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- Author or Editor: Reinout Boers x
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Abstract
A theory of the parameterization of the entrainment zone depth has been developed based on conservation of energy. This theory suggests that the normalized entrainment zone depth is proportional to the inverse square root of the Richardson number.
A comparison of this theory with atmospheric observations indicates excellent agreement. It does not adequately predict the laboratory data, although it improves on parcel theory, which is based on a momentum balance.
Abstract
A theory of the parameterization of the entrainment zone depth has been developed based on conservation of energy. This theory suggests that the normalized entrainment zone depth is proportional to the inverse square root of the Richardson number.
A comparison of this theory with atmospheric observations indicates excellent agreement. It does not adequately predict the laboratory data, although it improves on parcel theory, which is based on a momentum balance.
Abstract
Cloud-top entrainment instability was investigated using a mixing line analysis. Mixing time scales are closely related to the actual size of the parcel, so that local instabilities are largely dependent on the scales of mixing near the cloud top. Given a fixed transport velocity, variation over a small range of parcel length scales (parcel mixing velocities) turns an energy-producing mixing process into an energy-consuming mixing process. It is suggested that a single criterion for cloud-top entrainment instability will not be found due to the role of at least three factors operating more or less independently: the stability of the mixing line, the entrainment speed, and the strength of the internal boundary-layer circulation.
Abstract
Cloud-top entrainment instability was investigated using a mixing line analysis. Mixing time scales are closely related to the actual size of the parcel, so that local instabilities are largely dependent on the scales of mixing near the cloud top. Given a fixed transport velocity, variation over a small range of parcel length scales (parcel mixing velocities) turns an energy-producing mixing process into an energy-consuming mixing process. It is suggested that a single criterion for cloud-top entrainment instability will not be found due to the role of at least three factors operating more or less independently: the stability of the mixing line, the entrainment speed, and the strength of the internal boundary-layer circulation.
Abstract
The microstructure of a Pacific stratocumulus capped boundary layer was examined. The boundary layer flow was over an area of sea-surface temperature gradient, so that no steady state was present. Conserved variable diagrams showed a complex structure with one branch of the mixing line extending, from the mixed layer saturation point to the ocean surface saturation point, a second branch in the cloud layer and a third upward into the inversion. They correspond well with a conceptual model for the unstable, radiatively cooled cloud topped boundary layer. Saturation point pairs for ascending and descending branch of the internal boundary layer circulation were isolated by a simple conditional sampling technique. Pair separation decreased downward towards the surface, as did the variance around the mean ascent and descent values. Also, the variance was smaller for the ascending branch than for the descending branch. Spectral analysis of the saturation pressure showed that the primary circulation scale was 5 km, the same scale that was observed by lidar observations of cloud tops. A budget diagram for the time-dependent boundary layer is used to derive the time evolution of the layer and the entrainment rate using radiative flux estimates from a model incorporating cloud top heights from the lidar, and cloud liquid water and temperature from a mixing line model. A reasonable cloud top entrainment rate of 1 cm s−1 was obtained. The internal convective circulation velocity was found to be an order of magnitude higher corresponding to an internal circulation time scale of about 1 hour. The external adjustment time of the layer was 10 hours and the lag of the layer mean from equilibrium agreed with the advection over a 3°C warmer sea surface in 10 hours.
Abstract
The microstructure of a Pacific stratocumulus capped boundary layer was examined. The boundary layer flow was over an area of sea-surface temperature gradient, so that no steady state was present. Conserved variable diagrams showed a complex structure with one branch of the mixing line extending, from the mixed layer saturation point to the ocean surface saturation point, a second branch in the cloud layer and a third upward into the inversion. They correspond well with a conceptual model for the unstable, radiatively cooled cloud topped boundary layer. Saturation point pairs for ascending and descending branch of the internal boundary layer circulation were isolated by a simple conditional sampling technique. Pair separation decreased downward towards the surface, as did the variance around the mean ascent and descent values. Also, the variance was smaller for the ascending branch than for the descending branch. Spectral analysis of the saturation pressure showed that the primary circulation scale was 5 km, the same scale that was observed by lidar observations of cloud tops. A budget diagram for the time-dependent boundary layer is used to derive the time evolution of the layer and the entrainment rate using radiative flux estimates from a model incorporating cloud top heights from the lidar, and cloud liquid water and temperature from a mixing line model. A reasonable cloud top entrainment rate of 1 cm s−1 was obtained. The internal convective circulation velocity was found to be an order of magnitude higher corresponding to an internal circulation time scale of about 1 hour. The external adjustment time of the layer was 10 hours and the lag of the layer mean from equilibrium agreed with the advection over a 3°C warmer sea surface in 10 hours.
Abstract
In situ aircraft data and lidar data are used to analyze a transition in the boundary layer thermodynamic structure from a clear boundary layer through small cumulus and broken stratocumulus to a deck of solid stratocumulus. The data was collected in conjunction with a Landsat overpass on 7 July 1987 off the coast of southern California. A steady progression in mixing line stability is seen associated with the change in cloudiness. The (empirically based) stability threshold for the breakup of this stratocumulus is that the slope of the mixing line is 0.66 ± 0.04 of the slope of the wet virtual adiabat (the stability threshold for cloud-top entrainment instability). We propose a simple linear parameterization for cloud fraction in terms of mixing line stability. Surface flux measurements are consistent with bulk aerodynamic estimates. We present mean profiles for the four cloudiness regimes for further analysis.
Abstract
In situ aircraft data and lidar data are used to analyze a transition in the boundary layer thermodynamic structure from a clear boundary layer through small cumulus and broken stratocumulus to a deck of solid stratocumulus. The data was collected in conjunction with a Landsat overpass on 7 July 1987 off the coast of southern California. A steady progression in mixing line stability is seen associated with the change in cloudiness. The (empirically based) stability threshold for the breakup of this stratocumulus is that the slope of the mixing line is 0.66 ± 0.04 of the slope of the wet virtual adiabat (the stability threshold for cloud-top entrainment instability). We propose a simple linear parameterization for cloud fraction in terms of mixing line stability. Surface flux measurements are consistent with bulk aerodynamic estimates. We present mean profiles for the four cloudiness regimes for further analysis.
Abstract
Two cold-air outbreaks were studied during the Genesis of Atlantic Lows Experiment. A lidar system was operated to observe the boundary layer evolution and the development of clouds. On the first day (30 January 1986) boundary layer rise was less than 50% of the value for the second day (2 March 1986). On the first day only a thin broken cloud cover formed, while on the second day a thick solid cloud deck formed—although the average moisture content was 60% of that on the first day. A trajectory slab model was employed to simulate the evolution of the layer over the ocean near the cast Atlantic shore. The model allows for vertical gradients in conservative variables under neutrally buoyant conditions. The primary effect of these assumptions, which are based on observed thermodynamic profiles, is to reduce cloudiness to be more in line with observations. Boundary layer depth was reasonably well predicted as was sensible and latent heat flux.
Abstract
Two cold-air outbreaks were studied during the Genesis of Atlantic Lows Experiment. A lidar system was operated to observe the boundary layer evolution and the development of clouds. On the first day (30 January 1986) boundary layer rise was less than 50% of the value for the second day (2 March 1986). On the first day only a thin broken cloud cover formed, while on the second day a thick solid cloud deck formed—although the average moisture content was 60% of that on the first day. A trajectory slab model was employed to simulate the evolution of the layer over the ocean near the cast Atlantic shore. The model allows for vertical gradients in conservative variables under neutrally buoyant conditions. The primary effect of these assumptions, which are based on observed thermodynamic profiles, is to reduce cloudiness to be more in line with observations. Boundary layer depth was reasonably well predicted as was sensible and latent heat flux.
Abstract
A Nd:YAG lidar system was flown aboard NASA's ER-2 high altitude aircraft. Observations of cloud top height were made with 70 m along-track and 7.5 m vertical-height resolution. The lidar data observed from an East Pacific stratocumulus cloud height deck revealed large cloud variability on 1–5 km scales. The cloud deck sloped upward from 700 to 1000 m in a northeast-southwest direction over a distance of 120 km. Vertical cloud top distributions were negatively skewed indicating flat-topped clouds. The dominant spectral peak of the cloud top variations was found at 4.5 km, which is 5 to 7 times the depth of the local boundary layer. No other peaks were significant in the average spectrum, The cloud layer was stable with respect to cloud top entrainment instability. The southwestern region of the study area was more prone to shear instability at cloud top than the northeastern region. The results of this study show that a lidar system is ideal to provide the topography of clouds and local boundary layer depth. This information is useful in the study of cloud top radiation and parameterization of clouds in numerical models.
Abstract
A Nd:YAG lidar system was flown aboard NASA's ER-2 high altitude aircraft. Observations of cloud top height were made with 70 m along-track and 7.5 m vertical-height resolution. The lidar data observed from an East Pacific stratocumulus cloud height deck revealed large cloud variability on 1–5 km scales. The cloud deck sloped upward from 700 to 1000 m in a northeast-southwest direction over a distance of 120 km. Vertical cloud top distributions were negatively skewed indicating flat-topped clouds. The dominant spectral peak of the cloud top variations was found at 4.5 km, which is 5 to 7 times the depth of the local boundary layer. No other peaks were significant in the average spectrum, The cloud layer was stable with respect to cloud top entrainment instability. The southwestern region of the study area was more prone to shear instability at cloud top than the northeastern region. The results of this study show that a lidar system is ideal to provide the topography of clouds and local boundary layer depth. This information is useful in the study of cloud top radiation and parameterization of clouds in numerical models.
Abstract
A dataset of 9 years in duration (2009–17) of clouds and radiation was obtained at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Cloud radiative forcings (CRF) were derived from the dataset and related to cloud cover and temperature. Also, the data were compared with RCM output. Results indicate that there is a seasonal cycle (i.e., winter, spring, summer, and autumn) in longwave (CRF-LW: 48.3, 34.4, 30.8, and 38.7 W m−2) and shortwave (CRF-SW: −23.6, −60.9, −67.8, and −32.9 W m−2) forcings at CESAR. Total CRF is positive in winter and negative in summer. The RCM has a cold bias with respect to the observations, but the model CRF-LW corresponds well to the observed CRF-LW as a result of compensating errors in the difference function that makes up the CRF-LW. The absolute value of model CRF-SW is smaller than the observed CRF-SW in summer, mostly because of albedo differences. The majority of clouds from above 2 km are present at the same time as low clouds, so the higher clouds have only a small impact on CRF whereas low clouds dominate their values. CRF-LW is a function of fractional cloudiness. CRF-SW is also a function of fractional cloudiness, if the values are normalized by the cosine of solar zenith angle. Expressions for CRF-LW and CRF-SW were derived as functions of temperature, fractional cloudiness, and solar zenith angle, indicating that CRF is the largest when fractional cloudiness is the highest but is also large for low temperature and high sun angle.
Abstract
A dataset of 9 years in duration (2009–17) of clouds and radiation was obtained at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Cloud radiative forcings (CRF) were derived from the dataset and related to cloud cover and temperature. Also, the data were compared with RCM output. Results indicate that there is a seasonal cycle (i.e., winter, spring, summer, and autumn) in longwave (CRF-LW: 48.3, 34.4, 30.8, and 38.7 W m−2) and shortwave (CRF-SW: −23.6, −60.9, −67.8, and −32.9 W m−2) forcings at CESAR. Total CRF is positive in winter and negative in summer. The RCM has a cold bias with respect to the observations, but the model CRF-LW corresponds well to the observed CRF-LW as a result of compensating errors in the difference function that makes up the CRF-LW. The absolute value of model CRF-SW is smaller than the observed CRF-SW in summer, mostly because of albedo differences. The majority of clouds from above 2 km are present at the same time as low clouds, so the higher clouds have only a small impact on CRF whereas low clouds dominate their values. CRF-LW is a function of fractional cloudiness. CRF-SW is also a function of fractional cloudiness, if the values are normalized by the cosine of solar zenith angle. Expressions for CRF-LW and CRF-SW were derived as functions of temperature, fractional cloudiness, and solar zenith angle, indicating that CRF is the largest when fractional cloudiness is the highest but is also large for low temperature and high sun angle.
Abstract
In the Agile Way of Working (AoW), a group of developers jointly work to efficiently realize a project. Here we report on the application of AoW in meteorological research and development (R&D) outside of the software engineering environment. Three projects were formulated, derived from the observations strategy (2015) of the Royal Netherlands Meteorological Institute (KNMI). An initial phase of preparation consisted of breaking down the workload into tasks to be accomplished by individual project members and achievable in two one-week sprints. Sprints consisted of daily stand-ups, where accomplishments, work intentions, and obstacles were discussed, followed by project work in a joint working environment. The three projects identified were 1) flying a drone to detect boundary layer evolution, 2) monitoring the quality of the precipitation measurement system, and 3) realizing a platform for merging third-party data with meteorological observations. The preparation phase proved to be vitally important to each of the projects. The roles of the product owner and Scrum master in streamlining and guiding these projects were essential to the success of the sprint weeks, but the joint group settings worked well for only two of the three projects. While team members were positive about their experience with the AoW, the challenge remains to fuse the traditional individual work practice of researchers with that of software engineers, who are experienced in working in a group setting.
Abstract
In the Agile Way of Working (AoW), a group of developers jointly work to efficiently realize a project. Here we report on the application of AoW in meteorological research and development (R&D) outside of the software engineering environment. Three projects were formulated, derived from the observations strategy (2015) of the Royal Netherlands Meteorological Institute (KNMI). An initial phase of preparation consisted of breaking down the workload into tasks to be accomplished by individual project members and achievable in two one-week sprints. Sprints consisted of daily stand-ups, where accomplishments, work intentions, and obstacles were discussed, followed by project work in a joint working environment. The three projects identified were 1) flying a drone to detect boundary layer evolution, 2) monitoring the quality of the precipitation measurement system, and 3) realizing a platform for merging third-party data with meteorological observations. The preparation phase proved to be vitally important to each of the projects. The roles of the product owner and Scrum master in streamlining and guiding these projects were essential to the success of the sprint weeks, but the joint group settings worked well for only two of the three projects. While team members were positive about their experience with the AoW, the challenge remains to fuse the traditional individual work practice of researchers with that of software engineers, who are experienced in working in a group setting.
