Abstract

Sanders and Gyakum's (1980) 24-hour criterion (a central pressure fall that averages at least 1 mb h⁻¹ for 24 h) of a rapidly deepening extratropical surface cyclone is adopted to investigate explosive surface cyclogenesis associated with cyclones in polar air streams and frontal waves induced by cyclones in polar air streams. A case is described in detail in which both the cold air vortex and frontal wave meet the criterion. Other examples of rapid cyclogenesis in which only the cold air cyclone or frontal wave satisfy the criterion are given. These examples confirm that cyclones in polar streams and frontal waves induced by cyclones in polar air streams can exhibit rapid enough deepening rates to fulfill Sanders and Gyakum's criterion.

These events are located downstream of a mobile upper-tropospheric trough, within or poleward of the jet stream axis, and within or ahead of a planetary-wave trough. A quasi-geostrophic diagnosis of the case described in detail reveals computed pressure falls much less than those observed. It appears likely that quasi-geostrophic deepening rates for the other examples would also be far short of observed rates. These findings are in accordance with Sanders and Gyakum's (1980) observations.

Abstract

A polar air stream cyclogenesis over the wintertime North American Continent is documented. Numerous similarities are found between this land case and oceanic polar lows studied Harrold and Browning (1969), Reed (1979) and Mullen (1979). The main difference between the land case and oceanic polar lows is that the land cyclone develops in an air mass with negligible water vapor content whereas oceanic ones clearly do not since they are characterized by enhanced cumulus activity during their early stage of development Hence, it is concluded that cumulus convection and its associated latent heat release are not essential for cyclone development in a polar air stream. It is also shown that the surface occlusion which forms from the merger of the polar trough with the main polar front does not follow the typical surface occlusion process as described by the Norwegian cyclone model.

Abstract

The large-scale environment of small synoptic-scale cyclones that occur in polar air streams over the wintertime North Pacific behind or poleward of major frontal bands is objectively documented by the compositing of meridional (latitude-height) cross sections for 22 cases. The cold air mass cyclones are found to be associated with deep baroclinity throughout the troposphere and are located on the low-pressure side of well-developed jet streams in regions of strong cyclonic wind shear. The lower troposphere is conditionally unstable and in the early stage of development is strongly heated from below by the warmer ocean.

Based on the results of the composites, and theoretical considerations, it is concluded that the oceanic polar air cyclones are a baroclinic instability phenomenon whose small scales relate to low values of the Richardson number near the surface and whose large upper level amplitudes relate to the effects of latent heat release on baroclinic wave development. Barotropic instability cannot be ruled as a possible contributing factor in their formation.

Abstract

The net forcing of blocking flows by transient eddies having synoptic time scales is examined within the framework of quasi-geostrophic theory. Temporal filtering is used to distinguish the effects of transients having synoptic lime scales (i.e., periods less than approximately seven days) from those having longer time scales. Convergences of transient eddy transports of heat and vorticity during blocking conditions are computed from highpass-filtered time series. The eddy flux convergences for individual blocking events are then composited according to the locations of the blocks. The quasi-geostrophic potential vorticity equation relating geopotential tendency and the eddy flux convergences of heat and vorticity is solved for composite blocking conditions. The diagnosis is performed for both observed blocks and blocks simulated by a general circulation model.

The net quasi-geostrophic geopotential tendencies due to transports by the synoptic-scale transient eddies exhibit a quadrature relationship with the blocking pattern throughout the troposphere, with anticyclonic eddy forcing being located about one-quarter wavelength upstream of the blocking anticyclone. The net quasi-geo- strophic temperature tendencies due to transports by the synoptic-scale transient eddies tend to be out of phase with the temperature perturbations of the block. These relationships were reproducible in all of the observed and model blocking events examined.

The effects of transient disturbances on blocking flows as depicted by our diagnostic technique are contrasted with those for earlier studies. Reasons for the inconsistencies in the results of earlier studies are discussed.

Abstract

Blocking anticyclones that appear in perpetual January simulations of a spectral general circulation model are examined. Blocks in three geographical regions are studied: the North Pacific, the North Atlantic and western North America. Local time-averaged balances of vorticity and heat are evaluated for composite cases of blocking. The following common relationships emerged from these budgets.

The time-mean divergence term is, in general, a flat-order term in the vorticity balance throughout the troposphere and its pattern over severe orography is closely related to the underlying topography. Above the surface layer, the horizontal advection of time-mean absolute vorticity by the mean wind mainly balances the divergence term with the net effect of the time-mean vorticity forcing being a tendency for the blocking pattern to propagate downstream. The transient eddy vorticity transports act to shift the block upstream and hence they mainly offset the downstream tendency due to the time-mean flow; the magnitude of the eddy vorticity term is typically one-third to one-half that of the divergence or advection terms alone. Frictional dissipation is negligible everywhere except near the ground where it primarily offsets the divergence term.

The horizontal advection of the time-mean temperature field by the mean wind throughout the troposphere is a first-order term in the beat balance and is mainly responsible for maintaining the block's thermal perturbations; it is predominately balanced by adiabatic heating in the free troposphere and by diabatic heating near the surface. Transient eddy heat transports act to dissipate the block's thermal perturbations at all levels, while diabatic heating does not exhibit a systematic relationship with the temperature field at any level.

A quasi-geostrophic diagnosis of the ageostrophic motion field suggests that dynamical processes which strongly affect the vorticity balance may be more important to the maintenance of model blocks than processes which strongly affect the heat balance. The mountains appear capable of influencing the shape of the model blocks, but preliminary results indicate that orographic forcing may not be absolutely essential for the blocking process to occur in the model.

Abstract

Sensitivity experiments with a perpetual January version of a low-resolution general circulation model are conducted to investigate the influence of mountains on blocking in the Northern Hemisphere. Two 1200-day integrations, one with and the other without mountains, are compared.

The absence of orography severely reduces the total number of days on which blocking is present anywhere over the Northern Hemisphere. It decreases the number of blocking episodes and shortens the duration of an average event. Strong blocking events do occur in the no-mountains simulation, however, and their structure and local maintenance characteristics are similar to strong blocks in the control run.

It is concluded that orographic forcing greatly aids the blocking process in the model but is not crucial for the existence of model blocks.

Abstract

Sensitivity experiments with a perpetual January version of a low-resolution general circulation model are conducted to investigate the influence of Pacific sea surface temperature (SST) anomalies on blocking in the Northern Hemisphere. Six 1200-day integrations, each of which contains a different specified SST anomaly superimposed upon a fixed climatological SST distribution, are compared with a 1200-day control simulation to examine the impact of both equatorial Pacific and midlatitude North Pacific anomalous forcing. An ensemble of eight independent 90-day realizations is extracted from each simulation. Distributions of the ensemble-mean differences in the total number of blocking days from the control simulation are presented, along with estimates of the statistic significance of these differences.

Pacific SST anomalies do not strongly affect the total number of days in which a block is present anywhere over the North Pacific basin, but they apparently can affect the preferred regions over the North Pacific basin where model blocking occurs as follows: A warm tropical Pacific SST anomaly tends to enhance blocking activity along the west coast of North America and suppress it near the Aleutian Islands, while a cool midlatitude North Pacific SST anomaly near the dateline tends only to suppress blocking activity near the Aleutian Islands. The combination of a warm tropical SST anomaly and a cool extratropical SST anomaly is more effective in modifying blocking activity over the midlatitude North Pacific than either SST anomaly acting alone. The inclusion of either a tropical or extratropical Pacific SST anomaly does not produce a change in the blocking activity deemed significant over other regions of the Northern Hemisphere.

Based on the results of this paper and previous studies, it is concluded that certain Pacific SST anomaly patterns having realistic amplitudes can favor the onset of the recurrence of blocking in a particular geographical region in the model, but they do not appear to affect strongly either the persistence of an individual blocking event once initiated or the dynamical processes important to its maintenance as reflected in local balances of heat and vorticity.

Abstract

Sensitivity experiments with a perpetual January version of a low-revolution general circulation model (GCM) are conducted to investigate the influence of different, nonzero specifications of orography on low-frequency variability (LFV) and blocking in the Northern Hemisphere. Two 1200-day integrations are compared to examine the impact of an enhanced “envelope” orography. An ensemble of eight independent 90-day realizations is extracted from each simulation. Distributions of ensemble-mean statistics for the two simulations are presented, along with estimates of the statistical significance of the differences.

The use of an envelope orography leads to significant changes in the distribution of the LFV (periods 10–90 days) over the Northern Hemisphere. When the LFV is partitioned into contributions from intramonthly ( 10–30-day periods) and intermonthly (30–90-day periods) fluctuations, it is found that the envelope orography significantly alters the distributions of intramonthly scale variability over the North Atlantic Ocean.

The impact of envelope orography on blocking, as measured by an objective criterion, is then examined. Significant changes in its spatial distribution are found over the North Atlantic but not in the total number of blocking days occurring anywhere within the Atlantic basin. No significant changes are found over the North Pacific. The changes in blocking distribution over the North Atlantic make the model's climatology more consistent with observations.

The GCM results are interpreted in light of results from simple modeling studies. Based on this comparison, it is hypothesized that the changes in LFV and blocking over the North Atlantic are a response to differences in the orographic forcing downstream of the Rocky Mountains.

It is concluded that a modest change in the representation of orography can significantly affect local distributions of intramonthly variability and blocking in a low-resolution GCM.

Abstract

The systematic error and uncertainty of surface analyses produced by the National Meteorological Center (NMC) over the North Pacific Ocean is estimated. Differences in cyclone position, central pressure, and 12-h deepening rate between an operational manual analysis for the Northern Hemisphere (NH) and the automated initial analysis for the aviation run (AVN) of NMC's Global Forecast Model are compared, and their statistical significance is judged.

The AVN central pressures are, on average, 2.2 mb higher than the NH values. The AVN initial maps underestimate cyclone intensity at all central pressures values, but the error is biggest (3.9 mb) for deep systems (central pressures ≤980 mb). The absolute displacement error averages 210 km with standard deviation of 200 km. Position errors are largest for weak systems (central pressures ≥1000 mb), averaging 300 km and being more than double the 140 km value for deep lows. The aggregate mean vector displacement of 10 km is negligible.

Cyclone positions for the AVN initial maps agree to within 130 km of the NH locations for rapid deepening lows (12 h pressure change ≤ −12 mb), the smallest mean displacement error for any category examined. The life cycle of cyclones, as portrayed by the AVN initial charts, is too slow, with the AVN maps underestimating the 12-h deepening rate for rapid deepening lows by 5.4 mb, for all other deepening lows by 1.5 mb, and the 12-h filling rate for all cyclolytic lows by 2.4 millibars.

Uncertainty in the central pressure and 12-h deepening rate, as measured by the width of confidence intervals, is largest for deep lows and for rapidly deepening cyclones, respectively, but the uncertainty in cyclone position is smallest for such systems. Weak lows or those undergoing little 12-h pressure change exhibit the greatest uncertainty in their position but they are more consistently defined in terms of central pressure.

Implications of the uncertainty estimates on forecast verification and the generation of initial perturbations for ensemble forecasts are discussed.

Abstract

The statistical character of precipitation events from hourly stage IV analyses is documented for the eastern United States during the cool [December–February (DJF)] and the warm [June–August (JJA)] seasons for the four years of 2002–05. Isotropic e-folding distances and in situ e-folding times are computed for mesh sizes that vary from 4 km (the minimal stage IV pixel size) to 32 km for two thresholds: light (1 mm h⁻¹) and heavy (5 mm h⁻¹) precipitation rates. Marked seasonal variability characterizes the e-folding times. They typically run between 2 and 3 h during winter and 1 and 2 h during summer for light events, and they run an hour shorter for heavy rainfall during both seasons. Spatial decorrelation estimates also reveal considerable seasonal and geographical variability; e-folding distances typically lie between 60 and 180 km during the winter and between 30 and 60 km during the summer for light episodes, and they are approximately a factor of 2 to 3 shorter for heavy events. Anisotropic statistics are estimated by a simple geometric model. Hourly precipitation patterns show a preference for a southwest–northeast orientation during both seasons with greater elongation during the winter. Mean propagation velocities of precipitating systems are faster and are more closely aligned with the dilatation axis during the winter. These statistics should provide useful guidance for diagnosing and improving the spatiotemporal variance characteristics of precipitation for downscaling algorithms and numerical models of hydrometeorological prediction systems.