Geostrophic Forcing Research Articles

Mesoscale and large‐eddy simulation model studies of the Martian atmosphere in support of Phoenix

In late May of 2008, the NASA/JPL Phoenix spacecraft will touch down near its targeted landing site on Mars (68.2°N, 126.6°W). Entry, descent, and landing (EDL) occurs in the late afternoon (∼1630 hours local solar time (LST)) during late northern spring (Ls ∼ 78°). Using a mesoscale and a large‐eddy simulation (LES) model, we have investigated the range of conditions that might be encountered in the lower atmosphere during EDL. High‐resolution (∼18 km) results from the Oregon State University Mars MM5 (OSU MMM5) are used to understand the hazards from the transient circulations prominent during this season. Poleward of ∼80°N these storms produce strong winds (∼35 m s−1) near the ground; however, owing to the synoptic structure of these storms, and the deep convective mixed layer equatorward of the seasonal cap boundary during EDL, our modeling suggests the spacecraft would not be in winds stronger than ∼20 m s−1 at parachute separation. The storm‐driven variability is much weaker at Phoenix latitudes than it is poleward of the seasonal cap edge (result from an extensive sensitivity study). The OSU MLES model is used to explicitly simulate the hazards of convection and atmospheric turbulence at very high resolution (100 m). This modeling suggests that an upper bound for the maximum expected horizontal‐mean atmospheric turbulent kinetic energy (TKE) is ∼12 m2 s−2, seen ∼3 km above the ground at ∼1430 hours LST. TKE amplitudes are greatest when the horizontal mean wind is large (shear production) and/or the surface albedo is low (a lower albedo enhances buoyancy production, mimicking decreased atmospheric stability after a storm advects colder air into the region). LES simulations predict deep mixed layers (∼6–7 km), ∼1.5 km deeper than the mesoscale model (∼5 km). Mesoscale modeling suggests that the actual landing site differs meteorologically from other longitudes (larger‐amplitude diurnal wind cycle), a consequence of the strong thermal circulations that are excited by the very large regional topography. The OSU MLES model was modified for this work to utilize time‐ and height‐dependent geostrophic wind forcing (constructed from OSU MMM5 results). With this forcing, the OSU MLES provides a site‐specific simulation, where the time/height variability of the horizontal mean LES wind field is in good agreement with the OSU MMM5. On the basis of some statistical analysis, we have good confidence that the “full‐spectrum” wind field is within engineering guidelines for Phoenix EDL.

Mesoscale to Submesoscale Transition in the California Current System. Part II: Frontal Processes

Abstract This is the second of three papers investigating the regime transition that occurs in numerical simulations for an idealized, equilibrium, subtropical, eastern boundary, upwelling current system similar to the California Current. The emergent upper-ocean submesoscale fronts are analyzed from phenomenological and dynamical perspectives, using a combination of composite averaging and separation of distinctive subregions of the flow. The initiating dynamical process for the transition is near-surface frontogenesis. The frontal behavior is similar to both observed meteorological surface fronts and solutions of the approximate dynamical model called surface dynamics (i.e., uniform interior potential vorticity q and diagnostic force balance) in the intensification of surface density gradients and secondary circulations in response to a mesoscale strain field. However, there are significant behavioral differences compared to the surface-dynamics model. Wind stress acts on fronts through nonlinear Ekman transport and creation and destruction of potential vorticity. The strain-induced frontogenesis is disrupted by vigorous submesoscale frontal instabilities that in turn lead to secondary frontogenesis events, submesoscale vortices, and excitation of even smaller-scale flows. Intermittent, submesoscale breakdown of geostrophic and gradient-wind force balance occurs during the intense frontogenesis and frontal-instability events.

Aeolian processes in Proctor Crater on Mars: Mesoscale modeling of dune‐forming winds

Both atmospheric modeling and spacecraft imagery of Mars are now of sufficient quality that the two can be used in conjunction to acquire an understanding of regional‐ and local‐scale aeolian processes on Mars. We apply a mesoscale atmospheric model adapted for use on Mars (the Mars MM5) to Proctor Crater, a 150 km diameter crater in the southern highlands. Proctor Crater contains numerous aeolian features that indicate wind direction, including a large dark dune field with reversing transverse and star dunes containing three different slipface orientations, small and older bright bedforms that are most likely transverse granule ripples, and seasonally erased dust devil tracks. Results from model runs spanning a Martian year, with a horizontal grid spacing of 10 km, predict winds aligned with two of the three dune slipfaces as well as spring and summer winds matching the dust devil track orientations. The primary (most prevalent) dune slipface orientation corresponds to a fall and winter westerly wind created by geostrophic forces. The tertiary dune slipface orientation is caused by spring and summer evening katabatic flows down the eastern rim of the crater, influencing only the eastern portion of the crater floor. The dunes are trapped in the crater because the tertiary winds, enhanced by topography, counter transport from the oppositely oriented primary winds, which may have originally carried sand into the crater. The dust devil tracks are caused by light spring and summer westerly winds during the early afternoon caused by planetary rotation. The secondary dune slipface orientation is not predicted by model results from either the Mars MM5 or the Geophysical Fluid Dynamics Laboratory Mars general circulation model. The reason for this is not clear, and the wind circulation pattern that creates this dune slipface is not well constrained. The Mars MM5 model runs do not predict stresses above the saltation threshold for dune sand of the appropriate size and composition. As with previous work, the calculated wind velocities are too low, which may be caused by too large of a grid spacing.

Vertical density currents: a review of their potential role in the deposition and interpretation of deep-sea ash layers

Marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology. Recent experiments, and field data from the 1991 Pinatubo eruption, challenge tacit assumptions that, following atmospheric fallout on the ocean surface, marine sedimentation of ash-grade material occurs by Stokes Law settling of individual grains. Laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates: further experiments, described herein, involving sustained particle fluxes entering stratified environments, demonstrate that ash particles can decouple from the transporting fluid, whose vertical motions are limited by the stable density gradient. In the ocean, vertical density currents generated by ash-loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1–3 orders of magnitude faster than possible by Stokesian settling, greatly reducing their residence time. Sea-bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column, plus suspension fallout from the polydisperse turbid layer delivered by convective plumes, yielding normal distribution grading. The turbid layer may continue laterally as a (nepheloid) current under the influence of bottom slopes and/or geostrophic forces, either mechanism possibly modifying the grain-size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations. Consequently, and regardless of any subsequent reworking, deep-sea ash isopachs may not directly reflect an eruptive signal, and the ash layers may not directly record the order, rate, or location of arrival of ash particles at the ocean surface.

Mean Structure of the Nocturnal Boundary Layer under Strong and Weak Wind Conditions: EPRI Case Study

Abstract The major objective of this study was to analyze the mean structure and evolution of the nocturnal boundary layer (NBL) under strong and weak wind conditions. Meteorological data collected during the plume-validation experiment conducted by the Electric Power Research Institute (EPRI) over a flat homogeneous terrain at Kincaid, Illinois (39°35′N, 89°25′W), were utilized. A one-dimensional meteorological boundary layer model originally developed by R. A. Pielke, modified with turbulent kinetic energy mixing-length closure, a layer-by-layer emissivity-based radiation scheme, and nonlinear nondimensional temperature and wind profiles in the surface layer, was used. In the four cases that were considered, ranging from strong to weak geostrophic forcing, the model reproduced the observed mean profiles, their evolutions in the NBL, and the inertial oscillations reasonably well. The NBL developed into three layers wherein 1) very close to the surface, radiative cooling dominated over turbulence cooling;...

Upper- and lower-troposphere coupling processes involved in the FASTEX IOP16 frontal cyclone

This paper investigates processes involved in the development of the secondary low sampled on 17 February 1997 during the field phase of the Fronts and Atlantic Storm-Track EXperiment (FASTEX). This low is a fast- moving frontal wave that emerged from the trailing cold front of a parent low situated over Greenland. The dynamics of this cyclone and its environment are described using the European Centre for Medium- Range Weather Forecasts analysis corresponding to Intensive Observing Period 16 of FASTEX. The ageostrophic circulation is retrieved and analysed in the upper and in the lower levels. In the upper levels it is found that the curvature of the upper-level jet plays a crucial role in suppressing or amplifying the vertical motions. In the lower levels, in the region where the cloud head emerges, the vertical motion is scrutinized in the light of forcings which are acting. It is found that the vertical motions are the response to a geostrophic forcing reinforced by diabatic processes. The conjunction of vertical motions in the lower levels associated with vertical motions in the upper levels leads, in the developing period, to a coupling process that seems to be (following theoretical calculations) at the origin of the subsequent development of the cyclone. The instability processes involved in the development of the studied secondary cyclone are also investigated. The atmosphere appears neutral for upright convection while the region of the coupling corresponds to a region which is potentially unstable with respect to slantwise convection. The tendency of the atmosphere is to relax this instability. The vertical motions of the coupling processes are then reinforced by the relaxation of slantwise instability. Copyright © 2002 Royal Meteorological Society

Formation process of intermediate water in baroclinic current under cooling

The formation process of intermediate water in baroclinic current under cooling is investigated using a nonhydrostatic numerical model. After baroclinic instability develops into finite amplitude in a short time, strong downdrafts with a horizontal scale of 1 km are generated near the density front and subduct surface water to depths (∼400 m) along isopycnals. As a result, patches of ventilated water of 10∼20 km horizontal scale with anticyclonic circulation are formed at intermediate depths. Combined effects of baroclinic instability and convection are key dynamics for these phenomena. Convection acts as an initiator for baroclinic instability at the onset and accelerates its subsequent growth by reducing stratification. Developed baroclinic wave forms an intense density front, and downdraft along isopycnals is generated through the frontogenetic process. Density change due to convection intensifies this frontal downdraft by strengthening the geostrophic forcing (tendency to destroy the geostrophic balance) and by reducing potential vorticity (static stability). Further, symmetric instability induced by the density change due to convection and intensified by the frontogenetic process drives slantwise convection, which in turn, enhances the frontal downdraft. Consequent downward velocity becomes >20 times as large as that of the frontal downdraft without convection (cooling) and twice larger than that of pure convection. Since the intensified frontal downdraft moves its position with time and induces divergent flow at depths, a patch of ventilated water with a horizontal scale much larger than that of the frontal downdraft is formed. The role of convection (cooling) in the formation process of intermediate water in this context is to enhance the frontal downdraft rather than to deepen the mixed layer. This scenario is quite different from the one realized when baroclinic instability and convection do not coexist.

The Contribution of Organized Roll Vortices to the Surface Wind Vector in Baroclinic Conditions

Abstract Observations show that, for a given geostrophic forcing, baroclinity acting on the planetary boundary layer produces a nearly sinusoidal modification of the near-surface wind. Compared to barotropic conditions the speed is enhanced in the direction of the thermal wind and the cross-isobar angle increases (decreases) in cold (warm) advection. These modifications are asymmetric with respect to the thermal wind orientation. Two-layer similarity models that match a stratification-dependent surface layer to a stratification and baroclinity dependent Ekman layer simulate aspects of this asymmetric baroclinic modification if the cold advection conditions are more unstably stratified than the warm advection conditions. The authors demonstrate that roll vortices in a baroclinic planetary boundary layer produce an asymmetric surface wind modification in neutral stratification that can work in concert with the coupling between stratification and baroclinity to enhance the net effect of baroclinity on the su...

The omega equation: Q‐G interpretations of simple circulation features

AbstractQuasi‐geostrophic (Q‐G) theory can be used to infer the vertical circulation associated with a synoptic distribution of pressure and temperature in much the same sense as geostrophic theory is used to infer a three‐dimensional distribution of the horizontal circulation. For the purposes of subjective analysis, a diagnosis of the vertical circulation and implied surface development can be based on identifying the sign and magnitude of a geostrophic forcing function associated with the particular distributions of geopotential and thickness shown on a conventional 500 hPa analysis chart. Examples of idealised circulation features are used to demonstrate both the strengths and the weaknesses of this approach, with reference to an analysis based on three different methods of evaluating the geostrophic forcing. Copyright © 1997 Royal Meteorological Society

Evaluation of the RAMS model for estimating turbulent fluxes over the Chesapeake Bay

This study has examined the ability of a mesoscale model to compute pollutant deposition velocity and surface fluxes over water with the accuracy needed for air pollution studies. The Regional Atmospheric Modeling System (RAMS) was evaluated against buoy data over the Chesapeake Bay. Turbulence and surface layer variables (e.g., surface fluxes, friction velocity) and ultimately deposition velocities were primarily examined against the buoy data for 23 cases in 1994. Sensible heat fluxes and temperatures compared well with observations when the model horizontal resolution was reduced to 5 km or less and the first model level were 12 m. Other turbulent variables compared poorly especially under stable stratifications. Overall, latent heat fluxes and deposition velocities were significantly under predicted even with these finer resolutions. Results degraded when the effects of clouds on short- and long-wave radiation were incorporated. Results were also insensitive to increase in the initial sea surface temperature (SST) over the observed daytime range (1°C), and to the use of an improved roughness length parameterization over water. Errors were much larger during stably stratified conditions, therefore, substantial under prediction of pollutants to the bay would be expected by RAMS during stable periods. However, RAMS would be a reliable predictor of fluxes and deposition velocity during unstable conditions. Further experiments were done to help explain the poor predictions during the daytime for stable conditions. Fluxes over water were strongly sensitive to the soil moisture content of the surrounding land masses for stable atmospheric conditions and improved when geostrophic forcing terms were added to the momentum equations. However, the improved results given with geostrophic forcing were either representative of synoptic forcing or a decoupled layer from the surface that could not be resolved.

The force balance of sea ice in a numerical model of the Arctic Ocean

The balance of forces in the sea ice model of Hibler [1979] is examined. The model predicts that internal stress gradients are an important force in much of the Arctic Ocean except in summer, when they are significant only off the northern coasts of Greenland and the Canadian Archipelago. A partition of the internal stress gradient between the pressure gradient and the viscous terms reveals that both are significant, although they operate on very different timescales. The acceleration term is generally negligible, while the sum of Coriolis plus sea surface tilt is small. Thus the seasonal average force balance in fall, winter, and spring is mostly between three terms of roughly equal magnitudes: air drag, water drag, and internal stress gradients. This is also true for the monthly average force balance. However, we find that there is a transition around the weekly timescale and that on a daily basis the force balance at a particular location and time is often between only two terms: either between air drag and water drag or between air drag and internal stress gradients. The model is in agreement with the observations of Thorndike and Colony [1982] in that the correlation between geostrophic wind forcing and the model's ice velocity field is high. This result is discussed in the context of the force balance; we show that the presence of significant internal stress gradients does not preclude high wind‐ice correlation. A breakdown of the internal stress gradient into component parts reveals that the shear viscous force is far from negligible, which casts strong doubt on the theoretical validity of the cavitating fluid approximation (in which this component is neglected). Finally, the role of ice pressure is examined by varying the parameter P*. We find a strong sensitivity in terms of the force balance, as well as ice thickness and velocity.

Evaluation and diagnosis of surface currents in the National Centers for Environmental Prediction's ocean analyses

Ensemble average currents from the 15 m depth level of the National Centers for Environmental Prediction's (NCEP) analyses of the tropical Pacific Ocean are evaluated against surface mixed layer current observations obtained from an extensive set of satellite‐tracked drifting buoys. These averages display many climatological characteristics of the region but are not intended to serve as a climatology because the data from the analyses are trimmed to match the time‐space distribution of the observations. Substantial discrepancies between the analyses and the observations are revealed. First, the near‐equatorial meridional currents and divergence have approximately twice the magnitude in the analyses as in the observations. This discrepancy is largely independent of whether temperature profile data are assimilated or not and is attributed to the parameterization of vertical viscosity. Second, the zonal flow in both the North Equatorial Countercurrent (NECC) and the South Equatorial Current (SEC) is much stronger in the analyses than in the observations, especially in the western Pacific. This discrepancy is associated with assimilation of temperature profile data. It arises because salinity is an active variable in the underlying analysis model but is not controlled by boundary fluxes or other observations. Under the uncontrolled influence of advection and strong horizontal diffusion the salinity distribution becomes nearly homogeneous. Consequently, the analyses do not account for observed temperature‐salinity correlations when density is computed following assimilation of temperature profile data. This leads to erroneous pressure gradients that drive excessively strong geostrophic currents and force large accelerations near the western boundary. Our results indicate that it is important to consider the consequences on the density structure of neglecting salinity during the assimilation of temperature data. We recommend that surface salinity observations from drifting buoys and volunteer observing ships be initiated to improve the ocean analyses.

Mesoscale Atmospheric Circulations over the Southwestern Ross Sea Sector, Antarctica

In this study the mesoscale atmospheric circulation over the southwestern Ross Sea sector during winter is examined. The hydrostatic meso-gamma-scale atmospheric model MAR (Modele Atmospherique Regional) is used. Polar night is assumed, and an idealized large-scale situation is prescribed, with zero geostrophic forcing. The impact of a partial sea-ice cover on the atmospheric circulation is assessed by prescribing lead fractions in the range of the observed values (i.e., between 0% and 30%). Simulations show that the propagation of katabatic airstreams over Terra Nova Bay is facilitated by the presence of leads because the identity of cold, dense, katabatic air is better marked in warmer environmental maritime conditions. Boundary layer fronts and mesocyclone activity are associated with the katabatic airstreams. They are enhanced by the presence of leads. In particular, when the lead fraction is prescribed to be between 20% and 30%, the model simulates mesocyclone intensities comparable to those observed. Taking into account that such a lead fraction is situated in the upper range of the observed values in the central Ross Sea during winter, these results suggest that winter Ross Sea mesocyclones could nor always result from a pure mesoscale forcing. In contrast to the summer situation, no snow precipitation occurs for the simulated winter case, probably because of the too-low absolute humidity content of the air. Such model behavior is in agreement with the observations, which reveal a summer precipitation maximum at McMurdo Station on Ross Island. It is also found that the position of the simulated mesocyclone over Terra Nova Bay is nor sensitive to the lead fraction. Furthermore, this meteorological situation favors the advection of relatively mild and moist maritime air over a long distance in the ice-sheet interior. This process, which is referred to as a moist-air intrusion, could affect the Antarctic ice-sheet mass balance.

Frontal substructures within the planetary boundary layer

A two-dimensional mesoscale model, extended by a TKE closure for the subgrid-scale terms and coupled with a soil model, is used to investigate the role of the Planetary Boundary Layer (PBL) for the development and the substructures of two different types of cold fronts. The effects of turbulent friction, large-scale (geostrophic) forcing and the diurnal variation of the terms of the surface energy balance (SEB) equation on the frontal development are studied by 10 different model runs. The ageostrophic cross-frontal circulation in the lowest two kilometres of a cold front results from friction as well as from large-scale forcing. The first one dominates the PBL processes and causes a special boundary-layer structure, which becomes apparent through the existence of seven characteristic zones defined for the x-z cross sections of potential temperature. The arrangement of these characteristic zones depends on the sense of rotation of the frictionally induced part of the ageostrophic circulation and hence on the direction of the along-front jet within the boundary layer. The daytime increase of the terms of the SEB equation for a midlatitude midsummer case leads to a strong enhancement of the frictionally induced cross-frontal circulation. The arrangement of the seven characteristic zones, however, is approximately conserved.

Variations in Atlantic surface ocean paleoceanography, 50°‐80°N: A time‐slice record of the last 30,000 years

Eight time slices of surface‐water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea‐ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep‐sea cores, 20 of which are accelerator mass spectrometry (AMS)‐14C dated and 30 of which have high (>8 cm /kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The δ18O and δ13C distributions depict three main modes of surface circulation: (1) The Holocene‐style interglacial mode which largely persisted over the last 12.8 14C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice‐free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S‐N flow across the eastern Iceland‐Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface‐water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene‐style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during δ18O event 3.1 and the Bölling. Meltwater phases markedly influenced the European continental climate by suppressing the “heat pump” of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell.An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95PA01453, Variations in Atlantic surface ocean paleoceanography, 50°‐80°N: A time‐slice record of the last 30,000 years, M. Sarnthein et al.) Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.

Pvectors as a Diagnostic Tool for Synoptic-Scale Circulations

Abstract The quasigeostrophic geopotential tendency equation is derived using P vectors in both pressure and entropy coordinates. This vector form of the geostrophic forcing in the geopotential tendency equation is similar to the Q-vector form of the ω equation. It is shown that the horizontal components of P are the advection of geostrophic momentum and the vertical component of P is the horizontal temperature advection. The P vectors are shown to be related to Q and C vectors, as well as the ageostrophic circulation. The three-dimensional pseudocurl of the P vector gives the C vector that equals the ageostrophic pseudovorticity in the quasigeostrophic model. The horizontal components of the pseudocurl of P are perpendicular and proportional to the Q. The horizontal divergence of the P vector is the geostrophic absolute vorticity advection while the three-dimensional divergence of the P vector is the geostrophic quasigeostrophic potential vorticity advection. The ageostrophic wind can be partitioned into...

Mesoscale Response to a Meandering Surface Temperature Interface

Mesoscale variation of the boundary-layer (BL) front produced by a surface temperature interface depends upon the scale of meander along that interface. For a relatively large-scale meander, the circulations are quasi two-dimensional relative to the local interface boundary, and a meander signature appears in the BL structure. For a relatively small-scale meander, alongfront blending eliminates organization about individual meanders to produce a quasi two-dimensional circulation and gradients oriented perpendicular to the mean front. The fundamental atmospheric scale controlling this transition is the mesoscale deformation radius, which depends upon the warm-side BL depth. With strong large-scale geostrophic forcing, however, the resulting alongfront advection length scale increases the meander size required to approach the large-scale limit. Large-scale meanders typically create two local maxima of vertical velocity, whereas small-scale meanders develop a single maximum on the warm side of the frontal zone. At intermediate scales, variations of the vertical velocity maximum are particularly complex when large-scale geostrophic winds are relatively weak.

A Three-Dimensional Numerical Investigation Of a Carolina Coastal Low-Level Jet during GALE IOP 2

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the structure and dynamics of three low-level jets (LLJs) observed during the second intensive observation period of the Genesis of Atlantic Lows Experiment: 1) a Piedmont LLJ along the east slope of the Appalachians, 2) a coastal LLJ (the focus of this study) along the Carolina coastline, and 3) an LLJ to the rein of a cold-frontal system positioned over the Gulf Stream. Geostrophic forcing was important for the formation of the LLJs. Shallow local baroclinity near the top of the cold dome associated with the cold air dammed to the east of the Appalachian Mountains forced the Piedmont LLJ. An analysis of the model momentum tendencies reveals that the coastal LLJ developed and was maintained by strong geostrophic forcing associated with the coastal baroclinic zone, and its strength was modulated by strong, inertial accelerations. Significant horizontal structure in the coastal LLJ developed during the daytime as a result of the different vertical mixing properties associated with continental and maritime parcel source regions. Model sensitivity experiments indicate that diabatic processes substantially influence the evolution of the coastal and cold-frontal LLJs, Latent heating associated with banded precipitation over the Gulf Strum to the rear of the front was the primary Forcing mechanism for the frontal LLJ. Sensible heating within the marine atmospheric boundary layer acted to enhance the coastal baroclinic zone and low-level geostrophic forcing, and to subsequently strengthen the coastal LLJ. Cold-air damming and strong lower-tropospheric sensible and latent heating in the vicinity of the Gulf Stream, which frequently occur during autumn and winter months along the East Coast, combine to produce a favorable mesoscale environment for LLJ formation with a wind direction parallel to the coastline.

The Impact of the Sea Surface Temperature Resolution on Mesoscale Coastal Processes during GALE IOP 2

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the sensitivity of the structure and evolution of mesoscale coastal phenomena to the sea surface temperature (SST) distribution in the vicinity of the Gulf Stream during intensive observation period 2 (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE). Experiments are performed with three different SST analyses: A 1) a high-resolution 14-km analysis, 2) a medium-resolution 275-km analysis, and 3) a coarse-resolution 381-km analysis. The results indicate that numerical simulations of mesoscale phenomena embedded in the marine atmospheric boundary layer (MABL) in the vicinity of the Gulf Stream are very sensitive to the SST distribution. The total (sensible and latent) average heat fluxes differ by less than 15% among the three experiments; however, the mesoscale distributions of the oceanic surface heat fluxes differ substantially. As a result of large differences in the lower-tropospheric diabatic heating, significant dissimilarities occur among the three experiments in terms of the intensity and movement of the north-wall MABL front, MABL structure, coastal front, cyclone, and precipitation. The maximum values of diagnosed quantities (e.g., vorticity, divergence, thermal gradients, and frontogenesis) in the vicinity of the Gulf Stream vary by as much as a factor of 8 among the three simulations. Also, the lower-tropospheric geostrophic forcing along the coast is relatively weak in the two simulations that used lower-resolution SST analyses. This weak geostrophic forcing in the lower-resolution SST experiments results in the development of a low-level jet that is weaker than observed and simulated in the experiment with the high-resolution analysis. Among the three experiments, the high-resolution SST analysis simulation best captures the analyzed intensity, structure, and movement of the mesoscale coastal phenomena. Thus, the use of high-resolution SST analyses in research and operational mesoscale models may be essential in some cases for the accurate prediction of coastal cyclones and their associated mesoscale structures.

Ageostrophic Pseudovorticity and Geostrophic C-Vector Forcing—A New Look at the Q Vector in Three Dimensions

Abstract By combining the two Q-vector component equations with the third quasigeostrophic (QG) diagnostic equation (the vertical ageostrophic vorticity equation) a complete set of QG diagnostic equations is formed in a three-dimensional vector form with the ageostrophic pseudovorticity vector on the left-hand side and a newly defined geostrophic forcing vector (the C vector) on the right-hand side. The horizontal projection of the C vector is a rotated Q vector (by 90° to the right). The vertical C-vector component is proportional to the Gaussian curvature of the geopotential surface of constant pressure. Since C-vector streamlines can be viewed as ageostrophic pseudovortex lines, ageostrophic circulations can be easily inferred through three-dimensional “vorticity thinking,” which considers both the boundary effect and moist processes. The C vector is interpreted physically in terms of generation of Coriolis force curl and buoyancy curl due to the geostrophic advection alone. The basic techniques and po...