The effects of wind-shear on cirrus: a large eddy model (LEM) and radar case study

Gourihar Kulkarni* and Steven DobbieInstitute for Climate and Atmospheric Science, School of Earth and Environment,

University of Leeds, Leeds, UK*Now at Atmospheric Science & Global Change Division, Pacific Northwest

National Laboratory (PNNL), Richland, WA 99352, USA

For more information about this poster please contact Dr Steven Dobbie, Environment, School of Earth and Environment, The University of Leeds, Leeds, LS2 9JT Email: dobbie@env.leeds.ac.uk Tel:+44 (0) 113 343 6725

SummaryA new Thermal Gradient ice nucleation Diffusion Chamber (TGDC) has been designed and constructed to investigate ice nucleation characteristics of atmospherically important Ice Nuclei (IN) aerosols. This chamber has been specifically designed so that ice nucleation events can be observed optically whilst the aerosol sample is progressed through various conditions within the static diffusion chamber. The TGDC can establish a range of super-saturations with respect to ice (SSi) over the temperature range of -10 to -34°C and for sufficient time to allow for ice nucleation events of the aerosol particles. The ice nucleation event is determined through observation when an ice embryo forms on an aerosol, which is supported on the Teflon substrate; These events were observed and recorded using digital photography. The design of the TGDC allows for understanding time variations in ice nucleation events for a wide range of SSi conditions and different IN without the need to alter the TGDC conditions makes the TGDC a unique nucleation chamber. Calibration of the system is performed by observing (NH4)2SO4 deliquescence and the results are in good agreement with the literature data. Results are presented for mineral dust IN, taken from the AMMA project, properties such as critical SSi, onset dependence, and active fraction.

Acknowledgments: The authors would like to thank Dr Jim McQuaid, Prof Mike Smith, Dr Justin Lingard, Dr Sarah Walker and Dr Barbara Brooks for support during experiments in the aerosol laboratory at Leeds.

Motivation:

-Very limited data exists for IN nucleation and the nucleation process is poorly understood

-Cloud modelling studies need accurate information about the characteristics of IN, nucleation properties, variations with source regions, transport, and aging

-Development of a process chamber that can be adapted not only to determine nucleation critical super-saturations/rates and active fractions but also isolate the effects of specific processes, such as the hysteresis effects

ICASInsitute for Climate and Atmospheric Science

Figure 1: Shows the diagram of saturation vapour pressure (e) against the temperature (T in °C). Tp and Bp represent the conditions at the top and bottom plates, respectively. Ice supersaturation is the region enclosed between the straight line TpBp and ice saturation. (Not to scale).

Ice Supersaturation

Water Saturation

Tp

Bp

A

e

T

Ice Saturation

0o C Tb Tt

Basic Theory:

The basic design of a TGDC consists of two parallel plates with the inner walls coated with ice. With both plates at sub-zero conditions and one plate at a colder temperature, the result is that linear profiles establish in water vapour and temperature between the plates. Ice super-saturation profiles establish between the plates, as indicated in Figure 1.

New TGDC design: This TGDC is designed to focus on the nucleation processes of individual aerosol particles. Consequently, we place aerosol particles on a sample holder and insert the holder into the parallel plate system. The nucleation process is observed with an optical system observing through a port in the upper plate. The design is illustrated in Figure 2.

Figure 2. The new TGDC design

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-32 -28 -24 -20 -16 -12

Temperature (deg C)

Sat

urat

ion

vapo

ur p

ress

ure

w.r

.t. ic

e (P

a)

1

2

3

Figure 4. Chamber plates with the similar temperature values are warmed up at a constant rate from point 1 to the point 3. In the path from point 1 to point 3, the plates temperature crosses the point 2 (-19.35 ˚C) where the deliquescence of the AS is observed, validating the temperature.

Figure 5. Deliquescence of AS is used to calibrate the SSi profile compared to theory. We use the TGDC range where both lines overlap. These results were also confirmed with Fluent.

The TGDC is housed in the Environmental Cold room at ICAS, Aerosol Facility, University of Leeds.

Figure 3. TGDC in the Cold Room

Calibration

Results: Mineral dustESEM/EDX

Source Regions

Atomic Percent

% Si % Al % Mg % Ca % Na % Fe

Dakar Airport

47 13 3 14 5 12

Dakar 51 13 3 8 5 11

Nigeria 65 15 1 3 3 8

Spain 22 7 2 60 3 4

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90

-38 -34 -30 -26 -22 -18 -14 -10 -6 -2

Temperature (deg C)

Rel

ativ

e H

umid

ity w

ater

(%

)

Present work

Kanji and Abbatt, 2006

Braban et al., 2001

Onasch et al., 1999

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112

-35 -30 -25 -20 -15 -10

Temperature (deg C)

RH

i (%

)

Figure 7: Plot of onset RHi as a function of temperature for dust samples collected from Dakar

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-40 -35 -30 -25 -20 -15 -10

Temperature (deg C)

RH

i (%

)

Spanish dustAirportNigeriaDakar

Figure 8: Relationship of onset RH with temperature. The different dust samples from four locations are examined in the onset ice nucleation experiments

Figure 6

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109 110 111 112 113 114 115 116 117RHi (%)

Ac

tiv

e F

rac

tio

n (

f)

Dakar

Nigeria

Figure 7.7: The active IN fraction of dust particles from two different locations, Dakar and Nigeria, as function of the RHi. The active fraction error is each point is ±0.05.

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109 110 111 112 113 114 115 116 117

RHi (%)

Tim

e La

g (s

ec)

Temp = -30 deg C

Temp = -20 deg C

Figure 7.8: Showing the relationship between the time-lag and RHi for two different temperatures, -30 and -20 ˚C for Dakar location.

Table 7.4: Comparison of number of ice particles produced by two expressions, Pruppacher and Klett (1997) and Meyers et al., (1992) at different RHi values.

Location RHi(%)

nice (particles L-1), Meyers (1992) scheme, nice

(particles L-1),-20 ˚C -30 ˚C

Dakar 110 10 11 2

116 13 15 5

Nigeria 110 21 22 2

116 24 24 5

AverageJhet

Dust Location

Temperature (˚C)

RHi ( % )

(events cm-2 sec-1)

Dakar -20 110 121

-30 110 134

-20 116 153

-30 116 182

Nigeria -20 110 258

-30 110 273

-20 116 291

-30 116 298

Table 7.3: Comparison of average heterogeneous rates at different temperatures and RHi from Dakar and Nigeria locations. Uncertainties in the average Jhet are less than 10 % of the Jhet value.

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Temperature (deg C)

RH

i (%

)

Figure 7.1: Plot of onset RHi as a function of temperature for dust samples collected from Nigeria

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Temperature (deg C)

RH

i (%

)

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i (%

)

Figure 7.3: Plot of onset RHi as a function of temperature for dust samples collected from Dakar airport.

Figure 7.4: Plot of onset RHi as a function of temperature for dust samples collected from Spain

8.2 Summary of resultsThe following is a summary of the major findings of this thesis and refers back to the thesis aims outlined in Chapter 1:

1) In this thesis a new experimental system was developed to investigate the ice nucleation properties of mineral dust particles. The set-up includes a newly designed ice nucleation thermal gradient diffusion chamber (TGDC) which allows experimental determination of the time evolution of nucleation events, and a heating and optical system. The temperature and ice supersaturation thermodynamic conditions developed inside the TGDC are calibrated and validated by observing ammonium sulfate deliquescence

within RHi of ±1.5% accuracy. 2) The morphology and elemental composition of individual and bulk particles is determined using a scanning electron microscope equipped with an energy dispersive (SEM-

EDX) system. It is observed that the surface of dust particles consist of an irregular surface with many cracks and steps. The elemental composition from each dust sample collected from four different locations is similar, but the relative percentage of each element is observed to be different. It is also observed that dust particles do not contain any soluble compounds and since current ice nucleation experiments are performed below water saturation the ice nucleation experiments are therefore carried out in the

deposition ice nucleation mode. An interesting finding using the SEM-EDX system is that the elemental composition across individual particles is homogeneous. Since we notice the nucleation beginning at locations such as cracks and steps to initiate the nucleation on the dust particle, we surmise that these specific surface characteristics are likely to favour ice nucleation as

opposed to spatial variations in composition. 3) Experiments were performed to investigate the heterogeneous ice nucleation of dust particles for temperatures between -10 ˚C and -34 ˚C and RHi which is ranged

between ice and water saturation. In these experiments, the formation of ice on genuine dust particles, which were collected at the ground from the Saharan desert (during AMMA) and along the South East coast of Spain, is observed by optical microscopy. The following are the major findings related to the ice nucleation properties:

3i) The onset nucleation results show that Saharan mineral dust particles nucleate at low RHi values of 104%. The spread in the onset RHi distribution for Saharan dust (104 to 110%) is found to be wider than the Spanish dust (106 to 110%). From the elemental analysis it is most likely that the Calcium in Spanish dust is causing the slightly raised

onset RHi and altering the surface properties. 3ii) The active IN fraction of dust particles from two different source regions is determined. It is observed that the fraction is quite similar for Saharan and Spanish regions

and is greater at higher RHi (116%) compared to lower RHi (110%). The maximum fraction found at these RHi is approximately 0.7 and 0.48 respectively. 3iii) The determination of the nucleation time-lag shows that the time-lag is lower at warmer temperatures by approximately 30 seconds. The nucleation time-lag is observed

to vary between 70 to 170 seconds. The calculations also show that the time-lag span is larger at lower RHi compared to higher RHi by 50 seconds. 4) Experiments were carried out to investigate the ice nucleation properties of genuine mineral dust particles from different source regions. It is observed that the spatial

variation of dust source regions does not appear to have significant impact on the ice nucleation properties. 5) The ice nucleation rates are calculated for various temperatures and RHi. The rates are calculated at time = t1 where the onset nucleation is observed. It is observed that

nucleation rates are sensitive to the source region of the mineral dust particles. Further these rates were then used to estimate the number density of ice particles (nice) using classical nucleation theory. The result show agreement with other previous studies but significant differences between these calculated nice and predicted by Meyers

(1992) scheme. The nice discrepancy between these parameterizations highlights the need for improved parameterization schemes and more work on the theoretical foundations in conjunction with experiments.

In conclusion the real strength of the system is the ability to observe individual nucleation events over time. The new TGDC has been shown to be useful for studying ice nucleation properties of aerosols such as onset RHi, active fraction and nucleation time-lag as well as other properties such as deliquescence. These are useful quantities for

aerosol transport models and atmospheric model simulations of ice and mixed phase cloud processes.