I awakened this morning to, 9 inches of fresh snow and 9F. I suspect it will be a bit warmer in Israel. I am not looking forward to this long flight. I arrive in Tel-Aviv at 1535 which is 7:35 MST, or 26.5 hrs after I got up this morning. This includ es a 3 hour layover at Dulles and over 2 hours at Paris. Why do I put myself through such torture?
1547 EST--I used my remaining free upgrade certificates that die the end of this year to upgrade to business. I wish I could have those big seats the rest of the trip. Weather in Washington is 3000 ft ceilings, and light rain; not very nice. I prefer t he cold dry snow to this!
7:14
The flight over went well except that it was long. I also had difficulty finding my way to the proper terminal at Paris DeGaulle airport. After arriving at the hotel I walked along the coast with Harry Ochs and watched him eat as I fasted on the Sabb ath.
Breakfast at the hotel was quite an impressive spread of varieties of salads, breads, boiled eggs, fish, cheeses, and cereals. It was a bit better than the 'continental' breakfast I got in Rawlins, Wyoming consisting of cereal and toast.
I decided to take a tour of Jerusalem with Bob today. Our first stop was at the holocaust Museum--a real downer. Then they took us to Bethlehem and the Church of the Nativity, supposedly the oldest Christian Church in the world, first built in 300AD. Next as with tours worldwide they stopped at a tourist junk shop. These tour agencies must get a kickback for such stops. They saw Bob coming as he bought a bunch of woodcarving things. When he stepped outside the Arab street salesmen came after him like "flies to dead meat." He is loaded with necklaces, cards, and what-have-you. They even followed him into the bus!
After a rather mediocre lunch we visited old Jerusalem. We entered by way of the Arab section, past a bazaar. The streets are very narrow not much wider than sidewalks. Now imagine them being filled with wall-to-wall tourists, and Arab shoppers. The n imagine in the center of those streets fruit and vegetables, and what have you. Then imagine Arab kids aggressively pushing empty and loaded handcarts up and down those streets. In addition, our tour guide was trying to keep us all together. When we did stop to wait for the slow ones, we formed a great road block in the thoroughfare.
The narrow streets wandered though a series of little shops with hawkers pushing their wares. Besides being noisy, the streets were littered with trash. It was definitely a very colorful place!
We progressed onward through what was more like a dark cellar than open streets, as over the streets were arches bracing the walls to keep them from collapsing into the streets. Eventually we came to a security checkpoint much like at airports even with metal detectors. This was the entrance into the Jewish section. Instantly, the streets became clean and the walls straightened and the buildings restored. Although, the streets are still narrow and winding, they seem more open and brighter.
Including our visit to Bethlehem, we visited at least five churches; more than I've been in the last ten years! A few of those were almost 1700 years old, one being the oldest Christian church in the world. These churches followed the life of Christ including his birth, his crucifixion, and burial. A crypt in one was purported to be his burial place; no body! Oops that’s right, he ascended to heaven complete with body.
We also visited the Wailing Wall which I had visited before, and an excavated section that descended 50 feet or so below the current old city, to the city of 2200 years ago. Eventually, we made our way to another gate and our guide spent 20 minutes o r so collecting all or our group.
It took us to 7:45 to get back to the hotel and we were then picked up by Zev Levin, Tamir Reisin, and Shalva Tzivion to go to supper. Each brought their wives to liven the party. I had a great opportunity to chat with Sussie Levin who we have known f or years. Sussie, who never exercised in her life, and who always gave me some grief about my exercise habits has turned into an exercise addict. She swims regularly and works out on a treadmill, and feels and looks great! Zev has a new wife!
To bed by 11:30 and up by six for breakfast and a 2.5hr drive to the Kibbutz where we will have our meeting.
It seems like everyone in Israel has a cellular phone. Wherever you go people are talking on their cellular phones. Even on the bus, I hear the cellular phones ringing. Early in our first session a cellular went off.
After a short run with Bob and a not short enough lunch, the first session of the workshop began. Alex Khain and I shared the session chairing job.
Jean Louie Brenguir talked about measuring cloud droplet broadening using ASSP. Measurement errors with the system were discussed and he showed convincing evidence of rapid drop broadening faster than current theories can explain.
Professor Mazin presented an overview about the need for modelers to more objectively test their models. He used an analog between modelers and models as puppeteers and puppets where different strings are pulled to make models fit data. I began the d iscussion session imitating a puppet on a string. It looks like it will be "pick on modelers" workshop and I seem to be the one to pick on.
Zev Levin discussed observations and simulations of the effects of giant, sulfate-covered dry particles on droplet spectral broadening. He made a convincing case that they can be quite important.
Harry Ochs presented observations and kinematic modeling of rain in Hawaiian rain bands. His kinematic model gave good correspondence between observed and simulated patterns of radar reflectivity. He argues that the particular flow properties of the clouds growing in little sheer contributed to localized growth of large highly reflective drops.
Alex Khain gave an overview of his recent work with Pinsky on the effects of turbulence on collection kernels and droplet concentration fluctuations on scales of a few centimeters. I think his work is explaining enhancement of droplet collection of la rger cloud droplets but does not explain the process of initial broadening.
T. Elperin discussed the effects of turbulence on accumulations of aerosol concentrations. He discussed one mechanism which was important to aerosols greater than one centimeter that would contribute to aerosol accumulation in the vicinity of inversi ons. It is not clear how this process which he discussed relative to molecular diffusion relates in the atmosphere when large eddy diffusion would prevail. He also discussed aerosol accumulations on centimeter scales by the action of vortices such as Khai n discussed for droplets. I think this is the same model as Wang and Maxey’s.
Pinsky focused further on turbulence influence on collision efficiency as it pertains to collection kernel enhancement. I expect there will be considerable discussion about this topic in the future in this workshop.
I had planned to go running before breakfast but awoke to rain. The morning colors in the overcast light-raining sky were surrealistic--very pretty! Yesterday we were in the tropics and the presence of date palms, frangipanni, hibicus, and other trop ical flowers did nothing to dispel this image. Today, with the passage of a cold front, we are reminded that we are really in middle latitudes.
Nadine Chaumeliac showed results of cloud model simulations of cloud chemistry. She also showed simulations of transports of chemical species over Africa using RAMS. She finished with work underway simulating and observing clouds over the Puy de Dome, an example of laboratory and modeling studies.
Sabina Wurtzler discussed, modeling studies of aerosol cloud interactions. She discussed numerical experiments with the Clarke/Hall/Flossman model processing of aerosol through clouds.
Y. Rudich discussed the uptake of nitrate radicals in cloud drops. Emphasis on isolating various steps in the process and measuring the rates in the lab.
Discussion of previous papers.
J. Joseph-Modeling of Cloud Radiation Interaction. Talked about 3D radiative transfer and satellite observations including fractal stuff.
Bott modeling clouds radiation interaction.
Danny Rosenfeld-Remote Sensing of Clouds from space. Using multi-spectral imagery shows that inferred cloud top effective radius is consistent with occurrence of precipitation. Shows shift of cloud top effective radius to smaller sizes in regions of sm oke plumes over Indonesia implying suppression of rain there.
John-Louie Brenguier--retrieval of effective radius via satellite- Shows increase of optical thickness of clouds with height increases with height to the 2/3.
After dinner we had a talk by Samuel Kantor, senior advisor to Mekoroth, the water company to Israel. He discussed the water problems and policies in Israel. He showed projections of water needs through the middle of the next century. He also discusse d agreements with Jordan that actually places further constraints on the water system. Plans for building dams in Jordan for optimizing joint water resources were discussed. The cost of desalinization relative to other options was also discussed. Cloud se eding is not part of the water resource strategy but considered a bonus if it proves to be a reliable resource.
I began the day with a before breakfast jog. The day is overcast but it appears that it will warm up. It is surprising that at this latitude they are able to grow bananas. My banana plants died when temperatures dropped to the upper forties in my off ice and my greenhouse. Maybe they have a tougher variety. The fruit are small like ladyfingers.
The kibbutz has that look of a commune. The buildings are grayish, not painted for some time. Bits of trash are about the yards making them look like student renters live there. Of course this occurs in privately owned neighborhoods, but the uniformit y of such an appearance is what stands out. There just aren't the freshly painted houses, well-kept yards, and nicely landscaped lawns. Everything has that gray look of communist Poland and Russia.
I gather life is really changing in the kibbutz. In this one, 60% of the income comes from tourism such as the hotel and only 40% from agriculture. Many communes have established industries of one form or another. Our guide on a tour of the kibbutz p ointed out that they are deliberating whether or not to introduce salaries. Currently everyone earns the same stipend regardless of what they do or don't do. There are lawyers and doctors who earn the same as someone shoveling cow manure! As a result youn g people are leaving the kibbutzes. If a lawyer wants to have a fancy car to impress his potential clients, he must make a special request to a committee who may or may not approve it, in spite of the fact that all his high earnings go back into the kibbu tz pool. This change from a pure agrarian lifestyle is putting a lot of pressure on the entire system. Bjorn are you listening?
This morning’s session will start off with my talk followed by Bob Walko. I will talk about our experiences with LES/CRMs coupled to bin-resolving microphysics models. The extended text of my talk is appended with this report. Bob will talk about our recent work in microphysics parameterization schemes.
Beheng also talked about parameterization of warm cloud microphysics.
Tamir Reisin presented a more philosophical talk focusing on open issues. He described the need to consider habits, and mixed phase particles, perhaps using Bob's thermo energy approach. His talk was overall rather pessimistic.
Shalva Tzivion discussed the accuracy of stochastic collection equation. He showed that a new two moment scheme using 36 bins does very well compared to the 144 bin two moment scheme, and much better than Berry and Reinhart’s 72 and 104 bin model.
Pinhas Albert shows how African dust can contribute to model forecast temperature errors in the lower troposphere.
Tamir--Parallelization of microphysics models. Shows an example of physical parallelization of bin microphysics.
Yoav Yair--Modeling Thunderstorms and Lightening in Jupiter-
Applied temperature reversal graupel/ice model to Jovian atmosphere in axisymmetric model.
Y. Yin discussed the sensitivity of cloud microphysics and precipitation to CCN size distribution. Respondek et al 95, ref to fitting Hobbs et al 85 aerosol size distribution with 3 modes of log-normal distributions.
Today is tour day. We are being taken by tour bus to the Golan Heights and other sights. It is a nice sunny day but when I ran this morning I saw towering cu over the surrounding hills. I expect we will encounter clouds in the higher terrain. Bob, sa ys the road gets up to 5000 ft in Israeli territory in the Golan Heights; whether we get up there is another thing.
Our first stop was a pumping station which is the main water supply for western and southern Israel. In addition to direct water supply they also recharge the aquifers on the coastal plains during the winter season.
We next visited a church supposedly located where Christ fed 5000 with three fishes and five loaves of bread. There were mosaics on the floor dating back to something like 400 AD.
Along the road in piles of rocks were brown, long-tailed animals something like slim marmots. At one place I saw about 6 of them in a small tree. I also saw a gazelle lying beneath a tree looking much like a goat.
We visited an overlook of the ancient city of Gamla which was destroyed by the Romans after a long siege. Had I known the layout I would have jogged down to the ruins since the roundtrip was only 1.2 miles. I would have missed the old stone olive pre ss, however. Still this what I don't like about tours. Everything is done on someone else’s schedule. This is the sort of place I like to sit in silence (no noisy tourists) and contemplate what life was like here thousands of years ago. Located on a narro w ridge, you know that they located here because of its strategic location for defense. Has life changed all that much in the last two thousand years? They built aqueducts to transport water from the highlands to the village. Along the entrance road are r ocks stacked like a table, called Domens, which are ancient burial sites of nomadic peoples 4000 years ago.
It looked like good soaring conditions as a dozen or so vultures were soaring several thousand feet above. After a while they disappeared from view, probably off on a cross country flight searching for carcasses. Although it is probably often good s oaring here, I doubt it is permitted to fly gliders, since it is so close to the border. A few vultures lingered near the canyon and soared low along the ridge crest. How can such birds so beautiful in flight be so ugly looking when viewed close up? The a ltitude of the Golan Heights is about 1700 ft. Although it looks very dry here there were a number of wild flower species blooming among the weeds. Some of these are cyclamen which I have never seen in the wild, and other flowers which looked familiar but I can't identify.
While we were looking over the canyon, we were frequently startled by cannon fire from tanks. The loud boom echoed through the canyon. Further reminders of the tenuous peace were jet fighters over-flying us.
Our next stop was 3800 ft Mount Kental. It has bunkers and armaments on top and overlooks Syria to the east. To the north is Mount Herman the highest peak in Israel. It is located on the border of Israel, Syria, and Lebanon. A Kibbutz is located at t he foot of the mountain and apple and cherry orchards are sitting almost on the border. To the east is a destroyed Syrian village in the U.N. neutral zone. Further east is a newly built (since the Yon Kipper war) Syrian city. This border is quiet now as t he Syrians choose to stir things up along the Lebanon border rather than jeopardize their own lands.
As I stood on the summit surrounded by bunkers, it makes one think how precarious life is here. Why do we humans hate each other so? Races, color, religion all are divisive. Orthodox Jews, orthodox Moslems, the religious right, all believe that their way is the only way. Here in Israel they all claim this territory as a holy place which they should own. The Golan Heights is one rocky, arid place that no farmer in the U.S. in his right mind would farm let alone fight over.
We had a bit of a panic upon leaving the mountain top as our head count was missing one person. Have we lost someone? If so, it was at an earlier stop. I guess not, it was a bad head count to start with.
Lunch was in a Druez village where we had pitas, falafel, humus and salad stuff. Great vegetarian fare! The owner was huge and wore a white jacket, pants that looked like culottes, wore a white knit hat, and had a handlebar mustache. He was something to see. In the middle of the dining area was a kerosene stove that was cylindrical, with a chrome tube running from a fuel container on a rod overhanging it. It was covered with blue porcelain paint with white specs.
The Druez' are a 1000 year old separate Moslem sect. They seem more liberal that the standard Moslems as the woman bare their heads, not having to wear shawls. Moreover, they are not required to pray three times a day. They believe that when they di e they are born-again as a Druez. They do very well under Israeli rule as they have a large market for the apples they grow. According to our guide they publicly claim to want to return to Syrian rule, but secretly prefer Israeli control. If they don't in dicate a desire to be Syrian they fear that if the territory is returned to Syria they will be persecuted. In the past they didn't receive any support from Syria as they were on the frontier.
After more riding in the bus we returned to the Melody Hotel in Tel-Aviv. Still feeling full from our large lunch, none of us felt like having supper. So, after making arrangements for a 4:30 AM taxi pickup, I took a shower and went to bed early.
Smoking--over the years I have found it harder and harder to be around smokers. The second-hand smoke makes me cough and often gives me a headache. Also, I hate the smell of my clothes after being around smokers--PU! The problem is I really like a nu mber of smokers, but find myself avoiding them if it looks like they will have a chance to light up. At the meeting and in the bus they weren't allowed to smoke, but at breaks or bus stops lookout! Even at some restaurants, they were permitted to smoke. I f I sat near a smoker and they lit up, I usually found an excuse to move elsewhere. I always feel bad about that, but I feel so uncomfortable around their smoke I feel I have no other choice.
It was time to leave Israel, to say good bye to my many friends who I probably won't see until the next meeting. It is always a strange feeling to leave a country that you visit only a few times in a lifetime. You have seen much, but there is so much that you have not experienced. Moreover, as you get older and see friends and colleagues die, you realize there may be some of your friends that you may not meet again.
We began the marathon day at 4:00 AM with a 4:30 cab ride and a 7:15 flight. The flight to Paris-DeGaulle was uneventful with nice views of the Alps. Arriving at Paris we were off-loaded mid-field and picked up by a bus. This time, when I got off the bus and immediately upon entering terminal two, I asked for directions. There just are no big signs explaining how or where to go for connections. In fact, it took the woman some time to find out we had to go to terminal one. I was a little anxious since we only had 1.5 hrs to catch our connection. To the right of entering the terminal and well hidden from view as one enters was a waiting place for terminal one. Flights were posted on a terminal screen but only for an hour. So, we still didn't know which gate. After more than 20 minutes Bob and I caught the shuttle bus to terminal one which first stopped at all four terminal two satellites. Entering terminal one, still no listing for our flight. So, I asked a clerk at a desk and got a rather sharp reply pointing us in the right direction. After three times through security checks we made it to the check-in counter just as they were loading. Bob had to get his ticket straightened out as his ticket for this leg was inadvertently collected on the way over. He made it!
Taking off from overcast Paris we broke through into sunshine at 6000 ft or so. This is the fourth flight in or out of Paris this time of year and I have yet to see the sun. Over the central Atlantic all I can see is solid cirrus.
The reminder of the flight through Washington-Dulles and on to Denver was uneventful, arriving home about 10:30 PM.
Cloud-Resolving Simulations with Bin-Resolving Microphysics
William R. Cotton
Colorado State University
Dept. of Atmospheric Science
Fort Collins, CO 80523-1371
cotton@isis.atmos.colostate.edu
1.0 Introduction
A commonly used tool in the investigation of the fundamental physics
and dynamics of boundary layer clouds and even deep convective clouds
and the development of parameterization schemes is a cloud-resolving
model (CRM). Cloud-resolving models are generally two-dimensional
models which explicitly resolve the large eddies in boundary layer
clouds and deep convective clouds. The most faithful representation of
the large eddies in clouds and the atmospheric boundary layer is
large-eddy-simulation (LES) models. The two-dimensional CRM is a
compromise in LES models to enable performing a greater number of
simulations or performing simulations with more complex physics than
can be generally used in a LES model.
For the most part CRMs and LES models of the cloudy boundary layer
make use of simplified bulk microphysics schemes. Such schemes employ
one or two prognostic variables when applied to warm clouds and five
or 10 variables when applied to mixed-phase clouds. However, use of
bulk microphysics scheme in CRMs is not really consistent with the
philosophy of such an approach. The concept of CRMs is to
explicitly-resolve the dominant energy containing eddies in the
atmospheric boundary layer(ABL) or the cloudy boundary layer and to
use as little parameterization of the motions in the ABL as possible.
Thus the approach that is most consistent with the CRM methodology is
to use a bin-resolving microphysics scheme in a CRM. Bin-resolving
microphysics models parameterize cloud microphysical processes as
little as possible and explicitly resolve the size-spectra of
hydrometeors rather than parameterize them.
In this paper I summarize are experience using the regional
atmospheric modeling system(RAMS; see Pielke et al,1993)deployed as a
CRM or LES model( coupled to the University of Tel-Aviv bin-resolving microphysics model(Tzivion et al., 1987; Reisin et al., 1996). I will begin by first discussing the limitations of the cloud resolving model/bin-microphysics(CRM/BM) approach, then s ummarize what we have learned from such models, and conclude by discussing our plans for the future.
2.0 Limitations of the CRM/BM modeling approach
LES models are inherently three-dimensional and when used with
fine enough grid spacings, they are generally believed and have been
shown under a few limited conditions to represent the large-eddy
structure of the clear and cloudy boundary layer rather well. There is
a perception in the community that LES models are rather insensitive
to the sub-grid closure scheme and the particular grid spacings
selected. At least this is the view when they are applied to the
convective boundary layer, not the stable boundary layer. Recently the
GEWEX Cloud System Study Program(GCSS)(ref Brownings paper) Working
Group#1 performed a series of model intercomparison studies to examine
the performance of LES models in simulating the cloud-topped marine
boundary layer. After performing somewhat realistic marine boundary
layer simulations and finding quite a bit of variablility in the
simulated entrainment rates among the models(Moeng et al, 199?) it was
decided to perform intercomparison simulations for an idealized smoke
filled boundary layer case. Variability among the models was still
found. During the recent GCSS Working Group#1 workshop in Seattle in
July 1997 Dave Lewellen reported that his model exhibited little
sensitivity in predicted bulk entrainment rates to prescribed grid
spacings over quite a large range. Stevens(personal communication)
reported to me that several other groups have attempted to replicate
Lewellen's results with some agreeing with him and others not. The
difference appears to be related to the sub-grid closure schemes with
the TKE-based Deardorff schemes exhibiting little sensitivity to grid
spacings while the Smagorinsky type schemes being very sensitive to
grid spacings. Apparently the advection of TKE in the TKE prognostic
schemes compensate for the variation in grid spacings. In summary the
LES models are sensitivity to at least the form of sub-grid scale
closure used even for the convective boundary layer and certainly for
stable boundary layers.
Another limitation of LES models is that they do not
explicitly-resolve turbulent eddies on the scales of droplets or other
hydrometeors in clouds. There is considerable evidence that turbulence
on scales of a few centimeters can affect the rate of initial
broadening of droplet spectra by vapor deposition(Manton,1979;
Copper,1989; Mazin,1968; Sedunov,1966; Baker and Latham,1979;Baker et
al,1980;1984; Telford and Chai,1980;Telford et al,1984;Shaw et
al,1997) and/or collision and coalescence(de Almeida,1975;1976;Koziel
and Leighton,1996;Pinsky and Khain,1997; Khain and Pinsky,1997). The
details of the magnitudes and importance of turbulence effects on
these scales is still being hotly debated in the literature.
Nonetheless, turbulence influences on such small scales must be
parameterized in CRMs and LES models.
Another problem that is endemic to CRM and LES models
regardless of whether standard finite difference operators or
pseudospectral techniques are used, is that spurious cloud-edge
supersaturations can be produced when coupled to explicit microphysics models.
Use of conservative thermodynamic variables and monotonic advection
operators only minimizes the problem but does not completely eliminate
it. Likewise, the problem cannot be fixed by increasing resolution.
Instead, higher resolution just sharpens the cloud-edge gradients
which makes the problem worse(Grabowski,1989--in stevens et al 96).
Stevens et al,(1996) conclude that the problem is caused by the
inability of Eulerian models to represent the location of the boundary
of a cloud with a resolution better than the grid scale itself. Thus
some droplets are spuriously produced or destroyed at the leading edge
of the cloud causing an artificial imbalance between the rate of
production and dissipation of supersaturation. This can lead to
activation of aerosol over large regions of the cloud boundary that has
little relation to actual physical mechanisms of activation such as
found at cloud base.
Besides the limitations we have discussed for the 3D LES models 2D
CRMs are also subject to limitations imposed by the constraint to
two-dimensional dynamics. As noted by Stevens et al(1997), the lack of
energy cascade in two dimensions results in eddies which are larger in
horizontal scale than in 3D and which become increasingly vigorous as
the grid separation becomes smaller. This results in 2D models being
very sensitive to small amplitude variations in initial conditions.
Nonetheless, the magnitudes of velocities predicted by 2D models are
comparable to those obtained in 3D, so that they provide a useful
environment testing and development of coupled microphysics/dynamics
models and for performing sensitivity experiments to variations cloud
microstructure.
Neither are bin-resolving microphysics models free of problems
associated with a finite-difference representation of the
vapor-deposition, collection, and breakup equations. As noted by
Tzivion et al,(1998), while Berry and Reinhardt's(1974) scheme does
well in representing Golovin's analytical solutions for the collection
equations for small time steps, it fails to conserve mass when
realistic kernels are used. Likewise Bleck's(1970) single-moment
solution to the collection equation artificially accelerates the
collection process. Tzivion et al`s(1987) exactly conserves total mass
independent of the number of bins, time step, initial conditions, or
kernel used. That is why we selected this scheme for incorporation
into the RAMS CRM and LES models. Nonetheless it is a rather expensive
procedure and moreover, it is rather cumbersome to alter the physics in
the model.
Finally, when bin-microphysics models are incorporated into
finite-difference CRM or LES models, it is not clear to what extent
the interactions of advection and turbulent diffusion with
bin-resolved vapor deposition and collection represent true physical solutions or perhaps induce some form of artificial broadening of droplet spectra. The
problem at that point is so nonlinear that there is no way to
systematically analyze the complicated numerical/physical system.
In spite of these difficulties, CRMs and LES models still provide the
greatest opportunity for simulating as realistically as possible the
complete precipitation lifecycle of a variety of cloud systems.
Single-column models with bin-microphysics do not provide realistic
updraft/downdraft environments to drive cloud supersaturations, and
precipitation sedimentation rates(Brown, 1997; Bott et al,1997;Stevens
et al, 1998). Likewise, mesoscale and general circulation models are
not capable of supplying microphysics models the appropriate
velocity scales. If used with proper caution, as one should with any
model, coupled CRM/BM models can teach us a great deal about the
interactions of microphysics and dynamics of a wide range of cloud types.
In the next section, we summarize what the CRM/LES versions of RAMS
coupled to the UTV bin-resolving microphysics model has taught us.
3.0 Simulations with coupled cloud-resolving models and bin-resolving
microphysics models
3.1 Marine Stratocumulus Clouds
Our greatest experience in simulating coupled bin-resolving
microphysics and CRMs or LES models has been with marine stratocumulus
clouds. Stevens(1996; Stevens et al,1997) examined the influence of
drizzle on the dynamics of marine stratocumulus. RAMS was run as both
a 2D CRM and 3D LES with about 25 m grid spacing in the vertical and
55m spacing in the horizontal. The UTV bin-resolving microphysics was
run with 25 bins spanning from 3 to 1000 micrometers. Consistent with
observational studies (Grost et al, 1982;Paluch and Lenschow,1991) and single-column modeling studies(Chen and Cotton, 1987; Nicholls, 1987; Wang and Wang,
1994) they found that a precipitating stratocumulus layer is
stabilized with respect to the cloud layer. That is precipitation
produces a moister a cooler sub-cloud layer and warmer and drier
cloud layer. The above studies have interpreted this to mean that a
precipitating marine stratocumulus layer becomes decoupled; that is
turbulent kinetic energy and moisture and heat fluxes go to zero
somewhere in the sub-cloud layer. Our results suggest that a precipitating cloudy boundary layer actually remains coupled in the sense that boundary layer fluxes are maintained or even increase, but the nature of the coupling changes.
Instead, turbulence becomes more intermittent and cumulus like, feeding the
overlying stratus layer with moisture.
We also found that a drizzling boundary layer does not always result
in lower cloud fractional coverage than a non-drizzling boundary layer
as suggested by Albrecht(1989). Simulations of the ASTEX Lagrangian 1
case with drizzle exhibited larger cloud fractional coverage than the
no-drizzle runs. We did find that with somewhat unrealistically large
precipitation efficiencies, drizzle resulted in warming and drying of
the cloud-layer, and as a consequence lower cloud coverage.
3.2 Warm-season Arctic Stratus
We have also applied the CRM/BM model to the simulation of
warm-season Arctic stratus(Olsson et al, 1997; Harrington, 1997). In
this case we used 60m horizontal grid spacing and vertical spacing stretched from 30 to 45m in the boundary layer. The major unique feature of summertime Arctic stratus compared to sub-tropical marine stratocumulus is the
persistent solar heating in the cloud layer. The other feature that
differs from marine stratocumulus is that the surface is not generally
a source for moisture and/or heat fluxes( the exception is over
leads). Instead the main supply of moisture is through horizontal
advection. As a consequence, lower CCN concentrations which support
drizzle formation, stabilizes the subcloud layer and produces weaker
cloud circulation. Moreover, owing to the presence of persistent
solar radiation, droplet distributions with broad tails absorb more
solar radiation than those with narrow distributions, which acts as a
stabilizing force on the cloud turbulence. The solar absorption
stabilizing affect is actually stronger than the stabilizing affect of drizzle.
3.3 Radiative influences on droplet vapor deposition growth in Arctic stratus
Harrington(1997) implemented the radiative contribution to the heat
budget of droplets growing by vapor deposition in the UTV BM model.
If the droplets are strongly radiating in the longwave, this contributes to enhanced vapor deposition of drops, particularly for larger droplets. On the other had, absorption by solar radiation warms the droplet in a relative
sense and thereby reduces the rate of vapor deposition of droplets.
Calculations with this term added to the BM model were made in two
ways. First, a standard CRM run was performed similar that described
above. Then, a number of Lagrangian parcels were released below cloud
base and allowed to advect vertically and horizontally by the
explicitly-resolved flow in the CRM. In each parcel droplet growth by
vapor deposition and collection was calculated. In addition, simulations were performed with the added radiation term in a fully coupled CRM/BM simulation. Parcels that spent at least 12 minutes near cloud top(where they radiate strongly to space) prod uce enhanced drop spectral broadening and reduce the time needed to initiate drizzle by as much 30 minutes. This effect is not as
pronounced in the fully coupled CRM/BM with the radiation term as
droplet broadening as a result of the spurious cloud top
supersaturation mentioned earlier and/or turbulence diffusion masked
its influence.
3.4 Transition season Arctic stratus
Harrington (1997) emulated transition-season Arctic stratus by cooling
the warm-season sounding first by 5C and then by 10C, while
maintaining the original relative humidity profile. In this case Reisin
et al's. (1996) mixed phase version of the UTV bin-resolving microphysics model was used.
The simulations with 5C cooling produced a mixed-phase cloud that
was dynamically similar to the warm-season case. When he doubled the
ice crystal concentration for this case, the optical depth of the
cloud diminished. Furthermore, the evaporation of precipitation that
settled out of the cloud layer moistened the layer below driving it to
saturation. The moistened lower layer was able to radiate to space
through the now thinned upper cloud deck and cool to water saturation,
thus creating a multi-layered cloud structure. This we believe is
another possible mechanism for forming the frequently observed
multi-layered structure of the Arctic stratus.
When the sounding was cooled by 10C, the resultant higher crystal concentrations and more vigorous vapor deposition rates depleted the liquid water in the cloud layer to such an extent that cloud top radiation cooling
reduced and the cloud collapsed as a dynamic entity. Only if higher
fall velocity ice crystal habits were assumed could the cloud maintain
itself.
4.0 Summary
We have shown that a coupled CRM/BM model is by no means a perfect
model and has a number of limitations associated with its grid spacing
and finite difference techniques. However, when used with caution, we
have shown that this approach can provide some new and unique insights
into the interactions between cloud microphysics and cloud dynamics of
a number of cloud systems.
