Numerical simulation of cirrus cloud radiative forcing using lidar
backscatter data. Preliminary results
Simulación numérica de forzamiento radiativo de nubes cirros usando datos de
retrodispersión lidar. Resultados preliminares
Boris Barja
(*), Juan Carlos Antuña
Camagüey Lidar Station. Camagüey Meteorological Center, INSMET, Cuba.
* bbarja@cmw.insmet.cu
Recibido / Received: 20 – Jul – 2007. Versión revisada / Revised version: 23 – Oct – 2007. Aceptado / Accepted: 10 – Nov – 2007
ABSTRACT:
Cirrus clouds radiative forcing effects on shortwave radiation have been studied combining lidar measurements with a state of the art radiative transfer code. For this purpose 132 profiles of backscatter lidar observations at 532 nm were used. The selected profiles were collected during 1993 – 1998 period. The radiative transfer code was adapted to the local conditions at Camagüey, Cuba, using water vapor and temperature profiles as well as locally measured surface albedo from the site. Runs were conducted both under the presence of cirrus clouds and in clear sky conditions. The calculated shortwave broadband cloud radiative forcing values have negative sign and vary according to the different optical depths of cirrus. A close correlation between the negative cirrus radiative forcing and optical depth (anticorrelation) was found at the top of the atmosphere and surface when broadband solar irradiances calculations are analyzed. This close correlation was found also in different intervals of wavelength in the solar spectrum (near infrared, visible, and ultraviolet). The major effect of cirrus clouds on the solar radiation is in the near infrared wavelengths. Limitations of the current lidar dataset are discussed.
Keywords: Cirrus Clouds, Cloud Radiative Forcing.
RESUMEN:
El forzamiento radiativo de las nubes cirros sobre la radiación de onda corta ha sido estudiado, combinando mediciones de lidar con un modelo de transferencia radiativa de la actual generación. Para este propósito fueron utilizados un total de 132 perfiles de coeficiente de retrodispersión obtenido con lidar a 532 nm de longitud de onda. Estos perfiles fueron obtenidos durante el periodo de 1993 a 1998, en el sitio de mediciones de la Estación Lidar Camagüey. El código de transferencia radiativa fue adaptado a las condiciones locales de Camagüey, Cuba. Para este objetivo se utilizaron perfiles verticales de vapor de agua y temperatura, obtenidos a partir de sondeos aerológicos en el sitio de mediciones, así como valores de superficie de las variables meteorológicas y del albedo de superficie. Se realizaron cálculos de los flujos de radiación considerando la presencia de nubes cirros y en condiciones de cielo claro, para la determinación del forzamiento radiativo de las nubes cirros. Los valores del forzamiento radiativo por nubes cirros de la radiación solar tienen signo negativo y varían de acuerdo a los diferentes espesores ópticos. En el tope de la atmósfera y en la superficie fue encontrada una correlación entre el forzamiento radiativo negativo por cirros y el espesor óptico, cuando se analizó el forzamiento en la banda total del espectro solar. Esta correlación fue observada también en los diferentes intervalos de longitud de onda in el espectro solas (infrarrojo cercano, visible, y ultravioleta). El principal efecto de las nubes cirros sobre la radiación solar se encuentra en el intervalo de longitudes de onda del infrarrojo cercano. Las limitaciones del actual conjunto de datos de mediciones lidar de nubes cirros son discutidas.
REFERENCES AND LINKS
[1] Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell and C. A. Johnson Edts., Cambridge University Press, Cambridge (UK), pp. 881 (2001).
[2] IPCC, 2007. Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H. L. Miller Edts., Cambridge University Press, Cambridge (UK), pp. 996 (2007).
[3] K.-N. Liou, “The influence of cirrus on weather and climate processes: A global perspective”, Mon. Weather Rev. 114, 1167-1199 (1986).
[4] I. Schlimme, A. Macke, J. Reichardt, “The impact of ice crystal shapes, size distributions, and spatial structures of cirrus clouds on solar radiative fluxes”, J. Atmos. Sci.62, 2274-2283 (2005).
[5] C. Poetzsch-Heffter, Q. Liu, E. Ruprecht, C. Simmer, “Effect of cloud types on the earth radiation budget calculated with the ISCCP C1 dataset: Methodology and initial results”, J. Climate8, 829–843 (1995). [6] T. Chen, W. B. Rossow, Y. Zhang, “Radiative effects of cloud-type variations”, J. Climate13, 264–286
(2000).
[7] Y. Zhang, A. Macke, F. Albers, “Effect of crystal size spectrum and crystal shape on stratiform cirrus radiative forcing”, J. Atmos. Res.52, 59–75 (1999).
[8] V. Ramaswamy, V. Ramanathan, “Solar absorption by cirrus clouds and the maintenance of the tropical upper troposphere thermal structure”, J. Atmos. Sci.46, 2293-2310 (1989).
[9] V. Ramaswamy, S. M. Freidenreich, “Solar radiative line-by-line determination of water vapor absorption and water cloud extinction in inhomogeneous atmospheres”, J. Geophys. Res.96, 9133–9157 (1991). [10] S. M. Freidenreich, V. Ramaswamy, “A new multiple-band solar radiative parameterization for general
circulation models”, J. Geophys. Res.104 (D24), 31389-31409 (1999).
[11] J. C. Antuña, B. Barja, “Propiedades ópticas de nubes cirros medidas con lidar en Camagüey, Cuba”, Opt. Pura Apl.39, 11-16, (2006).
[12] T. J. Garrett, H. Gerber, D. G. Baumgardner, C. H. Twohy, E. M. Weinstock, “Small, highly reflective ice crystals in low-latitude cirrus”, Geophys. Res. Lett.30, 2132 (2003).
[13] J. C. Antuña, “Lidar measurements of stratospheric aerosols from Mount Pinatubo at Camagüey, Cuba, after some volcanic eruptions”, Atmos. Environ.30, 1857-1860 (1996).
[14] J. C. Antuña, D. Marin, R. Aroche, “Statistical model of some atmospheric parameters at Camagüey Meteorological Site”, (in Spanish, Unpublished Manuscript, available from the Camagüey Meteorological Centre Library), pp. 24 (1991).
[15] B. Barja, “Caracterización de las nubes cirros y su efecto sobre la radiación solar en El Caribe”, Doctoral Thesis , in preparation (2007).
[16] Q. Fu, “An accurate parameterization of the solar radiative properties of cirrus clouds for climate models”, J. Climate9, 2058–2082 (1996).
[17] J. C. Antuña, A. Fonte, R. Estevan, B. Barja, R. Acea, J. C. Antuña Jr., “Solar radiation data rescue at Camagüey, Cuba”, submitted to Bull. Amer. Met. Soc. (2007).
[18] V. I. Khvorostyanov, K. Sassen, “Microphysical processes in cirrus and their impact on radiation”, pp. 397-432 in Cirrus, D. Lynch and K. Sassen Edts., Oxford University Press (2002).
[19] G. M. McFarquhar, A. J. Heymsfield, J. Spinhirne, B. Hart, “Thin and subvisual tropopause tropical cirrus: Observations and radiative impacts”, J. Atmos. Sci.57, 1841-1853 (2000).
1. Introduction
The scientific community identifies cloud effects on the earth-atmosphere radiation balance as an important factor in climate change. These effects are associated to many uncertainties due to the extreme variability and complexity of clouds [1,2]. Particularly, cirrus clouds are widely recognized to play an important role in the radiation budget of the
change sign depending on ice crystal geometry and size [7]. Hence, in contrast to low and midlevel clouds, the sign of the net cirrus cloud radiative forcing is determined by the microphysical composition of the clouds. The solar absorption by the cirrus clouds is important in the maintenance of the thermal structure of the upper troposphere [8]. The cirrus clouds radiative forcing is not well understood due the incomplete knowledge of such properties. The aim of this study is to quantify the effect of cirrus cloud, measured in Camagüey with lidar, on the solar radiative fluxes.
To achieve this objective we used a state of the art radiative transfer code [9,10]. This radiative transfer model was adapted to the local conditions at Camagüey, Cuba. The cloud structures taken from lidar measurements in our site [11] allow us to investigate the influence of vertical cloud inhomogeneity on the solar broadband fluxes. Also the effective particle size was theoretically determined from the analytic results reported for our geographic area [12]. The lidar measurements were conducted at Camagüey Lidar Station (CLS), located at the Camagüey Meteorological Center, Cuba, at 21,4ºN and 77,9ºW, between 1993 and 1998. Measurements at the CLS were conducted using an aerosol-backscatter lidar operating at 532 nm. Details about the instrument have been reported earlier [13].
The calculated shortwave broadband cirrus cloud radiative forcing values vary according to the different optical depths of cirrus. A close correlation between the negative cirrus radiative forcing and optical depth (anticorrelation) was found at the top of the atmosphere and surface when broadband solar irradiances calculations are analyzed. This close correlation was found also in different intervals of wavelength in the solar spectrum (near infrared, visible, and ultraviolet). The major effect of cirrus clouds on the solar radiation is in the near infrared wavelengths. Limitations of the current lidar dataset are discussed.
2. Data and methods
The radiative transfer code used in the present work is a state of the art shortwave radiative transfer model. This code was developed in the Geophysical Fluid Dynamics Laboratory. The model includes particulate scattering and absorption, Rayleigh scattering, and gaseous absorption by O2, O3, CO2,
and H2O. The solar spectrum is divided into 25
pseudo-monochromatic bands between 0 and 57,600 cm-1. Aerosol and cloud single-scattering properties
are integrated into the 25 band radiative transfer
model using a solar-flux weighted averaging scheme. Reflection and transmission for homogeneous layers is calculated using the d-Eddington method. The ‘‘adding’’ technique is used to solve for vertical fluxes and heating rates. Compared to reference calculations without cloud or aerosol, the calculations of heating rates are accurate to within 10%, and within 2% for atmospheric absorbed flux. The model is discussed in detail in the references [9,10].
The adaptation of the code to our local condition was performed by substitution of the temperature and water vapour mixing ratio profiles. Values to the pressure, temperature, water vapour mixing ratio, surface albedo at the surface were obtained from the Camagüey Meteorological Surface Station reports dataset from 1988 to 2001. In order to obtain the temperature and water vapour mixing ratio vertical profiles we used the mean aerological sounding in the site with the dataset from 1981 to 1988 [14]. The procedure used for this and adjust of the code is discussed in detail in previous publication [15].
Fu´s parameterization was used to represent the solar radiative properties of cirrus clouds in the code calculations. This is based assuming the hexagonal crystals shapes of cirrus clouds. We need introduce to the code the value of the optical depths and the generalized effective size [16]at the levels where the cirrus is there. Also, in the code are used two methods to obtain the drop coalbedo, referred to as thin–averaging and thick–averaging techniques. The first one defines de coalbedo as the solar weighted mean value over the band spectral interval. This technique will be used, due to the small geometrical depth of our cirrus clouds dataset.
Cirrus clouds dataset contains 132 individual extinction profiles in 36 days of measurements from 1993 to 1998. The Lidar system uses a doubled frequency Nd–YAG laser (532 nm, 50 Hz, 300 mJ/pulse) and in cirrus operation mode the altitude resolution is changed to 75 m. The receiving telescope has 34 cm of diameter and the field of view is 3 mrad. The number of laser shots for cirrus measurements are 1000. Details about the instrument can be found in a former paper [13].
backscattering coefficient of cirrus clouds measured in Camagüey site are showed in Fig. 1. The value of the lidar ratio used to convert the backscattering coefficient profile in extinction coefficient profile was 10 sr. The method employed to obtain the value of the lidar ratio for our dataset has been described in an earlier paper [11]. The author used this value because the few values of lidar ratios reported in the literature for the subvisual and thin cirrus clouds in the subtropical region.
Fig. 1. Examples of backscattering coefficient profiles obtained with lidar in Camagüey site.
We use the extinction profile of the cirrus clouds derived from lidar backscattering profile to obtain the optical depth profile of the cirrus cloud that will be introduced in the radiative transfer code. The resolution of the lidar measurements is smaller than the resolution of the code, at the altitude of the cirrus clouds. Thus, one layer of the code has various lidar layers. We adjusted the measurements vertical resolution to the layers used in the code summing the extinction coefficients contained in each code layers. It was the method to produce the structure of the cirrus clouds to be introduced to the radiative transfer code. Quality control of the procedure was performed. We compare the total optical depth of the cirrus clouds profile to be used in the model with the original optical depth of the cirrus clouds. The comparison showed a 100 % of the agreement between the optical depth values from both different resolution profiles.
Also, we estimate the profile of the ice crystal generalized effective size (Dge) to the cirrus clouds particles in order to be introduced in the model. For this, we use an experimental relation reported to our region [12] to estimate the effective radius (re). This equation related the effective radius with the temperature at the altitude of the cirrus clouds. Also, is reported a relation between Dge and re [16]. Then, we combining both equation and acquire the following relation to Dge:
⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = 39 75 exp 698 , 7 T
Dge , (1)
where T is the temperature with units of Celsius degree (ºC) at the height where there are the particles. Temperature was calculated via interpolation from the reanalysis temperature profile from the nearest point to our site the same day each measurement was conducted.
In order to evaluate the radiative impact of the cirrus clouds measured in Camagüey by lidar we investigate the solar radiative forcing of cirrus. To derive these quantities we used the expression:
clear net cloud net F F
CF= − . (2)
Fnet is the net flux, difference between downward and upward fluxes, “cloud” mean the presence of the cloud in the calculations and “clear” mean clear sky conditions, that is no cloud in the calculations.
The downward and upward fluxes, heating rates, and cloud forcing profiles obtained for each case of cirrus measurements are instantaneous values corresponding to cosine of solar zenith angle of 0,89264. This number is the annual mean value of the cosine of solar zenith angle at the noon in our site.
The average value of surface albedo obtained from actinometric measurements at CLS site [17], 0,1212 ± 0,0152 was used in calculations.
3. Results and discussion
Figure 2 shows two heating rates profiles calculated from one cirrus cloud measurement case, when the assumptions of homogeneous and inhomogeneous cirrus are considered. Homogeneous clouds means the value of the cloud characteristics are constant in the vertical. Meanwhile, inhomogeneous cloud means that the characteristics of the cirrus clouds change with the altitude. The cirrus cloud case showed in Fig. 2 has an optical and geometrical depth of 2,75 and 5 km respectively, between 10 km and 15 km of altitude.
Fig. 2. Comparison of the Heating Rates profile considering homogeneous and inhomogeneous cirrus in
calculation.
Thus, the vertical structure of the cirrus optical properties has a significant effect on the vertical distribution of the solar radiative fluxes. Smaller and thus more reflective ice particles near the cloud top and high altitude cirrus lead to larger reflectivities, smaller transmissivities and slightly larger absorptivities.
The cloud heterogeneity plays an important role in determining heating rate distributions in the clouds. We can see that generally we have a solar heating in the cirrus clouds, but it is more significant in the upper part of the cloud. The solar radiation is reflected in the upper part of the cloud, where there are the smallest crystal ice particles in the clouds, and high concentrations of ice crystal. Because the behaviour of the heating rate out of the cloud boundaries is similar for both considerations, it is depth and the generalized effective size, cause differences in the flux values in the calculations.
The case showed is the largest vertical extended cirrus in our dataset. Another cirrus cases show a similar behaviour but its values and variability are much smaller due to small optical thickness.
Figure 3 display the shortwave radiative forcing of cirrus clouds in the broadband and different bands (infrared, visible, and ultraviolet) of the solar spectrum at the top of the atmosphere (TOA) and surface (SFC), in the CLS site. These values of radiative forcing are instantaneous values corresponding to cosine of solar zenith angle of 0.89264, near noon, as mentioned previously. The sign of solar radiative forcing is negative both at TOA and SFC. These indicate that the cirrus clouds cool both locations, in the solar radiation region of the spectrum. That is, the outgoing shortwave radiation in the presence of cirrus clouds is larger
than in the clear sky conditions. Albedo effect exceeds the greenhouse effects in solar radiation due to abundance of the small crystals in the upper part and high altitude cirrus clouds.
The values of the radiative forcing range from -0.1 W/m2 to around -100 W/m2, in the broadband case.
Only 10 % of the sample has values greater than 10 W/m2. Cirrus cloud forcing varies according to
different optical depths, it increases as the cirrus optical depth raise in both place, TOA and SFC. We find a close linear correlation between negative cirrus cloud forcing and cirrus optical depth (anticorrelation). The correlation coefficients are in the broadband 0,99 in both locations at the TOA and SFC.
Cirrus radiative forcing in the three spectral bands has similar behaviour that in the broadband. The main effect takes place on the infrared wavelength interval of the solar spectrum. This behaviour corresponds with the theoretical calculations reported in the literature [8].
A previous work [11], characterized our dataset. It is composed of the three cirrus cloud types, Opaque (τ>0,3), thin (0,03<τ<0.3) and subvisual (τ<0.03), with mean optical depth of 0,498, 0,074 and 0,016, respectively. But because the conditions in which the measurements were conducted it is more representative of fine cirrus cloud. That is it is mainly composed of thin and subvisual cirrus clouds. Mean values of the instantaneous cirrus radiative forcing at noon have values of −24,7 W/m2, −2,9 W/m2 and −0,5 W/m2, for opaque, thin
and subvisual cirrus clouds respectively. The values of cirrus radiative forcing in TOA and SFC are very close.
Khvorostyanov and Sassen [18] report values of cirrus radiative forcing in solar spectrum for the opaque cirrus clouds in the interval from −40 W/m2
to −120 W/m2 near noon, at TOA and SFC
respectively. Also in the case of thin cirrus clouds, the behaviour is similar to the opaque clouds regarding to the sign of the radiative forcing, but its values are much smaller due to the small optical thickness of the thin cirrus clouds. The values of the instantaneous forcing of thin cirrus are −4 W/m2 and −1 W/m2, at SFC and TOA, respectively, near noon.
In comparison with the values mentioned previously for opaque clouds in our dataset we have mean value of −24,7 W/m2 and −106,8 for maximum value. Also
for the thin cirrus clouds the value mentioned previously for the mean value −2,9 W/m2. The
Fig. 3. The solar radiative forcing of cirrus clouds are plotted versus optical depth for the whole cirrus clouds dataset. Different regions of the solar spectrum are shown as well as the broadband. Cosine of solar zenith angle used in calculations
was 0,89264.
The geometrical properties and the temperature of the base and top of the cirrus clouds reported in the cited work, show similar characteristics for the three cirrus clouds types that we have studied at the CLS [11].
The behaviour of the midlatitude cirrus clouds radiative forcing in the solar spectrum for the single case of the opaque clouds is similar to the values reported in the present study, for opaque cirrus clouds.
A second work reported the behaviour of the instantaneous solar radiative forcing of the cirrus cloud during the Central Equatorial Pacific Experiment (CEPEX) [19]. Dataset is composed principally by subvisual cirrus clouds, although there are some cases of thin cirrus clouds. The averages characteristics for the dataset obtained with lidar were 0,004 to the visible optical depth, and altitude of near 14 km. For particular cases analyzed by the authors [19] the optical depth values range from 0,0002 to 0,15, with a mean value of 0,0036. This data have similar features than the thin and subvisual cirrus clouds in our dataset.
McFarquhar et al [19] report mean value of instantaneous radiative forcing in the solar radiation region at TOA of −0,24 W/m2, for a case of
subvisual cirrus measured. The range of the radiative forcing values for this case was between about −0,1 W/m2 and −1,5 W/m2. Also is reported the results to
the some cases analyzed, the mean of instantaneous values of the cloud radiative forcing in solar channels is −0,61 W/m2. In comparison, our results
for radiative forcing of subvisual cirrus clouds mentioned previously is −0,5 W/m2 as mean value.
The results discussed in the present work conducted at the CLS site for thin and subvisual type of cirrus clouds are in agreement with the results reported for the tropical cirrus clouds cases in CEPEX experiment.
4. Conclusions
The behaviour of the cloud radiative forcing at CLS is in agreement in general with the cases reported in the literature for the different types of cirrus clouds. Opaque cirrus clouds radiative forcing values in CLS are in concordance with two reported cases of opaque midlatitude cirrus clouds. Thin and subvisual cirrus clouds radiative forcing are in agreement with thin and subvisual tropical cirrus clouds radiative forcing reported in the literature.
Due to the limited amount of data and bias in the dataset, the preliminary results are predominantly representative to the thin and subvisual cirrus clouds.
Acknowledgements
This work is supported by the Cuban Program of Climate Change and Environment under grant No. 01303177.
Authors want to express it deep gratitude to Prof. V. Ramaswamy and S. M. Freidenreich for providing the radiative code and their advice during the code implementation for PC.
