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- Author or Editor: Johannes Schmetz x
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Abstract
Subdividing the Indian Ocean domain into three areas: (i) a moist cloudy area due to tropical deep convection, (ii) a moist clear area fed by the evaporation of hydrometeors from adjacent high clouds, and (iii) a dry area represented by descending air over the subtropics, the relationships between upper-tropospheric humidity over these three areas and tropical convections are examined using the European Geostationary Meteorological Satellite (Meteosat-5) observations. It is observed that the clear dry area shrinks and becomes drier in response to expansion of the cloudy area in the Tropics and vice versa. This change in upper-tropospheric humidity over the subtropics appears to mitigate the increase (decrease) in water vapor greenhouse effect caused by the expansion (contraction) of moist convective areas.
A simple sensitivity test shows that the strength of the water vapor feedback due to changes in the spatial extent of tropical convection is benign, though slightly negative, if the changes in subtropical dryness are considered.
Abstract
Subdividing the Indian Ocean domain into three areas: (i) a moist cloudy area due to tropical deep convection, (ii) a moist clear area fed by the evaporation of hydrometeors from adjacent high clouds, and (iii) a dry area represented by descending air over the subtropics, the relationships between upper-tropospheric humidity over these three areas and tropical convections are examined using the European Geostationary Meteorological Satellite (Meteosat-5) observations. It is observed that the clear dry area shrinks and becomes drier in response to expansion of the cloudy area in the Tropics and vice versa. This change in upper-tropospheric humidity over the subtropics appears to mitigate the increase (decrease) in water vapor greenhouse effect caused by the expansion (contraction) of moist convective areas.
A simple sensitivity test shows that the strength of the water vapor feedback due to changes in the spatial extent of tropical convection is benign, though slightly negative, if the changes in subtropical dryness are considered.
Abstract
Outgoing longwave radiative fluxes (OLR) and the longwave cloud-radiative forcing at the atmosphere are retrieved from METEOSAT radiance observations in the thermal infrared window (IR: 10.5–12.5 μm) and water vapor (WV: 5.7–7.1 μm) channels for April 1985. The analysis exploits an operationally preprocessed radiance dataset that includes a scene identification of clear sky, low level, medium level and high level clouds. Monthly means of the OLR and the longwave cloud-radiative forcing are inferred for areas of about 200 km × 200 km. Extended regions with a forcing larger than 60 W m−2 are found within the intertropical convergence zone (ITCZ) over southern Sudan and around 5°S over Brazil and the adjacent Atlantic Ocean.
The contribution of three levels of cloud to the longwave radiative forcing is estimated: high level coulds (≤400 hPa) contribute about 80% to the total longwave forcing in regions with strong convective activity (ITCZ). Medium level coulds (700 ≤ cloud top < 400 hPa) induce a maximum forcing of 15–20 W m−2 over the Ethiopian highland, while low level cloud forcing reaches values of 5–10 W m−2 over the marine stratocumulus regions and within the midlatitude westerlies.
Systematic errors in the longwave cloud-radiative forcing due to calibration errors, cloud contamination of clear sky radiances and a dry bias in the humidity of the upper troposphere, which may occur as a result of minimizing the cloud contamination, are discussed; it is concluded that the present study underestimates maximum values of the longwave cloud-radiative forcing by about 10 W m−2.
Abstract
Outgoing longwave radiative fluxes (OLR) and the longwave cloud-radiative forcing at the atmosphere are retrieved from METEOSAT radiance observations in the thermal infrared window (IR: 10.5–12.5 μm) and water vapor (WV: 5.7–7.1 μm) channels for April 1985. The analysis exploits an operationally preprocessed radiance dataset that includes a scene identification of clear sky, low level, medium level and high level clouds. Monthly means of the OLR and the longwave cloud-radiative forcing are inferred for areas of about 200 km × 200 km. Extended regions with a forcing larger than 60 W m−2 are found within the intertropical convergence zone (ITCZ) over southern Sudan and around 5°S over Brazil and the adjacent Atlantic Ocean.
The contribution of three levels of cloud to the longwave radiative forcing is estimated: high level coulds (≤400 hPa) contribute about 80% to the total longwave forcing in regions with strong convective activity (ITCZ). Medium level coulds (700 ≤ cloud top < 400 hPa) induce a maximum forcing of 15–20 W m−2 over the Ethiopian highland, while low level cloud forcing reaches values of 5–10 W m−2 over the marine stratocumulus regions and within the midlatitude westerlies.
Systematic errors in the longwave cloud-radiative forcing due to calibration errors, cloud contamination of clear sky radiances and a dry bias in the humidity of the upper troposphere, which may occur as a result of minimizing the cloud contamination, are discussed; it is concluded that the present study underestimates maximum values of the longwave cloud-radiative forcing by about 10 W m−2.
Abstract
In this paper, the amount of satellite-derived longwave cloud radiative forcing (CRF) that is due to an increase in upper-tropospheric water vapor associated with the evolution from clear-sky to the observed all-sky conditions is assessed. This is important because the satellite-derived clear-sky outgoing radiative fluxes needed for the CRF determination are from cloud-free areas away from the cloudy regions in order to avoid cloud contamination of the clear-sky fluxes. However, avoidance of cloud contamination implies a sampling problem as the clear-sky fluxes represent an area drier than the hypothetical clear-sky humidity in cloudy regions. While this issue has been recognized in earlier works this study makes an attempt to quantitatively estimate the bias in the clear-sky longwave CRF. Water vapor amounts in the 200–500-mb layer corresponding to all-sky condition are derived from microwave measurements with the Special Sensor Microwave Temperature-2 Profiler and are used in combination with cloud data for determining the clear-sky water vapor distribution of that layer. The obtained water vapor information is then used to constrain the humidity profiles for calculating clear-sky longwave fluxes at the top of the atmosphere. It is shown that the clear-sky moisture bias in the upper troposphere can be up to 40%–50% drier over convectively active regions. Results indicate that up to 12 W m−2 corresponding to about 15% of the satellite-derived longwave CRF in tropical regions can be attributed to the water vapor changes associated with cloud development.
Abstract
In this paper, the amount of satellite-derived longwave cloud radiative forcing (CRF) that is due to an increase in upper-tropospheric water vapor associated with the evolution from clear-sky to the observed all-sky conditions is assessed. This is important because the satellite-derived clear-sky outgoing radiative fluxes needed for the CRF determination are from cloud-free areas away from the cloudy regions in order to avoid cloud contamination of the clear-sky fluxes. However, avoidance of cloud contamination implies a sampling problem as the clear-sky fluxes represent an area drier than the hypothetical clear-sky humidity in cloudy regions. While this issue has been recognized in earlier works this study makes an attempt to quantitatively estimate the bias in the clear-sky longwave CRF. Water vapor amounts in the 200–500-mb layer corresponding to all-sky condition are derived from microwave measurements with the Special Sensor Microwave Temperature-2 Profiler and are used in combination with cloud data for determining the clear-sky water vapor distribution of that layer. The obtained water vapor information is then used to constrain the humidity profiles for calculating clear-sky longwave fluxes at the top of the atmosphere. It is shown that the clear-sky moisture bias in the upper troposphere can be up to 40%–50% drier over convectively active regions. Results indicate that up to 12 W m−2 corresponding to about 15% of the satellite-derived longwave CRF in tropical regions can be attributed to the water vapor changes associated with cloud development.
