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45 O 04 In-cloud measurements of effective droplet radius
J. Aerosol Sci., Vol. 24. Suppl. I, pp. $583-$584, 1993 Printed in Great Britain.
0021-8502/93 $6.00 + 0.00 Pergamon Press Lid
45 0 04 IN-CLOUD
MEASUREMENTS
OF
EFFECTIVE
DROPLET
RADIUS
H. Gerber Gerber Scientific
Inc.,
1643 Bentana Way,
Reston,
VA 22090,
U.S.A
KEYWORDS
effective d r o p l e t radius,
climate change,
aircraft m e a s u r e m e n t s
METHODS
A probe for m e a s u r e m e n t s from an aircraft of the cloud p r o p e r t i e s of LWC (liquid water content) and of PSA (total particle surface area) was developed, and was used during last summer's 4-week investigation of Atlantic stratocumuli (ASTEX). The new probe (PVM-100A, Gerber et al. 1993), see Fig. i, is an a d a p t a t i o n of a g r o u n d - b a s e d instrument (PVM-100) for m e a s u r i n g LWC (Gerber, 1991; Arends, et al., 1992.) The operating principle of the aircraft probe is illustrated in Fig. 2. Droplets f o r w a r d - s c a t t e r light out of a laser beam into a lens that focuses the light onto two detectors. Filters, with c o n c e n t r i c areas of variable transmission, are placed in front of the sensors, and weight the scattered light of p o l y d i s p e r s e droplets with radii r to produce Fig. 1 - A i r c r a f t probe for m e a s u r e m e n t outputs proportional to ~ r 3 (LWC) and ~ r 2 of LWC and re: Dimensions in inches, welght ~4 lb. (PSA). The ratio LWC/PSA is proportional to the effective droplet radius, re, which is SPI'ITTER used in the parameteri-Jl i zation of the radiative . ~ . - . . . . . . -. ,~_ - ~ - - . . . . . . . . . . ~'c effect of clouds, and is __ an indicator of anthropogenic climate in,~.s i I -~tr~, fluences (Slingo, 1990). The probe was caliSE,SO, brated in the fog/wind tunnel facility at ECN Petten, where it was Fig. 2 - Principal optical components compared to the filter of the aircraft probe.
~
$583
$584
H (;~:RB~ ~
method for LWC, (Gerber et al.,
and to corrected 1993).
FSSP-I00
measurements
for PSA
RESULTS The probe generated a continuous data set of LWC and r during the ~i00 hrs. of in-cloud measurements from the U. o~ Washington's C-131A aircraft during ASTEX, with negligible drift in probe optics and electronics. Interpretation of this data set resulted in findings, some of which follow: i) Large reductions in r e were noted near stratocu cloud top when air trajectories brought a European air mass into the vicinity of the Azores. 2) In most clouds values of r e were precisely invariant in entrainment features near cloud top, indicating the presence of inhomogeneous mixing, and confirming the results of Nicholls (1989). Infrequently, enhanced values of r e suggested the presence of entity mixing described by Telford and Wagner (1981). 3) Comparison of r~ measured 2O with an FSSP-100 and t~e PVM-100A is shown by the open circles in 18 Fig. 3; horizontal bars are the 16 predicted behavior of FSSP-IO0, Baumgardner and Spowart (1990). 14 Fig. 3 represents 500 Km of incloud data with a wide range in LWC and droplet concentration. The approximately constant factor difference between r e measurements suggests the following procedure for retrieving accurate FSSP-100 droplet spectra when both instruments are co-located, and '2 ~ PVM = 1.5101,ESSP - 0.,5114 J given that the PVM-100 produces accurate results. The factor 2 4 6 8 10 12 14 i6 18 20 difference in r_ corrects the FSSP, R Err. ( u m ) relative FSSP-I~0 size spectrum, and the adjustment of FSSP-100 Fig. 3 - Comparison of r e LWC to PVM-100A LWC corrects measured with FSSP-100 the FSSP-100 concentration and PVM-100A in marine tion e r r o r s . stratocumuli (circles).
o//j! j/J
":/y
?
REFERENCES
Arends, B.B., G.P.A. Kos, W. Wobrock, D. Schell, K.J. Noone, S. Fuzzi, S. Pahl (1992), Tellus, Vol. 44B, 604-611. Baumgardner, D., M. Spowart (1990), J. Atmos. Oceanic Technol., Vol. 7, 666-672. Gerber, H. (1991), AD_pl. Q ~ . , Vol. 30, 4824-4831. Gerber, H., B.G. Arends, A. Ackerman (1993), Atm. R ~ . , submitted. Nicholls, S. (1989), O. J. R. Meteoro. SOU., Vol. 115, 487-511. Slingo, A. (1990), Natu;~, Vol 343, 49-51. Telford, J.W., P.B. Wagner (1981), Pure and ADpl. GeODhVS., Vol. 119, 934-965.
0021-8502/93 $6.00 + 0.00 Pergamon Press Lid
45 0 04 IN-CLOUD
MEASUREMENTS
OF
EFFECTIVE
DROPLET
RADIUS
H. Gerber Gerber Scientific
Inc.,
1643 Bentana Way,
Reston,
VA 22090,
U.S.A
KEYWORDS
effective d r o p l e t radius,
climate change,
aircraft m e a s u r e m e n t s
METHODS
A probe for m e a s u r e m e n t s from an aircraft of the cloud p r o p e r t i e s of LWC (liquid water content) and of PSA (total particle surface area) was developed, and was used during last summer's 4-week investigation of Atlantic stratocumuli (ASTEX). The new probe (PVM-100A, Gerber et al. 1993), see Fig. i, is an a d a p t a t i o n of a g r o u n d - b a s e d instrument (PVM-100) for m e a s u r i n g LWC (Gerber, 1991; Arends, et al., 1992.) The operating principle of the aircraft probe is illustrated in Fig. 2. Droplets f o r w a r d - s c a t t e r light out of a laser beam into a lens that focuses the light onto two detectors. Filters, with c o n c e n t r i c areas of variable transmission, are placed in front of the sensors, and weight the scattered light of p o l y d i s p e r s e droplets with radii r to produce Fig. 1 - A i r c r a f t probe for m e a s u r e m e n t outputs proportional to ~ r 3 (LWC) and ~ r 2 of LWC and re: Dimensions in inches, welght ~4 lb. (PSA). The ratio LWC/PSA is proportional to the effective droplet radius, re, which is SPI'ITTER used in the parameteri-Jl i zation of the radiative . ~ . - . . . . . . -. ,~_ - ~ - - . . . . . . . . . . ~'c effect of clouds, and is __ an indicator of anthropogenic climate in,~.s i I -~tr~, fluences (Slingo, 1990). The probe was caliSE,SO, brated in the fog/wind tunnel facility at ECN Petten, where it was Fig. 2 - Principal optical components compared to the filter of the aircraft probe.
~
$583
$584
H (;~:RB~ ~
method for LWC, (Gerber et al.,
and to corrected 1993).
FSSP-I00
measurements
for PSA
RESULTS The probe generated a continuous data set of LWC and r during the ~i00 hrs. of in-cloud measurements from the U. o~ Washington's C-131A aircraft during ASTEX, with negligible drift in probe optics and electronics. Interpretation of this data set resulted in findings, some of which follow: i) Large reductions in r e were noted near stratocu cloud top when air trajectories brought a European air mass into the vicinity of the Azores. 2) In most clouds values of r e were precisely invariant in entrainment features near cloud top, indicating the presence of inhomogeneous mixing, and confirming the results of Nicholls (1989). Infrequently, enhanced values of r e suggested the presence of entity mixing described by Telford and Wagner (1981). 3) Comparison of r~ measured 2O with an FSSP-100 and t~e PVM-100A is shown by the open circles in 18 Fig. 3; horizontal bars are the 16 predicted behavior of FSSP-IO0, Baumgardner and Spowart (1990). 14 Fig. 3 represents 500 Km of incloud data with a wide range in LWC and droplet concentration. The approximately constant factor difference between r e measurements suggests the following procedure for retrieving accurate FSSP-100 droplet spectra when both instruments are co-located, and '2 ~ PVM = 1.5101,ESSP - 0.,5114 J given that the PVM-100 produces accurate results. The factor 2 4 6 8 10 12 14 i6 18 20 difference in r_ corrects the FSSP, R Err. ( u m ) relative FSSP-I~0 size spectrum, and the adjustment of FSSP-100 Fig. 3 - Comparison of r e LWC to PVM-100A LWC corrects measured with FSSP-100 the FSSP-100 concentration and PVM-100A in marine tion e r r o r s . stratocumuli (circles).
o//j! j/J
":/y
?
REFERENCES
Arends, B.B., G.P.A. Kos, W. Wobrock, D. Schell, K.J. Noone, S. Fuzzi, S. Pahl (1992), Tellus, Vol. 44B, 604-611. Baumgardner, D., M. Spowart (1990), J. Atmos. Oceanic Technol., Vol. 7, 666-672. Gerber, H. (1991), AD_pl. Q ~ . , Vol. 30, 4824-4831. Gerber, H., B.G. Arends, A. Ackerman (1993), Atm. R ~ . , submitted. Nicholls, S. (1989), O. J. R. Meteoro. SOU., Vol. 115, 487-511. Slingo, A. (1990), Natu;~, Vol 343, 49-51. Telford, J.W., P.B. Wagner (1981), Pure and ADpl. GeODhVS., Vol. 119, 934-965.