ALBERT

Beschreibung: A recent field campaign was conducted to measure the properties of thin, warm convective clouds forming under conditions of weak updrafts. During the campaign, short-lived clouds (on the order of minutes) with droplets' effective radius of 1–2 μm and low liquid water path (~ 500 mg m–2) were measured. These low values are puzzling, since in most studies an effective radius of 4 μm is reported to serve as the lower bound for clouds. A theoretical cloud model designed to resolve the droplet-activation process suggested conditions that favor the formation of such clouds. Here we show that these clouds, which mark the transition from haze to cloud, are highly sensitive to the magnitude of the initial perturbation that initiated them. We define these clouds as "transition-zone clouds". The existence of such clouds poses a key challenge for the analysis of atmospheric observations and models, since they "further smooth" the transition from dry aerosol through haze pockets to cumulus clouds.

Beschreibung: An air parcel model was developed to study the formation of small convective clouds that appear under conditions of weak updraft and a strong thermal inversion layer above the clouds. Observations suggest that these clouds are characterized by a cloud base height far lower than the lifting condensation level. Considering such atmospheric conditions, the air parcel model shows that these clouds cannot be the result of classical thermals or plumes that are caused by perturbations in the temperature near the surface. We suggest that such clouds are the result of perturbations in the relative humidity of elevated air pockets. These results explain the existence of small clouds that standard methods fail to predict and shed light on processes related to the formation of convective clouds from the lowest end of the size distribution.

Beschreibung: A recent field campaign was conducted to measure the properties of thin, warm, convective clouds forming under conditions of weak updrafts. During the campaign, short-lived clouds (on the order of minutes) with droplets' effective radius of 1–2 μm and low liquid water path (~500 mg m−2) were measured. These low values are puzzling, since in most studies an effective radius of 4 μm is reported to serve as the lower bound for clouds. A theoretical cloud model designed to resolve the droplet-activation process suggested conditions that favor the formation of such clouds. Here we show that these clouds, which mark the transition from haze to cloud, are highly sensitive to the magnitude of the initial perturbation that initiated them. We define these clouds as "transition-zone clouds". The existence of such clouds poses a key challenge for the analysis of atmospheric observations and models, since they "further smooth" the transition from dry aerosol through haze pockets to cumulus clouds.

Beschreibung: Clouds play a critical role in the Earth's radiative budget as they modulate the atmosphere by reflecting shortwave solar radiation and absorbing long wave IR radiation emitted by the Earth's surface. Although extensively studied for decades, cloud modelling in global circulation models is far from adequate, mostly due to insufficient spatial resolution of the circulation models. In addition, measurements of cloud properties still need improvement, since the vast majority of remote sensing techniques are focused in relatively large, thick clouds. In this study, we utilize ground based hyperspectral measurements and analysis to explore very thin water clouds. These clouds are characterized by liquid water path (LWP) that spans from as high as ~50 g m−2 and down to 65 mg m−2 with a minimum of about 0.01 visible optical depth. The retrieval methodology relies on three elements: a detailed radiative transfer calculations in the longwave IR regime, signal enhancement by subtraction of a clear sky reference, and spectral matching method which exploits fine spectral differences between water droplets of different radii. A detailed description of the theoretical basis for the retrieval technique is provided along with a comprehensive discussion regarding its limitations. The proposed methodology was validated in a controlled experiment where artificial clouds were sprayed and their effective radii were both measured and retrieved simultaneously. This methodology can be used in several ways: (1) the frequency and optical properties of very thin water clouds can be studied more precisely in order to evaluate their total radiative forcing on the Earth's radiation budget. (2) The unique optical properties of the inter-region between clouds (clouds' "twilight zone") can be studied in order to more rigorously understanding of the governing physical processes which dominate this region. (3) Since the optical thickness of a developed cloud gradually decreases towards its edges, the proposed methodology can be used to study the spatial microphysical behaviour of these edges. (4) A spatial-temporal analysis can be used to study mixing processes in clouds' entrainment zone.

Beschreibung: The height of a cloud in the atmospheric column is a key parameter in its characterization. Several remote sensing techniques (passive and active, either ground-based or on space-borne platforms) and in-situ measurements are routinely used in order to estimate top and base heights of clouds. In this article we present a novel method that combines thermal imaging from the ground and sounded wind profile in order to derive the cloud base height. This method is independent of cloud types, making it efficient for both low boundary layer and high clouds. In addition, using thermal imaging ensures extraction of clouds' features during daytime as well as at nighttime. The proposed technique was validated by comparison to active sounding by ceilometers (which is a standard ground based method), to lifted condensation level (LCL) calculations, and to MODIS products obtained from space. As all passive remote sensing techniques, the proposed method extracts only the height of the lowest cloud layer, thus upper cloud layers are not detected. Nevertheless, the information derived from this method can be complementary to space-borne cloud top measurements when deep-convective clouds are present. Unlike techniques such as LCL, this method is not limited to boundary layer clouds, and can extract the cloud base height at any level, as long as sufficient thermal contrast exists between the radiative temperatures of the cloud and its surrounding air parcel. Another advantage of the proposed method is its simplicity and modest power needs, making it particularly suitable for field measurements and deployment at remote locations. Our method can be further simplified for use with visible CCD or CMOS camera (although nighttime clouds will not be observed).

Beschreibung: Clouds play a critical role in the Earth's radiative budget as they modulate the atmosphere by reflecting shortwave solar radiation and absorbing long wave IR radiation emitted by the Earth's surface. Although extensively studied for decades, cloud modelling in global circulation models is far from adequate, mostly due to insufficient spatial resolution of the circulation models. In addition, measurements of cloud properties still need improvement, since the vast majority of remote sensing techniques are focused in relatively large, thick clouds. In this study, we utilize ground based hyperspectral measurements and analysis to explore very thin water clouds. These clouds are characterized by liquid water path (LWP) that spans from as high as ~50g m−2 and down to 65 mg m−2 with a minimum of about 0.01 visible optical depth. The retrieval methodology relies on three elements: a detailed radiative transfer calculations in the longwave IR regime, signal enhancement by subtraction of a clear sky reference, and spectral matching method which exploits fine spectral differences between water droplets of different radii. A detailed description of the theoretical basis for the retrieval technique is provided along with a comprehensive discussion regarding its limitations. The proposed methodology was validated in a controlled experiment where artificial clouds were sprayed and their effective radii were both measured and retrieved simultaneously. This methodology can be used in several ways: (1) the frequency and optical properties of very thin water clouds can be studied more precisely in order to evaluate their total radiative forcing on the Earth's radiation budget. (2) The unique optical properties of the inter-region between clouds (clouds' "twilight zone") can be studied in order to more rigorously understanding of the governing physical processes which dominate this region. (3) Since the optical thickness of a developed cloud gradually decreases towards its edges, the proposed methodology can be used to study the spatial microphysical behaviour of these edges. (4) A spatial-temporal analysis can be used to study mixing processes in clouds' entrainment zone.

Beschreibung: The height of a cloud in the atmospheric column is a key parameter in its characterization. Several remote sensing techniques (passive and active, either ground-based or on space-borne platforms) and in-situ measurements are routinely used in order to estimate top and base heights of clouds. In this article we present a novel method that combines thermal imaging from the ground and sounded wind profile in order to derive the cloud base height. This method is independent of cloud types, making it efficient for both low boundary layer and high clouds. In addition, using thermal imaging ensures extraction of clouds' features during daytime as well as at nighttime. The proposed technique was validated by comparison to active sounding by ceilometers (which is a standard ground based method), to lifted condensation level (LCL) calculations, and to MODIS products obtained from space. As all passive remote sensing techniques, the proposed method extracts only the height of the lowest cloud layer, thus upper cloud layers are not detected. Nevertheless, the information derived from this method can be complementary to space-borne cloud top measurements when deep-convective clouds are present. Unlike techniques such as LCL, this method is not limited to boundary layer clouds, and can extract the cloud base height at any level, as long as sufficient thermal contrast exists between the radiative temperatures of the cloud and its surrounding air parcel. Another advantage of the proposed method is its simplicity and modest power needs, making it particularly suitable for field measurements and deployment at remote locations. Our method can be further simplified for use with visible CCD or CMOS camera (although nighttime clouds will not be observed).