Alongfront Variability Research Articles

Comments on “A CloudSat–CALIPSO View of Cloud and Precipitation Properties across Cold Fronts over the Global Oceans”

Naud et al. constructed satellite-based composite analyses of clouds and precipitation across cold fronts. However, their approach does not exclude occluded fronts, does not separate anafronts from katafronts, does not separate frontlike phenomena primarily identified by thermal gradients from those primarily identified by wind changes, and smooths over alongfront variability. By lumping these disparate frontal structures together, the front-centered composite cross sections reveal forward-sloping structures and weak gradients across them, raising questions about how to interpret their composite cross sections.

Variability of Precipitation along Cold Fronts in Idealized Baroclinic Waves

AbstractPrecipitation patterns along cold fronts can exhibit a variety of morphologies including narrow cold-frontal rainbands and core-and-gap structures. A three-dimensional primitive equation model is used to investigate alongfront variability of precipitation in an idealized baroclinic wave. Along the poleward part of the cold front, a narrow line of precipitation develops. Along the equatorward part of the cold front, precipitation cores and gaps form. The difference between the two evolutions is due to differences in the orientation of vertical shear near the front in the lower troposphere: at the poleward end the along-frontal shear is dominant and the front is in near-thermal wind balance, while at the equatorward end the cross-frontal shear is almost as large. At the poleward end, the thermal structure remains erect with the front well defined up to the midtroposphere, hence updrafts remain erect and precipitation falls in a continuous line along the front. At the equatorward end, the cores form as undulations appear in both the prefrontal and postfrontal lighter precipitation, associated with vorticity maxima moving along the front on either side. Cross-frontal winds aloft tilt updrafts, so that some precipitation falls ahead of the surface cold front, forming the cores. Sensitivity simulations are also presented in which SST and roughness length are varied between simulations. Larger SST reduces cross-frontal winds aloft and leads to a more continuous rainband. Larger roughness length destroys the surface wind shift and thermal gradient, allowing mesovortices to dominate the precipitation distribution, leading to distinctive and irregularly shaped, quasi-regularly spaced precipitation maxima.

Numerical Study of Horizontal Shear Instability Waves along Narrow Cold Frontal Rainbands

Abstract The effects of variations in low-level ambient vertical shear and horizontal shear on the alongfront variability of narrow cold frontal rainbands (NCFRs) that propagate into neutral and slightly unstable environments are investigated through a series of idealized cloud-resolving simulations. In cases initialized with slightly unstable sounding and weak ambient cross-frontal vertical shears, core-gap structures of precipitation along NCFRs occur that are associated with wavelike disturbances that derive their kinetic energy mainly from the mean local vertical shear and buoyancy. However, over a wide range of environmental conditions, core-gap structures of precipitation occur because of the development of a horizontal shear instability (HSI) wave along the NCFRs. The growth rate and amplitude of the HSI wave decrease significantly as the vertical shear of the ambient cross-front wind is reduced. These decreases are a consequence of the enhancement of the low-level local vertical shear immediately behind the leading edge. The strong local vertical shear acts to damp the vorticity edge wave on the cold air side of the shear zone, thereby suppressing the growth of the HSI wave through the interaction of the two vorticity edge waves. It is also noted that the initial wavelength of the HSI wave increases markedly with increasing horizontal shear. The local vertical shear around the leading edge is shown to damp long HSI waves more strongly than short waves, and the horizontal shear dependency of the wavelength is explained by the decrease in the magnitude of the vertical shear relative to that of the horizontal shear.

Alongfront Variability of Precipitation Associated with a Midlatitude Frontal Zone: TRMM Observations and MM5 Simulation

Abstract On 19 February 2001, the Tropical Rainfall Measuring Mission (TRMM) satellite observed complex alongfront variability in the precipitation structure of an intense cold-frontal rainband. The TRMM Microwave Imager brightness temperatures suggested that, compared to the northern and southern ends of the rainband, a greater amount of precipitation ice was concentrated in the middle portion of the rainband where the front bowed out. A model simulation conducted using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is examined to explain the distribution of precipitation associated with the cold-frontal rainband. The simulation reveals that the enhanced precipitation ice production and the implied mean ascent along the central part of the front were associated with a synergistic interaction between a low-level front and an upper-level front associated with an intrusion of high-PV stratospheric air. The low-level front contributed to an intense bow-shaped narrow cold-frontal rainband (NCFR). The upper-level front was dynamically active only along the central to northern portion of the NCFR, where the upper-level PV advection and Q-vector convergence were most prominent. The enhanced mean ascent associated with the upper-level front contributed to a wide cold-frontal rainband (WCFR) that trailed or overlapped with the NCFR along its central to northern segments. Because of the combination of the forcing from both lower- and upper-level fronts, the ascent was deepest and most intense along the central portion of the front. Thus, a large concentration of precipitation ice, attributed to both the NCFR and WCFR, was produced.

Numerical Study of Precipitation Core-Gap Structure along Cold Fronts

Abstract The mechanism responsible for the core-gap structure of precipitation along narrow cold-frontal rainbands (NCFRs) is investigated through analyses of idealized cloud-resolving simulations of cold fronts. The control simulation, in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy (CAPE) of ∼60 J kg−1, reproduces the typical alongfront variability of observed NCFRs. The simulated NCFR is broken up into regularly spaced, ellipsoidal cores oriented at a clockwise angle to the cold front. While horizontal-shear instability (HSI) has frequently been proposed as a mechanism leading to the alongfront variability of NCFRs, no characteristic features of HSI are recognized in the simulated vertical vorticity field at the leading edge of the cold front. The alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front. The wave phase lines are oriented nearly perpendicular to the direction of mean vertical shear, with enhanced (suppressed) precipitation occurring at the wave updrafts (downdrafts). An analysis of the eddy kinetic energy budget indicates that the wavelike disturbance derives most of its energy from the mean vertical shear and the buoyancy. Sensitivity experiments reveal a systematic relationship between the alongfront variability of NCFRs and the stability of the prefrontal thermal stratification. Simulated precipitation cores remain essentially parallel to the cold front when the prefrontal environment is absolutely stable or almost neutral to surface parcel ascent. The typical alongfront variability of NCFRs is reproduced for weakly unstable environments with small amounts of CAPE (≤140 J kg−1). On the other hand, simulations with sufficiently unstable environments produce precipitation cores oriented counterclockwise to cold fronts.

Research Aircraft Observations of the Mesoscale and Microscale Structure of a Cold Front over the Eastern Pacific Ocean

The structure of an oceanic cold front is described on the basis of research aircraft observations taken during the Ocean Storms field experiment. Synoptic and mesoscale analyses compare the structure of an upper-level jet-front system observed slightly downstream from the wind speed maximum to its structure in the upstream entrance region. Stratospheric potential vorticity and ozone were found within the frontal zone down to about 800 mb. Microscale analyses of the front near the sea surface were carried out for a portion of the front having the signature of a 'rope' cloud in satellite imagery. A narrow (less than 1 km) zone of upward motion (about 4 m/s) and of horizontal shear (about 0.01/s) characterized the front near the surface. Significant alongfront variability was found, including lateral displacements in the frontal zone where there were weaker updrafts.

Linear Stability Models of Shelfbreak Fronts

Abstract The stability of inviscid frontally trapped waves along a shelfbreak is examined to determine whether frontal instabilities may contribute to the alongfront variability frequently observed. Three different basic states with increasingly complex stratification are treated. In each case, the shelfbreak occurs where a constant depth shelf meets a slope region with linearly increasing depth. The first basic state consists of an unstratified flow with a velocity discontinuity located at the shelfbreak. The resulting shear waves are stable for the velocity shear and topography typical of the Middle Atlantic Bight. Next, a two-layer, geostrophically balanced front located at the shelfbreak is considered. This is similar to the configuration of Flagg and Beardsley, but with an unbounded bottom topography offshore beyond the frontal region. For parameters typical of the Middle Atlantic Bight in winter, the most unstable wave is surface-trapped with a typical e-olding growth time scale of 9.5 days. The mos...

Stability of a coastal upwelling front: 2. Model results and comparison with observations

A two‐layer shallow water equation model is used to investigate the linear stability of a coastal upwelling front. The model features a surface front parallel to a coastal boundary and bottom topography which is an arbitrary function of the cross‐shelf coordinate. The existence of unstable waves on the model coastal upwelling front, as suggested by the general stability theorem developed in a companion paper, is confirmed by solving directly the linearized equations of motion. The unstable wave motions are frontally trapped and dominant in the upper layer. The wave propagates phase in the direction of the basic state flow, and the primary energy conversion is via baroclinic instability. The effect of varying the model parameters is presented. Moving the front closer than ∼2 Rossby radii to the coastal boundary results in a decrease in the growth rate of the fastest growing wave. Increasing the overall vertical shear of the basic state flow, by either decreasing the lower layer depth or increasing the steepness of the interface, results in an increase in the growth of the fastest growing wave. A bottom sloping in the same sense as the interface results in a decrease of the growth rates and alongfront wave numbers of the unstable waves in the system. Linearized bottom friction is included in the stability model and results in a decrease in the growth rates of the unstable waves by extracting energy from the system. Since the unstable mode is strongest in the upper layer, bottom friction will not stabilize the upwelling front. A comparison between the predictions from the simple two‐layer model and observed alongfront variability for three areas of active upwelling is presented. Reasonable agreement is found, suggesting that the observed alongfront variability can be interpreted in terms of the instability of a coastal upwelling front.

Variability of Frontal Structure in the Southern Norwegian Sea

Abstract A hydrographic survey (CTD) was conducted in the vicinity of the Iceland–Faeroe Island oceanic front (IFF) north of the Faeroe Islands during October 1980. It consisted of CTD transects on three horizontal scales ranging from kilometers to hundreds of kilometers. Intense interleaving of different water masses is found in the IFF in the presence of horizontal current shear. Significant alongfront variability on scales of about 50 km is present, consistent with earlier findings. Estimates of cross-front heat flux of 5.16 × 104 W m−2 and salt flux of 1.58 g m−2 s−1 are greater than those found for the Antarctic Polar Front but are of the same order as eddy heat flux across the IFF found by Willebrand and Meincke. Evidence suggests that intrusive interleaving in the IFF on 50 m vertical scales is driven by double-diffusive convection.