Depart Thursday morning from Fort Collins/Loveland.
Flight to Dulles uneventful. Overnight flight to Amsterdam was hard to
sleep on because many people talked continuously and loudly. Also a 300
pounds plus person behind me kept bumping my seat and flopping a newspaper
he was reading on my head. I don't know how he fit in the seat!
Arrived Amsterdam Sunday 0700 and bike arrived on over-sized luggage
pickup area intact. As instructed I sought out the train station which
is the most conveniently located train station in an airport I have ever
Took the train to transfer point and as I was told it would arrive in
15 minutes, decided to put the bike together. Train arrived before I had
a chance to get everything packed, but got everything on the train. I got
the bike adjusted and all finalized by the time I arrived in Utrecht.
Then I spent about an hour trying to locate travel information (VVV)
but the Utrecht area map they gave me was worth it.
Following the VVV's directions I pedaled to DeBilt. It was only 3 or
4 miles and after asking directions I finally found the hotel and arrived
by 10:00 a.m. After a shower I took a nap to 12:30 p.m.
Then I pedaled to KNMI and had lunch at their cafeteria with Aad Van
Ulden. Finding everything ready for next week's meeting, I decided to take
a ride to help keep awake. Aad had mentioned there was a glider port only
10 km west of DeBilt. I decided to check it out. I found a nice forested
area near the airport, but a high barbed wire fence looking much like a
military area surrounding it. If it looks military, smells military, it
must be military. After riding around 3/4 of the perimeter on some scenic
winding bike paths, I finally found an information headquarters. As I suspected
it was an airport with glider operations but only for military.
So continuing around the perimeter I headed back only to get into a
full downpour. I got out my rain poncho which kept me dry except for my
feet. After returning to my room to get dry shoes I pedaled toward Utrecht
to find supper.
The prices in my hotel ran about $30 U.S. per meal. I found an Indonesian
restaurant whose owner found me a room next door to park my bike. The food
was interesting with many tastes I had not experienced. It ranged from
mild to very spicy and cost about $30! Pedaled back to hotel, watched a
U.S. TV program with Dutch subtitles and after reading went to sleep by
10:30 p.m. I slept through to 9:30 a.m.-- Vollie I slept in!
After breakfast at the hotel I pedaled to DeBilt to
pick up some bread, cheese and tomatoes. My destination today is a supposed
glider port near Nieuw-Loosdrecht. Bicycling in The Netherlands is great,
just like Denmark. Well-defined bikeways with separate stop lights and
signs. Most people pedal bikes about like what I bought in 1954 as the
newest thing--three-speed, English style with hugh wheels, an iron frame,
kick stand, and chain-guard. They must weigh 50 lbs. Families were out
biking, some with camping gear. There were bikers everywhere. Riding clubs
all in the same color shirts rode high-performance road bikes. I even
saw one guy riding a TREK 7000 series mountain bike.
The only problem I had was the bike trails don't always follow the
major roadways. Also, since there are no mountains, I frequently got
my directions fouled up. At one point I found myself at Soest about
10 km east of my destination.
After getting help from numerous people (all of whom were very helpful)
I finally found the Hilversum Airport. There I met a glider club who
were operating a two-winch system plus aerotows. The winching operation
was very impressive with light signals and a portable control tower.
I got a ride with an instructor in a ASK 13 for the price of the instructor
($10). After a great launch to approx. 200 m we found a thermal and
soared in it to approx. 1200 m. Two other planes were in it, flying
closer than I have ever been in a thermal. The instructor continued
to ask me to flatten out my bank in turns in the thermals contrary to
what I have been taught here. Rolf experienced the same thing flying
in Switzerland. I had trouble making coordinated turns at first as it
was quite different from the sailplanes I have flown so far. The rudder
pedals, in particular, were a pain. As you placed your entire foot into
metal frames and had to move your whole leg, instead of just wiggle
your toes with your heal fixed in place. It was great ride and I got
some great views of the area.
I biked back to DeBilt through some forests and farmland, a pleasant
Then I biked to Altrecht to a pizza place and got a pizza and beer
for about $10.
The Dutch seem about as conservative about soaring as they are about
biking. All the sailplanes I saw were canvas covered--no glass ships
here! Aside for lower maintenance costs the old ships fly slow (approx.
30 kts) and have low sink rates so they can stay in the weak thermals
and low cloud bases pretty well.
On biking, besides the standard old style bikes being the norm, no
one wears helmets, and they look peculiarly at mine, especially with
the mirror. Few wear shorts, but instead standard long pants.
Today I decided to bike to Terlet to the east (approx.
70 km) where I was told that a very impressive commercial soaring operation
is located. I began about 10:00 a.m. after sleeping in to 8:30 (Vollie
take note!). I went through Zeist and beyond passed over a hilly, forested
area and on to Wordenberg. As usual I lost the bikeway in the towns where
the highway bi-passed but not the bikes. It seems the biking signs all
pointed to the same town which much later (on my return) I found meant
through traffic! Oh well! In one town I believe Schurensee, I asked directions
of a man dressed in a suit with four children in hand also dressed in
suits. The man said he didn't speak English but proceeded to tell me the
Lord made six days for work and one Sunday for rest. Well I knew I was
lost, but not that bad! So, I proceeded on, lost as I was, and eventually
found the bike route which again paralleled the main road.
Along the way I passed many small farms; dairy, I believe, judging
from the manure smells. Many of the farm homes as well as the homes
in town had very nice flower gardens. I passed through another forest
area so thick there was no underbrush and cars on the highway were instructed
to turn on lights.
After 3.5 hours I found my way to Terlet. The area is a highland area
with strong winds, more open land scattered with heather about and smaller
trees and bushes.
The gliderport is on top of a ridge. Here was an impressive commercial
glider operation comparable to Turf near Phoenix. The difference was
Turf operates a fleet of 6 or more tow planes. At Terlet they had a
single tow plane, an old 2-place motorized glider, and a fleet of high-performance
2-place and single place glass sailplanes. They were all lined up in
six side-by-side rows, with several behind the lead planes like the
start to the Indy car race.
The trick was a superwinch that operated six winch lines. A truck hauled
the six lines up from the winch like the start to a sulky horse race.
The lines were dropped parachutes and weak links attached, then hooked
on the planes when they were ready to launch. The planes were individually
launched but in rapid succession.
The weak link by the way in a metal contraption holding a piece of
metal rated to break upon stressed for a two-place or single-place plane
(different metal pieces).
I asked the cost for a flight and since it was 2.5 times what it cost
at Hilversum. I decided to pass. I liked the club atmosphere better
at Hilversum anyway.
I had my lunch and at 3:00 p.m. started back. The ride back was tiring
as the winds picked up to 12 to 15 kts dead on the nose.
Oh, I forgot, on the way out and again on my return I passed a windy
meadow area near Ede. There there was a kite convention. There must
have been 100 or more kites flying in the area, some huge, some big
and many small ones. Some contest maybe? Loads of cars were parked along
the roadside. It was the only area I had trouble biking because the
bikeways were cluttered with family groups on bikes. Mom, pop, the kids,
grandparents, etc. all moving at snail's paces and me without a bell
yelling "bike passing on left" again and again. I guess they
got the idea.
Finally I broke out of the pack and headed along. I was getting pretty
tired toward the end, especially my (sore) END. I decided I didn't want
to wait and pay a big price for food besides, so I took a 3km (out and
return) side-track and stopped at McDonald's--yes me at McDs! I had
2 fish sandwiches, fries, and a vanilla shake for about $6.50 US. Lots
of fat and energy to make up for seven hours of continuous biking! When
I stepped outside it was raining, so I had to suit-up. Got back to the
hotel and ordered a cheesecake to take to my room.
The I relished in a hot shower and then ate my cheesecake with hot
drinks prepared from the hot water faucet. Now I'm pooped and going
to bed. Hopefully I won't wake up after an hour of sleep like last night.
I read from 11:00 p.m to 11:30 p.m. Hence the reason I feel like sleeping
in--All part of the adjustment process.
I slept the night through--hooray! Today I thought
I'd take a train to Amsterdam and then bike to Volendam. I got going by
9:00 a.m. and hardly left DeBilt when it started raining. I put on my
poncho over may rain jacket to keep my pack dry. Took the train to Amsterdam
and ferry to the north side. I followed a river for a while when it really
started pouring. After 4 or 5 miles along a pretty riverside trail I decided
to turn back. Returning to Amsterdam I biked around town and even took
a glass-topped tourist boat. Besides the views from the boat, I enjoyed
the "greenhouse" effect. Remember today is Monday, my fast day,
so I am kind of lazy. Also, my butt is still sore from yesterday.
I particularly enjoyed the old boats along the canals, especially the
old working sailboats with huge lee boards that people have converted
I returned to DeBilt and got charged $6 for my bike on the train. I'll
have to fold it and put it in my bag to fool them in future.
I found a message from Bjorn at the hotel, so I know he is about but
haven't seen him yet.
After a nap I contacted Bjorn and walked into DeBilt with him to keep
him company while he ate (remember it is Monday, my fast day). We found
a Ching-Indo restaurant, what ever that means where he had a dish that
sounded Indian, but tasted more Chinese.
After a full nights sleep and breakfast, I put Bjorn
on the cross bar of my bike with a towel for cushioning and headed for
Bilthoven which is purported to have bike rental at the train station.
It's about a 3 km ride and I worked up a sweat in my rainsuit pedaling
Bjorn and me. I also got a taste of Bjorn's ponytail! Bjorn started getting
a charley horse in his leg, holding it out from the bike.
He was able to rent a standard Dutch ironsides single-speed with a
coaster bake for about $19 per week.
We then headed southeast on a cool and drizzly morning. Bjorn wasn't
so sure he wanted to do this since he neglected to bring rain gear--to
The Netherlands? I told him to wear shorts and loaned him my parka which
he never used as it never got beyond light drizzle.
We took to narrow, paved roads south and east of Dricebergen-Rijsenmberg.
The road had a canal (like our irrigation canals) running along at least
one side, with boats (row boats) in front of most houses. Every so often
we encountered a castle, some looking pretty old, others probably 19th
century. It was a pretty pastoral landscape with plenty of cousheisa
We finally reached the river a top a dyke that was put in place for
flood prevention. Eventually we walked our bikes across the gates of
a lock, then along dam gates across the river. We then came to a closed,
Now what do we do? We saw no signs that we interpreted as "keep
out" getting there. Of course, that doesn't mean there weren't
any. I found a call box and pushed the button asking if we could get
through? On the other side a recreational park was noted on the map.
After some discussion, we in English and the guy on the speaker in Dutch,
I gathered we were supposed to put 5 Kl (approx. $3) in some box before
he opened it. Couldn't find the box and just as we started to turn back
across the dam the gate opened about 3 ft and I walked through. They
promptly closed it with me out and Bjorn still inside. Now what? After
Bjorn and the voice in the box had an English/Dutch discussion, the
gate opened again and he quickly slid through.
Once in the recreation area we began exploring it. I found a sailboat
rental place and was tempted to rent one. But they wanted 65 Kl for
the afternoon or approx. $40 and it was cool, windy and it looked like
it would rain. So we pedaled on to a marina that was closed up and moved
some chairs under an overhanging roof and had lunch while it rained.
Our lunch consisted of cheese and bread which I had purchased earlier
at DeBilt, some apples from our breakfast, and some pears which I picked
up beneath a tree along the road. They were great!
We headed back by way of Wijk bij Duurstede, taking a ferry across
the river. From there we took some single-lane paved side roads through
Cothen (drove through a windmill), Doorn, and again on side roads to
Drieberge-Rijsenburg. We headed north through wooded countryside and
bike lanes to Austerlitz, the rough woods to Zeist. Zeist is quite a
wealthy looking city. Actually I haven't seen anything run down or close
to slums here. But Zeist really stands out. I got a pastry and Bjorn
cashed some money and, bucking the wind, we headed back to DeBilt. Ah!
a nice hot shower again.
I figure I have ridden over 400 km since Friday. My butt is a bit sore,
otherwise OK. Bjorn did well on the single speed and kept with me all
0830 Begin meeting. Welcome B. Cotton. Chris Bretherton
plan for meeting.
Bjorn overview on entrainment.
Taylor's entrainment hypothesis origin of this model.
Separation boundary "laminar super-layer" - Townsend
Basic lab experiments - Deardorff's Tank experiments - 2 layer fluid
- sharing grids. Summarizes selected results from lab exps.
Issue of similarity - lab
Stratocumulus -- how does it differ from lab exps?
Debate of quantitative value of field stratocu obs vs. lab exps.
Recent attempts at synthesis. Breidenthal's work.
- No consensus
- A = 0.2 law not well established, but doesn't matter
- Much less known about stratocu
- Previous lab and theoretical work often contradictory
- Need for more observations
- Shows Stull's fig. of passive, forced, and active convection
- Approaches to simulating cloud population - Arakawa Schubert etc.
- What is the horizontal mesoscale convection (10-50 km) based on
Kain and Fritsch (1993)
- Hyrbrid mesoscale
- How to separate cloudiness by convective scheme and stratiform cloudiness?
- Shows a fractional cloudiness parameterization scheme derived from
LES a function of not defined--may be related to variance? I guess
its Mellor's definition of
Joao Teixeira Describes ECMWF single column model. Shows prog.
eqs. for liquid water and cloud fraction and parameterizations of
cloud coverage entrainment rates, dissipation of cloud fraction,
etc. Describes Monin-Obukov surface layer implementation. Too many
eqs. certainly can't absorb all!
Model is available by writing letter to director request it (ECMWF).
Describes his "nonlinear" subgrid scale scheme. Overcomes
deficiencies of linear models.
- No backscatter
- Can't do normal stress effects of secondary flows
- Inconsistent with respect to invariant transformation
Nonlinear model has 2 more terms added to standard Smagorinsky
closure due to strain rate tensor and rotation rate tensor. Simplest
nonlinear scheme that has all rotational properties.
Nonlinear model extracts energy better from the mean flow and also
TKE is a little different between linear and nonlinear SCS schemes
but temperature fluxes are identical. Smoke fluxes are slightly
different. In TKE budget, largest difference in transport term near
Overall nonlinear model has small effect for smoke case but it
may be more important in a sheared boundary layer case.
It is not known if coefficients are stability dependent--they may
Effect of cloud top cooling on entrainment rate. She varies downwelling
radiative flux 60, 40, 20. Potential temperature flux varies linearly
with FR. TKE doesn't vary linearly. Cloud top height or varies with
is approx. linearly proportional to average TKE.
Stratocu to trade cu transitions. Showed slides of different cloud
regimes over Porto Santo. Discusses his 2D cloud-resolving model
and application to transition from solid stratus to cu.
More stratocumulus to cumulus transitions. PM mesoscale cellular
convection. Agee (1987) fig. showing regions of MCC.
Explanations of MCC's
- constant flux BC's coupled with low (Fiedler, 1994; Qu and Randall,
- Narrow up and broad downdrafts in cu (Bjerknes, 1939; Van Delden,
- Clouds preferentially???
- Cooperative instability with deep gravity waves. Clark, Hauf,
Hsu and Sun (1991) 3D with turbulence model doing transports
Chris shows 3D simulation for ASTEX 4 June -- nocturnal based on
Porto Santo sounding 500 m . Cloud spacing approx. 50 km controlled
by up moist/down dry.
- Broad aspect ratios driven by latent heating (up moist/down
- Commented that so far cloud fraction is same whether or not
- Suggests that open vs. closed cells are the same dynamically
except that you can see cu while the other is obscured by shallow
Andreas Chlond - Max Planck Institute
LES model. Added a single-moment bin model of microphysics. Include
radiative effects in droplet growth. Have 12 classes - ice particles
1-64 to simulate contrails. For boundary layer clouds they use a
bulk microphysics scheme (Lupkes et al., 1989) this scheme is tested
for closed-box type calculations.
Radiation scheme. Use spherical harmonic spatial grid method
(low truncations 2?; simplified version of Frank Evans scheme).
Solutions obtained using conjugant gradient method? Bounded cascade
model to describe spatial distribution of cloud liquid-water. Claims
errors of approx. 2% for truncations of 5 to 7.
Shows variation in "A" with . Shows strong sensitivity
to . Bulk of smoke fluxes come from reasonable scales even for very
coarse . Little sensitivity to sub-grid parameterization once m.
Smoke cooling varied from 10 varied linearly with was constant.
Absorption coefficient (var. thickness of cooling) falls off as
layer deepens, but not too much and nearly constant.
Then he lowered cooling rate (made top of smoke transparent) --
entrainment values dropped sharply, but was flat, and dropped.
Cloud with fixed cooling
Complication of radiation coupled to LWC. Set radiative cooling
as in smoke, but otherwise cloud could do its thing. With ; 50 200
m all give same fluxes, etc.
Varied moisture amounts, e.g., Randall-Deardorff criteria and increases
to a max. at their point and remains constant after that and cloud
fraction goes from 1 to smaller values.
For fully coupled cloud (radiation and cond/evp) higher resolution
was need than fixed radiation case. But not a lot of higher resolution.
- Malcolm MacVean
- "Smoke Case" showed results of his calcs - with 3.2
km and 6.4 km domain - not much sensitivity so smaller domain
- Showed hour-to-hour variability for his runs--some differences
in amplitude but shapes some.
- Somewhat bigger variations in smoke flux.
- Residual in energy budget near inversion were large, otherwise
Rate of rise of smoke top varies with method of calculating buoyancy
in equation. Hour-to-hour variations in total smoke flux were very
Showed plots for all participants--huge scatter!
"A" values varied greatly from monotone to non-monotone
schemes. Variability (hour-to-hour) greater in non-monotone schemes.
Clear differences in smoke topped height between monotone and non-monotone.
Sub-grid contributions to smoke fluxes are consistently small.
Horizontal velocity variances agree among monotone schemes, less
so for non-monotone. Same with .
Theta flux shows more scatter but sub-grid fluxes are very small
compared with total.
Skewness values are more consistent except above inversion where
monotone schemes yield small values (near zero) of .
Eddy viscosity (diagnosed) varies quite a bit with MacVean and
Bjorn showing a max near inversion while others don't. Reasonable
consistency in resolved TKE.
High Vertical Resolution
values smaller. Smoke heights closer. Layer average TKE shows more
spread but converging? Total smoke fluxes showed most variation.
Radiative cooling still appears to extend one grid point above inversion.
Restricted domain results less statistical significance in 2D and
3D. is consistently larger in 3D than 2D. Peter Bechtold did only
Bjorn's Summary of Many Experiments
- Changed eddy Prandtle number - No effect.
- Changed diffusion length scale - No effect.
- Moved point where K is calculated - No effect.
- Used non-monotone operator - Same as group.
- Monotonic one direction - No change except reduced cost.
- Ran compressible model - Little change
- Ran anelastic model - Little change.
Increased resolution increased entrainment if only. Increase gave
about same resolution.
P. Mason - If simulations are correct model suggests what is important
is local distribution of . You can't distinguish numerical diffusion
from Smagorinsky diffusion by scaling.
Shows differences in predictions of for smoke case for cases using
centered differences and using monotonic schemes.
Shows case where most radiative cooling occurring at smoke top
and case where most is within the mixed layer.
- Location of radiative cooling important
- High resolution runs--gradients better preserved placing more
radiative cooling in mixed layer
- Real cloud exhibits different budgets - v not conserved
- Evolving concept - differences in coarse resolution runs attributable
to placement of cooling and not to differences in "dynamical
resolution" of entrainment.
Sensitivity of dry entrainment to turbulent Prandtl number and
mixing length in 1D and 3D simulations.
New model code, non-hydrostatic built from grid up. 1D simulation
can be tweaked to compared well with LES using in 1D gives better
Shows dependence of A.
Shows values of for cases of surface heating and cloud top cooling
things fall on line if
ASTEX Lagrangian I and 2 cases
Chris and Peter Duynkerke describe the Lagrangian experiments.
Peter - Lagrangian 1 -
Drizzle rate: a lot of scatter about 1 mm/day. Estimates of . Stephan
de Roode--Turbulent structure of 1st Lagrangian SST increased 4.5
C in 4 days subsidence 0.5 cm s.
KNMI sponsored a fancy full course dinner in Utrecht. It took
4 hrs! Got back to my room at 11:30 p.m.
- Chris Bretherton
2D simulations of ASTER Lagrangian with HUSCI.
Lagrangian 1 -- Forced with observed subsidence, geostrophic
winds SST's. Exhibited a deepening of BL with time and transition
to cumulus with a decoupled BL.
At end of simulation cloud fraction remained approx. 100% near
top of BL which is an overestimate of cloud cover.
Lagrangian 2 -- Subsidence assumed constant throughout period.
Simulation gives a drop in BL height followed by a growth while
it was observed to rise steadily. Was it the model or data driving
2D simulation using the UUCEM.
Lagrangian 1 -- Results comparable to Chris except that
cloud fraction was much less than 100%. Shows substantial difference
in simulation due to radiation.
Lagrangian 2 -- Steve's results more similar to Chris' than
either are to observations.
Lagrangian 1 -- BL depth varied reasonably. Cloud cover
not bad. LWP ok earlier on but poor later on. What maintained the
cloud was not the stratocu para. but large scale condensation from
Sensitivity -- Not sensitive to radiation scheme frequency
of activation. Small time steps smooth oscillations but cloud disappears.
Lagrangian 2 -- Produces convection immediately and produces
stratus cloud similar to Chris and Steve.
Erik van Meijgaard
Single column model for global-wide ECHAM4. Nonlocal diffusion
(Troen and Mahrt, 1986; Holtslag and Moeng, 1991. Cloud scheme generally
based on Sundqvist's scheme.
Lagrangian 2 -- LWC's predicted much less than observed
partly because mass flux scheme very active. Cloud cover looked
Lagrangian 1 -- Cloud base very low. Cloud cover solid stratus
early on changed to cu and lower cover later as observed. LWC looks
good (at 10 hrs later on it creates too high a cloud. Sensitivity
experiments with mass flux scheme. Modified entrainment rate which
results in mass flux decreasing with height. Increasing entrainment
rates doesn't improve results.
LWC underestimated by standard scheme. Increase of lateral entrainment
puts increased RH at low levels, but this is in wrong direction.
Mostly discussion of representativeness of sounding. Diagnosed
fractional entrainment;detrainment rates much larger than generally
used in GCMs and detrainment is larger than entrainment.
Presents Don Lenschow's proposed measurement strategy for entrainment.
Lagrangian 1 -- Drizzle rates reasonably consistent. Sensible
heat fluxes are consistent latent heat fluxes are more variable.
Lagrangian 2 -- Lots of disagreement on cloud fraction.
Drizzle rates vary a lot with obs being close to zero. Some models
First Lagrangian: Cases 1,3 -- General agreement with obs
Second Lagrangian: Cases 2,4 -- General agreement with cloud
but not cloud evolution. Reasons not clear. Candidates: Initial
soundings and spin-up time.
- Clarify; standardize simulation procedures (relaxation; winds)
- Increase participation; submit results.
- Sensitivity issues; Case 2 initialization. Role of drizzle?
- Assemble more extensive aircraft etc. obs for verification.
- Group paper; Summarize results (deadline date for inclusion?)
The Smoke Cloud Case Study of the GCSS Boundary Layer Cloud WG
Summary of Results and Ongoing Work from Aug 95 Workshop
- Even if the subgrid fluxes are at all levels smaller than the
resolved fluxes, LES results are still sensitive to the numerical
algorithm and subgridscale turbulence scheme.
- Centered difference advection schemes produced negative
smoke, spurious temperature rises, and substantial spurious
heat fluxes in the layer above the inversion. On the positive
side, they also tended to produce relatively small entrainment
rates, with typical A values varied from 0.5-1.
- In the control run, monotone schemes tended to have 'A'
of 0.8-1.6, 50-100 they produce more numerical diffusion.
The numerical diffusion appears to be comparable to the explicit
turbulent diffusion. In one model (D. Stevens), no explicit
diffusion was used but the results were comparable to other
- A simple model of advection of an oscillating interface
coupled to a mixed layer (B. Stevens) below produces entrainment
rates comparable to those found in the models. This purely
numerical entrainment mechanism as opposed to a more physical
explanation such as wave-breaking, eddy rebound, etc. may
be responsible for much of the entrainment seen in the LES
- It was generally agreed that the vertical grid spacing specified
in the control run (25 m) was too large to fully resolve the important
motions at the entrainment interface. Enhanced vertical resolution
decreases 'A' and the entrainment rate. D. Lewellen's results,
corroborated by B. Stevens, suggest 'A' decreases to 0.4-0.5 if
a 10 m or less vertical grid spacing is used and is insensitve
to further increases in vertical resolution. The entrainment process
was not found to be sensitive to changes in the horizontal resolution
between 50 and 320 m. However, 2D high resolution simulations
by B. Stevens and D. Stevens suggest that if vertical and horizontal
grid spacing are both increased, the decrease in 'A' may be much
smaller than simply increasing vertical resolution alone, confusing
the issue a bit. Two considerations may contribute to the need
for higher vertical resolution:
- Radiative cooling in the inversion and top of the smoke
layer is too vertically concentrated to be resolved.
- Scaling arguments provide a height scale zu = wstar**2/delta-b
for undulations of the inversion. For the smoke cloud case,
zu is approximately 3 m. The LES cannot resolve these undulations
unless dz is comparable or less than zu, possibly leading
to a poor representation of the entrainment-forcing motions
at the entrainment interface.
- From 1 + 2 and cross sections, it was clear that implicit or
explicit diffusion, rather than resolved overturning, was the
entrainment mechanism in all of the LES runs. Both horizontal
and vertical resolution would have to be increased 5-10 fold to
see reolved scale overturning, which is not computationally feasible
in the immediate future. There was debate about how important
a drawback this was.
- M. MacVean showed that most models had large residuals in the
kinetic energy budget at the inversion. He traced this to an energetic
inconsistency between the usual finite-difference representation
of the buoyancy term in the vertical momentum equation and the
vertical advection term in the potential temperature equation.
The implied conversion between potential and kinetic energy due
to buoyancy flux are different in the two equations, leading to
a nonconservation of total energy that can be quite severe in
a poorly resolved inversion such as occurs in the control run.
He showed how the buoyancy could be rewritten to remove this inconsistency
in a scheme using centered time differencing, and found that this
significantly decreased A in the control simulation and almost
entirely removed the residual in the KE budget. He could not find
an analogous way to improve energy conservation at the inversion
for forward-in-time schemes,
- Sensitivity studies of the control results to changes in forcing
strength and distribution and inversion strength were performed
by several groups (Lewellen, MacVean, B. Stevens, Siebesma and
Cuijpers, Moeng, Rand and Bretherton). The main purpose of these
studies was to test whether the parameterization we/wstar = A/Ri
did correctly describe the effects of changing the forcing. Contradictory
results ranging from scaling laws that we/wstar = c*Ri**(-0.6)
to we/wstar = c*Ri**(-2.5) were obtained. Differences may partially
be due to temporal variability in entrainment rate and turbulent
transports, especially for high Ri. High vertical resolution studies
of Lewellen supported an Ri**(-1) scaling. The entrainment rate
also decreased markedly if more of the cooling took place below
the entrainment region.
- Despite the above sensitivities, many resolved features of the
convective turbulence, such as skewness profiles, were at least
qualitatively similar between all LES models. Not surprisingly,
most differences between models appear to be associated with the
dynamics and subgridscale processes at the entrainment interface.
- Several groups also performed 2D simulations, and all found
much higher values of A between 3-5 and of entrainment rate, compared
to the 3D runs.
- Two groups also tried one-dimensional models (P. Bechtold and
J. Cuxart). Bechtold required enhanced vertical diffusion of smoke
to maintain a well-mixed smoke profile. Cuxart found A = 2.3,
lowering to 1.5 with 5 m vertical resolution, with a stability
dependent Prandtl number. More general conclusions could be drawn
if more 1D modelling groups tried to run this case.
- Some groups (Cuijpers, Lewellen) also considered the impact
of adding moist thermodynamics back into the control run without
mkaking the cloud radiatively active and found this tended to
Planned Papers and Future Work
- A group paper, with M. MacVean as lead author, will be prepared
during this winter, including the above conclusions and sample
results. This paper will focus exclusively on results presented
at the workshop. (Malcolm intends to write a separate note about
point 4 above, as well).
- One or more further papers in which we try to resolve some of
the issue raised in this workshop, in particular points 2, 3,
and 5. These papers will involve further simulations and analysis,
with limited intercomparison of results between groups, and further
work will be coordinated using email, WWW and phone. Groups indicating
some interest in further participation included Lewellen, MacVean,
B. Stevens, Siebesma and Cuijpers, Rand and Bretherton and D.
Principal topics to investigate further:
- Nail down effect of not resolving radiative cooling and placing
too much cooling in the inversion above the actual turbulent mixing.
It was agreed to remove cooling in the inversion from future simulations
by only allowing points with S 0.5 to contribute to the smoke
path and radiative cooling. This is analogous to a stratocumulus
cloud, where intense cooling requires the presence of condensate
in a mixture, which typically requires that the mixture be at
least 90 air.
- Continue investigating the effects of vertical and horizontal
resolution, particularly to nail down whether low horizontal resolution
but very high vertical resolution is sufficient and how the required
vertical resolution can be predicted a priori. One tool that may
be useful here is simulations with a nested region of both horizotnal
and vertical refinement in the entrainment zone (could be investigated
bu D. Stevens and B. Stevens)
- Try to get agreement between groups on the dependence of we/wstar
on the forcing, perhaps using higher vertical resolution in the
- By increasing the radiative forcing dramatically, do a simulation
with much lower Ri (on the order of 10) in which the entrainment
process should be fully resolvable and test whether A agrees with
lab experiments for this case. For such a case, a direct numerical
simulation with constant viscosity and diffusivity should also
Group agreed to met at Clermont-Ferrand next August.
After the meeting Steve Krueger (on Bjorn's single speed rental)
and I took a bike ride to the north. Steve had a hard time with
the brisk head winds.
We followed winding back country roads with the usual canals along
side. In Westbroek we took pictures of a large operating windmill.
Very picturesque. From there we found a recreational lake with many
sailboarders on it. Steve wants to get back there to sail Sunday
We then followed a bike path around the lake and then back to the
Bjorn, Steve, the student from Penn State working with John Wyngaard
and I went to dinner in DeBilt. Had a good meal and a few real great
- Start of the long day back to the Fort. Bicycled to Utrecht, broke
down my bike and headed by train to the airport. It is convenient
to have the train go right into the station. Too bad DIA doesn't do