e d i m a g e s w e r e a n a l y z e d u s i n g i m a g e -p r o c e s s i n g s o f t w a r e t o c r e a t e b i n a r y ( p u r e b l a c k a n d w h i t e ) i m a g e s f o r s t a t i s t i c a l a n a l y s i s. S t e r e o l o g i c p a r a m e t e r s w e r e c o m p u t e d a u t o m a t i c a l l y b y p a s s i n g t e s t l i n e s a c r o s s t h e b i n a r y i m a g e a n d u s i n g e q u a t i o n s f o r c o u n t i n g s t a t i s t i c s, a s d e s c r i b e d b y D o z i e r e t a l. ( ). T h e p o i n t d e n s i t y s t a t i s t i c p r o v i d e d t h e t w o -d i m e n s i o n a l l i t t e r c o v e r a g e ( % ) a t t h e s n o w s u r f a c e, b u t a s i m p l e r a t i o o f b l a c k t o w h i t e a r e a s c o u l d p r o v i d e t h e s a m e r e s u l t s. I n a d d i t i o n t o m e a s u r i n g s n o w p r o p e r t i e s a t p e a k a c c u m u l a t i o n ( s e e a b o v e ), s n o w s a m p l e s a p p r o x i m a t e l y 0. 3 m w i d e, 0. 5 m l o n g, a n d 0. 1 m d e e p w e r e c o l l e c t e d t o q u a n t i f y t h e l i t t e r i n t h e s n o w p a c k ( M e l l o h e t a l. i n p r e p ). Ea c h o f t h e f i v e s a m p l e s w a s m e l t e d t o s e p a r a t e l i t t e r f r o m s n o w a n d d r a i n e d o f e x c e s s w a t e r ; t h e l i t t e r w a s d r i e d i n a s o i l -d r y i n g o v e n f o r 2 4 h o u r s a n d w e i g h e d ( R o b i n s o n e t a l. i n p r e p ). Snow process model S N T H E R M i s a n i n t e r n a t i o n a l l y r e c o g n i z e d, o n e -d i m e n s i o n a l m a s s a n d e n e r g y b a l a n c e m o d e l f o r p r e d i c t i n g s n o w p a c k p r o p e r t i e s a n d p r o c e s s e s ( J o r d a n ), a n d i s t h e f o u n d a t i o n f o r t h i s s n o w m o d e l l i n g e f f o r t. S N T H E R M c a l c u l a t e s e n e r g y e x c h a n g e a t t h e s u r f a c e a n d b o t t o m o f t h e s n o w c o v e r, i n -p a c k p r o c e s s e s, g r a i n g r o w t h, d e n s i f i c a t i o n a n d s e t t l e m e n t, m e l t i n g a n d l i q u i d w a t e r f l o w, h e a t c o n d u c t i o n a n d v a p o r d i f f u s i o n ( J o r d a n ). H a r d y e t a l. ( , ) s u c c e s s f u l l y a p p l i e d S N T H E R M t o f o r e s t e d e n v i r o n m e n t s b y l i n k i n g i t t o a g e o m e t r i c-o p t i c, r a d i a t i v e -t r a n s f e r m o d e l ( L i e t a l , N i e t a l ). S N T H E R M c o m p u t e s r e f l e c t e d s o l a r r a d i a t i o n u s i n g a n a l b e d o r o u t i n e t h a t a c c o u n t s f o r c h a n g e s i n a l b e d o d u e t o g r a i n g r o w t h, s u n a n g l e, a n d c l o u d c o v e r ( M a r k s ) a n d m o s t r e c e n t l y, r e d u c e s a l b e d o t o a c c o u n t fo r t h e o p t i c a l t h i n n i n g o f s n o w ( H a r d y e t a l ). S i n c e c l e a n s n o w i s u n l i k e l y b e l o w a f o r e s t c a n o p y, H a r d y e t a l. ( ) i n t r o d u c e d a p r o v i s i o n a l r o u t i n e t o e s t i m a t e l i t t e r f a l l o n t h e f o r e s t f l o o r t h a t r e d u c e d t h e s u b c a n o p y s n o w a l b e d o a s f o r e s t l i t t e r a c c u m u l a t e s. T h e r o u t i n e a l l o w s t h e f r a c t i o n a l c o v e r a g e o f l i t t e r i n o r o n t h e s n o w p a c k t o i n c r e a s e t h r o u g h o u t t h e e n t i r e s n o w s e a s o n a c c o r d i n g t o e q. 1 : lc = 1. 0 ( 1. 0 lr ) d a y ( 1 ) w h e r e lc i s t h e f r a c t i o n a l l i t t e r c o v e r a g e ( m 2 m 2 ), lr i s t h e d a i l y l i t t e r r a t e ( m 2 m 2 d 1 ) o f t h e f o r e s t, a n d day i s t h e n u m b e r o f l i t t e r a c c u m u l a t i o n d a y s f o r w h i c h t h e l i t t e r c o v e r a g e i s c a l c u l a t e d. T h e r e s u l t i n g a l b e d o i s a w e i g h t e d a v e r a g e o f l i t t e r a l b e d o a n d c l e a n s n o w a l b e d o i n p r o p o r t i o n t o t h e i r a r e a l c o v e r a g e. T h i s l i t t e r r o u t i n e w a s i n t e n d e d a s a p r e l i m i n a r y a t t e m p t a t c o n s i d e r i n g t h e w e l l-k n o w n e f f e c t l i t t e r h a s o n r e d u c i n g a l b e d o. Snow model initialization W e r a n t h e m o d e l f r o m 1 M a r c h ( d a y 6 0 ) t h r o u g h 2 3 A p r i l ( d a y ). T h e m e t e o r o l o g i c a l f i l e u s e d f o r t h e d u r a t i o n o f t h e m o d e l r u n i n c l u d e s a i r t e m p e r a t u r e, r e l a t i v e h u m i d i t y, a n d w i n d s p e e d d a t a f r o m t h e c o n i f e r s t a t i o n. W e u s e d a n a i r t e m p e r a t u r e t h r e s h o l d o f C t o s e p a r a t e p r e c i p i t a t i o n e v e n t s a s s n o w o r r a i n. U s i n g t h e i n c o m i n g s o l a r r a d i at i o n f r o m t h e o p e n s i t e a n d t h e m e a n o f t h e f o u r -u p l o o k i n g P S P s f r o m t h e c o n i f e r s i t e, w e e s t a b l i s h e d t h e t r a n s m i s s i v i t y f o r e a c h t i m e s t e p. B e c a u s e o f r a p i d l y c h a n g i n g s u n a n g l e s i n t h e s p r i n g, w e d i v i d e d t h e m o d e l l i n g p e r i o d i n t o t w o p e r i o d s a n d m u l t i p l i e d t h e m e a n t r a n s m i s s i v i t y f o r e a c h p e r i o d b y t h e u n o b s t r u c t e d p y r a n o m e t e r d a t a t o o b t a i n a v a l u e f o r t h e s u b c a n o p y i n c o m i n g s o l a r r a d i a t i o n. W e a l l o w e d S N T H E R M t o e s t i m a t e r e f l e c t e d r a d i a t i o n b a s e d o n s n o w g r a i n s i z e, a g e, a n d l i t t e r c o v e r a g e. T h e m e a n o f t h e f o u r-i n v e r t e d P S P s p r o v i d e d r e f l e c t e d r a d i a t i o n d a t a f o r a l b e d o v a l i d a t i o n. W e a v e r a g e d t h e l o n g w a v e d a t a f r o m b o t h P I R s f o r l o n g w a v e i n p u t. D a t a f o r t h e s n o w p r o p e r t i e s r e q u i r e d b y t h e m o d e l ( s n o w l a y e r t h i c k n e s s, t e m p e r a t u r e, d e n s i t y, a n d g r a i n s i ze ) w e r e o b t a i n e d f r o m s n o w p i t o b s e r v a t i o n s o n 9 M a r c h a n d a d j u s t e d t o r e p r e s e n t t h e s n o w c o n d i t i o n s f o r 1 M a r c h. L i t t e r a c c u m u l a t e d o n t h e s n o w s u r f a c e b e g i n n i n g w i t h
5 t h e f i r s t s n o w f a l l. W h e n a m o d e l r u n b e g i n s a t p e a k a c c u m u l a t i o n, t h e l i t t e r c o n t e n t o f t h e i n i t i a l s n o w l a y e r s a r e c o m p u t e d u s i n g e q. 1 b a s e d o n t h e n u m b e r o f d a y s b e t w e e n s n o w e v e n t s l e a d i n g t o m a x i m u m a c c u m u l a t i o n. I n o r d e r t o r u n t h e l i t t e r a l g o r i t h m w i t h S N T H E R M, S N T H E R M r e q u i r e s a n a d d i t i o n a l c o l u m n c o n t a i n i n g e s t i m a t e s o f l i t t e r c o n t e n t i n t h e s n o w i n p u t f i l e. W e u s e d p r e c i p i t a t i o n d a t a f r o m t h e o p e n s i t e t o d e t e r m i n e t h e t i m i n g o f s n o w f a l l e v e n t s l e a d i n g u p t o 1 M a r c h a n d t h e r e f o r e c o m p u t e d l i t t e r c o v e r a g e f o r e a c h s n o w l a y e r. T o v a l i d a t e t h e m o d e l o u t p u t w e c o m p a r e d m a n u a l a n d c o n t i n u o u s m e a s u r e m e n t s o f s n o w d e p t h i n t h e c o n i f e r f o r e s t ; p e r c e n t l i t t e r c o v e r a g e d e r i v e d f r o m d i g i t a l i m a g e s ; a n d o u r m e a s u r e m e n t s o f f o r e s t a l b e d o, w i t h S N T H E R M o u t p u t. RESULTS D r y s p r i n g w e a t h e r ( i. e., c l e a r r a d i o m e t e r s ) p e r m i t t e d n e a r l y c o n t i n u o u s m e a s u r e m e n t o f t h r o u g h -c a n o p y i n c o m i n g r a d i a t i o n a n d t h a t r e f l e c t e d f r o m t h e s n o w-c o v e r e d f o r e s t f l o o r i n t h e s e s t a n d s f r o m l a t e w i n t e r t h r o u g h t h e s n o w m e l t s e a s o n ( M e l l o h e t a l. i n p r e p. ). S u b c a n o p y i n c o m i n g s o l a r r a d i a t i o n w a s c o m p a r e d w i t h t h a t m e a s u r e d i n t h e o p e n s i t e, l e s s t h a n o n e k i l o m e t e r a w a y. M e a n c o n i f e r t r a n s m i s s i v i t y f o r t h e m o d e l l e d p e r i o d w a s L o n g w a v e r a d i a t i o n d a t a m e a s u r e d b y t h e t w o P I R s w e r e i n v e r y c l o s e a g r e e m e n t ( r 2 = ) a n d w e r e a v e r a g e d t o r e p r e s e n t t h e i n c i d e n t l o n g w a v e c o m p o n e n t i n t h e f o r e s t. A s e x p e c t e d, t h e l o n g w a v e c o m p o n e n t i n t h e f o r e s t h a d h i g h e r v a l u e s b u t m u c h l o w e r t e m p o r a l v a r i a t i o n t h a n m e a s u r e d i n t h e o p e n s i t e. L o w w i n d s p e e d s i n t h e c o n i f e r s i t e s u g g e s t t h a t t h e t u r b u l e n t f l u x e s a r e l o w a n d t h a t t h e r a d i a t i on b a l a n c e d o m i n a t e s t h e s u r f a c e e n e r g y f l u x e s. Photos of snow surface T h e e l e c t r o n i c d i g i t a l c a m e r a s y s t e m s u c c e s s f u l l y m o n i t o r e d s n o w s u r f a c e c o n d i t i o n s r e m o t e l y u s i n g r a d i o c o m m u n i c a t i o n s ( M e l l o h e t a l. i n p r e p ). T h e t w i c e -d a i l y d i g i t a l i m a g e s o f t h e s n ow s u r f a c e d o c u m e n t l i t t e r c o n d i t i o n s f r o m 4 M a r c h t h r o u g h t h e c o m p l e t i o n o f m e l t o n 2 1 A p r i l ( e. g., F i g. 2 ). D u r i n g t h i s p e r i o d, 5 0 i m a g e s w e r e o f s u f f i c i e n t q u a l i t y t o p r o c e s s, o f w h i c h 3 0 p r o v i d e d u s e f u l i n f o r m a t i o n. T w e n t y o f t h e i m a g e s e i t h e r w e r e f r e e o f l i t t e r d u e t o a r e c e n t s n o w f a l l, s u g g e s t e d n o c h a n g e f r o m t h e p r i o r i m a g e, o r c o n t a i n e d b a r e g r o u n d. A n a l y s e s o f t h e d i g i t a l i m a g e s p r o v i d e d p e r c e n t s u r f a c e l i t t e r c o v e r a g e f r o m m a x i m u m s n o w d e p t h t h r o u g h s p r i n g a s l i t t e r l a y e r s w e r e r e-e x p o s e d d u r i n g m e l t o u t. T h e l a t e s t i m a g e w i t h c o m p l e t e s n o w c o v e r w a s t a k e n 1 6 A p r i l w i t h l i t t e r c o v e r a g e o f 2 5 %. T h e n e x t i m a g e w a s f r o m 1 9 A p r i l a n d h a s a l i t t e r c o v e r a g e o f 5 6 % a n d i n c l u d e s s o m e b a r e g r o u n d. B y 2 1 A p r i l a l l b u t 3 % o f t h e i m a g e w a s s n o w - f r e e. T h e r e l a t i o n s h i p b e t w e e n l i t t e r c o v e r a g e d e r i v e d f r o m t h e d i g i t a l i m a g e s a n d t h e d a i l y a v e r a g e a l b e d o m e a s u r e d i n t h e f o r e s t e d s t a n d i s d e s c r i b e d b y a s e c o n d o r d e r p o l y n o m i a l ( r 2 = ) ( M e l l o h e t a l. i n p r e p. ). a b
6 Figure 2. Two examples of digital images obtained from the remote camera system and the corresponding binary image. The images were obtained on 1 April (a) and 12 April (b) and represent 7% and 24% litter coverage, respectively. Model run using litter routine W e c o m p a r e d m e a s u r e m e n t s o f s n o w d e p t h, s n o w s u r f a c e l i t t e r c o v e r a g e, a n d s n o w s u r f a c e a l b e d o b e n e a t h t h e c o n i f e r c a n o p y w i t h r e s u l t s o f a S N T H E R M m o d e l r u n u s i n g t h e l i t t e r e q u a t i o n ( e q. 1 ). W e a s s u m e d a l i t t e r r a t e o f d 1 f o r t h i s c o n i f e r s i t e ( a n i n c r e a s e o f d 1 a s p r e vi o u s l y u s e d f o r a b l a c k s p r u c e s t a n d ). T h e a l b e d o o f t h e c o n i f e r l i t t e r ( ) w a s b a s e d o n r e s u l t s o f O k e ( ) a n d a l b e d o m e a s u r e m e n t s o f t h e f o r e s t f l o o r a f t e r c o m p l e t e s n o w m e l t ( M e l l o h e t a l. i n p r e p ). T h e g r o u n d i n t h e c o n i f e r s i t e w a s c o n t i n u o u s l y s n o w c o v e r e d f o r approximately 125 d a y s, o f w h i c h 4 8 d a y s w e r e m o d e l l e d. T h e s n o w i n p u t f i l e c o n t a i n e d e s t i m a t e s o f l i t t e r c o v e r a g e, c o m p u t e d f r o m e q. 1, i n e a c h e x i s t i n g s n o w l a y e r f r o m d a y 3 5 0, , t o d a y 6 0, ( T a b l e 1 ). T h e s u m o f t h e l i t t e r c o v e r a g e i n T a b l e 1 i s , w h i c h e q u a l s t h e d a i l y l i t t e r r a t e ( d 1 ) m u l t i p l i e d b y t h e n u m b e r o f s n o w d a y s ( 7 5 ) b e t w e e n d a y 3 5 0, , a n d d a y 6 0, T h e r e a r e t w o w a y s t h e f r a c t i o n o f l i t t e r o n t h e s n o w s u r f a c e i n c r e a s e s. E i t h e r e x i s t i n g s n o w l a y e r s me l t, l e a v i n g b e h i n d t h e i r l i t t e r o n t h e s n o w s u r f a c e, o r a d d i t i o n a l l i t t e r a c c u m u l a t e s o n t h e s n o w s u r f a c e t h r o u g h t i m e. S i m i l a r l y, l i t t e r l e a v e s t h e s n o w p a c k l a t e i n t h e a b l a t i o n s e a s o n a s t h e l o w e s t s n o w l a y e r s m e l t a n d l i t t e r b e c o m e s p a r t o f t h e f o r e s t f l o o r. T h e l i t t e r e q u a t i o n e s t i m a t e s t o t a l l i t t e r c o v e r a g e o f 5 3 % i n t h i s c o n i f e r f o r e s t a t t h e e n d o f t h e s n o w a b l a t i o n p e r i o d. O n 9 M a r c h, w h i l e a n a l y z i n g t h e s n o w p a c k p r o f i l e i n t h e c o n i f e r f o r e s t, s i g n i f i c a n t l i t t e r h o r i z o n s w e r e n o t e d a t s n o w h e i g h t s o f a p p r o x i m a t e l y 3 1 c m, 3 9 c m, a n d c m, w i t h t h e m o s t p r o n o u n c e d l i t t e r h o r i z o n a t 3 1 c m. T h e c o l l e c t e d s n o w a n d l i t t e r s a m p l e s t h a t w e r e s e p a r a t e d, d r i e d, a n d w e i g h e d c o n f i r m e d t h o s e o b s e r v a t i o n s ( F i g. 3 a ). F o r c o m p a r i s o n, w e p l o t t h e m o d e l l e d l i t t e r c o v e r a g e i n t h e s n o w p a c k, a s d e t e r m i n e d b y S N T H E R M f o r d a y 6 8, a n d b i n n e d a c c o r d i n g t o F i g u r e 3 a ( F i g. 3 b ). T h e m o d e l l e d l i t t e r c o v e r a g e s h o w s a s i m i l a r p r o f i l e o f l i t t e r a c c u m u l a t i o n a s t h e m e a s u r e d s a m p l e s w i t h t h e g r e a t e s t a c c u m u l a t i o n m i d-s n o w p a c k a n d t h e m o s t s i g n i f i c a n t l i t t e r l a y e r a t b e t w e e n a n d 3 3-c m h e i g h t. T h e m o s t p r o n o u n c e d l i t t e r h o r i z o n s o c c u r w h e n t h e p e r i o d b e t w e e n s n o w e v e n t s i s l o n g e s t. Table 1. Snow property input file used to initialize SNTHERM. The input file is based on snow property measurements made in the conifer site on 9 March and adjusted for 1 March. Litter coverage is the percent coverage for the respective snow layer. The surface snow layer is at the top of the table. Temp. (K) Snow height (m) Density (Kg m 3 ) Grain size (m) Litter coverage (%)
7 43-59 Day Weight of Dry Litter (g) Day Litter Coverage % Figure 3. Profile of dry litter weights in the snowpack as collected on day 68 and processed by Robinson et al. (in prep) (Fig. 3a). Profile of SNTHERM-determined litter in the snowpack on day 68 (Fig. 3b). These data were binned to compare with field data. L i t t e r c o v e r a g e p r o f i l e s d e t e r m i n e d b y S N T H E R M d u r i n g t h e m o d e l r u n w e r e b i n n e d i n t o 5-cm i n t e r v a l s a n d p l o t t e d f o r e v e r y t e n d a y s o f m o d e l o u t p u t ( F i g. 4 ). T h e s e r i e s o f p l o t s s h o w s t h e d i s t r i b u t i o n o f l i t t e r i n t h e s n o w p a c k t h r o u g h t i m e a n d t h e i n c r e a s e i n l i t t e r c o n c e n t r a t i o n a s t h e s n o w m e l t s. S n o w d e p t h s f o r d a y s 6 0, 7 0, 8 0, 9 0, a n d w e r e , , , , a n d 4. 5 c m, r e s p e c t i v e l y. B e c a u s e S N T H E R M o u t p u t s l i t t e r i n n o d e s o f v a r y i n g t h i c k n e s s a n d c o m b i n e s a n d d i v i d e s n o d e s, i t i s n o t p r a c t i c a l t o t r a c k i n d i v i d u a l l i t t e r h o ri z o n s. F i g u r e 4 e s u g g e s t s a s u r f a c e l i t t e r c o v e r a g e o f 6 2 % o n d a y 1 0 0, c o m p a r e d w i t h m o d e l e s t i m a t e o f t o t a l l i t t e r c o v e r a g e o f 5 3 % a t t h e e n d o f t h e s n o w a b l a t i o n p e r i o d. T h i s d i s c r e p a n c y i s e a s i l y e x p l a i n e d b y t h e w a y S N T H E R M a c c o u n t s f o r l i t t e r i n a m e l t i n g s n o w p a c k. O n d a y t h e s n o w p a c k i s 4. 5 c m d e e p a n d c o n t a i n s a s u m o f 6 2 % l i t t e r i n f i v e r e m a i n i n g s n o w l a y e r s. J u s t b e f o r e c o m p l e t e a b l a t i o n o n d a y 1 0 7, t h e s n o w s u r f a c e l i t t e r c o n t e n t i s 5 3 %. T h e m i s s i n g l i t t e r i s i n c o r p o r a t e d i n t o t h e f o r e s t f l o o r a n d n o t c o n s i d e r e d snow surface l i t t e r.
8 Snow Height (cm Day 60 Snow Height (cm Day Litter Coverage (%) Litter Coverage (%) Snow Height (cm Day 80 Snow Height (cm) Day Litter Coverage (%) Litter Coverage (%) Day 100 Snow Height (cm) Litter Coverage (%) Figure 4. Time series of modelled litter coverage in the snowpack from day 60 to day 100, in 10-day intervals. F i g u r e 5 c o m p a r e s m e a s u r e m e n t s o f s n o w d e p t h, s n o w s u r f a c e l i t t e r c o v e r a g e, a n d a l b e d o i n t h e c o n i f e r f o r e s t t o r e s u l t s f r o m S N T H E R M. M o d e l l e d s n o w d e p t h c o m p a r e s f a v o r a b l y w i t h s n o w s e n s o r d a t a a n d m a n u a l m e a s u r e m e n t s ( F i g. 5 a ), b u t m e l t e d t h r e e d a y s e a r l i e r. M o d e l l e d s n o w s u r f a c e l i t t e r c o v e r a g e t r a c k e d t h e m e a s u r e d l i t t e r c o v e r a g e e x t r e m e l y w e l l t h r o u g h t h e a b l a t i o n s e a s o n ( F i g. 5 b ). J u s t p r i o r t o t h e m o d e l p r e d i c t i n g c o m p l e t e s n o w a b l a t i o n, t h e l i t t e r c o v e r a g e i n c r e a s e d d r a m a t i c a l l y t o c l o s e l y m a t c h t h e m e a s u r e d l i t t e r c o v e r a g e. F i n a l l y, S N T H E R M - p r e d i c t e d s n o w s u r f a c e a l b e d o g r a d u a l l y d e c r e a s e s a s l i t t e r a c c u m u l a t e s o n t h e s n o w s u r f a c e, r e a c h i n g t h e a l b e d o o f t h e l i t t e r ( ) a t t h e t i m e o f c o m p l e t e s n o w a b l a t i o n a n d t h r e e t o f o u r d a y s e a r l i e r t h a n t h e s n o w u n d e r t h e c a m e r a ( F i g. 5 c ). E a r l y i n t h e m o d e l l i n g p e r i o d ( d a y s 6 0 t o 7 5 ), t h e m e a s u r e d a l b e d o s h o w s m u c h v a r i a b i l i t y a s i t r e s p o n d s t o s n o w e v e n t s, w h i l e t h e m o d e l l e d
9 r e s p o n s e i s l e s s p r o n o u n c e d. B e t w e e n d a y s 8 9 a n d 1 0 3, S N T H E R M p r e d i c t s a h i g h e r -t h a n - m e a s u r e d a l b e d o b e n e a t h t h e c o n i f e r c a n o p y, a l t h o u g h f o r m u c h o f t h a t t i m e t h e m o d e l i s w i t h i n t h e range of the 15 -m i n u t e m e a s u r e m e n t s. Figure 5. Results from SNTHERM model runs incorporating the litter algorithm and compared with a) measured snow depth, b) snow surface litter coverage, and c) measured albedo from the snow surface.
10 Influence of winds on litter coverage W e i n v e s t i g a t e d t h e i n f l u e n c e o f w i n d s o n s n o w s u r f a c e l i t t e r c o v e r a g e t o a s s e s s t h e i m p o r t a n c e o f i n c l u d i n g t h i s m e t e o r o l o g i c a l p a r a m e t e r i n t h e l i t t e r a l g o r i t h m. W e c o n d u c t e d t h i s a n a l y s i s o n l y f o r t h e p e r i o d p r i o r t o r a p i d s n o w ab l a t i o n ( u p t o d a y 8 8 ), a t w h i c h p o i n t i t i s d i f f i c u l t t o s e p a r a t e t h e r e -e x p o s u r e o f b u r i e d l i t t e r l a y e r s d u r i n g s n o w a b l a t i o n f r o m t h e a c c u m u l a t i o n o f a d d i t i o n a l l i t t e r. T w o-m e t e r w i n d s p e e d s a v e r a g e d o v e r t h e 1 5 -m i n u t e i n t e r v a l s i n t h e c o n i f e r s i t e d u r i ng the s n o w a b l a t i o n s e a s o n n e v e r e x c e e d m s 1. B y s u m m i n g t h e s u b c a n o p y w i n d s p e e d s f o r d a y s b e t w e e n l i t t e r o b s e r v a t i o n s, v i a s n o w s u r f a c e p h o t o s ( u s u a l l y e v e r y o n e t o t h r e e d a y s ), a n d d i v i d i n g b y t h e n u m b e r o f d a y s b e t w e e n l i t t e r o b s e r v a t i o n s, w e o b t a i n e d a m e a n d a i l y t o t a l w i n d s p e e d. T h i s d a i l y t o t a l w i n d s p e e d w a s c o m p a r e d w i t h t h e c o r r e s p o n d i n g c h a n g e i n p e r c e n t l i t t e r c o v e r a g e ( F i g. 6 ), w h e r e p o s i t i v e v a l u e s r e p r e s e n t e d a n i n c r e a s e i n l i t t e r c o v e r a g e s i n c e t h e l a s t o b s e r v a t i o n a n d n e g a t i v e v a l u e s r e p r e s e n t a d e c r e a s e. N o t e t h e t i m e s c a l e d o e s n o t r e p r e s e n t c o n s e c u t i v e d a y s, b u t r a t h e r w a s b a s e d o n d a y s w i t h o b s e r v a t i o n s o f l i t t e r a c c u m u l a t i o n o n t h e s n o w s u r f a c e. F o r d a y s 6 5 t h r o u g h 6 7, w i n d s w e r e s t e a d y w h i l e l i t t e r a c c u m u l a t e d. F o r d a y s 6 8, 7 0, a n d 7 1, t h e i n c r e a s i n g w i n d s c o r r e s p o n d e d t o l e s s l i t t e r o n t h e s n o w s u r f a c e b e c a u s e o f t h e p r e v i o u s p r e c i p i t a t i o n e v e n t. D a y s 7 7 a n d 7 8, a s w e l l a s d a y s 8 5 t h r o u g h 8 7, s h o w e d t h e o p p o s i t e i n f l u e n c e e x p e c t e d i f t h e s e w i n d s w e r e t o s i g n i f i c a n t l y i n c r e a s e l i t t e r f a l l o n t h e s n o w s u r f a c e. A l e a s t s q u a r e s r e g r e s s i o n t h r o u g h a s c a t t e r d i a g r a m o f l i t t e r c h a n g e v s. d a i l y w i n d s p e e d y i e l d s a n r 2 = , c o n f i r m i n g t h i s w e a k r e l a t i o n s h i p Day in Year Change in Litter Coverage Daily Total Wind Speed Figure 6. Mean daily total wind speed in the conifer forest compared to change in litter accumulation for the days prior to the onset of rapid snow ablation. DISCUSSION M e a s u r e m e n t s o f s n o w s u r f a c e l i t t e r c o v e r a g e a n d s u b c a n o p y a l b e d o f r o m a c o n i f e r f o r e s t i n n o r t h e r n V e r m o n t, U S A ( M e l l o h e t a l. i n p r e p ) p r o v i d e d v a l i d a t i o n d a t a f o r t h e l i t t e r a l g o r i t h m d e v e l o p e d b y H a r d y e t a l. ( ). I m a g e s o b t a i n e d f r o m t h e d i g i t a l c a m e r a s y s t e m w e r e k e y t o p r o v i d i n g n e a r -d a i l y i n f o r m a t i o n o n t h e s n o w s u r f a c e l i t t e r a c c u m u l a t i o n f o r m o d e l v a l i d a t i o n. M u l t i p l e m ea s u r e m e n t s o f t h e s u b c a n o p y s n o w a l b e d o w e r e r e q u i r e d t o h a v e c o n f i d e n c e i n t h e s o l a r c o m p o n e n t s b e n e a t h a f o r e s t c a n o p y. S N T H E R M, c o u p l e d w i t h t h e l i t t e r a l g o r i t h m, e s t i m a t e d f o r e s t l i t t e r c o n c e n t r a t i o n s i n a l l s n o w l a y e r s, f o r e v e r y t i m e s t e p a s o u t p u t b y t h e m o d e l. S N T H E R M e s t i m a t e s o f i n -p a c k f o r e s t l i t t e r c o m p a r e d f a v o r a b l y w i t h m e a s u r e d l i t t e r i n t h e s n o w p a c k o n 9 M a r c h, a n d s h o w a l o g i c a l p r o g r e s s i o n o f c o n c e n t r a t i o n s t h r o u g h t h e a b l a t i o n s e a s o n. T h e l i t t e r a l g o r i t h m, a s c o u p l e d w i t h S N T H E R M, p r e d i c t e d s n o w d e p t h, l i t t e r c o v e r a g e, a n d s n o w s u r f a c e a l b e d o b e n e a t h t h i s f o r e s t c a n o p y. B o t h t h e p r e d i c t e d s n o w d e p t h a n d l i t t e r c o v e r a g e
11 c o m p a r e d w e l l w i t h m e a s u r e m e n t s a n d f i e l d o b s e r v a t i o n s. S n o w d e p t h p r e d i c t i o n s m a y i m p r o v e b y c o n s i d e r i n g t h e s h a d i n g e f f e c t s o f l i t t e r o n t h e s n o w s u r f a c e l a t e i n t h e a b l a t i o n s e a s o n. S n o w s u r f a c e a l b e d o p r e d i c t i o n s d u r i n g t h e e a r l y p o r t i o n o f t h e m o d e l r u n a r e l e s s v a r i a b l e t h a n t h e m e a s u r e m e n t s. D u r i n g t h i s p e r i o d, s n o w o n t h e r a d i o m e t e r s r e q u i r e d r e m o v a l o f s o m e a l b e d o d a t a, s o t h e s e d a i l y a v e r a g e v a l u e s i n c l u d e s o m e m i s s i n g t i m e s t e p s. T h e h i g h e r m o d e l p r e d i c t i o n s o f s n o w s u r f a c e a l b e d o i n t h e f o r e s t f r o m d a y s 8 9 t o i s e x p l a i n e d b y t h e n a t u r e o f t h i s p a r t i c u l a r c o n i f e r f o r e s t. T h e c a m e r a c a p t u r i n g s n o w s u r f a c e l i t t e r i m a g e s v i e w s a 1 -? 1 -m a r e a t h a t i s c l e a r o f f a l l e n t r e e s t e m s. T h e p r e d i c t e d l i t t e r c o v e r a g e o n t h e s n o w s u r f a c e c a p t u r e s t h a t c o n d i t i o n w e l l. H o w e v e r, t h r e e o f t h e f o u r d o w n l o o k i n g P S P s h a d o n e o r m o r e l a r g e f a l l e n t r e e s t e m ( s ) i n t h e f i e l d o f v i e w ( s e e F i g. 1 ), t h e r e b y r e d u c i n g t h e a l b e d o s i g n i f i c a n t l y c o m p a r e d t o s n o w w i t h o u t p r o t r u d i n g l o g s ( P o m e r o y e t a l ). T h e a b s e n c e o f t h e s e l o g s w o u l d i n c r e a s e m e a s u r e d a l b e d o a s v i e w e d b y t h e d o w n l o o k i n g P S P s. A d d i t i o n a l l y, t h e m o d e l p r e d i c t s a t h i n ( l e s s t ha n 1 0 -c m ) s n o w p a c k f r o m d a y 9 6 o n, y e t f o r t h e a l b e d o e s t i m a t e s, t h e m o d e l c o n s i d e r s l i t t e r o n t h e s n o w s u r f a c e o n l y. R a d i a t i o n p e n e t r a t i o n a n d s u b s u r f a c e l i t t e r c o v e r a g e c o n s i d e r a t i o n s w o u l d f u r t h e r r e d u c e a l b e d o p r e d i c t i o n s a n d a r e p a r t o f f u t u r e m o d e l re f i n e m e n t s. I n i t i a l l y, o u r i n t e n t w a s t o t e s t t h e l i t t e r e q u a t i o n ( e q. 1 ), w h i c h p r e v i o u s l y w a s n o t v a l i d a t e d w i t h f o r e s t l i t t e r a n d a l b e d o d a t a. W e t h e n p l a n n e d t o m o d i f y t h e r o u t i n e b y i n c o r p o r a t i n g e f f e c t s o f w e a t h e r e v e n t s t o i m p r o v e t h e l i t t e r a l g o r i t h m. T h i s c o m p l e x, y e t p o o r r e l a t i o n s h i p ( r 2 = ; m = ) b e t w e e n d a i l y t o t a l w i n d s p e e d u n d e r t h e c a n o p y a n d c h a n g e i n l i t t e r c o v e r a g e o n t h e s n o w s u r f a c e s u g g e s t t h a t, f o r t h i s f o r e s t a n d a b l a t i o n s e a s o n, i n c o r p o r a t i n g w i n d e v e n t s i n t o t h e a l g o r i th m w i l l n o t i m p r o v e t h e r e s u l t s. CONCLUSIONS T h e f i e l d d a t a p r o v i d e d b y M e l l o h e t a l. ( i n p r e p ) a l l o w e d t h e f i r s t v a l i d a t i o n o f a p r e v i o u s l y p r o p o s e d a l g o r i t h m t o i n c o r p o r a t e t h e e f f e c t s o f f o r e s t l i t t e r o n s n o w s u r f a c e a l b e d o i n t o a s n o w m e l t m o d e l. T h e s i m p l e a l g o r i t h m a l l o w s l i t t e r t o a c c u m u l a t e o n t h e s n o w s u r f a c e a s a f u n c t i o n o f t i m e a n d a g i v e n l i t t e r r a t e. O u r m o d e l l i n g r e s u l t s s u g g e s t t h e a l g o r i t h m i s s u c c e s s f u l i n a c c u r a t e l y e s t i m a t i n g t h e f r a c t i o n a l c o v e r a g e o f f o r e s t l i t t e r o n a s n o w s u r f a c e. T h i s e s t i m a t e improved SNTHERM s a b i l i t y t o p r e d i c t t h e s n o w s u r f a c e a l b e d o a n d s u b s e q u e n t r a t e o f s n o w a b l a t i o n. T o f u r t h e r v a l i d a t e t h e a l g o r i t h m, t h e m o d e l s h o u l d r u n f o r a v a r i e t y o f f o r e s t t y p e s a n d w i t h m o r e P S P s t o c a t c h t h e i n h e r e n t v a r i a b i l i t y o f t h e s o l a r c o m p o n e n t s b e n e a t h a f o r e s t c a n o p y. C u r r e n t r e s e a r c h ( a b l a t i o n s e a s o n ) e x t e n d s t h e w o r k o f M e l l o h e t a l. 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