Showing papers in "Journal of Applied Meteorology in 1982"

A High-Resolution Model of the Planetary Boundary Layer—Sensitivity Tests and Comparisons with SESAME-79 Data

Abstract: A one-dimensional, planetary boundary layer (PBL) model is presented and verified using April 10, 1979 SESAME data. The model contains two modules to account for two different regimes of turbulent mixing. Separate parameterizations are made for stable and unstable conditions, with a predictive slab model for surface temperature. Atmospheric variables in the surface layer are calculated with a prognostic model, with moisture included in the coupled surface/PBL modeling. Sensitivity tests are performed for factors such as moisture availability, albedo, surface roughness, and thermal capacity, and a 24 hr simulation is summarized for day and night conditions. The comparison with the SESAME data comprises three hour intervals, using a time-dependent geostrophic wind. Close correlations were found with daytime conditions, but not in nighttime thermal structure, while the turbulence was faithfully predicted. Both geostrophic flow and surface characteristics were shown to have significant effects on the model predictions

Breakup of Temperature Inversions in Deep Mountain Valleys: Part I. Observations.

Abstract: The breakup of temperature inversions in the deep mountain valleys of western Colorado has been studied by means of tethered balloon observations of wind and temperature structure on clear weather days in different seasons. Vertical potential temperature structure profiles evolve following one of three patterns. Two of the patterns are special cases of the third pattern, in which inversions are destroyed by two continuous processes-upward growth of a convective boundary layer (CBL) into the base of the valley inversion, and descent of the inversion top. The three idealized patterns are described and 21 case studies of inversion breakup following the patterns are summarized. Inversion breakup begins at sunrise and is generally completed in 3½–5 h, unless the valley is snow covered or the ground is wet. Warming of the inversion layer is consistent with subsidence heating. An hypothesis is offered to explain the observations, stressing the role of the sensible heat flux in causing the CBL to grow an...

Objective Criteria for Rejecting Data for Bowen Ratio Flux Calculations

Abstract: In addition to the inherent problem of accumulating errors of measurement of net radiation and subsurface heat flux, the Bowen ratio energy balance method often produces totally unacceptable sensible and latent heat fluxes: wrong signs (directions) and extremely inaccurate magnitudes of the fluxes, or both. These problems are due to resolution limits of the instruments. Objective criteria to eliminate undesirable data are derived in general forms. An example is graphically presented for the common case of the psychometric tower with a 0.05°C resolution limit of temperature measurement.

Fitting Models to Daily Rainfall Data

Abstract: A range of Markov chain models have been used in the past to describe rainfall occurrence. Gamma distributions are commonly used for modeling rainfall amounts. These are all examples of generalized linear models. This unified view allows a regression-type approach to be used to fit and test alternative models. The approach is illustrated by fitting first- and second-order Markov chains in which the transition probabilities vary with time of year to data from sites in Jordan, Niger, Botswana and Sri Lanka. Gamma distributions with parameters varying with time are also fitted.

A Simple Parameterization of the Surface Fluxes of Sensible and Latent Heat During Daytime Compared with the Penman-Monteith Concept

Abstract: A comparison is made between two methods for determining the surface fluxes of sensible and latent heat during daytime. The first method, known as the Penman-Monteith approach, incorporates a more complete description of the physics. However, it needs a relatively large number of input parameters, which is inconvenient in many applications. The second method is a modification of the Priestley-Taylor evaporation model, which needs only net radiation, air temperature and an indication of the moisture condition at the surface. Both models are compared on the basis of hourly micro-meteorological data above short grass obtained in the Netherlands during the summer of 1977. The experiments were performed under predominantly unstable conditions [0 ≥ z/L0 ≥ −0.3z = (mean) measuring height, L0 = Obukhov length] with weak or no advection. It appears that, under these environmental conditions, the models have a similar skill. Therefore, the simple parameterization is preferred for practical purposes. It rev...

Barrier Winds Along the Sierra Nevada Mountains

Abstract: Observational evidence from instrumented aircraft, Doppler radar and rawinsondes suggest low-level, mountain-parallel jets are a common wintertime feature along the western slope of the Sierra Nevada Range and extending into the California Valley. It is proposed that the formation and maintenance of the low-level jet is a result of the pressure field created by the damming of stable air as it is forced up against the steep mountain barrier. Numerical experiments, using a two-dimensional (x, z) primitive equation model incorporating terrain representative of the Sierra Nevada Mountains, are carried out to test this assertion.

Automatic classification of clouds on Meteosat imagery - Application to high-level clouds

Abstract: A statistical classification method based on clustering on three-dimensional histograms is applied to the three channels of the Meteosat imagery. The results of this classification are studied for different cloud cover cases over tropical regions. For high-level cloud classes, it is shown that the bidimensional IR-water vapor histogram allows one to deduce the cloud top temperature even for semi-transparent clouds.

Atmospheric Turbidity in the Polar Regions.

Abstract: Analysis is presented of 800 measurements of atmospheric monochromatic aerosol optical depth made poleward of ∼65° latitude. The atmosphere of the southern polar region appears to be uncontaminated but is charged with a background aerosol having a mean size of 0.1 μm radius, an almost constant mixing ratio throughout the troposphere, a sea level optical depth (λ = 500 nm) of ∼0.025 and an inferred columnar mass loading of 4-15 × 10−7 g cm−2. At around the time of spring equinox the northern polar region (all longitudes) is invaded with Arctic Haze, an aerosol showing a strong anthropogenic chemical fingerprint. The optical depth anomaly introduced by this man-caused haze is τ0 ≈ 0.110 and the associated columnar mass loading is ∼1.5 × 10−6 g cm−2. Turbidity measured seven decades ago at the solar observatory at Uppsala (60°N), suggests that Arctic optical depth has been rising at a rate of dτ/dt ≈ 0.01 ± 0.005 per decade.

Breakup of Temperature Inversions in Deep Mountain Valleys: Part II. Thermodynamic Model

Abstract: A thermodynamic model is developed to simulate the evolution of vertical temperature structure during the breakup of nocturnal temperature inversions in mountain valleys. The primary inputs to the model are the valley floor width, sidewall inclination angles, characteristics of the valley inversion at sunrise, and an estimate of sensible heat flux obtained from solar radiation calculations. The outputs, obtained by a numerical integration of the model equations, are the time-dependent height of a convective boundary layer that grows upward from the valley floor after sunrise, the height of the inversion top, and vertical potential temperature profiles of the valley atmosphere. The model can simulate the three patterns of temperature structure evolution observed in deep valleys of western Colorado. The well-known inversion breakup over flat terrain is a special case of the model, for which valley floor width becomes infinite. The characteristics of the model equations are investigated for several ...

On the Use of Satellite Data to Infer Surface Fluxes at Meteorological Scales

Abstract: For numerical meteorological prediction, the determination of energy, momentum and moisture fluxes at the earth's surface requires knowledge of surface properties at a very coarse spatial resolution. A procedure is described for estimating the important surface properties (diurnal heat capacity, moisture availability) through analysis of high spatial resolution visible and thermal infrared data from satellites. An averaging method is then specified for aggregating or parameterizing local estimates of surface properties to a relatively coarse grid spacing. The method considers grid scale fluxes between ground and atmosphere, specifying that parameterized surface values should yield fluxes equal to those produced by integration of the high spatial resolution description of the surface. This allows estimation of latent and sensible heat exchange with the atmosphere at the large scales of general circulation models. The procedure is illustrated by application to a data set from the Heat Capacity Mapp...

A Z-R Relationship for Hurricanes

Abstract: Drop-size measurements taken during hurricane research flight missions at altitudes at or below 3 km were used to derive a relationship between reflectivity factor and rainfall rate. The instrument used in this study is the Knollenberg Particle Measurement System (PMS) Optical Array Spectrometer Probe, model number OAP–2D–P. The overall Z–R relationship based on data from four flights into three storms at three altitudes is Z = 300R1.35. No important differences were noted when the data were stratified by region (eyewall versus outside of eyewall) or altitude. The importance of using a Z–R relation that is representative of stratiform and convective regimes to estimate precipitation is illustrated by the large extent of the stratiform rain in hurricanes as revealed by radar. Comparisons of the hurricane Z–R relation derived in this study with the classic Marshall-Palmer (1948) relation and the relation used by the National Weather Service for convective rain reveal differences at the high rainfal...

Total Ozone Determination from the Backscattered Ultraviolet (BUV) Experiment

Abstract: The algorithm used to derive total ozone from the Nimbus 4 Backscattered Ultraviolet (BUV) experiment is described. A seven-year global data set with more than one million retrievals has been produced and archived using this algorithm. The algorithm is a physical retrieval scheme using accurate radiative transfer computations. Error sources are discussed and verified using Dobson network comparisons and the statistics of the BUV A- and B-pair derived ozone values.

Numerical Simulation of the Effects of St. Louis on Mesoscale Boundary-Layer Airflow and Vertical Air Motion: Simulations of Urban vs Non-Urban Effects

Abstract: A three-dimensional mesoscale computer model is used to assess the importance of urban effects, relative to non-urban effects, on mesoscale boundary-layer vertical air motion and on the height of the boundary layer downwind of St. Louis, Missouri. Simulations are made for south, southwest, west and northwest winds, with urban land uses replaced by rural land uses, both with and without topography. Simulations including urban effects indicated mesoscale upward air motion downwind of the city for all wind directions, strongest for southwest winds and weakest for northwest winds. With urban effects excluded, much weaker upward motion was found downwind for south, southwest and west winds, and downward vertical velocities occurred in the downwind areas for northwest winds. The results of this study imply that mesoscale boundary-layer upward air motion occurs downwind of St. Louis, primarily as a result of urban effects. Local geographic influences may tend to enhance or suppress this upward air motio...

An Apparent Relationship between Eurasian Spring Snow Cover and the Advance Period of the Indian Summer Monsoon

Abstract: Satellite images and Northern Hemisphere snow cover charts have been used to derive the Eurasian spring (March-May) snow cover area for the period 1967–78. The advance period of summer monsoon from the extreme southern tip of peninsular India to northwest India was derived from the Indian Daily Weather Reports. Year-to-year variations of the advance period of the Indian summer monsoon were compared with the Eurasian spring snow cover area and also with the Eurasian snowmelt, from March to May. This preliminary study revealed that springs with extensive snow cover area over Eurasia were followed by a longer advance period of the Indian summer monsoon. Further, an inverse relationship (negative correlation) is found between spring snowmelt and the advance period, that is, springs with more snowmelt over Eurasia were related with a shorter advance period of the Indian summer monsoon.

Microwave Radiometric Observations Near 19.35, 92 and 183 GHz of Precipitation in Tropical Storm Cora

Abstract: Observations of rain cells in the remains of a decaying tropical storm were made by Airborne Microwave Radiometers at 19.35,92 and three frequencies near 183 GHz. Extremely low brightness temperatures, as low as 140 K were noted in the 92 and 183 GHz observations. These can be accounted for by the ice often associated with raindrop formation. Further, 183 GHz observations can be interpreted in terms of the height of the ice. The brightness temperatures observed suggest the presence of precipitation sized ice as high as 9 km or more.

Spectral and diurnal variations in clear sky planetary albedo

Abstract: Spectral and diurnal variations in the clear sky planetary albedo of the earth are calculated using a radiative transfer model to obtain January and July values for a 5 deg x 5 deg global grid. The model employs observed climatological values of temperatures, humidities, snow and sea-ice cover. The diurnal cycle of clear sky albedo is calculated in the following intervals: 0.2-0.5, 0.5-0.7, and 0.7-4 microns. Observed ozone distribution is specified as a function of latitude and season. The 0.2-0.5 micron spectral albedo is 10-20% higher than the total albedo for all latitudes because of Rayleigh scattering; the 0.5-0.7 micron albedo differs from the total albedo by 1-2% for most latitudes, while the 0.7-4 micron albedo is 5-10% lower than the total because of strong atmospheric absorption. Planetary albedo decreases from morning to local noon, with diurnal variations being particularly strong over water.

Evaluation of a New Operational Technique for Producing Clear Radiances

Abstract: To produce atmospheric temperature profiles from measurements of infrared data, it is necessary to obtain infrared radiances for clear areas. Clear radiances are obtained either by identifying spots that are completely clear or by extracting clear values from areas that are partly overcast. Until 10 June 1980, clear radiances were obtained using an algorithm described by Smith and Woolf (1976). At this time, the algorithm was replaced by a new algorithm using current techniques. The new algorithm produced more accurate results which resulted in improved consistency between retrievals from the clear and partly cloudy areas. In addition, the new algorithm produced many partly cloudy retrievals in areas where the former method had produced less accurate retrievals using only microwave channels. During the study, it was discovered that the crucial assumption of a single layer or even a two- or three-layer cloud is seldom satisfied. The new algorithm uses various tests to identify pairs of spots with ...

Potential Errors in the Application of Principal Component (Eigenvector) Analysis to Geophysical Data

Abstract: Principal component (PC) analysis performed on irregularly spaced data can produce distorted loading patterns. We provide an example to demonstrate some distorted patterns which can result from the direct application of PC analysis (or eigenvector analysis, factor analysis, or asymptotic singular decomposition) on irregularly spaced data. The PCs overestimate loadings in areas of dense data. The problem can be avoided by interpolating the irregularly spaced data to a grid which closely approximates equal-area.

An Operational Large-Scale Marine Planetary Boundary Layer Model

Abstract: A marine planetary boundary layer (PBL) model is presented and compared with data from sea-based experiments. The PBL model comprises two layers, the outer an Ekman-Taylor layer with stratification-dependent secondary flow, and the logarithmic surface layer corrected for stratification and humidity effects and variable surface roughness. Corrections are noted for air much warmer than water in stable conditions and for low wind speeds. The layers are analytically defined along with similarity relations and a resistance law for inclusion in a program. An additional interfacial layer correction is developed and shown to be significant for heat flux calculations. Experimental data from GOASEX were used to predict the windfield in the Gulf of Alaska, and JASIN data was used for windfields SE of Iceland. The JASIN-derived wind field predictions were accurate to within 1 m/sec and 10 deg in a 200 km triangle.

Determination of semi-transparent cirrus cloud temperature from infrared radiances - Application to Meteosat

Abstract: A bispectral technique was employed to determine the cirrus cloud-top temperature. Data were gathered from aircraft equipped with the ARIES radiometer, which on sixteen flights provided IR readings at 6.5 and 11.5 microns of cirrus tops. The flights were made on a NASA Convair to a maximum ceiling of 12,500 m. The brightness temperatures above the cirrus were found to be consistently colder, by up to 40 K, than the 11.5 microns window channel, and good spatial correlations were displayed over whole clouds. Effective emissivities are calculated for both channels and applied to Meteosat digital imagery. The subsequent derived temperatures were significantly colder than black-body temperatures, which is explained by taking into account the absorption of emitted radiation by water vapor.

Remote sensing of precipitable water over the oceans from Nimbus 7 microwave measurements

Abstract: Global maps of precipitable water over derived from scanning multichannel microwave radiometer (SMMR) data reveal salient features associated with ocean currents and the large scale general circulation in the atmosphere. Nimbus-7 SMMR brightness temperature measurements in the 21 and 18 GHz channels are used to sense the precipitable water in the atmospheric over oceans. The difference in the brightness temperature (T sub 21 -T sub 18), both in the horizontal and vertical polarization, is found to be essentially a function of the precipitable water in the atmosphere. An equation, based on the physical consideration of the radiative transfer in the microwave region, is developed to relate the precipitable water to (T sub 21 - T sub 18). It shows that the signal (T sub 21- T sub 18) does not suffer severely from the noise introduced by variations in the sea surface temperature, surface winds, and liquid water content in non rain clouds. The rms deviation between the estimated precipitable water from SMMR data and that given by the closely coincident ship radiosondes is about 0.25 g/ sq cm

Comparison of Dual-Polarization Radar Measurements of Rain with Ground-Based Disdrometer Measurements

Abstract: Dual-polarization radar measurements of ZH and ZDR, where ZDR = 10 logZH/ZV and ZH, ZV are the radar reflectivity factors for horizontal and vertical polarizations, respectively, are compared with values derived from raindrop-size distributions measured by a Joss-Waldvogel disdrometer. The measurements were made simultaneously, with the radar pulse volume situated 120 m above the ground-based disdrometer. Despite the difference in size of the respective sampling volumes, agreement between the radar and disdrometer measurements was generally good. The radar estimates of ZH exceeded the disdrometer estimates by 1.6 dB on average, well within the expected experimental error. The disdrometer estimates of ZDR exceeded the radar estimates by 0.3 dB on average. This was somewhat larger than could be accounted for by experimental error, and further analysis suggested that some modification to the theoretical ratios of horizontal and vertical backscattering cross sections was required, for drops of diamet...

The Isotopic Composition of Cyclonic Precipitation

Abstract: The deuterium/hydrogen ratios δD of sequential samples of precipitation at Palisades, New York were measured during four separate storms during January 1978. The values ranged from a low of −177‰ to a high of −30‰. For each of the storms there was a general increase of δD values with time. The changes of δD values are explained by means of detailed meteorological analysis assuming that the precipitation derives from a Rayleigh condensation process in which all condensation in the air column directly above the station is assumed to fall immediately to the ground. The resulting calculated values of δD usually agree to within 5‰ of the observed measured values.

Statistical Considerations in the Estimation of Divergence From Single-Doppler Radar and Application to Prestorm Boundary-Layer Observations

Abstract: Methods of statistical regression have been applied to single-radar radial velocity fields to map certain mesoscale (20–100 km) kinematic properties (e.g., divergence) of the convective boundary layer (CBL). Several methods, previously proposed, were found to produce estimates that were biased or whose variances were too large. When wind fields are linear on the meso- or larger scale, then single-Doppler velocity accuracies allow the estimation of horizontal divergence with an accuracy of about 4 × 10−5 s−1 and a resolution of ∼30 km, which may be sufficient to sense pre-thunderstorm convergence A case study for 19 June 1980 suggests that single-Doppler weather radars of modest sensitivity can map the mesoscale divergence patterns within the cloud-free CBL. For this day, convergence zones to the northeast seem to precede cloud development by 1–2 h, and to the west precede thunderstorms by 3–4 h.

Assessing the Value of Frost Forecasts to Orchardists: A Dynamic Decision-Making Approach

Abstract: The methodology of decision analysis is used to investigate the economic value of frost (i.e., minimum temperature) forecasts to orchardists. First, the fruit-frost situation and previous studies of the value of minimum temperature forecasts in this context are described. Then, after a brief overview of decision analysis, a decision-making model for the fruit-frost problem is presented. The model involves identifying the relevant actions and events (or outcomes), specifying the effect of taking protective action, and describing the relationships among temperature, bud loss, and yield loss. A bivariate normal distribution is used to model the relationship between forecast and observed temperatures, thereby characterizing the quality of different types of information. Since the orchardist wants to minimize expenses (or maximize payoffs) over the entire frost-protection season and since current actions and outcomes at any point in the season are related to both previous and future actions and outcom...

Estimating the Depth of the Daytime Convective Boundary Layer

Abstract: Three in-situ and five remote sensing techniques for measuring the height of the daytime convective boundary layer were compared. There was, as a rule, good agreement between the different systems when the capping inversion was steep and well defined, and some variability when the stratification was not so sharply defined. Two indirect methods for estimating boundary-layer heights from the length scales of convective motions in the layer are also discussed.

Empirical Orthogonal Function Analysis of Hawaiian Rainfall

Abstract: Empirical orthogonal function analysis was applied to monthly mean rainfall data at 63 stations in Hawaii encompassing a 37-year period. Major rainfall patterns in order to importance (E1–E3) proved to be trade wind, southwest wind and convective rainfall on an annual basis; trade wind, southwest wind and frontal rainfall during winter, spring and fall seasons; and trade wind, tropical disturbance and convective rainfall during summer. Trade wind rainfall (E1) explains most rainfall variance in summer and least variance in winter. Spectral analyses of the time-dependent coefficients for eigenvectors E1–E5 show annual, semi-annual, three-forths year, and 2–2½ year cycles. No spectral peaks relating to the 11- and 22-year sunspot cycles were found. Composite rainfall maps for wet and dry winter and summer half-years indicate the contributions that specific eigenvector patterns make to these anomalies. Comparisons between Hawaiian rainfall and E1 Ninos reveal that most (not all) E1 Nino winters in H...

Accuracy of Wind Velocity Determinations with Doppler Sodars

Abstract: The measurement of the radial wind velocity component in sodar systems is generally carried out by application of the first moment integral to the spectrum of the received echoes. In unfavorable signal-to-noise conditions this leads to a large systematic bias and lack of precision. In this paper the use of a two-step procedure is shown, whereby the spectrum of the echo is first localized and then the actual value of its center frequency is determined. Large improvements in accuracy and precision are thus obtained.

Toward parameterization of the stable boundary layer

Abstract: Wangara data is used to examine the depth of the nocturnal boundary layer (NBL) and the height to which surface-linked turbulence extends. It is noted that a linearity of virtual temperature profiles has been found to extend up to a significant portion of the NBL, and then diverge where the wind shear rides over the surface-induced turbulence. A series of Richardson numbers are examined for varying degrees of turbulence and the significant cooling region is observed to have greater depth than the depth of the linear relationship layer. A three-layer parameterization of the thermodynamic structure of the NBL is developed so that a system of five equations must be solved when the wind velocity profile and the temperature at the surface are known. A correlation between the bulk Richardson number and the depth of the linear layer was found to be 0.89.

A Single-Channel, Double-Viewing Angle Method for Sea Surface Temperature Determination from Coincident METEOSAT and TIROS-N Radiometric Measurements

Abstract: An experimental simulation of a single-channel, double-angle viewing technique for the determination of sea surface temperature from satellite is presented. This method relies upon the fact that the same area can be viewed simultaneously at two different angles (different air masses) by the geostationary satellite METEOSAT and by the polar orbiting satellite TIROS-N. Extrapolating the two air mass observations to zero air mass is shown to give a value of the temperature in good agreement with the true sea surface temperature. A discussion concerning the viewing angles is presented.