C. Zerefos (1), K. Eleftheratos (1), D. Balis (2), P. Zanis (3) C. Meleti (2), G. Tselioudis (4) and...
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C. Zerefos C. Zerefos (1) (1) , K. Eleftheratos , K. Eleftheratos (1) (1) , D. Balis , D. Balis (2) (2) , P. , P. Zanis Zanis (3) (3) C. Meleti C. Meleti (2) (2) , G. Tselioudis , G. Tselioudis (4) (4) and K. Kourtidis and K. Kourtidis (2) (2) (1) (1) Laboratory of Climatology & Atmospheric Environment, University of Athens, Laboratory of Climatology & Atmospheric Environment, University of Athens, Greece. Greece. (2) (2) Laboratory of Atmospheric Physics, University of Thessaloniki, Greece. Laboratory of Atmospheric Physics, University of Thessaloniki, Greece. (3) (3) Research Center for Atmospheric Physics & Climatology, Academy of Athens, Research Center for Atmospheric Physics & Climatology, Academy of Athens, Greece. Greece. (4) (4) Goddard Institute of Space Studies, NASA, USA. Goddard Institute of Space Studies, NASA, USA. Effects of natural perturbations on cirrus clouds Effects of natural perturbations on cirrus clouds EGS-AGU-EUG Joint Assembly EGS-AGU-EUG Joint Assembly Nice, France, 07-11 April 2003 In this study we examine on a global scale the variability of the cirrus cloud cover that can be attributed to known large- scale natural phenomena such as ENSO, QBO and the NAO. The analysis is based on the latest version of the ISCCP D2 cloud dataset and covers the period 1984-1998. Vertical velocities at 500 mb from the independently produced NCEP/NCAR reanalysis dataset were found to be consistent with the cirrus cloud data from the ISCCP, determining the highest part of the cloud natural variability in the tropics. The effect of ENSO, QBO and NAO has been examined separately. While QBO was found to have a negligible effect on cirrus clouds, the effect of ENSO and NAO is more significant and can be very important regionally. Cirrus clouds and ENSO are significantly correlated mainly in the tropics. During a warm (El Nino) event, cirrus clouds increase over the east tropical Pacific Ocean and decrease over its western part. The correlation between cirrus and NAO was found to be significant mainly over Europe suggesting that during the positive phase of NAO, cirrus clouds tend to increase over the northern Europe and to decrease as we move to the south. Abstract ISCCP data reveal geographical distributions of cirrus cloud amount quite similar to those published in the SAGE II climatology of Wang et al. (1996). The latitude-longitude distribution of cirrus cloud cover indicates very high cirrus amounts in the tropical upper troposphere, particularly over Micronesia (30-35%), central Africa (25-30%) and the northern part of South America (Figure 1). In these areas, cirrus cloud formation is strongly related to deep convective cloud systems by the rising moist air. These systems transport large amounts of ice water to the upper troposphere and in the presence of wind shear they generate extensive cirrus outflow anvils (Jensen et al., 1996). These processes are enhanced by the fact that cirrus clouds and vertical velocities are strongly correlated in the tropics (up to –0.8) and determine more than 50% of the variance in cirrus cloudiness over this area (Figure 3). The remaining 50% is probably attributed to other processes such as synoptic-scale uplifts of humid layers, which at the very low temperatures at the tropical tropopause generate ice crystals. As we move to higher latitudes, high cirrus amounts are concentrated over the long mountain chains of the northern and the southern hemisphere (Rockies, Appalachian, Sierra Nevada, Alps) and over Siberia. Their formation over these areas is presumably associated with orographic cloud formation and with frontal systems. Over the oceans of the southern hemisphere, the cirrus-free regions correspond to the persistent high-pressure regions (Zerefos et al., 2002). The seasonal averaged zonal mean distribution of cirrus cloudiness is in concert with the seasonal averaged zonal mean distribution of the vertical velocities (Figure 2). This explains the strong correlations between the two variables. Both zonal distributions reflect the seasonal shift of the ITCZ. Cirrus cloud natural variability can be significantly perturbed by large-scale modes of natural climate variability seen for example ENSO and the NAO. The effect of the QBO on cirrus clouds was not found to be negligible. ENSO effect: ENSO signal on cirrus clouds appears with a dipole structure in the tropics. From Figure 4b it appears that the enhanced rising warm moist air over the eastern tropical Pacific Ocean during the strong El Nino episode in 1997/98, is accompanied by sinking air over the western Pacific. Since cirrus clouds and vertical velocities are strongly correlated over these areas, this circulation caused significant changes in cirrus cloud cover. As can be seen from Figure 4a, cirrus cloudiness increased significantly from the normal conditions over the eastern tropical Pacific Ocean, about 10-20%, and decreased over Micronesia by the same amount. The reverse is observed during a cold (La Nina) episode. These features are consistent with past studies examining effects of El Nino on cirrus clouds based on satellite observations (Massie et al., 2000). NAO effect: The North Atlantic Oscillation affects both cirrus cloud cover and the vertical velocities (Figure 5). The higher correlations are found over the northern and the southern Europe Results and discussion 1. Jensen, E. J., O. B. Toon, H. B. Selkirk, J. D. Spinhirne, and M. R. Schoberl, On the formation and persistence of subvisible cirrus clouds near the tropical tropopause, J. Geophys. Res., 101(D16), 21,361-21,375, 1996. 2. Massie S, P. Lowe, X. Tie, M. Hervig, G. Thomas, and J. Russell III, Effect of the 1997 El Nino on the distribution of upper tropospheric cirrus, J. Geophys. Res., 105(D18), 22,725-22,741, 2000. 3. Wang, P.-H., P. Minnis, M. P. McCormick, G. S. Kent, and K. M. Skeens, A 6-year climatology of cloud occurrence frequency from Stratospheric Aerosol and Gas Experiment II observations (1985- 1990), J. Geophys. Res., 101(D23), 29,407-29,429, 1996. 4. Zerefos C. S., K. Eleftheratos, C. Meleti, D. Balis, P. Zanis and G. Tselioudis, Variability of cirrus cloud cover on global scale and possible impact from aviation, International Global References Figure 1. (a) Latitude-longitude map of annual mean cirrus cloud cover (in %) derived from the ISCCP satellite cloud dataset for the period 1984-1998. (b) Same as (a) but for the vertical velocities at 500 mb (in Pa/s) derived from the NCEP/NCAR reanalysis dataset for the same period. The negative and the positive values correspond to upward and downward air movements, respectively. Figure 2. (a) Seasonal averaged zonal mean distribution of cirrus cloud cover (in %) for the period 1984-1998 from ISCCP. (b) Same as (a) but for the vertical velocities at 500 mb (in Pa/s) from NCEP. Figure 3. Latitude-longitude map of correlation coefficients between cirrus cloud cover and vertical velocities at 500 mb. The highest correlations (about -0.8) are found in the tropics. Figure 4. (a) Latitude-longitude map of the difference in cirrus cloud cover between the 1997/98 ENSO years and the non-ENSO years 1984/85/95/96. (b) Same as (a) but for the vertical velocities at 500 mb. Figure 5. (a) Latitude-longitude map of correlation coefficients between cirrus cloud cover and the North Atlantic Oscillation index in the cold period. (b) Same as (a) but for the vertical velocities at 500mb. Acknowledgments This work was conducted within the project TRADE- OFF funded by the European Commission (Contract No EVK2-1999-00030). -1 8 0 -120 -6 0 0 60 120 180 L on gitu de -9 0 -6 0 -3 0 0 3 0 6 0 9 0 L atitud e 4 .0 8 .0 12 .0 16 .0 20 .0 24 .0 28 .0 32 .0 36 .0 40 .0 F igu re 1a.A n n u al m ean cirru s clo u d co v er (in % ) -1 8 0 -120 -6 0 0 60 120 180 L on gitu d e -9 0 -6 0 -3 0 0 3 0 6 0 9 0 L atitud e -0.05 -0.04 -0.02 -0.00 0.0 1 0.0 3 0.0 4 0.0 5 0.0 7 0.0 9 F ig u re 1 b . A n n u a l m ea n v ertica l v elo cities at 500 m b (P a/s) -9 0 -7 5 -6 0 -4 5 -3 0 -1 5 0 1 5 3 0 4 5 6 0 7 5 9 0 4 .0 8 .0 1 2 .0 1 6 .0 2 0 .0 2 4 .0 2 8 .0 3 2 .0 3 6 .0 4 0 .0 F igu re 2 a . S eason al v ariab ility o f m ean cirru s clo u d co v er (in % ) D D N J F M A M J J A S O -90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90 -0 .0 6 -0 .0 5 -0 .0 3 -0 .0 1 0 .0 0 0 .0 1 0 .0 3 0 .0 5 0 .0 6 0 .0 8 0 .0 9 F igu re 2b . S eason a l v a ria b ility o f m ean v ertica l v elo cities a t 5 0 0 m b (P a/s) D D N J F M A M J J A S O -180 -120 -6 0 0 60 120 180 L on gitu d e -90 -60 -30 0 30 60 90 L atitud e -3 6 -2 8 -2 0 -1 2 -4 4 12 20 28 36 F igu re 4 a . E ffect o f the 1997/98 E l N ino on cirru s clo u d s (% cover) -1 8 0 -1 2 0 -6 0 0 60 120 180 L on gitu d e -9 0 -6 0 -3 0 0 30 60 90 L atitud e -0.0 9 -0.0 7 -0.0 5 -0.0 3 -0.0 1 0.0 1 0.0 3 0.0 5 0.0 7 0.0 9 F igu re 4b . E ffect o f th e 1997/98 E l N ino on v ertica l v elo cities a t 5 0 0 m b (P a/s) -1 0 0 -8 0 -6 0 -4 0 -2 0 0 20 40 L on gitu d e 2 0 3 0 4 0 5 0 6 0 7 0 L atitu d e F igu re 5 a . C o rrela tio n m ap betw een cirru s clo u d cover an d N A O in d ex in DJFM -1 .0 -0 .8 -0 .6 -0 .4 -0 .2 0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 -1 0 0 -8 0 -6 0 -4 0 -2 0 0 20 40 L on gitu d e 2 0 3 0 4 0 5 0 6 0 7 0 L atitud e F igu re 5b . C o rrela tio n m ap b etw een v ertica l v elo cities a n d N A O in d ex in D JFM -1 .0 -0 .8 -0 .6 -0 .4 -0 .2 0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 -180 -1 2 0 -6 0 0 60 120 180 L on gitu d e -9 0 -6 0 -3 0 0 30 60 90 L atitu de -1 .0 -0 .8 -0 .6 -0 .4 -0 .2 0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 F igu re 3 . C o rrela tio n m ap b etw een cirru s clo u d cover an d v ertica l v elo cities a t 5 0 0 m b
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Transcript of C. Zerefos (1), K. Eleftheratos (1), D. Balis (2), P. Zanis (3) C. Meleti (2), G. Tselioudis (4) and...
C. ZerefosC. Zerefos(1)(1), K. Eleftheratos, K. Eleftheratos(1)(1), D. Balis, D. Balis(2)(2), P. Zanis, P. Zanis(3)(3)
C. MeletiC. Meleti(2)(2), G. Tselioudis, G. Tselioudis(4)(4) and K. Kourtidis and K. Kourtidis(2)(2)
(1) (1) Laboratory of Climatology & Atmospheric Environment, University of Athens, Greece.Laboratory of Climatology & Atmospheric Environment, University of Athens, Greece.
(2) (2) Laboratory of Atmospheric Physics, University of Thessaloniki, Greece.Laboratory of Atmospheric Physics, University of Thessaloniki, Greece.(3)(3) Research Center for Atmospheric Physics & Climatology, Academy of Athens, Greece. Research Center for Atmospheric Physics & Climatology, Academy of Athens, Greece.
(4) (4) Goddard Institute of Space Studies, NASA, USA.Goddard Institute of Space Studies, NASA, USA.
Effects of natural perturbations on cirrus cloudsEffects of natural perturbations on cirrus clouds
EGS-AGU-EUG Joint AssemblyEGS-AGU-EUG Joint AssemblyNice, France, 07-11 April 2003
In this study we examine on a global scale the variability of the cirrus cloud cover that can be attributed to known large-scale natural phenomena such as ENSO, QBO and the NAO. The analysis is based on the latest version of the ISCCP D2 cloud dataset and covers the period 1984-1998. Vertical velocities at 500 mb from the independently produced NCEP/NCAR reanalysis dataset were found to be consistent with the cirrus cloud data from the ISCCP, determining the highest part of the cloud natural variability in the tropics. The effect of ENSO, QBO and NAO has been examined separately. While QBO was found to have a negligible effect on cirrus clouds, the effect of ENSO and NAO is more significant and can be very important regionally. Cirrus clouds and ENSO are significantly correlated mainly in the tropics. During a warm (El Nino) event, cirrus clouds increase over the east tropical Pacific Ocean and decrease over its western part. The correlation between cirrus and NAO was found to be significant mainly over Europe suggesting that during the positive phase of NAO, cirrus clouds tend to increase over the northern Europe and to decrease as we move to the south.
Abstract
ISCCP data reveal geographical distributions of cirrus cloud amount quite similar to those published in the SAGE II climatology of Wang et al. (1996).
The latitude-longitude distribution of cirrus cloud cover indicates very high cirrus amounts in the tropical upper troposphere, particularly over Micronesia (30-35%), central Africa (25-30%) and the northern part of South America (Figure 1). In these areas, cirrus cloud formation is strongly related to deep convective cloud systems by the rising moist air. These systems transport large amounts of ice water to the upper troposphere and in the presence of wind shear they generate extensive cirrus outflow anvils (Jensen et al., 1996). These processes are enhanced by the fact that cirrus clouds and vertical velocities are strongly correlated in the tropics (up to –0.8) and determine more than 50% of the variance in cirrus cloudiness over this area (Figure 3). The remaining 50% is probably attributed to other processes such as synoptic-scale uplifts of humid layers, which at the very low temperatures at the tropical tropopause generate ice crystals. As we move to higher latitudes, high cirrus amounts are concentrated over the long mountain chains of the northern and the southern hemisphere (Rockies, Appalachian, Sierra Nevada, Alps) and over Siberia. Their formation over these areas is presumably associated with orographic cloud formation and with frontal systems. Over the oceans of the southern hemisphere, the cirrus-free regions correspond to the persistent high-pressure regions (Zerefos et al., 2002).
The seasonal averaged zonal mean distribution of cirrus cloudiness is in concert with the seasonal averaged zonal mean distribution of the vertical velocities (Figure 2). This explains the strong correlations between the two variables. Both zonal distributions reflect the seasonal shift of the ITCZ.
Cirrus cloud natural variability can be significantly perturbed by large-scale modes of natural climate variability seen for example ENSO and the NAO. The effect of the QBO on cirrus clouds was not found to be negligible.
ENSO effect: ENSO signal on cirrus clouds appears with a dipole structure in the tropics. From Figure 4b it appears that the enhanced rising warm moist air over the eastern tropical Pacific Ocean during the strong El Nino episode in 1997/98, is accompanied by sinking air over the western Pacific. Since cirrus clouds and vertical velocities are strongly correlated over these areas, this circulation caused significant changes in cirrus cloud cover. As can be seen from Figure 4a, cirrus cloudiness increased significantly from the normal conditions over the eastern tropical Pacific Ocean, about 10-20%, and decreased over Micronesia by the same amount. The reverse is observed during a cold (La Nina) episode. These features are consistent with past studies examining effects of El Nino on cirrus clouds based on satellite observations (Massie et al., 2000).
NAO effect: The North Atlantic Oscillation affects both cirrus cloud cover and the vertical velocities (Figure 5). The higher correlations are found over the northern and the southern Europe (up to 0.5 and –0.6, respectively). These correlations suggest that during a positive phase of NAO, the enhanced rising air from the deeper than normal Icelandic low, increases cirrus cloudiness over the northern part of Europe. Over the southern Europe, cirrus cloud decrease due to the sinking air caused by the stronger than normal high-pressure system at Azores.
Results and discussion
1. Jensen, E. J., O. B. Toon, H. B. Selkirk, J. D. Spinhirne, and M. R. Schoberl, On the formation and persistence of subvisible cirrus clouds near the tropical tropopause, J. Geophys. Res., 101(D16), 21,361-21,375, 1996.
2. Massie S, P. Lowe, X. Tie, M. Hervig, G. Thomas, and J. Russell III, Effect of the 1997 El Nino on the distribution of upper tropospheric cirrus, J. Geophys. Res., 105(D18), 22,725-22,741, 2000.
3. Wang, P.-H., P. Minnis, M. P. McCormick, G. S. Kent, and K. M. Skeens, A 6-year climatology of cloud occurrence frequency from Stratospheric Aerosol and Gas Experiment II observations (1985-1990), J. Geophys. Res., 101(D23), 29,407-29,429, 1996.
4. Zerefos C. S., K. Eleftheratos, C. Meleti, D. Balis, P. Zanis and G. Tselioudis, Variability of cirrus cloud cover on global scale and possible impact from aviation, International Global Atmospheric Chemistry (IGAC) Conference, Crete, Greece, 18-25 September 2002.
References
Figure 1. (a) Latitude-longitude map of annual mean cirrus cloud cover (in %) derived from the ISCCP satellite cloud dataset for the period 1984-1998. (b) Same as (a) but for the vertical velocities at 500 mb (in Pa/s) derived from the NCEP/NCAR reanalysis dataset for the same period. The negative and the positive values correspond
to upward and downward air movements, respectively.
Figure 2. (a) Seasonal averaged zonal mean distribution of cirrus cloud cover (in %) for the period 1984-1998 from ISCCP. (b) Same as (a) but for the vertical velocities at 500 mb (in Pa/s) from NCEP.
Figure 3. Latitude-longitude map of correlation coefficients between cirrus cloud cover and vertical velocities at 500 mb. The highest correlations (about -0.8) are found in the tropics.
Figure 4. (a) Latitude-longitude map of the difference in cirrus cloud cover between the 1997/98 ENSO years and the non-ENSO years 1984/85/95/96. (b) Same as (a) but for the vertical velocities at 500 mb.
Figure 5. (a) Latitude-longitude map of correlation coefficients between cirrus cloud cover and the North Atlantic Oscillation index in the cold period. (b) Same as (a) but for the vertical velocities at 500mb.
Acknowledgments
This work was conducted within the project TRADE-OFF funded by the European Commission (Contract No EVK2-1999-00030).
-1 8 0 -1 2 0 -6 0 0 6 0 1 2 0 1 8 0
L o n g itu d e
-9 0
-6 0
-3 0
0
3 0
6 0
9 0
Lat
itud
e
4 .0
8 .0
1 2 .0
1 6 .0
2 0 .0
2 4 .0
2 8 .0
3 2 .0
3 6 .0
4 0 .0
F ig u re 1 a . A n n u a l m ea n c irru s c lo u dco v er (in % )
-1 8 0 -1 2 0 -6 0 0 6 0 1 2 0 1 8 0
L o n g itu d e
-9 0
-6 0
-3 0
0
3 0
6 0
9 0
Lat
itud
e
-0 .0 5
-0 .0 4
-0 .0 2
-0 .0 0
0 .0 1
0 .0 3
0 .0 4
0 .0 5
0 .0 7
0 .0 9
F ig u re 1 b . A n n u a l m ea n v er tica l v e lo c itiesa t 5 0 0 m b (P a /s)
-9 0
-7 5
-6 0
-4 5
-3 0
-1 5
0
1 5
3 0
4 5
6 0
7 5
9 0
4 .0
8 .0
1 2 .0
1 6 .0
2 0 .0
2 4 .0
2 8 .0
3 2 .0
3 6 .0
4 0 .0
F ig u re 2 a . S ea so n a l v a r ia b ility o fm ea n c irru s c lo u d co v er (in % )
DD NJ F M A M JJ A S O-9 0
-7 5
-6 0
-4 5
-3 0
-1 5
0
1 5
3 0
4 5
6 0
7 5
9 0
-0 .0 6
-0 .0 5
-0 .0 3
-0 .0 1
0 .0 0
0 .0 1
0 .0 3
0 .0 5
0 .0 6
0 .0 8
0 .0 9
F ig u re 2 b . S ea so n a l v a r ia b ility o f m ea nv ertica l v e lo c ities a t 5 0 0 m b (P a /s)
DD NJ F M A M JJ A S O
-1 8 0 -1 2 0 -6 0 0 6 0 1 2 0 1 8 0
L on g itu d e
-9 0
-6 0
-3 0
0
3 0
6 0
9 0
Lat
itud
e
-3 6
-2 8
-2 0
-1 2
-4
4
1 2
2 0
2 8
3 6
F ig u re 4 a . E ffect o f th e 1 9 9 7 /9 8 E l N in oo n c irru s c lo u d s (% co v er)
-1 8 0 -1 2 0 -6 0 0 6 0 1 2 0 1 8 0
L o n g itu d e
-9 0
-6 0
-3 0
0
3 0
6 0
9 0
Lat
itud
e
-0 .0 9
-0 .0 7
-0 .0 5
-0 .0 3
-0 .0 1
0 .0 1
0 .0 3
0 .0 5
0 .0 7
0 .0 9
F ig u re 4 b . E ffec t o f th e 1 9 9 7 /9 8 E l N in o o nv er tica l v e lo c itie s a t 5 0 0 m b (P a /s)
-1 0 0 -8 0 -6 0 -4 0 -2 0 0 2 0 4 0
L o n g itu d e
2 0
3 0
4 0
5 0
6 0
7 0
Lat
itud
e
F ig u re 5 a . C o rre la tio n m a p b e tw een c irru sc lo u d co v er a n d N A O in d ex in D J F M
-1 .0-0 .8-0 .6-0 .4-0 .20 .00 .20 .40 .60 .81 .0
-1 0 0 -8 0 -6 0 -4 0 -2 0 0 2 0 4 0
L o n g itu d e
2 0
3 0
4 0
5 0
6 0
7 0
Lat
itud
e
F ig u re 5 b . C o rre la tio n m a p b etw een v er tica lv e lo c itie s a n d N A O in d ex in D J F M
-1 .0-0 .8-0 .6-0 .4-0 .20 .00 .20 .40 .60 .81 .0
-1 8 0 -1 2 0 -6 0 0 6 0 1 2 0 1 8 0
L o n g itu d e
-9 0
-6 0
-3 0
0
3 0
6 0
9 0
Lat
itud
e
-1 .0
-0 .8
-0 .6
-0 .4
-0 .2
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0
F ig u re 3 . C o rre la tio n m a p b etw een c irru s c lo u dco v er a n d v ertica l v e lo c itie s a t 5 0 0 m b
