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Donald C. Norquist

Abstract

Observed cloud characteristics, such as cloud cover, type, and base and top altitude, are of interest to the U.S. Air Force operational community for mission support. Predictions of such cloud characteristics are useful in support of a variety of mission activities. In this paper, a model output statistics approach to diagnosing these cloud characteristics from a forecast field generated by a mesoscale numerical weather prediction model is presented. Cloud characteristics information obtained from the air force RTNEPH cloud analysis supplied the cloud predictands, and forecast fields from the MM5 mesoscale numerical weather prediction model provided the weather variable predictors. Multiple linear regression (MLR) and multiple discriminant analysis (MDA) were used to develop the predictand–predictor relationships using 10 days of twice-daily cloud analyses and corresponding forecasts over a theater-scale grid. The consequent relationships were then applied to subsequent gridded forecast fields to obtain estimates of the distribution of the cloud characteristics at the forecast times. The methods used the most recent 10 days of cloud analyses and weather forecasts to develop the relationship for each successive application day.

The gridded cloud characteristics were diagnosed for 10 days in each of January and July of 1992 over a theater-scale region in southern Europe. The resulting diagnosed cloud predictions were verified against the RTNEPH analyses for forecast durations of 6–36 h at 6-h intervals. It is found that both the MLR and the MDA methods produced small mean errors in all the cloud variables. When compared with persistence, MLR showed skill in rmse in January, while MDA did not. On the other hand, MDA obtained a better score than MLR in percent diagnosed in the correct cloud amount category. Furthermore, the category selection method used with the MDA scheme effectively reproduced the cloud variables’ category frequency distribution compared with that of the verification data, while MLR did not. In July, both methods showed skill with respect to persistence in cloud amount. Verification results for cloud type, base altitude, and thickness did not show appreciable skill with respect to persistence. Cloud-ceiling altitude diagnoses showed consistent skill compared to persistence for both methods in both months.

Visual depictions of the total cloud amount distribution as diagnosed by the methods showed that the MDA algorithm is capable of generating useful cloud prediction products. The images produced by the MLR scheme had unrealistically flat gradients of total cloud amount and too many occurrences of partly cloudy skies. The multiple discriminant analysis scheme is considered to be a useful short-term solution to the U.S. Air Force need for predictions of cloud characteristics in theater-scale areas.

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Donald C. Norquist

Abstract

The U.S. Air Force has a long history of investment in cloud analysis and prediction operations. Their need for accurate cloud cover information has resulted in routine production of global cloud analyses (from their RTNEPH analysis model) and forecasts (using their ADVCLD cloud trajectory forecast model) over many years.

With the advancement of global numerical weather prediction technology and resulting forecast accuracy of noncloud meteorological quantities, it is of interest to determine if such technology could be used to benefit cloud cover forecasting. In this paper, a model output statistics approach to diagnosing cloud cover from forecast fields generated by a global numerical weather prediction model is presented. Cloud characteristics information obtained from the RTNEPH cloud analysis supplied the cloud predictands, and forecast fields from the U.S. Navy Operational Global Atmospheric Prediction System global weather prediction model provided the weather variable predictors. RTNEPH layer cloud cover was assigned to three cloud decks (high, middle, and low) based on reported cloud-base altitude, and RTNEPH total cloud cover was used as a separate predictand. Multiple discriminant analysis (MDA) was used to develop the predictand–predictor relationships for each cloud deck and total cloud using 5 days of twice-daily cloud analyses and corresponding forecasts for 30° latitude zones. The consequent relationships were applied to the forecasts fields from the forecast initialized on the day following each 5-day development period to diagnose cloud cover forecasts for the Northern or Southern Hemisphere.

In this study, cloud cover forecasts were diagnosed from global NWP model forecasts on hemispheric polar stereographic map projections with a grid spacing of 96 km. The diagnosed cloud forecasts (DCFs) were verified against the RTNEPH analyses for forecast durations of 12–72 h at 12-h intervals. Also verified were 12–48-h cloud cover forecasts (deck and total) from the ADVCLD cloud trajectory model, and from persistence (RTNEPH at initial forecast time). Biases from all three methods were generally small. The DCFs were significantly better than ADVCLD and persistence in all decks and total cloud, at almost all forecast durations in rmse and 20/20 score. ADVCLD scored better in these measures only at 12 h in total cloud, suggesting the possibility of a crossover in superior prediction skill from trajectory to diagnostic method somewhere between 12 and 24 h. DCF better preserved the cloud cover frequency distribution than did ADVCLD. ADVCLD displayed a greater degree of spatial variation inherent in RTNEPH cloud cover than did DCF. Both ADVCLD and DCF visual depictions of hemispheric total cloud cover appeared to provide useful cloud cover forecast information when compared with RTNEPH depictions. The advantages of the diagnosed cloud forecast algorithm presented in this study make it an excellent candidate for operational cloud cover prediction. It is expected that as cloud cover analyses are improved, the trajectory and diagnostic methods will prove complementary with the former more skillful at short-term predictions, and the latter better at long-term forecasts.

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Donald C. Norquist

Abstract

Global data analysis procedures were developed to perform data assimilation for observed geopotential heights wind components, and relative humidity. These procedures were implemented in conjunction with a global spectral forecast model (GSM) and normal mode initialization procedure to produce global analyses at six-hour intervals. A set of five-day experiments were conducted to assess the impact of several alternative sources of humidity analysis procedure. Satellite moisture retrievals, surface weather observations, and Air Force Global Weather Central 3-D nephanalysis cloud amounts were used as sources of upper-air relative humidity as to permit their use in data assimilation. The 3-D nephanalysis-inferred humidities, and to a lesser extent the surface weather observation-inferred humidities, were found to have a beneficial impact on the analyses. When used in a manner to maximize their impact, their inclusion in the analysis resulted in an analysis that agreed better with withheld radiosonde relative humidity than did the uncorrected first guess.

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Donald C. Norquist and Robert P. d'Entremont

Abstract

Vertical distributions of clouds have been a focus of many studies, motivated by their importance in radiative transfer processes in climate models. This study examines the horizontal distribution of cirrus clouds by means of satellite imagery analyses and numerical weather prediction model forecasts. A ground-truth dataset based on two aircraft mission periods flying particle probes through cirrus over a ground-based cloud radar is developed. Particle probe measurements in the cirrus clouds are used to compute ice water content and radar reflectivity averages in short time periods (25–30 s). Relationships for ice water content as a function of reflectivity are developed for 6-K ambient temperature categories. These relationships are applied to the radar-measured short-term-averaged reflectivities to compute vertical profiles of ice water content, which are vertically integrated over the depth of the observed cirrus clouds to form ice water path estimates. These and cloud-top height are compared with the same quantities as retrieved by the Geostationary Operational Environmental Satellite (GOES) level-2B algorithm applied to four channels of GOES-8 imagery measurements. The agreement in cloud-top height is reasonable (generally less than 2-km difference). The ice water path retrievals are smaller in magnitude than the radar estimates, and this difference grows with increasing cirrus thickness. Comparisons of a sequence of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) predictions and GOES level-2B retrievals of ice cloud tops for the convectively active second mission period showed that the MM5 cirrus areal extent was somewhat greater than the GOES depictions. Cloud-top height ranges were similar. MM5 is capable of producing ice water path magnitudes similar to the radar estimates, but the GOES retrievals are much more limited. Ninety-eight percent of the GOES grid points had ice water paths no greater than 60 g m−2, as compared with 74% for MM5. Ten percent of MM5 points had ice water content >200 g m−2, as compared with 0.07% for GOES retrievals. Based on this study, we conclude that GOES level-2B cloud-top retrievals are a reliable tool for prediction evaluations but the algorithm's retrievals of ice water path are not.

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Donald C. Norquist and Sam S. Chang

Abstract

Accuracy of humidity forecasts has been considered relatively unimportant to much of the operational numerical weather prediction (NWP) community. However, the U.S. Air Force is interested in accurate water vapor and cloud forecasts as end products. It is expected that the NWP community as a whole will become more involved in improving their humidity forecasts as they recognize the important role of accurate water vapor distributions in data assimilation, forecasts of temperature and precipitation, and climate change research.

As a modeling community, we need to begin now to identify and rectify the systematic humidity forecast errors that are present in NWP models. This will allow us to take full advantage of the new types of remotely sensed water vapor and cloud measurements that are on the horizon. The research reported in this paper attempts to address this issue in a simple, straightforward manner, using the Phillips Laboratory Global Spectral Model (PL GSM).

It was found that significant systematic specific humidity errors exist in the much-used FGGE [First CARP (Global Atmospheric Research Program) Global Experimental] (initialized) analyses. However, when a correction on the analyses was imposed and the PL GSM forecasts rerun, forecast errors similar to the forecast errors generated from the uncorrected analyses were observed. The errors were diagnosed through an evaluation of the tendency terms in the model's specific humidity prognostic equation. The results showed that systematic low-level tropical drying and upper-level global moistening could be attributed to the convective terms and the horizontal and vertical advection terms, respectively. Alternative formulations of the model were identified in an attempt to reduce or eliminate these errors. In general, it was found that the alternative formulations that do not modify the convection parameterization of the model reduced the upper-level moistening, while those that do modify the convection scheme reduced low-level tropical drying but introduced low-level and midlevel moistening in the summer hemisphere extratropics. The authors conclude that the nonconvective modifications could be instituted in the model as is. However, more work is needed on improving the way that convective parameterizations distribute water vapor in the vertical.

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Richard J. Reed, Donald C. Norquist, and Ernest E. Recker

Abstract

A compositing method is used to determine the average structure and properties of eight wave disturbances observed over west Africa and the eastern Atlantic during the period 23 August-19 September, 1974, a period marked by well-developed and regular wave activity. The disturbance centers propagated westward in the zone of cyclonic shear to the south of the 700 mb easterly jet, located at 16–17°N. The mean wave- length was about 25M km and the mean period 3.5 days. The mean zonal current satisfied the necessary condition for barotropic instability.

The composite disturbance was most intense at 650 mb, being cold core below and warm core above. Two circulation centers were evident at the surface, one located below the upper center and the other displaced 10° to the north at about the latitude of the monsoon trough. When separate composites were constructed for land and ocean stations, the dual centers were found to be primarily a land phenomenon. Distinctive features of the high-level (200 inb) circulation were a strong region of divergence located just ahead of the disturbance center and pronounced regions of anticyclonic and cyclonic vorticity situated several hundred kilometers to the north and south, respectively. Maximum low-level convergence and upward vertical motion were found in the region ahead and slightly south of the center. This was also the region of greatest convective cloud cover and largest precipitation amount.

Some minor differences are noted between wave behavior over land and sea. Over the ocean wavelengths were shorter, vorticities were greater at all levels, especially at the surface, and the horizontal wave axis was more tilted at levels close to the core of the mid-tropospheric jet stream. In association with the greater tilt, the northward momentum flux and transformation of zonal kinetic energy to eddy kinetic energy were stronger.

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Donald C. Norquist, Ernest E. Recker, and Richard J. Reed

Abstract

Fields of the meteorological variables in composite wave disturbance constructed for the region from IOOE to 31°W and 1°S to 26°N and for land and ocean subregions are used to diagnose energy transformations in African waves. The composites are based on data contained in the GATE Quick Look Data Set for the period 23 August to 19 September, 1974. The measurements indicate that for the region as a whole the kinetic energy of the waves is maintained almost equally by conversions from zonal kinetic energy and eddy available potential energy. Eddy available potential energy is supplied by the zonal available potential energy at a comparable rate. From the measured conversion rates it is estimated that in the absence of friction the kinetic energy of the waves would double in about 3 days.

Measurements for the subregions show that the conversion from zonal to eddy kinetic energy is stronger over the limited oceanic region considered than over the land, while conversely, the conversion of eddy available potential energy to eddy kinetic energy is stronger over the land than over the ocean. The conversion of zonal to eddy available potential energy differs little between the two regions. From these findings, and budgetary considerations, it is inferred that latent beat release in organized convection plays an important role in the wave growth and maintenance in west Africa but not over the adjacent ocean. This conclusion, however, must be regarded as tentative.

The distributions of the various energy conversion processes in meridional cross section are considered. The conversions of zonal kinetic and available potential energies to their corresponding eddy energies are characterized by concentrated regions of high values closely associated with the mid-tropospheric easterly jet stream. The conversion of eddy available potential energy to eddy kinetic energy exhibits a complex pattern in which the net conversion is a, small residual. Consequently this conversion cannot be regarded as being determined with the same high degree of reliability as the other two. However, major features of the pattern can be explained on physical grounds.

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Ross N. Hoffman, Christopher Grassotti, Ronald G. Isaacs, Jean-Francois Louis, Thomas Nehrkorn, and Donald C. Norquist

Abstract

A series of observing system simulation experiments (0SSEs) was conducted to assess the impacts on the Air Force Geophysics Laboratory (GL) global data assimilation system (GDAS) of a satellite Doppler wind lidar sounding system (WINDSAT) and of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave (SSM) T-1 and T-2 temperature and moisture retrievals. (The SSM/T-2 is expected to be launched in the early 1990s.) In simulating the SSM data, some horizontal correlations were induced because the simulated errors had different biases in different geophysical regimes. As an interpretative aid we calibrated our results to a series of real data experiments.

In an experiment in which the WINDSAT data is added to the observational database, the analyses and forecasts are improved relative to the control experiment. These improvements are large in the Southern Hemisphere extratropics. The addition of the SSM data improves the analysis of moisture particularly in the tropics and Southern Hemisphere extratropics.

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Donald C. Norquist, Paul R. Desrochers, Patrick J. McNicholl, and John R. Roadcap

Abstract

Future high-altitude laser systems may be affected by cirrus clouds. Laser transmission models were applied to measured and retrieved cirrus properties to determine cirrus impact on power incident on a target or receiver. A major goal was to see how well radiosondes and geostationary satellite imagery could specify the required properties. Based on the use of ground-based radar and lidar measurements as a reference, errors in cirrus-top and cirrus-base height estimates from radiosonde observations were 20%–25% of geostationary satellite retrieval errors. Radiosondes had a perfect cirrus detection rate as compared with 80% for satellite detection. Ice water path and effective particle size were obtained with a published radar–lidar retrieval algorithm and a documented satellite algorithm. Radar–lidar particle size and ice water path were 1.5 and 3 times the satellite retrievals, respectively. Radar–lidar-based laser extinction coefficients were 55% greater than satellite values. Measured radar–lidar cirrus thickness was consistently greater than satellite-retrieved thickness, but radar–lidar microphysical retrieval required detection by both sensors at each range gate, which limited the retrievals’ vertical extent. Greater radar–lidar extinction and greater satellite-based cirrus thickness yielded comparable optical depths for the two independent retrievals. Laser extinction–transmission models applied to radiosonde-retrieved cirrus heights and satellite-retrieved microphysical properties revealed a significant power loss by all models as the laser beam transits the cirrus layer. This suggests that cirrus location is more important than microphysics in high-altitude laser test support. Geostationary satellite imagery may be insufficient in cirrus detection and retrieval accuracy. Humidity-sensitive radiosondes are a potential proxy for ground-based remote sensors in cirrus detection and altitude determination.

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