Workshop on Cirrus Clouds and their Supersaturated Environment - Abstracts

Talks:
======


Robert Herman: Measurements of humidity in
wave clouds from MidCiX

During the NASA Middle Latitude Cirrus Experiment (MidCiX)
flight of 5 May 2004, the high-altitude NASA WB-57 aircraft
flew multiple times through a wave cloud at -50 to -60 C
over the Rocky Mountains. Relative humidity measurements
were made both inside and outside the wave cloud.

The evolution of humidity and particles in the cloud will be
discussed and compared with model predictions.




Andrew Heymsfield: Low temperature
supersaturations in wave clouds: Observation and
Model Comparisons. Observations and modeling of high
supersaturations in wave clouds

During the NASA Midlatitude Cirrus Experiment (MidCiX)
experiment in April-May 2004, the instrumented NASA WB-57F
made in-situ measurements within and upstream of a cold
(-50 to -60C) wave cloud. Measurements of relative humidity
were made from two research-grade hygrometers. Additional
measurements were made with several instruments that measure
the condensed water content and particle size distributions.
The observations are compared with modeling calculations of
the evolution of the humidity field and particle populations
during transport of air parcels through the wave cloud to
assess whether our current understanding of the homogeneous
ice nucleation process and the conversion of water vapor to
condensed water is consistent with observations.




Y Jaya Rao: Lidar observations of Cirrus clouds Over
Tropical Indian Station: Possible influence on Stratosphere
Troposphere Interaction

Nd:YAG lidar system, located at Gadanki(13.5N,79.2E) a
tropical station india,is being used to monitor aerosols
and cirrus clouds. Lidar observations clearly show occurace
of cirrus clouds over this station with varying thickness.
Scattering and depolarization ratios derived from lidar
observations have been analysed to study the cirrus cloud
characteristics including variation of cirrus cloud over
different temporal scales and alsoin conjunction with
the variation of heigh of the tropocal tropopause. An
attempt has been made to study the influence of cirrus
cloud on stratosphere troposphere exchange by making use
of simultaneous observations of Vertical velocity using
co-located Mesosphere-Stratospher-Troposphere (MST) radar
clearly show enhanced vertical wind in the presence of
thick cirrus clouds. Results clealy show the significant
contribution of cirrus cloud to the Stratosphere Troposphere
exchange at this tropical station.




Cornelius Schiller: Cirrus clouds and RHi in the TTL above
the Indian Ocean

Measurements of temperature, water vapour, total water,
ozone, and cloud properties were made in the TTL and lower
stratosphere above the western equatorial Indian Ocean in
February and March 1999 as part of the APE-THESEO project.
Individual profiles show a great deal of variability in
the altitude of the cold point and hygropause. Cirrus
clouds were observed up to the altitude of the cold point.
Characteristics, i.e. extension, number density, relative
humidity over ice of the cirrus clouds could be categorised
into two classes which allows us to infer their different
origin, but also their different impact on the H2O budget
of the TTL. Geometrically thicker clouds at relatively low
potential temperatures with only small supersaturations and
are embedded in dry air (i.e., around 30% relative humidity
over ice). Geometrically thin clouds at higher potential
temperatures were found to show large supersaturations and
are embedded in much more humid air. The thicker clouds are
formed by injection of moist air from convective outflow
into dry upper tropospheric air and thus moistening the
TTL, whilst the thinner clouds are formed in situ and have
thus a high potential for dehydration. The high relative
humidity over ice of the thin cirrus suggest an early stage
of its life cycle when the water vapour is not yet in
equilibrium with the ice phase. Vice versa, the low relative
humidity over ice of the thick clouds point to older clouds
equilibrated with water vapour or even in the dissipation
stage.




Peter Spichtinger: Formation and evolution of
ice-supersaturated regions

Case studies of the formation and evolution of
ice-supersaturated regions (ISSRs) are presented. These
ISSRs were detected by the Lindenberg radiosonde. Using
ECMWF analyses and Lagrangian trajectory calculations we can
study when the air parcels represented by the trajectories
got supersaturated and when they eventually got subsaturated
again. Additionally, METEOSAT infrared images are used
to distinguish between measurements in cloudy and cloud
free air. Finally, we use the measured relative humidity
to correct the negative bias of the ECMWF humidity and to
construct two-dimensional maps of ice supersaturation over
Europe during the considered periods. These maps led us
to a new perception of ISSRs as large dynamic regions of
supersaturated air where cirrus clouds can be imbedded at
some locations while there is clear air at others.




Claudia Stubenrauch: Persistent contrails and their
environment from TOVS Path-B

The TOVS Path-B dataset provides atmospheric temperature and
water vapor profiles as well as cloud and surface properties
over the globe, at a spatial resolution of 1° latitude x 1°
longitude. Their relatively high spectral resolution yields
reliable cirrus properties, day and night. Contrails form
when the hot and humid exhaust gases from the combustion of
fuels by an aircraft mix with the ambient, cold atmosphere.
Their persistence depends on upper tropospheric humidity
and temperature. We present a study on the evolution of
persistent contrails using TOVS Path-B upper tropospheric
humidity and effective high cloud amount.




Johannes K. Nielsen: Can humidity fluctuation inside cirrus
clouds be understood in terms of present condensation
theory?

Airborne humidity measurements carried out during the
CRYSTAL FACE campaign evidently shows that well inside
tropical cirrus clouds, the saturation ratio of water
vapor with respect to ice is not stabilized close to 1 as
expected, but rather fluctuating with a standard deviation
around 10 %, sometimes even more. In this contribution we
pursue the the following question: Can the fluctuations
in saturation ratio be due to fluctuations in wind,
temperature, particle density and humidity at lengthscales
below the instrumental resolution of around 100 m? 2-d
simulations with the MPC model, mimicking the statistical
properties of relevant parameters at the time of observation
suggest that this explanation is quite unlikely, since
small-scale variations in humidity is smeared out fast
compared to cloud age. We shall seek to answer the following
question: How much do the accommodation coefficient need to
be manipulated in order to make theory meet observations
through these simulations.




Markus Quante: Observations of Turbulence in Cirrus Clouds

The talk will review and discuss mainly the results of
airborne measurements of turbulence in cirrus gathered
since the late eighties over Europe and the United States
(mid-latitude). Turbulence is influencing the internal
structure, maintenance and dissipation of the clouds by
mixing and entrainment. The interaction between cloud scale
turbulence and waves and turbulence of the background flow
field will be emphasized. Typical values for length scales,
velocity variance, dissipation rates will be given along
with the presentation of spectral characteristics and
intermittency and isotropy measures. In addition, Doppler
radar measurements of cloud particle velocities made by the
GKSS cloud radar MIRACLE will be used to illustrate the
processes and their role in the overall cloud development.




Andrew Gettelman: Impacts of Supersaturation in the UT/LS
based on global models

The impact of supersaturation in the UT/LS is investigated
with the NCAR global modeling system, a state of the art
chemistry-climate model. The bulk formulation of ice
condensation is modified to allow supersaturation at
various levels, and the impacts of the changes on the UT/LS
region and the radiative balance are assessed. The model is
compared to new detailed satellite observations of humidity
in the UT/LS region, up to 150 hPa. The addition of a more
realistic assumption regarding the relative humidity for
the formation of clouds impacts the climate of the UT/LS.
Water vapor and radiation are affected significantly. These
changes indicate the important role of ice microphysics in
cirrus clouds in the radiation and chemical balance of the
UT/LS region.




Bernd Kärcher: Results from near-global, process-oriented
studies of relative humidity and cirrus and implications for
parameterization schemes

By using results from Lagrangian cirrus studies, the
talk addresses the following issues: generation of
supersaturation, distributions of relative humidity inside
and outside of cirrus, relation between probability to find
supersaturated air and cirrus coverage, fundamental needs
for improvements to make headway in parameterizing cirrus in
global models.




William Lahoz: Assimilation of Water Vapour in the UTLS
region

Water vapour is a key element in the composition and the
formation of clouds. Assimilation of water vapour in the
UTLS region is difficult due to the large range of values
between the troposphere and stratosphere and the sharp
gradients in the UTLS region. A number of operational
centres and research groups are developing techniques to
perform this assimilation. This talk will describe current
efforts at the DARC/MetOffice and will indicate possible
ways forward.




Adrian Tompkins: progress in the representation of ice
physics in the IFS

The ice microphysics of the ECMWF model is briefly
described, along with some of the current weaknesses. The
results from a simple parametrization for homogeneous ice
nucleation are then demonstrated, along with the impact
on the probability density function of upper tropical
troposphere relative humidities. Discussion will then focus
on future plans for the IFS cloud scheme.




Chuansen Ren: The parameterisation of cirrus clouds and the
role of ice nuclei

A parameterisation of cirrus clouds formed by homogeneous
nucleation is improved so that it can be easily used in
GCMs and climate models. The improved parameterisation
compares well with full microphysical model results
even when monodisperse aerosol particles are used in
the parameterisation to determine cirrus ice-crystal
number densities. However, the presence of ice nuclei
in the atmosphere can modify the formation of cirrus
clouds. If sufficient ice particles have been generated
by heterogeneous nucleation, the saturation ratio of the
air parcel will never reach that required for homogeneous
nucleation. Homogeneous nucleation will be suppressed only
when the number density of ice nuclei is above a critical
value, which depends on the temperature, the updraft
velocity, and the supersaturation at which ice nuclei
activate. We provide two new expressions to calculate the
critical number of ice nuclei and show which of these
expressions defines the critical number for the conditions
of the upper troposphere.




Johannes Quaas: Evaluation of cirrus microphysics in a GCM
using TOVS satellite data

A new microphysical scheme is developed for the use in the
Laboratoire de Météorologie Dynamique general circulation
model. It includes a detailed bulk description of the
processes in liquid and ice clouds, treating droplet and
ice crystal number concentrations and cloud liquid and ice
water prognostically and using rain, snow, and graupel as
precipitation species. Homogeneous crystal nucleation is
accounted for. In the study presented, the microphysical
dataset derived from TOVS satellite measurements by the ARA
group at the Laboratoire de Météorologie Dynamique is used
to evaluate different microphysical parameters of cirrus
clouds such as particle size and shape, ice water content
and emissivity.




Posters:
========


Bertrand Cadet: OPAR lidar sub-tropical cirrus cloud climatology compared to
others

Cirrus clouds have been identified as one important
regulator of the radiance balance of the earth-atmosphere
system. Since the last decade, their radiative, dynamical
or chemical impact in the global climate change is
still a major incertitude. Then, relationships between
microphysical, macrophysical and radiative properties,
deduced from cirrus clouds climatology, have to be
established, to better understand their role. The
Observatoire de Physique de l'Atmosphere de la Reunion
(OPAR) is located in the Indian Ocean just above the tropic
of Capricorn; it makes quasi-continuous lidar measurements
in the southern hemisphere where there is a dearth of
cirrus measurements. The lidar data set, between 1996 and
2001, have been processed to access to a sub-tropical
cirrus cloud climatology. Results show a large amount of
sub-visible cirrus clouds during austral summer and a good
correlation with the annual water vapour variation in the
atmosphere. Other, sub-tropical, tropical and mid-latitude,
climatologies have also been done, established on either
local or global measurements. We will present an overview
of the different climatologies compared to the OPAR
sub-tropical one.




Kostas Eleftheratos: Trends in Upper-Tropospheric Humidity

This study examines trends in Upper-Tropospheric Humidity
(UTH) based on the TOVS/HIRS global satellite observations
for the period 1979-1998. UTH data refer to a layer-averaged
relative humidity from approximately 200 to 500 hPa,
computed from satellite radiance observations at water
vapor channels near 6.7 μm (HIRS channel 12).
State-of-the-art analysis of global UTH data is performed
in order to examine the natural variability in UTH on time
scales from months to years. The effect of known atmospheric
fluctuations such as ENSO, QBO and NAO has been examined
separately. The effect of the fluctuations has been removed
from the data set based on simple regression statistical
modelling as described in Zerefos et al (2003). The new data
set, freed from all natural fluctuations and seasonality,
is used to calculate long-term changes of observed UTH in
space and time. Finally, our results are compared with
earlier studies examining long-term changes in UTH (Bates
and Jackson, 2001), in order to validate the accuracy
of our computed trend values. In our approach, we have
retained values of UTH in excess of ice saturation. This
allows looking for trends in the occurrence frequency of
ice-supersaturation as well.




Jean - Francois Gayet: Inter-hemispheric differences in
cirrus microphysical properties as a function of relative
humidity

Microphysical and optical measurements were performed in
mid-latitude cirrus clouds at temperatures between -33°C and
-60°C during southern and northern INCA field experiments
carried out at equivalent latitudes (53°S and 55°N) from
Punta Arenas in the Southern Hemisphere (Chile, SH) and
Prestwick in the Northern Hemisphere (Scotland, NH). For
similar environmental conditions (i.e. over a similar
range of temperature and vertical velocity) and for given
values of the relative humidity, the comparisons clearly
show distinct differences between the microphysical and
optical properties of cirrus clouds sampled in the SH and
NH field experiments. These differences may be related to
the contrasts in cirrus freezing thresholds in terms of
relative humidity over ice for onset of clouds which is
about 20% lower in NH than in SH, as reported in previous
INCA data analyses. Nevertheless, definite conclusions are
hampered by the large natural variability of cirrus cloud
properties and the limited number of flights performed, not
forgetting both the actual freezing mechanisms and potential
differences in small-scale dynamical variability which are
still insufficiently understood.




Michael Höpfner: Water vapour and clouds from MIPAS on
Envisat

MIPAS (Michelson Interferomter for Passive Atmospheric
Sounding) on Envisat is a polar orbiting limb sounding
instrument measuring the emitted (gases and clouds) and
scattered (clouds) mid-IR radiation with a very high
spectral resolution. Radiative transfer modelling in the
presence of clouds will be discussed by comparison of the
single scattering model KOPRA with the multiple scattering
code ARTS. Further, we present the capabilities of MIPAS
for deriving water vapour and cloud information on a global
scale in the UT/LS. Results of cloud occurrence and water
vapour distribution for the year 2003 will be shown.




Martina Krämer et al.: Quasi stationary relative humidities
and respective relaxation times in ice clouds

The quasi stationary relative humidities and respective
relaxation times in ice clouds are calculated for natural
cirrus (synoptically induced, gravity wave induced and
convective cirrus) and ice clouds nucleated at the AIDA
aerosol chamber for the temperature range 180-230K by using
simple equations provided recently by Korolev and Mazin (JAS
60, 2957-2974, 2003).
The quasi stationary relative humidities with respect to
ice (RH_ice_qs) increases with decreasing temperature to
values clearly above saturation for both natural cirrus
and AIDA ice clouds. The supersaturation raises with
increasing updraft velocity, increasing ice particle size
and decreasing ice particle number.
The relaxation times (Tau) are very fast (in the range of
seconds) for higher temperature, updraft velocity and for a
large number of small particles but increases to the range
of hours with decreasing temperature.
Thus, in ice clouds at temperatures below 200K, RH_ice
clearly exceeds saturation at ice formation and will reach
the -still supersaturated- quasi stationary state not until
some hours.




Martina Krämer et al: Nitric acid partitioning in cirrus
clouds: the role of aerosol particles and relative humidity

Owing to the potential of cirrus clouds to vertically
redistribute HNO3 in the upper troposphere, the uptake of
HNO3 on ice particles is in the focus of recent research.
Here, we investigate if HNO3 residing in freezing aerosol
particles could be a relevant source for HNO3 in ice clouds.
For this purpose, model studies on the sensitivity of the
uptake of HNO3 in aerosol particles on temperature, relative
humidity with respct to ice (RHice), particle chemistry and
amount of available HNO3 are performed. Combination of the
model results with the history of RHice in cirrus clouds
and comparison with field measurements indicates, that at
temperatures <200K a considerable part of the HNO3 in ice
originates from freezing particles containing HNO3. With
increasing temperature, adsorption of HNO3 molecules on the
ice surfaces becomes the main source of HNO3 in ice.




Alexander Mangold: Cirrus cloud microphysics and its
dependency on aerosol type, temperature and cooling rate

The microphysical properties of cirrus clouds like ice water
content, ice crystal number density, and size distribution
do not only affect the radiative impact of these clouds,
but also cloud lifetime, possible heterogeneous reactions
on the ice surface, or potential removal of substances by
sedimentation of bigger ice crystals. Experimental studies
which examine the conditions of homogeneous or heterogeneous
ice cloud formation and the resulting microphysical
properties are of major importance for the development and
validation of respective parameterisations and process
models.
We conducted laboratory measurements on the homogeneous and
heterogeneous ice nucleation and the water partitioning in
the temperature range between 238 and 185 K and at cooling
rates between ¿ 0.3 and ¿3.0 K min-1 in the large aerosol
chamber AIDA. A Lyman-alpha-fluorescence hygrometer was used
to measure the total water. Simultaneously the gas phase
water concentration was directly measured in situ by tunable
diode laser absorption spectroscopy (TDLAS) at 1370 nm.
The number concentration and size of the ice crystals was
obtained both from in situ FTIR extinction spectra and with
optical particle counters.
Pure sulphuric acid and ammonium sulphate solution droplets
were used to investigate the homogeneous freezing process.
Pure soot, soot coated with sulphuric acid or ammonium
sulphate, and pure mineral dust particles served as ice
nuclei for the heterogeneous ice cloud formation. The focus
of the presented results will be on the ice crystal number
concentration and size and the ice nucleation efficiency of
the different aerosol types for the various conditions.




Jack McConnell et al.: Program for study of the potential
impact of aviation emissions including aerosols on the upper
troposphere lower stratosphere region using GEM-AQ

Since the early 1970s there has been an ongoing concern
about the effects of emissions from large aircraft fleets
on the stratosphere. The 1999 IPPC report, Aviation and the
Global Atmosphere addressed mainly the climatic impacts of
subsonic aircraft on the atmosphere. It examined current
effects and investigated various scenarios of aircraft
fleets to the year 2050. The main emissions from aircraft
CO2 and water appeared to be of secondary importance, while
¿minor¿ emissions such as NOx and aerosols (including soot)
appeared to give rise to much more important perturbations
to the atmospheric climate. The main impacts were related
to the generation of upper tropospheric (UT) ozone: it was
estimated that the contemporary fleet contributed about 6%
to UT ozone and that this could double by 2050. Another
important climate impact was identified in the IPCC report
which was possibly much more serious and this was the
effects of aircraft on UT cirrus cloud. One aspect may be
described as the ¿direct¿ effect due to the formation of
condensation (con) trails as a result of supersaturated
air from the aircraft. The ¿indirect¿ effect is due to
the impact of an increase in FN (freezing nuclei) in the
UT regions due to aerosols/particulates from aircraft
emissions. These FN act as nuclei for ice crystals which
form cirrus clouds. Cirrus clouds act as warming agents for
the surface and lower atmosphere since they are relatively
transparent to solar radiation but absorb up-welling
terrestrial radiation from the lower atmosphere and re-emit
it at a much colder temperature which results in a warming
effect. However, this effect is much more difficult to
quantify. The primary objective of our work is to study
of the potential impact of aviation emissions (NOx, H2O,
SO4 and black carbon) on the upper troposphere/lower
stratosphere (UT/LS) region including the effects of cirrus
clouds on radiation and heterogeneous chemistry. GEM-AQ
is based on the Canadian Global Environmental Multiscale
weather forecast model and has air quality modules added,
which include both gas phase and aerosol chemistry with
anthropogenic and natural emissions, including estimates of
lightning emissions. The horizontal domain of this model
is global and extends vertically from the surface to about
30 km: however it can be extended vertically if necessary.
This model will be used to investigate chemical effects
of aircraft and can be modified to include up-to-date
parameterizations of cirrus formation.




Sophie Prud'homme: Mid-latitude cirrus cloud simulation and
comparison between observed and calculated lidar and radar
measurements

The aim of our work is to better understand the life cycle
of cirrus clouds and the interactions between microphysics,
radiative and dynamic processes by generating a field
of cirrus clouds and simulating ground-based lidar and
radar observations. The model used derives from the French
Meso-NH Atmospheric Simulation System and is updated for
cirrus clouds. A comparison between observed and simulated
lidar backscattering, radar reflectivity and radar Doppler
velocity measurements for a mid-latitude cirrus case will be
presented on this poster.




H.G.J. Smit, D. Kley, and M. Helten: Climatology of upper
tropospheric humidity over the north Atlantic: Seasonal and
inter annual variations

While it is clear that evaporation from the surface is
the ultimate source of water vapor in the troposphere and
precipitation is the sink, it is less clear what processes
control the upper tropospheric humidity (UTH). Whether UTH
is under thermodynamic or dynamic control is still one
of the unresolved questions regarding climate feedback,
important for a better understanding and prediction of the
global temperature changes. A large deficiency is that very
little is known about the spatial and temporal distribution
of UTH content.
Therefore, the poster presents a comprehensive climatology
of the UTH distribution over the north Atlantic obtained
from MOZAIC (Measurement of Ozone and Water Vapor by Airbus
In Service Aircraft) measurements made at 9 to 12 km cruise
altitude aboard five A340 aircraft during normal in-service
flights . The UTH measurements reveal a strong spatial and
temporal variability on different scales. Very pronounced
are the seasonal cycle and inter-annual variability of
specific humidity which are often positively correlated with
temperature. However, in summer a negative correlation of
relative humidity and temperature is observed.
The observed seasonal and inter-annual variations of the
MOZAIC-UTH climatology have been compared with results
obtained from ECMWF-analysis. Thereby, special attention has
been paid to the fact that a substantial fraction of the
MOZAIC UTH-measurements show supersaturation with respect to
ice which is not represented in the ECMWF-analysis.
The relative humidity frequency distribution in the tropics
and sub-tropics show to be clearly bi-modal with a dry
and a wet branch. Striking features in the tropics in the
outflow of cumulonimbus convection are: near cloud edge (t
= 0 h) uni-modal PDF (probability distribution) of RHI with
more than 60 percent ice supersaturation (ISS) downwards
the outflow (t > 0 h) the development from uni-modal into
bi-modal PDF of RHI (t > 0 h) with one dry-branch and one
wet branch:
 Dry branch: Fast drying within one day from 70-90 % RHI
             down to 40-50% RHI
             Clear sky subsidence
 Wet branch: ISS (100-150% RHI ) remains, even after t=24
             Dynamical lifting or radiative interaction of
             sub-visible clouds




Reinhold Spang: High-resolution limb-observations of trace
constituents and clouds in the UT/LS region

Despite the great importance of several trace constituents
(especially water vapour and clouds) in the UT/LS region for
the climate system, the distribution of these quantities
is far from being well understood. This is a result of the
existing lack of global observations with sufficiently high
vertical and horizontal resolution. The Cryogenic Infrared
Spectrometers and Telescopes for the Atmosphere (CRISTA)
instrument made a number of snapshots of the UT/LS during
its two Space Shuttle missions (STS 66 and STS 85) and
demonstrated the potential of the IR limb viewing technique
to provide information on several trace constituents
and subvisible cirrus clouds (SVC) with comparably high
spatial resolution. The presentation gives an overview of
CRISTA water vapor and cirrus observations in the UT/LS in
comparison with corresponding ECMWF data and modelled ice
water content by the CLaMS model (Chemical Lagrangian Model
of the Stratosphere). In addition, two follow-on instruments
with enhanced UT/LS measurement capabilities are introduced:
(1) CRISTA-NF, which is currently integrated for operation
on the Russian high-flying M55-Geophysica research aircraft
during the European SCOUT-tropics field campaign. (2) The
GLObal limb Radiance Imager for the Atmosphere (GLORIA)
instrument, proposed by FZ Juelich and FZ Karlsruhe in
response to DLRs call for a new German Earth observation
mission.




Florian Weidle and Heini Wernli: Analysis and validation of
3-dimensional ERA40 cloud ice field

The recently compiled reanalysis data set from the ECMWF,
ERA40, contains as a new product 3-dimensional fields of
cloud ice content and cloud liquid content. In this study,
a 1-year period is chosen, and the geographical, vertical
and seasonal distribution of these ERA40 variables is
analysed. Data from the POLDER satellite are then used for
a comparison and validation of the ERA40 cloud fields. This
comparison is made for specific time instances and should
provide valuable insight into the qualities and shortcomings
of the ERA40 cloud ice field.