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NASA-CR-201430 


Tellus ( 1995 ). 47 A. 575 596 Copyright r Munksgaard, 1995 

Printed in Belgium all rights reserved TELLUS 

ISSN 02X0 6495 


Planetary and synoptic-scale interactions during the life 
cycle of a mid-latitude blocking anticyclone over the 

North Atlantic 


By ANTHONY R LUPO* and PHILLIP J. SMITH, Department of Earth and Atmospheric Sciences , 
Purdue University, West Lafayette , IN 47907 , USA 

(Manuscript received 29 August 1994; in final form 6 March 1995) 


ABSTRACT 

The formation of a blocking anticyclone over the North Atlantic has been examined over its 
entire life-cycle using the Zwack Okossi (Z O) equation as the diagnostic tool. This blocking 
anticyclone occurred in late October and early November of 1985. The data used were provided 
by the NASA Goddard Laboratory for Atmospheres on a global 2.0° latitude by 2.5° longitude 
grid. The horizontal distribution of the atmospheric forcing mechanisms that were important to 
500 mb block formation, maintenance and decay were examined. A scale-partitioned form of the 
Z O equation was then used to examine the relative importance of forcing on the planetary and 
synoptic scales, and their interactions. As seen in previous studies, the results presented here 
show that upper tropospheric anticyclonic vorticity advection was the most important con- 
tributor to block formation and maintenance. However, adiabatic warming, and vorticity tilting 
were also important at various times during the block lifetime. In association with precursor 
surface cyclogenesis, the 300 mb jet streak in the downstream (upstream) from a long-wave 
trough (ridge) amplified significantly. This strengthening of the jet streak enhanced the anti- 
cyclonic vorticity advection field that aided the amplification of a 500 mb short-wave ridge. The 
partitioned height tendency results demonstrate that the interactions between the planetary 
and synoptic-scale through vorticity advection was the most important contributor to block 
formation. Planetary-scale, synoptic-scale, and their interactions contributed weakly to the 
maintenance of the blocking anticyclone, with the advection of synoptic-scale vorticity by the 
planetary-scale flow playing a more important role. Planetary-scale decay of the long-wave ridge 
contributed to the demise of this blocking event. 


1. Introduction 

Blocking anticyclones are large-scale phenomena 
which can have profound impact on mid-latitude 
weather and climatic conditions not only over 
the regions in which they occur, but also over 
upstream and downstream areas as well { Rex, 
1950, 1951; Illari, 1984; Agayan and Mokhov, 
1989; Mokhov, 1993). The formation of blocking 
anticyclones has proven to be a difficult problem 
for both operational forecasting and for various 
forecast models (Simmons, 1986; Tibaldi and 
Molteni, 1990; Tracton, 1990; Tibaldi et al., 1993, 


* Corresponding author. 


1994 ), even though their climatological behavior is 
w r ell known (Rex, 1950; Triedl et al., 1981; Lejenas 
and Okland, 1983; Lupo and Smith, 1995). Many 
studies have showm the importance of large-scale 
forcing, such as long-w r ave baroclinic processes or 
topography, in block formation and maintenance 
(Charney and Devore, 1979; Sperenza, 1986; Dole, 
1986). Other studies have described the configu- 
rations of and or the interactions between long 
waves in blocked flow's (Austin, 1980; Lejenas and 
Madden, 1992), In particular, a series of papers 
by Tung and Lindzen ( 1979) attempts to explain 
blocking as caused by the resonant amplification 
of large-scale planetary waves forced by those 
mechanisms named above. McWilliams (1980) 


Tellus 47 A (1995), 5, 1 



576 


A. R. LUPO AND P. J. SMITH 


demonstrates that blocking anticyclones have 
characteristics consistent with that of a solitary 
wave (“modon”), which in their simplest form 
consist of a high-low vortex pair. Modons are 
uniformly translating, shape preserving, non-linear 
analytic solutions of, in the case of the McWilliams 
study, the inviscid equivalent barotropic potential 
vorticity tendency equation. Frederiksen (1982) 
considers the problem of blocking as instabilities 
which grow in three-dimensional Hows. He studied 
the problem using a variety of static stabilities 
corresponding to differing flow- stabilities. All of 
these studies show plausible mechanisms for block 
formation and maintenance. However, they fail to 
address such issues as why blocks tend to occur in 
preferred geographic locations (Frederiksen, 1982), 
why blocks are observed to form and decay on 
time scales more consistent with synoptic-scale 
phenomena (Lupo and Smith, 1995), or why 
blocks are observed to ‘‘fluctuate” in intensity (or 
regenerate) during their life-cycle (McWilliams, 
1980). More recently, another set of studies has 
demonstrated the importance of mid-latitude 
transients on block formation (Alberta et al., 1991; 
Colucei, 1985, 1987; Mullen, 1987; Shutts, 1983, 
1986; Konrad and Colucci, 1988; Tsou and Smith, 
1990; Tracton, 1990). 

Diagnostic studies have utilized a variety of 
methods to examine the atmospheric forcing 
mechanisms that are important in block formation 
and maintenance. Hansen and Chen ( 1982) used 
atmospheric energetics equations in spectral space 
to examine both a Pacific and an Atlantic blocking 
case. They concluded that two processes that 
lead to block formation are the non-linear forcing 
of ultra long waves by intense cyclogenesis (mode 
1 blocking), and the baroclinic amplification of 
planetary-scale weaves (mode 2 blocking). Both 
scenarios were preceded by intense upstream sur- 
face cyclogenesis. Dole (1986) performed a com- 
posite analysis of persistent Pacific region positive 
(blocking anticyclones) and negative (“cutoff” 
lows) anomaly events to describe their vertical and 
thermal structures and their relationship to the 
zonal How in the jet region. He found that during 
development both classes of anomalies exhibited 
a highly baroclinic structure and that formation 
and decay often occurred very rapidly. He also 
found that no regional atmospheric precursor was 
evident until just prior to development. Mullen 
(1987) used the quasi-geostrophic potential 


vorticity equation to examine the importance of 
synoptic-scale eddy forcing on composite (time 
mean) Atlantic and Pacific blocking flows. He 
found that eddy vorticity transports, occurring 
one-quarter wave length upstream from the 
blocking ridge, and eddy heat transports were 
important during development, but that the 
former was more important than the latter in 
maintaining the composite blocks. Also, he found 
that baroclinic instability was a very important 
mechanism in the formation of synoptic-scale 
eddies associated with blocking. Alberta et al. 
(1991), Tsou and Smith (1990), and Tracton 
(1990) used various forms of the height tendency 
equation, the vorticity equation, or the omega 
equation as their diagnostic tools. They all deter- 
mined that anticyclonic vorticity advection was 
important in block formation. The first two studies 
also noted the importance of temperature advec- 
tion in block formation, while Tracton (1990) 
found this mechanism played a more indirect role. 
Alberta et al. ( 1991 ) noted that once the block 
was established, barotropic forcing processes 
dominated the maintenance period. Alberta el al. 
(1991) and Tsou and Smith (1990) also found 
block formation to be preceded by a rapidly- 
developing surface cyclone. Additionally, Tsou 
and Smith (1990) noted the importance of an 
associated jet streak just prior to block onset and 
just after the period of rapid surface cyclone 
development. 

Since the studies mentioned above show- blocking 
to involve both synoptic-scale and planetary-scale 
processes, some diagnostic studies have attempted 
to isolate the contributions of each and examine 
their interactions. Tsou and Smith (1990) use a 
partitioned form of the extended height tendency 
equation to examine the role each scale and their 
interactions play in block formation. They found 
that the interaction component, dominated by the 
advection of synoptic-scale vorticity by the 
planetary-scale winds, W'as the largest contributor 
to block formation. They also show that block 
formation is the result of the superposition of 
a mobile and amplifying synoptic-scale ridge 
and a large-scale stationary planetary-scale ridge. 
Following Tsou and Smith ( 1990 ), Tracton ( 1990) 
uses a spectral decomposition of 500 mb height 
fields to demonstrate that blocking reflects the 
superposition of synoptic-scale (and smaller scale) 
and planetary-scale wave modes. He used the 


Tellus 47 A (1995), 5, 1 



PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


577 


quasi-geostrophic vorticity and omega equations 
as the basic framework of his investigation and 
found results similar to the scale partitioned results 
of Tsou and Smith (1990). 

Many of the studies mentioned above focus on 
the development of blocking anticyclones, and a 
few examine their maintenance. Only one of these 
studies (Dole, 1986) examines the decay period of 
blocking anticyclones. Considering the difficulty 
that forecast models encounter during block for- 
mation and decay (Tibaldi and Molteni, 1990; 
Tracton, 1990; Tibaldi et al., 1993, 1994 ), there is 
a clear need for more studies involving the entire 
life-cycle of blocking anticyclones (Tracton, 1990). 
In this paper, a diagnosis of the entire life-cycle of 
an individual 500 mb blocking anticyclone event is 
performed. Using the Zwack-Okossi equation 
(Zwack and Okossi, 1986; Lupo et al., 1992) as the 
primary diagnostic tool, the horizontal distribu- 
tion of various atmospheric forcing mechanisms 
are studied. In addition, by partitioning the basic 
data into their planetary-scale and synoptic-scale 
components, the atmospheric forcing on each 
scale, as well as the interaction between the scales 
is also examined over the block life-cycle. Another 
unique aspect of this work is that a blocking 
anticyclone preceded by non-explosive cyclo- 
genesis is examined. Previous studies similar to 
this one, notably Tsou and Smith (1990) and 
Tracton (1990), examine blocking anticyclones 
that were preceded by explosive cyclogenesis. 
Finally, the primary objective of this diagnosis is 
to elaborate on the Tsou and Smith ( 1990) block 
formation mechanism, with particular attention 
given to the role that the intervening jet streak 
plays in block formation. 


2. Data 


The data used in this investigation were anal- 
yses obtained from NASA/Goddard Laboratory 
for Atmospheres (GLA) (Schubert et al., 1993). 
This assimilated data set covers a 5-year period 
from 1 March 1985 through 28 February 1990. 
Analyzed fields were provided on a 2.0° latitude by 
2.5 longitude grid at 14 mandatory pressure levels 
from 1000 to 20 mb at 6-h intervals for the entire 
globe. These fields include upper air parameters 
w and v (horizontal wind vector components in 
m s), r (geopotential height (m l), T (absolute tem- 


perature), rh (relative humidity), and q (mixing 
ratio (g/kg) ), all of which were then interpolated 
linearly in In (p) to 50 mb isobaric levels. Also 
included were a variety of surface parameters; a 
complete list of these can be found in Schubert 
et al. (1993). The analysis scheme incorporated 
data from a variety of sources. The basic com- 
ponents of the assimilation system, model physics, 
and parameterizations used are described in more 
detail by Baker et al. (1987) and Schubert et al. 
{ 1993). Finally, subsets of these data are available 
upon further request from the Goddard Distri- 
buted Active Archive Center ( DAAC) The subset 
of data utilized for this study covers the period 
from 26 October to 30 November 1985. 


3. Computational procedures 


The diagnosis was accomplished using the 
Zwack-Okossi (Z-O) equation (Zwack and 
Okossi, 1986) in its complete form (Lupo et al., 
1992). The Z-O equation is a geostrophic vorticity 
tendency (d£ g /dt) equation derived by coupling 
the vorticity and thermodynamic tendency equa- 
tions through the hydrostatic thickness equation 
(Lupo et al., 1992). The result is a generalization 
of the Petterssen-SutclifTe equation (Petterssen, 
1956, p.324) that allows for the diagnosis of 
geostrophic vorticity tendency at a near-surface 
pressure level as forced by dynamic and thermo- 
dynamic forcing mechanisms vertically integrated 
through the depth of the entire atmosphere. 

To use the Z-O methodology for diagnoses 
at pressure levels alolt (pi), it is necessary to 
formulate a geostrophic vorticity tendency equa- 
tion for level pi which includes the near-surface 
geostrophic vorticity tendency. This is easily 
accomplished by taking the horizontal laplacian 
and Eulerian time derivative of the hydrostatic 
thickness tendency equation, multiplying by the 
gravitational acceleration, and dividing by the 
Coriolis parameter (f) to yield the Z-O upper-air 
tendency equation: 




R . 

+ - V : 

J “pi 

pvr+^+Sw] 

/ 


d P 
P ' 


( 1 ) 


Teilus 47 A (1995), 5, 1 



578 


A. R. LUPO AND P. J. SMITH 


where, in this study, pi = 500 mb; p/ represents a 
near-surface level (the first 50 mb pressure level 
above the earth’s surface at any grid point ); V is 
the horizontal wind vector; Q the diabatic heating 
rate; S the static stability parameter (-T/0) 
{cQjdp)\ oj the vertical motion (dp/d/); V the del 
operator on an isobaric surface; and /?, < p , T, 
and (K the gas constant for dry air, the specific heat 
at a constant pressure, absolute temperature, and 
potential temperature, respectively. The near- 
surface geostrophie vorticity tendency (5f g /c/) p/ is 
then given by the complete Z-O near-surface 
tendency equation: 


) = PD 

■t V 




dC A dw 
- V • VC.. — (O + l - j — 
dp 4 op 


(a) (b) (c) 

vadv vvte dive 


./ 8V ' 

) + A • ( V x F) — 

CL \ 

— k Va> x — 


V f>/ 

/ 

ot J 

(dl 

(e) 

If) 

till 

fric 

ageo 


(PD)/?p j‘ r/ 

./ “ Pt *7’ 



-f V7> 


Q 



dp. 


( 2 ) 


(g) (hi (i) 

tadv diab adia 


In (2), F is the frictional force, £ a the absolute 
vorticity, and £ ag the ageostrophic vorticity. Also, 
PD is ( 1 /( pZ — />,)), where p t is the pressure at 
some sufficiently high pressure level (30 mb in this 
study ) chosen to encompass most of the atmo- 
spheric mass. Forcing mechanisms (a) through (f ) 
on the right-hand-side of (2) are the dynamic 
forcing mechanisms coming from the vorticity 
equation, while the remaining terms (g), (h), and 
(i) are the thermodynamic forcing mechanisms. 
Finally, ( 1 ) was relaxed to produce height ten- 
dencies at pi. These height tendencies were then 
filtered to remove all information below 5A.v 
( < 1000 km), thus removing subsynoptic-scale 
noise present due to data, model, and computa- 
tional errors. 


In (1) and (2), o was calculated using a 
complete form of the omega equation similar to 
that of Krishnamurti (1968): 


V 2 m%> 4- jl. 


crv) 

dp 2 



, . (IV „ 

x V VC a -A • V x F 4- — x Vo) 

x ' 1 op 


d~' r R f 

4- /w-rA + -v 2 v str- 
op- P \ 


(3) 


In (1), (2), and (3) the frictional force was 
restricted to the boundary layer and was calcul- 
ated using the Krishnamurti ( 1 968 ^ algorithm. 
Diabatic heating included convective and stable 
latent heat release (Fosdick and Smith, 1991; 
Lupo et al., 1 992 ). boundary-layer sensible heating 
(Lupo et al., 1992), and a longwave radiation 
parameterization (Sasamori, 1968) that assumes 
randomly overlapped clouds ( Harshvardhan et al., 
1987). The boundary-layer sensible heating cal- 
culation included contributions from all isobaric 
levels between the near-surface level to the first 
isobaric level above the planetary boundary layer 
( PBL ) as calculated by the Goddard Earth 
Observing System Atmospheric General Circula- 
tion Model (GEOS-1 AGCM ). The long-wave 
radiation parameterization included contributions 
due to water vapor, carbon dioxide, and ozone. 
The ageostrophic vorticity tendency was calculated 
from the ageostrophic wand, which is turn was 
calculated as the vector difference between the 
observed and geostrophie wind. All horizontal 
(vertical) derivatives were calculated using 4th 
(2nd)-order finite differencing. Vertical integrals 
were calculated using the trapezoidal rule, and all 
relaxation was accomplished using sequential 
over-relaxation (SOR) (Haltiner and Williams, 
1980, pp. 157-164). 

All the computations described in this chapter 
were carried out over the entire Northern 
Hemisphere in order to reduce the influence of 
the boundaries in the central region of the grid. 
However, all comparisons between Z O and 
observed height tendencies were made within a 
particular region. This region was chosen so 
as to encompass the blocking anticyclone and it’s 
immediate vicinity in the upstream and down- 
stream directions. For part of the life-cycle of the 


Tellus 47A (1995), 5, 1 



Response <• 


PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


579 


block (prior to 1200 GMT 2 November), this 
region was bounded by 70° N, 30 C N, 70° W, and 
10 W. This area covered most of the North 
Atlantic, Eastern North America, Greenland, and 
Western Europe. The remaining part of the block 
life-cycle was examined in a region bounded by 
70° N, 30° N, 90° W, and 30°W. This region 
covered the eastern third of North America and 
the Western North Atlantic. Comparisons were 
made using correlation coefficients between the 
Z-O equation height tendencies and observed 
± 6-h finite differenced height tendencies and mean 
absolute values of Z-O and observed tendencies, 
both over all grid points contained in the domains 
defined above. 

A scale partitioning procedure following that of 
Tsou and Smith ( 1990) was employed to partition 
the basic data fields into their planetary-scale and 
synoptic scale components. The scale partitioning 
was accomplished using a second-order two- 
dimensional Shapiro (1970) filter applied 1250 
times to the basic data fields. Applying the filter 
in this manner yields a response function (Fig. 1, 
curve a), which retains approximately 2%, 44%, 
and 80% of the signal for waves having a hori- 
zontal wavelength of 3000, 4500, and 6000 km 
at 45" N, respectively. This particular response 
function was chosen because blocking anticyclones 



typically have wavelengths of 6000 km. Also, since 
at 45°N 6000 km represents a hemispheric wave 
number between 4 and 5, the filtered data can be 
regarded as the planetary-scale component, and 
that part which was filtered out (the difference 
between the total and filtered component) becomes 
the synoptic-scale component. It should be noted 
here that the filtering procedure differs from that of 
Tsou and Smith ( 1990), who used a Barnes filtering 
procedure similar to Maddox (1980). While the 
filtering procedure was different in order to save 
computing time, the filtering parameters were 
chosen such that our response function (Fig. 1, 
curve a) was similar to that of Tsou and Smith 
(1990) (their Fig. 1 ). 

As noted previously, height tendencies were 
filtered to remove small scale (<1000 km) noise 
without significantly degrading the large-scale 
information. This was accomplished using a 
4th-order, 2-dimensional Shapiro filter applied 
1250 times to all height tendencies. The response 
function for this filter is curve b in Fig. 1. 

Finally, the filtered data fields were used in a 
partitioned form of the Z-O equation derived by 
substituting for each variable X: 

X=X+X\ (4) 

where X(X') represents the planetary-scale (syn- 
optic-scale) component. Using this technique in 
( 1 ) yields an equation of the form: 


at 


P + S + /, 


(5) 


where I\ S , and / represent the planetary-scale, 
synoptic-scale, and scale-interaction components, 
respectively, of the forcing processes on the right- 
hand-side. A similar equation for , occurs 

when the terms in (2) are partitioned. An example 
of the result of this process as applied to term (a) 
on the right-hand-side of (2), or the contribution 
to the total tendency by vorticitv advection, in 
both ( I ) and (2 ), is: 



fi* 

pd ) ( - v vc, - r ■ v;' 

‘/'i 

r s 


Fig. 1. Response curves for the (a) 2nd-order and (b) 
4th-order 2-dimensional Shapiro ( 1970) filler. 


- r vc:,- ■"•VCjd/;, (6) 

/, I, 


Tellus 47A (1995), 5. 1 



580 


A. R. LUPO AND P. J. SMITH 


where /, is the advection of synoptic-scale vorticity 
by the planetary-scale wind, / 2 is the advection 
of planetary-scale vorticity by the synoptic-scale 
wind, and I { + 1 2 = /. 

4. Synoptic discussion 

Fig. 2 is a graph of 500 mb central or maximum 
height values spanning the pre-block 500 mb ridge 
and blocking anticyclone period. Significant dates 
and periods within the blocking anticyclone's life 
cycle are labeled on the .v-axis or defined inside the 
figure. The procedures for choosing the central or 
maximum height values and time of block onset 
and termination are specified in Lupo and Smith 
(1995). In addition, the synoptic discussion that 
follows is based on the dates in Fig. 2. The pre- 
development 500 mb Atlantic region environment, 
represented by 1200 GMT 28 October 1985, 
shows a long-wave trough located over the western 
North Atlantic, eastern Canada, and the north- 
eastern United States (Fig. 3a). Embedded within 
this trough was a closed cyclone located over 
the Gulf of St. Lawrence (48°N, 65° W) and a 
short-wave ridge to the east of the closed center 
and south of Greenland. A stationary long-wave 
ridge was located over the eastern Atlantic and 
much of Europe. A surface cyclone located over 
Newfoundland had reached the end of its develop- 
ment phase with a central pressure of 990 mb. 

By 1800 GMT 29 October ( Fig. 3c), the 500 mb 
low was located over Newfoundland and had 
reached its minimum height 6-h earlier. The short- 



5600 I ■ i — 

12/29 06/30 00/31 


Tune (GMT V Dale (October November 19B5) 

Fig. 2. Plot of 500 mb maximum or central height values 
(gpm) for the 500 mb short-wave ridge or blocking 
anticyclone versus time. Important periods of time within 
the block’s lifecycle are separated by the vertical dashed 
lines. 


wave ridge, now located over southern Greenland 
and extending approximately 500 km into the 
North Atlantic, had propagated northeastward 
with a northwest-to-southeast oriented axis. This 
ridge became stationary at this time. Also, at this 
time a cyclonically curved jet streak at 300 mb w r as 
located along 50" W on the eastern ( western ) flank 
of the long-wave trough (ridge) (Fig. 3d). The 
surface cyclone noted above, with nearly the 
same central pressure, had moved northwestward 
slightly such that by this time there was little tilt 
with height. After this time the cyclone decayed 
rapidly and disappeared 12-h later. A second 
surface cyclone was just forming along a surface 
trough extending southward from the first low. 
This second cyclone first appeared as a closed 
system at this time with a central pressure of 
996 mb at 42 C N, 5(F W, or 700 km southeast of 
Newfoundland (Fig. 3b), and is of special interest 
because it is the precursor cyclone to the block 
development. References to a surface cyclone and 
cyclogenesis in the next two paragraphs apply to 
this cyclone. 

By 0600 GMT 30 October ( Fig. 3e ), the 500 mb 
short-wave ridge had intensified and a closed anti- 
cyclone center appeared southeast of Greenland 
(56 N, 32,5 r W), indicative of block onset. Also a 
closed high pressure area at the surface (not 
shown) appeared in approximately the same locale 
with a central pressure of 1026 mb. This surface 
anticyclone could be identified with the 500 mb 
anticyclone throughout the block lifetime. The 
300 mb jet streak (Fig. 3f) had also intensified 
w r ith the maximum wind speeds increasing approx- 
imately 25% in the first 12-h, 80% of which 
occurred in the first 6-h, following the start of 
surface cyclogenesis (1800 GMT 29 October). 
The short-wave amplification and intensifying jet 
streak upstream of the closed anticyclone, the 
latter associated with the developing surface 
cyclone, is a block development scenario similar 
to the conceptual model proposed by Tsou and 
Smith (1990). 

The blocking anticyclone remained nearly 
stationary for 42-h after onset. During this period 
it intensified most rapidly, reaching maturity at 
0000 GMT 31 October (Fig. 3g). At block onset, 
the surface cyclone was midway through its 
period of most rapid deepening and had achieved 
a central pressure of 991 mb. Slower deepening 
occurred over the next 12-h and the cyclone 


Tellus 47A (1995), 5, 1 



PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


581 


5<H) mb Heights (gpm) 1200 GMT 28 Oct 85 500 mb Wind Speed (m/s) 1200 GMT 28 Oct 85 



500 mb Heights (gpm) 1800 GMT 29 Oct 85 500 mb Wind Speed (m/s) 1800 GMT 29 Oct 85 



500 mb Heights (gpm) 0600 GMT 50 Oct 85 500 mb Wind Speed (m/s) 0600 GMT 50 Oct 85 



Fig. 3- Regional 500 mb height (gpm) and 300 mb windspced (m s' ’) maps for (a) and (b) 1200 GMT 28 October, 
(c) and (d) 1800 GMT 29 October, (e) and (f) 0600 GMT 30 October, (g) and (h) 0000 GMT 31 October. <i| and 
(j) 1200 GMT 01 November, and (k) and (1) 1200 GMT 04 November 1985. The contour intervals are 60 gpm for 
the height fields and 10 (m s' ') for the wind speeds. The surface cyclones are denoted by an “L” on the height maps. 
The shaded regions are wind speeds exceeding 40 (m s 1 ). 


Tellus 47 A (1995), 5, 1 







PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


583 


attained its lowest central pressure of 988 mb at 
1800 GMT 30 October (not shown). Throughout 
the remainder of the block lifetime, the occluded 
surface cyclone changed little in intensity and 
became associated with a 500 mb low that was 
part of the blocking dipole that was forming 
at 0000 GMT 31 October and is quite evident at 
1200 GMT 01 November (Fig. 3i). 

From 1200 GMT 1 November (Fig. 3i) through 
1200 GMT 2 November (not shown), the blocking 
anticyclone propagated westward across the 
North Atlantic and into extreme eastern Canada 
(58 N, 62.5 W). During the 60-h period ending 
with 1200 GMT 2 November, the central height 
values of the blocking anticyclone remained 
nearly constant (Fig. 2). A strong anticyclonically 
curved jet nearly encircled the 500 mb anticyclone, 
such that a strong easterly jet streak was located 
between the 500 mb anticyclone/cyclone dipole 
(Fig. 3j). After 1200 GMT 2 November, the block 
again became nearly stationary and remained 
so for the balance of its lifetime. The surface 
anticyclone associated with the block had slowly 
intensified reaching a maximum of 1037 mb by this 
time. 

The decay began after 1200 GMT 3 November 
(represented by Fig. 3k, 1200 GMT 4 November) 
and was characterized by the center of the anti- 
cyclone moving southward and the block losing 
its identity until it failed to meet the blocking 
criteria defined by Lupo and Smith (1995) on 
0000 GMT 5 November. Note from Fig. 2 that the 
block decay is not marked by a decrease in central 
height values. During the 36-h period preceding 
1200 GMT 3 November, the block underwent 
modest intensification (Fig. 2). After this time, 
the 500 mb cyclone that was part of the dipole 
became an open wave again as it moved east of 
the blocking ridge (see Fig. 3k ). In addition, 
another surface cyclone, located over the southeast 
Atlantic coast of the United States, developed in 
association with an advancing upper air trough. 
During this development, the surface cyclone w ? as 
located under a large region of 300 mb diffluent 
flow with jet maxima aligned in a configuration 
similar to that shown by Rodgers and Bosart 
(1991) (Fig. 31). Therefore, unlike the configura- 
tion at block onset, and unlike the conceptual 
model of Tsou and Smith (1990), no jet streak: 
was located on the eastern ( western ) flank of the 
500 mb trough( ridge). 


5. Results 

5,1. Z O diagnostic results 

To conserve space, the ensuing discussion will 
focus on the results for map times representative of 
the block development (pre-block ), intensification, 
maintenance, and decay periods, even though all 
6-h time periods during the block lifetime w^ere 
examined. Presented are total calculated 500 mb 
height tendencies and the contributions to the 
total height tendency by vorticity advechon, 
temperature advection, and adiabatic temperature 
change (map depictions). These mechanisms were 
chosen because they were consistently the largest 
contributors to the total tendency in the region 
studied. Also shown are the contribution of the 
individual forcing terms at the center point (the 
maximum 500 mb height value) of the 500 mb 
anticyclone (bar graphs), which is denoted by an 
“X” in each map. The bar graphs were used to 
represent the development component of the 
height tendency, since at the center point the 
propagation component is zero. Comparison of 
the calculated 500 mb height tendencies w ith the 
centered (±6-h) finite difference height changes 
for each map time resulted in average correlation 
coefficients of 0.794 and mean absolute values 
for the tw r o fields of 0.874 x10 3 (gpm/s) and 
1.029 x 10 " 3 (gpm/s), respectively. Thus, the cal- 
culated fields are reasonable representations of the 
observed height changes. 

Using 0000 GMT 30 October to represent the 
block development period. Fig. 4a shows that the 
region of ridge amplification east and southeast of 
Greenland was experiencing height increases, 
while height falls were prominent over the region 
of the precursor cyclone. From Figs. 4b, e it is seen 
that anticyclonic vorticity advection (AVA) was 
the primary contributor to ridge amplification at 
500 mb and had a spatial pattern similar to that 
of the total height tendency. The adiabatic tem- 
perature change also made a contribution to ridge 
amplification (Figs. 4d,e). The height rise maxi- 
mum due to this term was located just slig' tlv 
northeast of the ridge axis ( see Figs. 3c, e ). This 
maximum w r as due to a broad area of dow nward 
motion, whose profile (not shown) exhibited a 
maximum between 850 and 700 mb. The tem- 
perature advection term, like the remaining terms 
at the center point, was small (Figs. 4c, e). The 
other dynamic terms, with the exception of the 


Tellus 47A (1995), 5, 1 





Fig. 4. (a) Regional 500 mb calculated height tendency fields, and (b), { c }, and (d) are the contribution to the total 
tendency by vorticity advection, temperature advection, and adiabatic temperature change, respectively, for 0000 
GMT 30 October 1985. (e) Bar graphs showing the contribution of each individual forcing term to the total tendency 
at the block center marked with an “X” in (a), (b), (c) and (d). Contour interval: 0.5 units. Units: 10 1 gpm s '. 


Tellus 47A (1995), 5, 1 







PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


585 


vorticity tilting term, contributed to height falls 
along the ridge axis. 

An examination of the 300 mb wind field (Figs. 
3b, c, d) reveals that the ridge axis was located on 
the anticyclonic shear side of the 300 mb jet maxi- 
mum. Also, the jet maximum itself strengthened 
considerably in association with the low-level 
cyclogenesis. This jet maximum also changed in 
shape such that the curvature changed from 
cyclonic to anticyclonic during block develop- 
ment in response to changes in the shape of the 
height field. Therefore, it is likely that the strong 
anticyclonic vorticity advection found along the 
ridge axis throughout the development period 
was due to both the anticyclonic curvature and 
shear components of the vorticity field. This region 
of anticyclonic vorticity advection increased in 
strength throughout the development period coin- 
ciding with the strengthening of the jet maximum 
discussed in Section 4. Thus, it is likely that 
baroclinic processes, such as temperature advec- 
tion and latent heating played an indirect role in 
block formation the block by contributing to a 
strengthening of the jet maximum. This result and 
conclusion was also found by Tracton (1990). 

The blocking anticyclone intensification period 
is represented by 1200 GMT 30 October. At this 
time, 500 mb height rise maxima w r ere located 
northeast and west of the block center (Fig. 5a). 
Height rises over the block center had begun to 
diminish as had the forcing by each contributing 
term on the right-hand-side of (1) (Fig. 5e). By 
examining Figs. 5b, d, however, it is apparent that 
the height rise maxima induced by vorticity advec- 
tion and adiabatic temperature change were no 
longer in close proximity to the block center as had 
been the case 12-h earlier. Fig. 5e shows that AVA, 
temperature advection, adiabatic temperature 
change, and vorticity tilting contributed to block 
intensification, while all the other terms inhibited 
intensification over the block center. 

1200 GMT 1 November is representative of 
the 500 mb height tendencies during the block 
maintenance period. Fig. 6a shows that the height 
tendencies at the block center were generally 
weaker than at previous times, and the anti- 
cyclone center was near the zero tendency isopleth. 
Figs. 6b, c, and d also show r that the forcing due to 
each mechanism was not necessarily weaker in the 
region. Rather, like the total tendency, the block 
center was located close to the zero isopleth in 


each field. Therefore, during this period, the total 
forcing and the forcing contributions from each 
individual term at the block center were very 
weak, with no one term consistently dominating 
the others (Fig. 6e). The total tendency at this 
particular time exhibits height falls, but this 
reflects the minor oscillations (as seen in Fig. 2) 
of the zero-tendency line relative to the block 
center that prevailed throughout the maintenance 
period. Additionally, it was during the main- 
tenance period and after 1200 GMT 1 November 
that the block had moved westward, presumably 
encouraged by the height rise region west of the 
block. 

The decay of the blocking anticyclone, repre- 
sented by 1200 GMT 4 November, was marked by 
slow southward propagation of the center and the 
encroachment of height falls on the western flank 
and center of the block (Fig. 7a). The height fall 
maximum southwest of the block was associated 
with the advancing upper air trough mentioned 
previously. This height fall region was dominated 
by cyclonic vorticity advection, warm air advec- 
tion (Figs. 7b, c), and latent heat release (not 
shown ). From Figs. 7b, d, e, it can be seen that 
AVA, adiabatic warming, and vorticity tilting were 
still contributing to height rises at the block center 
at 500 mb, and prominent height rise maxima were 
located south-southwest of the block. However, a 
deep layer of upper tropospheric warm air advec- 
tion associated with the advancing trough con- 
tributed significantly to height falls at this time. 
Combined with the negative contributions from all 
the other dynamic forcing terms, this was sufficient 
to produce 500 mb height falls over the block 
center. 

5.2. Scale- partitioned results 
An overview^ of the scale-partitioned results will, 
again for brevity, apply to the four representative 
map times used in Subsection 5.1. In addition, a 
detailed discussion of the results is restricted to 
total partitioned 500 mb height tendency fields 
and anticyclone center statistics in order to IT ms 
on the influence of the partitioned forcing terms on 
block development. An overall comparison of the 
P, S, and I term contributions can be obtained 
from the absolute values of the partitioned height 
tendencies averaged over the region presented in 
Table 1. These reveal that the planetary, /synoptic- 
scale interaction height tendencies {I) and the syn- 


Tellus 47A (1995), 5, 1 

















PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


589 


Table 1 . Mean absolute values of the regional parti- 
tioned height tendency {10~ 3 gpm/s) for (a) the 
first three periods and (b) the decay period of the 
block 


^\^T*ime period: 



Scale 

(a) 

(b) 

planetary (P) 

0.1573 

0.4475 

synoptic (S) 

0.4924 

0.5449 

interactions (I) 

0.4899 

0.6110 


optic-scale height tendencies ( S ) were, on average, 
significantly larger than the planetary scale (P) 
height tendencies over the first three periods in the 
life cycle of the blocking event. While the / and S 
height tendencies were approximately equal in 
magnitude over these periods, the P height tenden- 
cies were only about one-third of the other two 
components, a result similar to that of Tsou and 
Smith ( 1990). Tracton (1990) found that the inter- 
action component (/) was the dominant compo- 
nent in his case study. During the decay period, the 



Fig. 8. Height tendency contributions to the Z-O total height tendency by the (a) planetary-scale, (b) synoptic-scale, 
and (c) planetary/synoptic-scale interaction components, (d) Bar graphs showing the contibution to the total 
tendency by each term at the block center point (X) on 0000 GMT 30 October 1985. Contour intervals: (a) 0.1 units 
and (b) and (c) 0.5 units. Units: 10 3 gpm s' 1 . 


Tellus 47 A (1995), 5. 1 




590 


A. R. LUPO AND P. J. SMITH 


Table 2. Abbreviations of the forcing processes represented in the interaction height tendencies figures 


Vadl -V-VC 
Vad2 -K'-Vf 
Tadl — V • VT' 
Tad2 -K'-Vf 
Adi 1 Sc</ 


Adi2 

Divl 

Div2 

Till 


S' to 


d 

dp 


do) 

dp 


dV „ , 
— — x Vco 
dp 


Til2 

Vvrl 

Vvr2 


„ 

— — — X VCD 
dp 

dp 


. dC 
> fy 


advection of synoptic-scale vorticity by the planctary-scale wind 
advection of planetary-scale vorticity by the synoptic-scale wind 
advection of synoptic-scale temperature by the planetary-scale wind 
advection of planetary-scale temperature by the synoptic-scale wind 
synoptic-scale vertical motion acting on the planetary-scale static stability field 
planetary-scale vertical motion acting on the synoptic-scale static stability field 

synoptic-scale divergence field interacting with the planetary-scale vorticity field 

planetary-scale divergence field interacting with the synoptic-scale vorticity field 

the cross product of the vertical shear of the planetarv-scale wind and the gradient of the 
synoptic-scale vertical motion 

the cross product of the vertical shear of the synoptic-scale wind and the gradient of the 
planetary-scale vertical motion 

the advection of planet ary -scale vorticity by the synoptic-scale vertical motion field 
the advection of synoptic-scale vorticity by the planetary-scale vertical motion field 


magnitude of the I and S terms remained nearly 
the same. However, the magnitude of the P com- 
ponent increased to a value that was nearly equal 
in magnitude to the other two components. 

The development period was dominated by 
500 mb height rises over the amplifying small scale 
ridge (see Fig. 4), forced primarily by the interac- 
tion height rises (Fig. 8c). An area of planetary- 
scale height rises (Fig. 8a) is located nearly coin- 
cident with the interaction height rises, while a 
synoptic-scale height rise maximum (Fig. 8b) is 
located west of the block center. An examination of 
the individual contributors (Fig. 8d) to height rises 
shows that the vorticity advection term, which 
dominated development, was forced by the S and 
both terms in the / vorticity advections. It should 
be noted here that Table 2 describes the abbrevia- 
tions found in Figs. 8d, 9d. lOd, and 1 Id. A com- 
parison with earlier map times (not shown ) reveals 
that it was the increase in amplitude of the inter- 
action component that accounted for the increase 
in vorticity advection associated with the streng- 
thening of the jet as discussed in Sections 4 and 5. 1 
The height rises due to adiabatic warming noted in 
Subsection 5.1 were contributed mainly by P and /. 
The / contribution was mostly due to vertical 


motion on the synoptic-scale interacting with the 
planetary-scale static stability field. 

The intensification period, in general, was 
characterized by smaller 500 mb height rises over 
the center of the block (see Fig. 5). The 1 height 
rise component, due mainly to the advection of 
synoptic-scale vorticity by the planetary-scale 
w'ind, was again the main contributor to the height 
rises (Fn:. 9di This period was also characterized 
by a lack of significant contributions from the 
thermodynamic forcing mechanisms (Fig. 9d). 
The remaining dynamic forcing mechanisms were 
largely due to synoptic-scale processes. As in 
Fig. 5, it can be shown by examining Figs. 8a -c 
that the height rise maxima were not weaker than 
they were during development. These maxima, 
in particular the interaction and planetary-scale 
component maxima, were just located farther from 
the block center than they were 12-h earlier. 

As discussed in Subsection 5.1, the maintenance 
period was marked by small 500 mb total tenden- 
cies and small contributions by each forcing 
mechanism. As was shown in Fig. 6, the block 
center w r as close to the zero tendency line for 
both the total height tendency and individual 
forcing tendencies. For most terms, the total parti- 


Tcllus 47 A (1995), 5, 1 



PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


591 



Fig. 9. As in Fig. 8 except for 1200 GMT 30 October 1985. 


tioned height tendencies also reflected this trend contributing to height rises over the block center; 

(Figs. lOa-c). However, the important role of but, on the planetary-scale, cyclonic vorticity 

the synoptic-scale vorticity advection by the advection (CVA) was the largest contributor to 

planetary-scale wind in maintaining the block is height falls. All of the other P forcing processes, 

still evident ( Fig. lOd ). with the exception of adiabatic temperature 

In the decay period, the largest net height falls change, were also producing height falls. In addi- 

near and over the block center were contributed by tion, most of the S and / terms were also forcing 

the P terms (Fig. 11). At this time, AVA was still height falls. The warm air advection contributing 


Tellus 47A (1995), 5, 1 




592 


A. R LUPO AND P. J. SMITH 



Fig . 10. As in Fig. 8 except for 1200 GMT 01 November 1985. 


to height falls in Fig. 7e was comprised mainly of the total height tendency. This qualitative observa- 
the interaction advections (Fig. lid), while the tion agrees with the findings of Tracton ( 1990) who 
dynamic mechanisms were mainly due to synoptic- found a high correlation between the advection 
scale processes. of synoptic-scale vorticity by the planetary-scale 

Throughout the block life-cycle, the interac- wind and the total vorticity tendency. Addi- 
tion component, which is clearly dominated by tionally, the 300 mb wind field can be partitioned 

the advection of synoptic-scale vorticity by the into a planetary-scale and synoptic-scale com- 
planetary-scale wind (not shown), most resembles ponents (Fig. 12). Correlating each component of 


Tellus 47 A (1995), 5, 1 






PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


593 




the wind field with each of the total partitioned 
tendencies reveals that the highest correlations 
were consistently between the synoptic-scale com- 
ponent of the total wind and the total interaction 
height tendencies (see Fig. 12). The correlations 
averaged 0.31 and ranged from 0.25-0.40. The 
other comparisons yielded correlations between 
— 0.1 and 0.1. Note in the example of Fig. 12 that 
the interaction height rise regions are located close 
to the synoptic-scale jet maxima. 


6. Conclusions 

The formation of a blocking anticyclone over 
the North Atlantic Ocean during the fall of 1985 
has been examined over its entire life cycle using 
the Z-O equation as the primary diagnostic tool 
and the Tsou and Smith (1990) block formation 
model as a guide. Similar diagnoses (Tsou and 
Smith, 1990; Tracton, 1990) have examined block- 
ing anticyclones that were preceded by explosive 


Tellus 47A (1995), 5, 1 





SF 7 


594 


A. R. LUPO AND P. J. SMITH 


300 mb Wind Speed (m/s) 



300 mb Wind Speed (m/s) 



(KMX) GMT 31 Oct 85 



Fig. 12. The (a) planetary-scale and (b) synoptic-scale component of the 300 mb wind speeds (ms ') and 
(c) planetary /synoptic-scale interaction component of the total height tendencies ( x 10 1 gpm s 1 ) for 0000 GMT 
31 October 1985. Contour intervals are: (a) 2, (b) 10. and (c) 0.5 units, respectively. 


cyclones. This paper examines a blocking anti- 
cyclone preceded by a non-explosively developing 
cyclone and, unlike other studies, examines the 
entire block life-cycle. This paper, like many others, 
demonstrates the importance of mid-latitude 
transients, especially extratropica! cyclones, in 
block formation and maintenance. Furthermore, 
as Dole (1986) found with Pacific positive and 
negative anomalies, this blocking anticyclone 
decayed just as rapidly as it developed, or on a 
time scale more consistent with synoptic-scale 
phenomena. Interestingly, in the case studied 
here, decay of the blocking anticyclone was also 


associated with an upstream developing cyclone. 
However, this cyclone was not accompanied by a 
jet streak favorably positioned as was the case for 
block development. More case studies are being 
examined to determine if this result can be found in 
other blocking anticyclones. 

The relationship between blocks and jet streaks 
following the results of Tsou and Smith ( 1990), in 
w'hich it is suggested that intervening jet streaks 
may play a role in the link between precursor 
cyclones and blocking anticyclones. In particular, 
the jet streak involved in this blocking anticyclone 
strengthened significantly in association with 


Tellus 47 A (1995), 5, 1 






PLANETARY AND SYNOPTIC-SCALE INTERACTIONS 


595 


surface cyclogenesis. The increased anticyclonic 
shear and curvature in turn strengthened the anti- 
cyclonic vorticity advection field that dominated 
the amplification of the downstream short-wave 
ridge. The location of jet streaks relative to the 
block center within the large-scale flow appeared 
to be important throughout the block life-cycle. As 
long as the jet maxima were located favorably, as 
in Tsou and Smith (1990), the block developed 
and was maintained. However, when they were 
located in an unfavorable configuration, or one 
favorable to cyclogenesis as in Rodgers and Bosart 
( 1991 ), the block decayed. 

In this diagnosis, it was found that anticyclonic 
vorticity advection was the largest contributor to 
block formation and maintenance at 500 mb. 
During much of the block life-cycle adiabatic 
warming resulting from downward motion, maxi- 
mizing between 850 mb and 600 mb, and vorticity 
tilting also contributed to 500 mb height rises over 
the anticyclone center. The other thermodynamic 
mechanisms w^ere very small until the decay 
period, when temperature advection contributed 
to height falls and the other dynamic forcing 
mechanisms also contributed to height falls over 
the block center. The block decayed when forcing 
processes contributing to height falls overwhelmed 
those contributing to height rises. 

The partitioned height tendencies over the anti- 
cyclone center demonstrated that the interac- 
tion component, largely due to the advection of 
synoptic-scale vorticity by the planetary-scale 
wind, dominated the total height tendency from 
development through maintenance. The adiabatic 
warming that contributed to block formation 
and maintenance described in Subsection 5.1, was 
dominated by the synoptic-scale vertical motion 


acting on the planetary-scale static stability field. 
During maintenance, the S and / components were 
jointly responsible for the height rise region west of 
the block, and this region probably encouraged 
the westward propagation ending at 1 200 GMT 02 
November 1985. During decay the planetary-scale 
forcing assumed a greater role in contributing to 
the total height tendency field. The regional MAVs 
shewed that the synoptic-scale and interaction 
components were nearly of equal magnitude 
throughout the block life-cycle. This result was 
similar to that of Tsou and Smith (1990), but dif- 
ferent from that of Tracton (1990) who showed 
that the interaction component w'as clearly domi- 
nant. Following and expanding on the suggestion 
of Tracton (1990), more case studies are being 
examined to determine if the relative importance 
the planetary-scale, synoptic-scale, and interaction 
components are dependent on season, flow' regime 
character, or geographical location. Finally, a high 
correlation between the synoptic-scale 500 mb 
wind field and the total interaction height tenden- 
cies was found. 


7. Acknowledgments 

The authors would like to thank co-workers 
Marty Rausch and Don Rolfson for their com- 
puter programming assistance and for their contri- 
butions during discussions. Also, we would like to 
thank Mike Seablom at the Goddard Laboratory 
for Atmospheres in Greenbelt, Maryland for his 
assistance in acquiring the data used for this study. 
Finally, the authors acknowledge the support of 
the National Aeronautics and Space Administra- 
tion through NASA grant # NAG8-915. 


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