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- Author or Editor: Toby N. Carlson x
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Abstract
A one-dimensional numerical model, capable of simulating surface temperature and heat flux, is described in terms of the effective atmospheric and terrain variables. The two model parameters which are most responsible for the formation of important temperature variations in the horizontal over the urban-rural complex are the thermal inertia (thermal property) and moisture availability, the former being most responsible for shaping the nighttime temperature pattern while the latter has a greater effect during the day.
The controlling substrate variables are not easily determinable by direct measurement over a surface consisting of an inhomogeneous agglomerate of elements. We present one method whereby surface temperature, a more readily obtainable quantity, can be used in conjunction with the surface model to determine by numerical or graphical inversion of the latter the effective values of moisture availability and thermal inertia and thereby provide a quantitative framework for analysis of a rough surface and for an evaluation of the surface energy budget.
Abstract
A one-dimensional numerical model, capable of simulating surface temperature and heat flux, is described in terms of the effective atmospheric and terrain variables. The two model parameters which are most responsible for the formation of important temperature variations in the horizontal over the urban-rural complex are the thermal inertia (thermal property) and moisture availability, the former being most responsible for shaping the nighttime temperature pattern while the latter has a greater effect during the day.
The controlling substrate variables are not easily determinable by direct measurement over a surface consisting of an inhomogeneous agglomerate of elements. We present one method whereby surface temperature, a more readily obtainable quantity, can be used in conjunction with the surface model to determine by numerical or graphical inversion of the latter the effective values of moisture availability and thermal inertia and thereby provide a quantitative framework for analysis of a rough surface and for an evaluation of the surface energy budget.
Abstract
The intense and prolonged heating of air passing over the Sahara during the summer and early fall months forms a deep mixed layer which extends up to 15–20,000 ft during July, the warmest month. The dust-laden heated air emerges from West Africa as a series of large-scale anticyclonic eddies which move westward over the tropical Atlantic above the trade-wind moist layer, principally in the layer between 5000 and 15,000 ft (600–800 mb). Measurements made during BOMEX show that this Saharan air is characterized by high values of potential temperature, dust and radon-222 which confirm a desert origin. As the parcels of air within the layer proceed across the Atlantic the continuous fallout of particulate matter and the mixing at the base of the layer cause dust to be transferred to the lower levels where its concentration may become sufficiently great to produce dense haze at the surface over wide areas over the Atlantic and Caribbean in the latitude belt 10–25N. Nevertheless, measurements indicate that the dust concentration and associated haziness are greater at 10,000 ft than at the surface.
The presence of Saharan air over the Caribbean can be recognized on conventional meteorological soundings as a virtually isentropic layer within which the potential temperature is about 40C; the mixing ratio within this layer generally remains fairly constant with height with typical mean values of 2–4 gm kg−1. The upper surface of the Saharan air layer, clearly visible from above as a sharply defined haze top, coincides with an inversion, above which the mixing ratio decreases rapidly with height. The lower portion of the isentropic Saharan air layer may be as much as 5–6C warmer than the normal tropical atmosphere; consequently, there is a strong suppressive inversion above the moist trade-wind layer. There is also a sharp horizontal temperature gradient between the Saharan dust plume and the normal tropical air mass; aircraft penetrations into Saharan air at heights of 700–800 mb show that the discontinuity between Saharan and non-Saharan air is front-like in character inasmuch as the potential temperature and mixing ratio may change by several degrees Celsius and several grams per kilogram, respectively, over a distance of just a few kilometers. Because of the steep (adiabatic) lapse rate in the Saharan air, the positive temperature anomaly diminishes rapidly with height and tends to vanish near 650 mb, above which the dusty air may be slightly cooler than the normal tropical environment. Associated with this large-scale temperature contrast is a wind maximum of up to 40–50 kt in the Saharan air layer, usually between 600 and 700 mb.
The westward speed of the Saharan air mass is usually about 15 kt, requiring about 5–6 days to cross the Atlantic. The leading edge of the Saharan air is often found immediately to the rear (east) of a large-amplitude African disturbance which also migrates from Africa to the Caribbean during the summer months at about the same forward speed. Normally the strongest winds and highest dust concentrations in the Saharan air are found in the southeasterly winds behind the disturbance and are therefore associated with the so-called “surges in the trades” which are often observed in the tropical Atlantic.
Saharan air pulses tend to leave the continent of Africa with a potential temperature of 43–44C, about 3–4C higher than that found in the Saharan layer over the Caribbean. This apparent cooling is due to net radiation losses within the Saharan air which amount to about 0.7C per day. At the same time the Saharan air sinks by 50–100 mb (a mean descending motion of 1–2 mm sec−1) between Africa and the Caribbean.
A model is proposed which depicts the movement of Saharan air from Africa to the Caribbean and its interaction with African disturbances. Although it is not known what effect, if any, the dust plume has upon the growth or suppression of disturbances, it is clear that the warmth of the Saharan air has a strong suppressive influence on cumulus convection and that, as a result, the advancing dust pulse is often marked by rapid clearing behind the disturbances.
Abstract
The intense and prolonged heating of air passing over the Sahara during the summer and early fall months forms a deep mixed layer which extends up to 15–20,000 ft during July, the warmest month. The dust-laden heated air emerges from West Africa as a series of large-scale anticyclonic eddies which move westward over the tropical Atlantic above the trade-wind moist layer, principally in the layer between 5000 and 15,000 ft (600–800 mb). Measurements made during BOMEX show that this Saharan air is characterized by high values of potential temperature, dust and radon-222 which confirm a desert origin. As the parcels of air within the layer proceed across the Atlantic the continuous fallout of particulate matter and the mixing at the base of the layer cause dust to be transferred to the lower levels where its concentration may become sufficiently great to produce dense haze at the surface over wide areas over the Atlantic and Caribbean in the latitude belt 10–25N. Nevertheless, measurements indicate that the dust concentration and associated haziness are greater at 10,000 ft than at the surface.
The presence of Saharan air over the Caribbean can be recognized on conventional meteorological soundings as a virtually isentropic layer within which the potential temperature is about 40C; the mixing ratio within this layer generally remains fairly constant with height with typical mean values of 2–4 gm kg−1. The upper surface of the Saharan air layer, clearly visible from above as a sharply defined haze top, coincides with an inversion, above which the mixing ratio decreases rapidly with height. The lower portion of the isentropic Saharan air layer may be as much as 5–6C warmer than the normal tropical atmosphere; consequently, there is a strong suppressive inversion above the moist trade-wind layer. There is also a sharp horizontal temperature gradient between the Saharan dust plume and the normal tropical air mass; aircraft penetrations into Saharan air at heights of 700–800 mb show that the discontinuity between Saharan and non-Saharan air is front-like in character inasmuch as the potential temperature and mixing ratio may change by several degrees Celsius and several grams per kilogram, respectively, over a distance of just a few kilometers. Because of the steep (adiabatic) lapse rate in the Saharan air, the positive temperature anomaly diminishes rapidly with height and tends to vanish near 650 mb, above which the dusty air may be slightly cooler than the normal tropical environment. Associated with this large-scale temperature contrast is a wind maximum of up to 40–50 kt in the Saharan air layer, usually between 600 and 700 mb.
The westward speed of the Saharan air mass is usually about 15 kt, requiring about 5–6 days to cross the Atlantic. The leading edge of the Saharan air is often found immediately to the rear (east) of a large-amplitude African disturbance which also migrates from Africa to the Caribbean during the summer months at about the same forward speed. Normally the strongest winds and highest dust concentrations in the Saharan air are found in the southeasterly winds behind the disturbance and are therefore associated with the so-called “surges in the trades” which are often observed in the tropical Atlantic.
Saharan air pulses tend to leave the continent of Africa with a potential temperature of 43–44C, about 3–4C higher than that found in the Saharan layer over the Caribbean. This apparent cooling is due to net radiation losses within the Saharan air which amount to about 0.7C per day. At the same time the Saharan air sinks by 50–100 mb (a mean descending motion of 1–2 mm sec−1) between Africa and the Caribbean.
A model is proposed which depicts the movement of Saharan air from Africa to the Caribbean and its interaction with African disturbances. Although it is not known what effect, if any, the dust plume has upon the growth or suppression of disturbances, it is clear that the warmth of the Saharan air has a strong suppressive influence on cumulus convection and that, as a result, the advancing dust pulse is often marked by rapid clearing behind the disturbances.
Abstract
A near-linear relationship is described between aerosol optical depth for Saharan dust over the eastern tropical Atlantic Ocean and radiance measured aboard the NOAA 3 satellite by the VHRR.
Abstract
A near-linear relationship is described between aerosol optical depth for Saharan dust over the eastern tropical Atlantic Ocean and radiance measured aboard the NOAA 3 satellite by the VHRR.
Abstract
This study outlines a method for the estimation of regional patterns of surface moisture availability (M 0) and fractional vegetation (Fr) in the presence of spatially variable vegetation cover. The method requires relating variations in satellite-derived (NOAA, Advanced Very High Resolution Radiometer) surface radiant temperature to a vegetation index (computed from satellite visible and near-infrared data) while coupling this association to an inverse modeling scheme. More than merely furnishing surface soil moisture values, the method constitutes a new conceptual and practical approach for combining thermal infrared and vegetation index measurements for incorporating the derived values of M 0 into hydrologic and atmospheric prediction models.
Application of the technique is demonstrated for a region in and around the city of Newcastle upon Tyne situated in the northeast of England. A regional estimate of M 0 is derived and is probably good for fractional vegetation cover up to 80% before errors in the estimated soil water content become unacceptably large. Moreover, a normalization scheme is suggested from which a nomogram, “universal triangle,” is constructed and is seen to fit the observed data well. The universal triangle also simplifies the inclusion of remotely derived M 0 in hydrology and meteorological models and is perhaps a practicable step toward integrating derived data from satellite measurements in weather forecasting.
Abstract
This study outlines a method for the estimation of regional patterns of surface moisture availability (M 0) and fractional vegetation (Fr) in the presence of spatially variable vegetation cover. The method requires relating variations in satellite-derived (NOAA, Advanced Very High Resolution Radiometer) surface radiant temperature to a vegetation index (computed from satellite visible and near-infrared data) while coupling this association to an inverse modeling scheme. More than merely furnishing surface soil moisture values, the method constitutes a new conceptual and practical approach for combining thermal infrared and vegetation index measurements for incorporating the derived values of M 0 into hydrologic and atmospheric prediction models.
Application of the technique is demonstrated for a region in and around the city of Newcastle upon Tyne situated in the northeast of England. A regional estimate of M 0 is derived and is probably good for fractional vegetation cover up to 80% before errors in the estimated soil water content become unacceptably large. Moreover, a normalization scheme is suggested from which a nomogram, “universal triangle,” is constructed and is seen to fit the observed data well. The universal triangle also simplifies the inclusion of remotely derived M 0 in hydrology and meteorological models and is perhaps a practicable step toward integrating derived data from satellite measurements in weather forecasting.
Abstract
A method for inferring the distribution of surface heat and evaporative fluxes and the ground moisture availability and thermal inertia (ground conductive capacity) is used to analyze two urbanized areas, Los Angeles and St. Louis. The technique employs infrared satellite temperature measurements in conjunction with a one-dimensional boundary-layer model. Results show that there is a marked reduction of evaporation and moisture availability and a corresponding elevation of sensible heat flux over urbanized areas and over cropped areas with low vegetative cover. Conversely, low heat flux and high evaporation characterize vegetated and, especially, forested areas. Warm urban centers appear directly related to a reduction in vegetation, which normally allows for a greater fraction of available radiant energy to be converted into latent heat flux. The distribution of thermal inertia was surprisingly ill-defined and its variation between urban and rural areas was quite small. Thus, the increased heat storage within the urban fabric, which has been proposed as the underlying cause of the nocturnal heat island, may be caused mainly by enhanced daytime surface heating which occurs because of surface dryness, rather than by large spatial variations in the conductivity of the surface.
Abstract
A method for inferring the distribution of surface heat and evaporative fluxes and the ground moisture availability and thermal inertia (ground conductive capacity) is used to analyze two urbanized areas, Los Angeles and St. Louis. The technique employs infrared satellite temperature measurements in conjunction with a one-dimensional boundary-layer model. Results show that there is a marked reduction of evaporation and moisture availability and a corresponding elevation of sensible heat flux over urbanized areas and over cropped areas with low vegetative cover. Conversely, low heat flux and high evaporation characterize vegetated and, especially, forested areas. Warm urban centers appear directly related to a reduction in vegetation, which normally allows for a greater fraction of available radiant energy to be converted into latent heat flux. The distribution of thermal inertia was surprisingly ill-defined and its variation between urban and rural areas was quite small. Thus, the increased heat storage within the urban fabric, which has been proposed as the underlying cause of the nocturnal heat island, may be caused mainly by enhanced daytime surface heating which occurs because of surface dryness, rather than by large spatial variations in the conductivity of the surface.
Abstract
A new approach to simulating the urban environment with a mesocale model has been developed to identify efficient strategies for mitigating increases in surface air temperatures associated with the urban heat island (UHI). A key step in this process is to define a “global” roughness for the cityscape and to use this roughness to diagnose 10-m temperature, moisture, and winds within an atmospheric model. This information is used to calculate local exchange coefficients for different city surface types (each with their own “local roughness” lengths); each surface’s energy balances, including surface air temperatures, humidity, and wind, are then readily obtained. The model was run for several summer days in 2001 for the New York City five-county area. The most effective strategy to reduce the surface radiometric and 2-m surface air temperatures was to increase the albedo of the city (impervious) surfaces. However, this caused increased thermal stress at street level, especially noontime thermal stress. As an alternative, the planting of trees reduced the UHI’s adverse effects of high temperatures and also reduced noontime thermal stress on city residents (and would also have reduced cooling energy requirements of small structures). Taking these results together, the analysis suggests that the best mitigation strategy is planting trees at street level and increasing the reflectivity of roofs.
Abstract
A new approach to simulating the urban environment with a mesocale model has been developed to identify efficient strategies for mitigating increases in surface air temperatures associated with the urban heat island (UHI). A key step in this process is to define a “global” roughness for the cityscape and to use this roughness to diagnose 10-m temperature, moisture, and winds within an atmospheric model. This information is used to calculate local exchange coefficients for different city surface types (each with their own “local roughness” lengths); each surface’s energy balances, including surface air temperatures, humidity, and wind, are then readily obtained. The model was run for several summer days in 2001 for the New York City five-county area. The most effective strategy to reduce the surface radiometric and 2-m surface air temperatures was to increase the albedo of the city (impervious) surfaces. However, this caused increased thermal stress at street level, especially noontime thermal stress. As an alternative, the planting of trees reduced the UHI’s adverse effects of high temperatures and also reduced noontime thermal stress on city residents (and would also have reduced cooling energy requirements of small structures). Taking these results together, the analysis suggests that the best mitigation strategy is planting trees at street level and increasing the reflectivity of roofs.
