Abstract

Wind-driven flow in the upper 90 meters during autumn–winter in the midlatitude North Pacific is investigated using satellite-traced drifting buoys (i.e., drifters) deployed nearly simultaneously but drogued at different depths. The difference in the relationship between drifter velocity and wind stress as a function of drogue depth is observed to change when its drogue entered the deepening mixed layer. This change is characterized by a sudden increase in the amplitude of the near-inertial motions observed by the drifters and by the onset of a persistent net displacement whose downwind component is approximately 3 times as large as its crosswind component. Attempts to model this downwind flow as a windage result in a large unexplained residual downwind velocity component. On the other hand, 80–90% of the observed crosswind displacement is explained by an Ekman slab model (i.e., with flow uniform over the mixed layer). This large residual downwind velocity component combined with the Ekman driven crosswind component results in drifter displacements whose angle with respect to the forcing wind is significantly greater than 0° and significantly less than 45° to the right (i.e., ∼30°).

Details of the flow obtained while the drogues were still below the deepening mixed layer suggests the presence of an Ekman-like spiral in the velocity vector beneath the mixed layer. Subsequent to all the drogues being in the mixed layer, the behaviour of all the drifters with respect to the local wind stress vector was essentially the same (i.e., independent of their drogue depth).

Abstract

The impacts of Asian orography on the wintertime atmospheric circulation over the Pacific are explored using altered-orography, semi-idealized, general circulation model experiments. The latitude of orography is found to be far more important than height. The Mongolian Plateau and nearby mountain ranges, centered at ~48°N, have an impact on the upper-level wintertime jet stream that is approximately 4 times greater than that of the larger and taller Tibetan Plateau and Himalayas to the south. Key contributing factors to the importance of the Mongolian mountains are latitudinal variations in the meridional potential vorticity gradient and the strength of the impinging wind—both of which determine the amplitude of the atmospheric response—and the structure of the atmosphere, which influences the spatial pattern of the downstream response. Interestingly, while the Mongolian mountains produce a larger response than the Tibetan Plateau in Northern Hemisphere winter, in April–June the response from the Tibetan Plateau predominates. This result holds in two different general circulation models. In experiments with idealized orography, varying the plateau latitude by 20°, from 43° to 63°N, changes the response amplitude by a factor of 2, with a maximum response for orography between 48° and 53°N, comparable to the Mongolian mountains. In these idealized experiments, the latitude of the maximum wintertime jet increase changes by only ~6°. It is proposed that this nearly invariant spatial response pattern is due to variations in the stationary wavenumber with latitude leading to differences in the zonal versus meridional wave propagation.

Abstract

The Antarctic circumpolar wave (ACW) is a nominal 4-yr climate signal in the ocean–atmosphere system in the Southern Ocean, propagating eastward at an average speed of 6–8 cm s⁻¹, composed of two waves taking approximately 8 years to circle the globe. The ACW is characterized by a persistent phase relationship between warm (cool) sea surface temperature (SST) anomalies and poleward (equatorward) meridional surface wind (MSW) anomalies. Recently, White and Chen demonstrated that SST anomalies in the Southern Ocean operate to induce anomalous vortex stretching in the lower troposphere that is balanced by the anomalous meridional advection of planetary vorticity, yielding MSW anomalies as observed. In the present study, the authors seek to understand how this atmospheric response to SST anomalies produces a positive feedback to the ocean (i.e., an anomalous SST tendency displaced eastward of SST anomalies) that both maintains the ACW against dissipation and accounts for its eastward propagation. To achieve this, we couple a global equilibrium climate model for the lower troposphere to a global heat budget model for the upper ocean. In the absence of coupling, the model Antarctic Circumpolar Current (ACC) advects SST anomalies from initial conditions to the east at speeds slower than observed, taking 12–14 years to circle the globe with amplitudes that become insignificant after 6–8 years. In the presence of coupling, eastward speeds of the model ACC are matched by those due to coupling, together yielding a model ACW of a nominal 4-yr period composed of two waves that circle the globe in approximately 8 years, as observed. Feedback from atmosphere to ocean works through the anomalous zonal surface wind response to SST anomalies, yielding poleward Ekman flow anomalies in phase with warm SST anomalies. As such, maintenance of the model ACW is achieved through a balance between anomalous meridional Ekman heat advection and anomalous sensible-plus-latent heat loss to the atmosphere. This balance requires the alignment of covarying SST and MSW anomalies to be tilted into the southwest–northeast direction, which accounts for the spiral structure observed in global SST and sea level pressure anomaly patterns around the Southern Ocean. Eastward coupling speeds of the model ACW derive from a beta effect in coupling that displaces a portion of the anomalous meridional Ekman heat advection, and its corresponding anomalous SST tendency, to the east of SST anomalies. Therefore, the ACW is an example of self-organization within the global ocean–atmosphere system, depending upon the spherical shape of the rotating earth for its propagation and the mean meridional SST gradient for its maintenance, and producing a net poleward eddy heat flux in the upper ocean that tends to reduce this mean gradient.

Abstract

Transformed Eulerian mean (TEM) equations and Eliassen–Palm (EP) flux diagnostics are presented for the general nonhydrostatic, fully compressible, deep atmosphere formulation of the primitive equations in spherical geometric coordinates. The TEM equations are applied to a general circulation model (GCM) based on these general primitive equations. It is demonstrated that a naive application in this model of the widely used approximations to the EP diagnostics, valid for the hydrostatic primitive equations using log-pressure as a vertical coordinate and presented, for example, by Andrews et al. in 1987 can lead to misleading features in these diagnostics. These features can be of the same order of magnitude as the diagnostics themselves throughout the winter stratosphere. Similar conclusions are found to hold for “downward control” calculations. The reasons are traced to the change of vertical coordinate from geometric height to log-pressure. Implications for the modeling community, including comparison of model output with that from reanalysis products available only on pressure surfaces, are discussed.

Abstract

Soil moisture has important implications for meteorology, climatology, hydrology, and agriculture. This has led to growing interest in development of in situ soil moisture monitoring networks. Measurement interpretation is severely limited without soil property data. In North Carolina, soil moisture has been monitored since 1999 as a routine parameter in the statewide Environment and Climate Observing Network (ECONet), but with little soils information available for ECONet sites. The objective of this paper is to provide soils data for ECONet development. The authors studied soil physical properties at 27 ECONet sites and generated a database with 13 soil physical parameters, including sand, silt, and clay contents; bulk density; total porosity; saturated hydraulic conductivity; air-dried water content; and water retention at six pressures. Soil properties were highly variable among individual ECONet sites [coefficients of variation (CVs) ranging from 12% to 80%]. This wide range of properties suggests very different behavior among sites with respect to soil moisture. A principal component analysis indicated parameter groupings associated primarily with soil texture, bulk density, and air-dried water content accounted for 80% of the total variance in the dataset. These results suggested that a few specific soil properties could be measured to provide an understanding of differences in sites with respect to major soil properties. The authors also illustrate how the measured soil properties have been used to develop new soil moisture products and data screening for the North Carolina ECONet. The methods, analysis, and results presented here have applications to North Carolina and for other regions with heterogeneous soils where soil moisture monitoring is valuable.

Abstract

The short-term forecast accuracy of six different forecast models over the western United States is described for January, February, and March 1996. Four of the models are operational products from the National Centers for Environmental Prediction (NCEP) and the other two are research models with initial and boundary conditions obtained from NCEP models. Model resolutions vary from global wavenumber 126 (∼100 km equivalent horizontal resolution) for the Medium Range Forecast model (MRF) to about 30 km for the Meso Eta, Utah Local Area Model (Utah LAM), and Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). Forecast errors are described in terms of bias error and mean square error (mse) as computed relative to (i) gridded objective analyses and (ii) rawinsonde observations. Bias error and mse fields computed relative to gridded analyses show considerable variation from model to model, with the largest errors produced by the most highly resolved models. Using this approach, it is impossible to separate real forecast errors from possibly correct, highly detailed forecast information because the forecast grids are of higher resolution than the observations used to generate the gridded analyses. Bias error and mse calculated relative to rawinsonde observations suggest that the Meso Eta, which is the most highly resolved and best developed operational model, produces the most accurate forecasts at 12 and 24 h, while the MM5 produces superior forecasts relative to the Utah LAM. At 36 h, the MRF appears to produce superior mass and wind field forecasts. Nevertheless, a preliminary validation of precipitation performance for fall 1997 suggests the more highly resolved models exhibit superior skill in predicting larger precipitation events. Although such results are valid when skill is averaged over many simulations, forecast errors at individual rawinsonde locations, averaged over subsets of the total forecast period, suggest more variability in forecast accuracy. Time series of local forecast errors show large variability from time to time and generally similar maximum error magnitudes among the different models.

Abstract

A number of improvements were implemented on 6 March 1991 into the National Meteorological Center's global model, which is used in the global data assimilation system (GDAS), the aviation (AVN) forecast, and the medium-range forecast (MRF):

• The horizontal resolution of the forecast model was increased from triangular truncation T80 to T126, which corresponds to an equivalent increase in grid resolution from 160 km to 105 km.

• The use of enhanced orography has been discontinued and replaced by mean orography.

• A new marine-stratus parameterization was introduced.

• A new mass-conservation constraint was implemented.

• The horizontal diffusion in the medium scales was reduced by adopting the Leith formulation.

• A new, more accurate sea-surface temperature analysis is now used.

In this note, we discuss each of the changes and briefly review the new model performance.

Abstract

Radar and rain gauge observations collected in coastal mountains during the California Land-Falling Jets Experiment (CALJET) are used to diagnose the bulk physical properties of rainfall during a wet winter season (January–March 1998). Three rainfall types were clearly distinguishable by differences in their vertical profiles of radar reflectivity and Doppler vertical velocity: nonbright band, bright band, and hybrid (seeder–feeder). The contribution of each rainfall type to the total rainfall observed at the radar site (1841 mm) was determined by a new, objective algorithm. While hybrid rain occurred most often, nonbrightband rain (NBB rain) contributed significantly (28%) to the total. This paper focuses on characterizing NBB rain because of the need to document this key physical process and because of its impact on Weather Surveillance Radar-1988 Doppler (WSR-88D) precipitation surveillance capabilities.

NBB rain is a quasi-steady, shallow rain process that does not exhibit a radar bright band, that occurs largely beneath the melting level, and that can produce rain rates exceeding 20 mm h⁻¹. Composite vertical profiles were produced for NBB rain using 1417 samples and brightband rain using 5061 samples. Although the mean rain rate for each composite was 3.95 mm h⁻¹, at all altitudes NBB rain had systematically weaker equivalent radar reflectivity (e.g., 20.5 dBZ _e vs 28.5 dBZ _e at 263 m above ground level) and much smaller Doppler vertical fall velocities (e.g., 2.25 m s⁻¹ vs 6.25 m s⁻¹ at 263 m) than did brightband rain. The reflectivity–rain-rate (Z–R) relationship for NBB rain (Z = 1.2R ^1.8) differs significantly from that of brightband/hybrid rain (Z = 207R ^1.1).

The meteorological context in which NBB rain occurred is described through case studies and seasonal statistics. NBB rain occurred in a wide variety of positions relative to frontal zones within land-falling storms, but three-quarters of it fell when the layer-mean, profiler-observed wind direction at 1250 m MSL (the altitude of the composite low-level jet) was between 190�� and 220°. The importance of orographic forcing during NBB rain, relative to all rain events, was indicated by a stronger correlation between upslope wind speed and coastal rain rates at 1250 m MSL (r = 0.74 vs r = 0.54), stronger low-level wind speeds, and wind directions more orthogonal to the mean terrain orientation.

Abstract

The early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is still debated. Using a two-box impulse response model, we demonstrate that multidecadal ocean variability was unlikely to be the driver of observed changes in global mean surface temperature (GMST) after AD 1850. Instead, virtually all (97%–98%) of the global low-frequency variability (>30 years) can be explained by external forcing. We find similarly high percentages of explained variance for interhemispheric and land–ocean temperature evolution. Three key aspects are identified that underpin the conclusion of this new study: inhomogeneous anthropogenic aerosol forcing (AER), biases in the instrumental sea surface temperature (SST) datasets, and inadequate representation of the response to varying forcing factors. Once the spatially heterogeneous nature of AER is accounted for, the MC period is reconcilable with external drivers. SST biases and imprecise forcing responses explain the putative disagreement between models and observations during the EW period. As a consequence, Atlantic multidecadal variability (AMV) is found to be primarily controlled by external forcing too. Future attribution studies should account for these important factors when discriminating between externally forced and internally generated influences on climate. We argue that AMV must not be used as a regressor and suggest a revised AMV index instead [the North Atlantic Variability Index (NAVI)]. Our associated best estimate for the transient climate response (TCR) is 1.57 K (±0.70 at the 5%–95% confidence level).