Studies of ageostrophic motion attending jet streaks and convection are examined. Various types of dynamical experiments are carried out with the hybrid isentropic-sigma coordinate model. The effects of vorticity and static stability distributions on the intensity of secondary circulations forced by inertial accelerations as well as the superposition of an amplifying baroclinic wave on the jet streak structure are investigated

Johnson, D. R.

English

NASA Technical Reports Server

MESOSCALE AGEOSTROPHIC CIRCULATIONS ASSOCIATED 
WITH BAROCLINIC JET STREAKS 
Donald R. Johnson 
Space Science and Engineer ing Center 
U n i v e r s i t y  o f  W i  sconsi n-Madi son 
Madison, W I  53706 
S i  gni f i cant Accompl i shments 
I n  our focus on s tud ies  of ageostrophic mot ion a t tend ing  j e t  s t reaks  and 
convection, var ious types of dynamical experiments have been c a r r i e d  out  wi th  
the  hyb r id  i sent r o p i  c -s i  gma coordi  nate model d u r i  ng t h e  past year . 
invo lved examination o f  t he  e f f e c t s  o f  v o r t i c i t y  and s t a t i c  s t a b i l i t y  
d i s t r i b u t i o n s  on t h e  i n t e n s i t y  o f  secondary c i r c u l a t i o n s  forced by i n e r t i a l  
acce le ra t ions  as we l l  as t h e  superpos i t ion  o f  an amp l i f y i ng  b a r o c l i n i c  wave on 
the  j e t  s t reak  s t ruc tu re .  
I n  one se r ies  o f  experiments, t he  ho r i zon ta l  wind shear o f  t he  j e t  s t reak  
was increased r e l a t i v e  t o  previous experiments (Fig.  1)  wh i l e  r e t a i n i n g  t h e  
same maximum wind speed and alongstream v a r i a t i o n  i n  t h e  wind. The r e s u l t  was 
an enhanced development of the  d i r e c t  and i n d i r e c t  c i r c u l a t i o n s  a t  t he  j e t  
entrance and e x i t  regions, respec t i  ve ly  , as revealed by increased p rec i  p i t a -  
t i o n  i n  t h e  j e t ' s  l e f t  e x i t  region. From quasi-geostrophic concepts, t he  
increase i n  p o s i t i v e  d i f f e r e n t i a l  v o r t i c i t y  advect ion enhanced the  v e r t i c a l  
motion and increased t h e  p r e c i p i t a t i o n .  The increase i n  p o s i t i v e  v o r t i c i t y  
advect ion was associated w i t h  the  l a r g e r  p o s i t i v e  v o r t i c i t y  on t h e  j e t ' s  
cyc lon i c  side. Another t e s t  was made i n  which t h e  i n i t i a l  a n t i c y c l o n i c  wind 
shear was increased on ly  on t h e  j e t ' s  a n t i c y c l o n i c  s ide  ( reducing t h e  absolute 
v o r t i c i t y  from 4 ~ 1 0 - ~  t o 1 . 5 ~ 1 0 - ~  s - l )  but  was not  increased on t h e  cyc lon i c  
s ide.  
increased. 
j e t ' s  l e f t  e x i t  reg ion  than fo r  t h e  o r i g i n a l  experiment. 
i ncreased ageostrophic response of t he  baroc l  i n i  c j e t  s t reak must be expla ined 
us ing  E l iassen 's  concepts i n  t h e  sense t h a t  a decrease i n  t h e  absolute 
v o r t i c i t y  r e s u l t s  i n  an enhanced response o f  t he  i n d i r e c t  c i r c u l a t i o n  a t  t h e  
j e t  e x i t .  Th is  i n  t u r n  leads t o  g rea ter  upward motion on t h e  cyc lon i c  s ide  
and thus t o  increased p r e c i p i t a t i o n .  
Another experiment invo lved decreasing t h e  s t a t i c  s t a b i l i t y  below t h e  j e t  
s t reak wh i l e  ma in ta in ing  e s s e n t i a l l y  t h e  same v e l o c i t y  s t ruc tu re .  
accomplished by i nc reas ing  t h e  ground temperature by 10 K along t h e  center  o f  
t h e  channel and inc reas ing  t h e  lapse r a t e  from 5 ~ 1 0 - ~  K/m t o  6 ~ 1 0 - ~  K/m. 
Areas of p r e c i p i t a t i o n  formed under t h e  r i g h t  entrance and l e f t  e x i t  regions 
w i t h  magnitudes th ree  t o  f o u r  t imes l a r g e r  than f o r  t h e  o r i g i n a l  experiment. 
b a r o c l i n i c  wave on t h e  j e t  s t reak.  The maximum wind and alongstream v a r i a t i o n  
o f  t he  j e t  s t reak  a t  340 K were near l y  t h e  same as i n  t h e  zonal experiments. 
The small c i r c u l a r  sur face low pressure center  (997 mb) was loca ted  downstream 
o f  the t rough a x i s  w h i l e  t h e  c i r c u l a r  h igh  pressure center  (1003 mb) was 
loca ted  upstream. 
conf igura t ions  a t  36 hours. 
f r o n t a l  regions south and east  o f  t h e  low. 
continuous band o f  p r e c i p i t a t i o n  a long t h e  b a r o c l i n i c  zone i s  shown i n  
These 
I n  t h i s  case, t h e  p o s i t i v e  d i f f e r e n t i a l  v o r t i c i t y  advect ion was no t  
F igure  2 shows t h a t  p r e c i p i t a t i o n  was s t i l l  50% greater  under t h e  
I n  t h i s  case, t h e  
This was 
The t h i r d  major experiment i nvo l ved  t h e  superpos i t ion  o f  an amp l i f y i ng  
F igure  38 shows the  sur face pressure and temperature 
Note t h e  apparent development o f  co ld  and warm 
The development o f  a r e l a t i v e l y  
171 
J 
https://ntrs.nasa.gov/search.jsp?R=19840026804 2020-03-22T10:58:57+00:00Z
Figure 38. Areas o f  maximum r a i n f a l l  tend t o  co inc ide  w i t h  t h e  areas o f  
greatest  p o s i t  i ve thermal advection . 
A comparison was made between experiments with s imi  1 a r  pseudo-geostr0phi.c 
and geostrophic i n i t i a l  wind d i s t r i b u t i o n s .  Subsequent evo lu t i on  of t h e  
ageostrophic wind may be assessed by decomposing t h e  ageostrophic acce le ra t ion  
i nto  terms descr i  b i  ng cent r i  p e t a l  accelerat ion,  k i  n e t i  c energy gradi en t  , and 
non-steadiness. The l a t t e r  e f f e c t  i s ,  o f  course, given by alJ/at. F igure  4 
dep ic ts  t h i s  acce le ra t ion  a t  6 hours i n t o  t h e  s imu la t ions  f o r  both t h e  pseudo- 
geostrophic and geostrophic experiments, The general sense of t h e  vector  
f i e l d  i n  t h e  pseudo-geostrophic case (towards h igher  pressure) i s  t h e  same as 
t h a t  which occurs a t  l a t e r  t ime  per iods i n  both runs a f t e r  t h e  imbalances o f  
i n i t i a l i z a t i o n  have subsided. 
acce le ra t ion  downstream o f  t h e  wind maximum a t  6 hours i s  almost reversed 
i n  d i r e c t i o n  (towards lower pressure). The cross product o f  k w i t h  a U / a t  
gives the  d i r e c t i o n  o f  t he  ageostrophic motion. 
6 hours, t h e  ageostrophic motion i n  t h i s  area i s  almost e n t i r e l y  reversed from 
t h e  f i e l d  t h a t  u l t i m a t e l y  develops. 
e l im ina ted  the  u n r e a l i s t i c  adjustment o f  t h e  geostrophic case. 
However, i n  t h e  geostrophic case t h e  
I n  t h e  geostFophic c k e  a t  
Pseudo-geostrophic i n i t i a l i z a t i o n  has 
Focus o f  Current Research --
The cur ren t  emphasis o f  e f f o r t  i s  on conso l ida t ion  o f  the  experimental 
model r e s u l t s  i n v o l v i n g  the  a t t r i b u t e s  o f  pseudo-geostrophic i n i t i a l i z a t i o n  o f  
b a r o c l i n i c  j e t s  and several s tud ies o f  t he  e f f e c t s  o f  ho r i zon ta l  wind shear, 
s t a t i c  s t a b i l i t y ,  and amp l i f y i ng  b a r o c l i n i c  c i r c u l a t i o n s  on ageostrophic 
motion associated w i t h  j e t  streaks. The in f l uence  o f  v i s c o s i t y  and ho r i zon ta l  
r e s o l u t i o n  i s  a lso  being analyzed. 
Recommendations -- f o r  New Research 
Future work should i n v e s t i g a t e  t h e  s t rengths and weaknesses o f :  
1) Determination o f  atmospheric s t r u c t u r e  from conventional and s a t e l l i t e  
observat i  ons u s i  ng a t i me-dependent i sent r o p i  c ass i  m i  1 a t i  on model . 
2) Tests o f  numerical mesoscale p r e d i c t i o n  comparing d i f f e r e n t  methods o f  
data ass i m i  1 a t  i on . 
3) I sen t  rop i  c model s f o r  numerical weat her p red i  c t i  on. 
Publ i cat  i ons S i  nce June 1983 
Black, T. L,, 1984: An i n v e s t i g a t i o n  o f  ageostrophic c i r c u l a t i o n s  o f  j e t  
---
streaks us ing a hybr i  d i sent rop i  c -s i  gma coordi nate numerical model . 
Ph .D . thes i  s ,  Uni vers i  t y  o f  W i  sconsi n, 
Johnson, U. K., 1984: The inducement o f  p lanetary  boundary l a y e r  mass 
convergence associated w i t h  vary ing v o r t i c i t y  beneath t ropospher ic  wind 
maximum. Proc, 2nd In t ,  Symp. on Nowcasting, Norrkoping, Sweden ( t o  be 
a v a i l a b l e  from Eur, Space Agency, Sci . and Tech . Publ . Branch) , 
Keyser, D. A. and 0. R. Johnson, 1984: E f f e c t s  o f  d i a b a t i c  heat ing  on the  
ageostrophi c c i  r c u l a t i  on o f  an upper t ropospher ic  j e t  streak. 
Condi t i onal l y  accepted f o r  pub1 i c a t i o n  i n  Mon . Wea . Rev. ---
172 
3 
A B 
m 
w f 
I 
I 
Figure  1. I n i t i a l  pseudo-geostrophic wind f i e l d  f o r  
zonal j e t ;  s o l i d  l i n e s  are iso tachs  (m/s). 
(A) Hor izonta l  cross sec t i on  a t  340 K; 
(8 )  v e r t i c a l  west-east cross sec t ion  through 
center  o f  domai n ; dashed 1 i nes are  i sentropes 
(K) 
F igure  2. Tota l  p r e c i p i t a t i o n  (10-1 mm) f o r  run 
w i t h  zonal j e t .  I n i t i a l  minimum 
absolute v o r t i c i t y  south o f  j e t  i s  
( A )  4 ~ 1 0 - ~  s - l ,  (8 )  1.5x1W5 s-l. 
0 I I 8 a 8 Tn a 8 8 " 8 ' 8 * " 
Surface cond i t ions  a t  36 hours fo r  
run  i n i t i a l i z e d  w i t h  wave s t r u c t u r e  
and c i  r c u l a r  pressure centers. 
(A) S o l i d  l i n e s  are isobars  (mb) and 
dashed l i n e s  are temperature (K)  , 
(B)  t o t a l  p r e c i p i t a t i o n  (mm). 
I = 
F igure  3. 
I;, . , ;.:, , , I , , . . I  . -  1 '''-ti -j . . .  .: . . , , 3 s  t; '4 I , , . .  . , ,+. . , 
F igure  4. Vectors descr ib ing  d i r e c t i o n  and r e l a t i v e  
magnitude o f  a y a t  a t  6 hours a t  340 K. 
Dashed l i n e s  are iso tachs  (m/s). 
( A )  Pseudo-geostrophi c i n i  t i  a1 i z a t i  on, 
(B )  geostrophic i n i  t i  a1 i zat ion.  
173 


Mesoscale ageostrophic circulations associated with baroclinic jet streaks

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