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Abstract
A four-layer three-dimensional model whose lowest layer is a time and space-dependent, well-mixed boundary layer is employed over artificial, irregular terrain on the mesoscale during a daytime heating cycle. Only if the surface heating and mixed-layer entrainment am suppressed does the Row field become steady as found previously using a shallow-water model. Unsteadiness is due both to diurnal effects, especially the relaxation of the frictional force as the mixed layer deepens irregularly, and to the presence of horizontal vacations in potential temperature. The latter can develop with time due to the negative feedback between mixed-layer depth and warming rate; after the early morning hours, however, this feedback causes a damping of the temperature anomalies to much smaller values by late afternoon.
Cool-air anomalies in the mixed layer are found to develop lesser mixed-layer depths than warm anomalies, yet to be accompanied by greater “reduced” surface pressures. As a result, a thermal-anomaly form drag occurs of very significant amplitude, since the cool air pools spend most of the day moving upslope, on the average, and the warm air pockets downslope. The thermal-anomaly form-drag coefficients are typically of greater magnitude than the shallow-water form-drag coefficients associated with a mixed layer of uniform potential temperature capped by a temperature jump. However, the former can on occasion become negative. Parameterizations for both types of form drag are offered as a function of terrain heights and slopes, mixed-layer wind speed and inversion strength, and horizontal temperature variability.
Abstract
A four-layer three-dimensional model whose lowest layer is a time and space-dependent, well-mixed boundary layer is employed over artificial, irregular terrain on the mesoscale during a daytime heating cycle. Only if the surface heating and mixed-layer entrainment am suppressed does the Row field become steady as found previously using a shallow-water model. Unsteadiness is due both to diurnal effects, especially the relaxation of the frictional force as the mixed layer deepens irregularly, and to the presence of horizontal vacations in potential temperature. The latter can develop with time due to the negative feedback between mixed-layer depth and warming rate; after the early morning hours, however, this feedback causes a damping of the temperature anomalies to much smaller values by late afternoon.
Cool-air anomalies in the mixed layer are found to develop lesser mixed-layer depths than warm anomalies, yet to be accompanied by greater “reduced” surface pressures. As a result, a thermal-anomaly form drag occurs of very significant amplitude, since the cool air pools spend most of the day moving upslope, on the average, and the warm air pockets downslope. The thermal-anomaly form-drag coefficients are typically of greater magnitude than the shallow-water form-drag coefficients associated with a mixed layer of uniform potential temperature capped by a temperature jump. However, the former can on occasion become negative. Parameterizations for both types of form drag are offered as a function of terrain heights and slopes, mixed-layer wind speed and inversion strength, and horizontal temperature variability.
Abstract
Numerical integrations using a potential enstrophy conserving scheme are presented for the flow within a mixed layer over hilly terrain using the hydrostatic shallow-water equations with a quadratic drag law. The mesoscale area treated is 150 km on a side; cyclic lateral boundary conditions are used. It is found that for the idealized conditions treated (no surface heating, no entrainment and no pressure adjustments aloft), the topography quickly induces a steady state flow pattern by means of surface friction. Unsteadiness does not occur unless a surface-friction Reynolds number, R = h̄/(CDL), exceeds ∼100, where h̄h is the mean mixed-layer thickness, CD is the surface drag coefficient and L is a representative horizontal terrain length scale. Effects of varying the Rossby number, Froude number and terrain-height parameter are examined.
Abstract
Numerical integrations using a potential enstrophy conserving scheme are presented for the flow within a mixed layer over hilly terrain using the hydrostatic shallow-water equations with a quadratic drag law. The mesoscale area treated is 150 km on a side; cyclic lateral boundary conditions are used. It is found that for the idealized conditions treated (no surface heating, no entrainment and no pressure adjustments aloft), the topography quickly induces a steady state flow pattern by means of surface friction. Unsteadiness does not occur unless a surface-friction Reynolds number, R = h̄/(CDL), exceeds ∼100, where h̄h is the mean mixed-layer thickness, CD is the surface drag coefficient and L is a representative horizontal terrain length scale. Effects of varying the Rossby number, Froude number and terrain-height parameter are examined.
Abstract
The National Meteorological Center's (NMC's) twice-daily, global 2.5° pressure analyses of temperature, relative humidity, and wind speed are compared, over the coterminous United States, to the National Climatic Data Center's twice-daily, upper-air rawinsonde observations and hourly, first-order, surface observations for the period 1 January 1988 through 31 December 1992. NMCs analyses have clearly improved during this time period. Still, there are some noticeable differences especially near the surface and at 1200 UTC. During the early morning there is a warm bias, relative humidity is too low, and the surface wind speed is too strong. Weaker systematic errors occur during the late afternoon: there is a cold bias, relative humidity is too high, and the surface wind speed is still too strong. Aloft, the bias is noticeably reduced except for the wind speed, which is somewhat too weak. The analysis wind speed also has too strong temporal variations near the surface and too weak temporal variations aloft. The analysis climatology can be corrected at each station by removing the bias. Transient variations can be corrected simply by multiplying the analysis anomalies by the ratio of the station standard deviation to the analysis standard deviation. Correcting for the biases and spatially interpolating the analysis and station collections to a 0.5° grid provides a useful guess for local conditions, especially if there is not a surface or rawinsonde station within about 200 km.
Abstract
The National Meteorological Center's (NMC's) twice-daily, global 2.5° pressure analyses of temperature, relative humidity, and wind speed are compared, over the coterminous United States, to the National Climatic Data Center's twice-daily, upper-air rawinsonde observations and hourly, first-order, surface observations for the period 1 January 1988 through 31 December 1992. NMCs analyses have clearly improved during this time period. Still, there are some noticeable differences especially near the surface and at 1200 UTC. During the early morning there is a warm bias, relative humidity is too low, and the surface wind speed is too strong. Weaker systematic errors occur during the late afternoon: there is a cold bias, relative humidity is too high, and the surface wind speed is still too strong. Aloft, the bias is noticeably reduced except for the wind speed, which is somewhat too weak. The analysis wind speed also has too strong temporal variations near the surface and too weak temporal variations aloft. The analysis climatology can be corrected at each station by removing the bias. Transient variations can be corrected simply by multiplying the analysis anomalies by the ratio of the station standard deviation to the analysis standard deviation. Correcting for the biases and spatially interpolating the analysis and station collections to a 0.5° grid provides a useful guess for local conditions, especially if there is not a surface or rawinsonde station within about 200 km.
