Abstract

The flux-form advection scheme of Bott is modified for the spherical coordinates, combined with the expanded-polar-zone (EPZ) technique to improve the overall performance of the advection calculations. With the EPZ technique, this Eulerian scheme has comparable efficiency as semi-Lagrangian methods for advection of nonreactive tracers on a sphere but with somewhat better overall numerical accuracy. The conservation of global tracer mass and the, positive definiteness of the algorithm are achieved to machine precision. For the test problem of solid body rotations on a sphere, this scheme shows small numerical diffusion, almost undetectable phase errors, and very little artificial deformation of the test shape even for cross-polar transport. In comparison with some semi-Lagrangian schemes and other high-order Eulerian methods, it shows very competitive performance. Numerical tests also indicate that, without any modifications, it performs just as well on slightly nonuniform Gaussian grid as on uniform grid. For the vertical advection, a fourth-order and two second-order versions of this scheme formulated on a nonuniform grid system have also been derived. The performance of these versions is tested with a nonuniform sigma grid system by using ideal one-dimensional test problems. This accurate numerical scheme is recommended for models where resolving the sharp vertical gradients of atmospheric trace species such as water vapor is important.

Abstract

A comprehensive air quality and meteorological monitoring project entitled the South-Central Coast Cooperative Aerometric Monitoring Program (SCCCAMP 1985) was conducted in the Santa Barbara Channel and adjacent areas from Point Sal to Point Dume during a five-week period in September–October 1985. As part of a larger study to analyze the SCCCAMP 1985 observations and related databases, an analysis has been conducted of a six-year historical ozone and meteorological database.

The objectives of the historical data analysis study were to 1) characterize meteorological and ozone concentration patterns during a six-year historical period (1979–1984), 2) identify relationships between meteorological variables and high ozone concentrations in the SCCCAMP region, and 3) compare the meteorological conditions and ozone concentrations observed during the SCCCAMP 1985 study with those during the same periods in the historical database in order to assess the representativeness of the SCCCAMP 1985 study period as a whole and individual high ozone events within the period.

The analysis indicated that high ozone concentrations in Santa Barbara County were associated with two conditions occurring simultaneously: 1) subsidence and limited mixing conditions, and 2) moderate easterly or southerly geostrophic flow. Although the actual flow fields and mixing conditions in the region are complex and variable, the 850-mb temperature and surface-pressure parameters were found to be useful, robust indicators of high ozone conditions in Santa Barbara. The seasonal distribution of high ozone events in Santa Barbara County, which peaks in September with a secondary peak in June, was found to be strongly related to the seasonal frequency of occurrence of favorable values of these meteorological variables. In contrast, the peak Ventura County ozone concentrations did not show the same sensitivity to surface pressure parameters, and the seasonal frequency of high ozone events, which peak in July, corresponds closely to that of the 850-mb temperature.

The SCCCAMP 1985 period was unusual in terms of its low frequency of occurrence of meteorological conditions associated with high ozone events in the region. As a result the observed average ozone concentrations were below historical values. However, several high ozone events did occur during the SCCCAMP 1985 study. The meteorological conditions during these individual events were found to be consistent with those typical of historical high ozone events.

Abstract

Recent observational studies have suggested that interactions between the atmosphere and the ocean play an important role in the pronounced annual cycle of the eastern equatorial Pacific and Atlantic Oceans. The key to this atmosphere–ocean interaction is a positive feedback between the surface winds and the local SST gradients in the cold tongue/ITCZ complex regions, which leads to an instability in the coupled system. By means of linear instability analyses and numerical model experiments, such an instability mechanism is explored in a simple coupled ocean-atmosphere system. The instability analysis yields a family of antisymmetric and symmetric unstable SST modes. The antisymmetric mode has the most rapid growth rate. The most unstable antisymmetric mode occurs at zero wavenumber and has zero frequency. The symmetric SST mode, although its growth rate is smaller, has a structure at annual period that appears to resemble the observed westward propagating feature in the annual cycle of near-equatorial zonal wind and SST. Unlike the ENSO type of coupled unstable modes, the modes of relevance to the seasonal cycle do not involve changes in the thermocline depth. The growth rates of these modes are linearly proportional to the mean vertical temperature gradient and inversely proportional to the depth of mean thermocline in the ocean. Because of the shallow thermocline and strong subsurface thermal gradients in the eastern Pacific and Atlantic Oceans, these coupled unstable modes strongly influence the seasonal cycles of those regions. On the basis of theoretical analyses and the observational evidence, it is suggested that the antisymmetric SST mode may be instrumental in rapidly reestablishing the cold tongues in the eastern Pacific and Atlantic Oceans during the Northern Hemisphere summer, whereas the symmetric SST mode contributes to the westward propagating feature in the annual cycle of near-equatorial zonal winds and SST.

Abstract

The climatology of oceanic rain column height derived from 12 years (July 1987–June 1999) of Special Sensor Microwave Imager (SSM/I) data is presented. The estimation procedure is based on a technique developed by Wilheit et al. In the annual mean, the SSM/I-derived oceanic rain height shows a maximum of about 4.7 km in the Tropics and decreases toward the high latitudes to less than 3.5 km at 50°. Interannual variations exhibit seasonal dependency and show maxima of about 200–300 m in the oceanic dry zones and in the midlatitude storm track regions. The rain heights estimated from the morning passes of the SSM/I are lower than those computed from the afternoon passes by about 60 m in the Tropics but are higher north of 40°N. This small difference cannot change the conclusion about the morning maximum in rain rate. The nonsystematic error increases with decreasing rain column height and is estimated to be about 120 m for rain heights of 4–5 km and 200 m at 3.5 km. Comparison with the height of the 0°C isotherm derived from the Goddard Laboratory for Atmospheres general circulation model (GCM) results shows a mean zonal low bias (SSM/I lower than GCM freezing height) of about 200 m in the Tropics. Outside the Tropics, the SSM/I rain column heights are much higher, reaching a difference of 2 km at 50°N. The small bias in the Tropics is consistent with the notion that the melting layer extends over hundreds of meters below the freezing level. Outside the Tropics, the sampling of the SSM/I rain height and the inclusion of nonraining observations in GCM calculations may contribute to the large discrepancy. The freezing height is interpreted as the columnar water content and found to be consistent with columnar water vapor maps retrieved from SSM/I data.

Abstract

Special Sensor Microwave/Imager (SSM/I) retrieved rainfall rates were assimilated into a limited-area numerical prediction model in an attempt to improve the initial analysis and forecast of a tropical cyclone. Typhoon Flo of 1990, which was observed in an intensive observation period of the Tropical Cyclone Motion Experiment-1990, was chosen for this study. The SSM/I retrieved rainfall rates within 888 km (8° latitude) of the storm center were incorporated into the initial fields by a reversed Kuo cumulus parameterization. In the procedure used here, the moisture field in the model is adjusted so that the model generates the SSM/I-observed rainfall rates. This scheme is applied through two different assimilation methods. The first method is based on a dynamic initialization in which the prediction model is integrated backward adiabatically to t = −6 h and then forward diabatically for 6 h to the initial time. During the diabatic forward integration, the SSM/I rainfall rates are incorporated using the reversed Kuo cumulus parameterization. The second method is a forward data assimilation integration starting from t = −12 h. From t = −6 h to t = 0, the SSM/I rainfall rates are incorporated, also using the reversed Kuo scheme. During this period, the momentum fields are relaxed to the initial (t = 0) analysis to reduce the initial position error generated during the preforecast integration. Five cases for which SSM/I overpasses were available were tested, including two cases before and three after Flo's recurvature. Forecasts at 48 h are compared with the actual storm track and intensifies estimated by the Joint Typhoon Warning Center. For the five cases tested, the assimilation of SSM/I retrieved rainfall rates reduced the average 48-h forecast distance error from 239 km in the control runs to 81 km in the assimilation experiments. It is postulated that the large positive impact was a consequence of the improved forecast intensity and speed of the typhoon when the SSM/I rain-rate data were assimilated.

Abstract

About 10 yr (July 1987–December 1997 with December 1987 missing) of oceanic monthly rainfall based on data taken by the Special Sensor Microwave/Imager (SSM/I) on board the Defense Meteorological Satellite Program satellites have been computed. The technique, based on the work of Wilheit et al., includes improved parameterization of the beam-filling correction, a refined land mask and sea ice filter. Monthly means are calculated for both 5° and 2.5° latitude–longitude boxes.

Monthly means over the latitude band of 50°N–50°S and error statistics are presented. The time-averaged rain rate is 3.09 mm day⁻¹ (std dev of 0.15 mm day⁻¹) with an error of 38.0% (std dev of 3.0%) for the 5° monthly means over the 10-yr period. These statistics compare favorably with 3.00 mm day⁻¹ (std dev of 0.19 mm day⁻¹) and 46.7% (std dev of 3.4%) computed from the 2.5° monthly means for the period January 1992–December 1994. Examination of the different rain rate categories shows no distinct discontinuity, except for months with a large number of missing SSM/I data.

An independent estimate of the error using observations from two satellites shows an error of 31% (std dev of 2.7%), consistent with the 38% estimated using (a.m. and p.m.) data from one satellite alone. Error estimates (31%) based on the 5° means by averaging four neighboring 2.5° boxes are larger than those (23%) estimated by assuming the means for these neighboring boxes are independent, thus suggesting spatial dependence of the 2.5° means.

Multiple regression analyses show that the error varies inversely as the square root of the number of samples but exhibits a somewhat weaker dependence on the mean rain rate. Regression analyses show a power law dependence of −0.255 to −0.265 on the rain rate for the 5° monthly means using data from a single satellite and a dependence of −0.366 for the 5° monthly means and −0.337 for the 2.5° monthly means based on two satellite measurements. The latter estimate is consistent with that obtained by Bell et al. using a different rainfall retrieval technique.

Abstract

The central question discussed here is how the rate at which drifter positions are determined and the position errors affect the calculation of velocity, acceleration and velocity gradients such as divergence and vorticity. The analysis shows that the mean-square velocity and acceleration errors each are composed of two terms. One arises from the position uncertainty and the discrete sampling rate. The other term is an alias resulting from sampling a continuous velocity or acceleration spectrum discretely. Effects at low and high frequencies and sampling intervals are examined by asymptotic expansions of the spectra. Then optimum smoothing and derivative filters are obtained for the velocity and accelerations, respectively. The efficiency of these filters is determined by comparison with the errors previously established.

The calculation of divergence and vorticity from drifter clusters typically neglects the position error, in which case the errors in the velocity gradients are proportional to the velocity errors. Our analysis shows that this procedure produces estimates of the velocity gradients whose magnitudes are less than the true values. This bias is easily removed. The analysis is concluded with a derivation of formulas for unbiased estimates of the variance and covariance of the velocity gradients.

Abstract

A laboratory model of the tornado-like vortex near the ground is developed and studied. The circulation is produced by a rotating cylindrical screen and the updraft is produced by an exhaust fan at the opening of the top hood. By means of kerosene smoke, the vortex core and a reverse flow zone were observed in the experiment. The profiles of velocity and pressure were measured at three different circulation strengths. The maximum inward radial velocity in the boundary layer is approximately proportional to the circulation strength. Outside the vortex core, the top hood and ground boundary layers, the flow is a potential vortex flow with a very small inward radial velocity. The vertical velocity distribution generally has a Gaussian profile except that it is slightly downward in the annular reverse flow region. The diameter of the reverse flow region is controlled by the opening size of the outlet on the top hood. The reverse flow region extends to the top of the ground boundary layer only when the circulation is strong enough. The maximum downward flow speed observed in the experiments was less than 30 cm sec⁻¹. A minimum pressure occurs at 1.27 cm from the ground on the vortex axis and shows the complexity of the flow in the conjunction region of the vortex core and the ground boundary layer.

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.

Abstract

Using aircraft data collected during the University of Chicago Lake-Effect Snow Storm project, the results of a case study of the convective thermal internal boundary layer (TIBL) over Lake Michigan are presented. An intense cold air outbreak on 20 January 1984 featured a rapid growth of the convective TIBL thickness and the concurrent development of cloud and snow. The average slope of the TIBL top over a fetch of 123.7 km was 1.0%. Microphysical characteristics of cloud and snow along with the TIBL development are also presented. Results of the TIBL integrated budgets of heat and total water (including cloud and snow water) are given in detail. Over the surface of Lake Michigan the average downward snow flux (snow precipitation rate) was 0.79 mm (water) per day. The average sensible and latent heat fluxes at the water surface were 323 and 248 W m⁻², respectively. About 13 percent of the total warming of this cloud-topped TIBL was due to radiation.