Canadian Atlantic Storms Program Research Articles

Microwave Properties of Frozen Precipitation around a North Atlantic Cyclone

Abstract Microwave brightness temperatures emanating from a North Atlantic cyclone were measured by the Special Sensor Microwave/Imager (SSM/I) on the Defense Meteorological Satellite Program satellite. As other investigators have found before, low 85.5-GHz brightness temperatures (215 ± 20 K) were observed from cumulonimbus clouds along the squall line; however, 85.5-GHz microwave brightness temperatures observed from the nimbostratus clouds north of the low center were significantly higher (255 ± 20 K). In situ measurements from aircraft during the Canadian Atlantic Storm Program II showed that heavy snowfall consisting of large tenuous aggregates existed in the nimbostratus clouds at the time of the SSM/I overpass. Distributions of snow, rain, liquid cloud water, and cloud ice mass were computed from a modified version of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model. That model employed a mixed-phase ice microphysics (MPIM) scheme that only considered one type of frozen hydro...

Observations and Numerical Simulations of the Origin and Development of Very Large Snowflakes

Abstract The Canadian Atlantic Storms Program (CASP II) field experiment was conducted near St. John’s, Newfoundland, Canada, during January–March 1992, and it focused on the nature of winter storms. Analyses of CASP II aircraft, surface, satellite, and radar observations collected during an intensive study of the origin and development of 9 mm h−1 precipitation containing 4–5-cm diameter snowflakes are compared in this article with results of the MM5 (mesoscale) and Mitchell (microphysical) models. MM5 simulations of the thermal, kinematic, and bulk microphysical fields were in good agreement with the observations; this comparison provided the basis for extending the spatial and temporal scales of the aircraft observations to a larger-scale domain using the model results. The Mitchell analytical–numerical model was used to improve the understanding of the microphysical processes that led to the development of the very large snowflakes. A synthesis of results using the different techniques leads to the co...

Low‐frequency currents at the northern shelf edge of the Grand Banks

The structure and generation mechanisms of low‐frequency currents at the northern shelf edge region of the Grand Banks are investigated using data from a bottom mounted acoustic Doppler current profiler (ADCP) and an array of conventional current meters collected during the Canadian Atlantic Storms Program II. The vertical structure of the current is determined from an empirical orthogonal function analysis of the ADCP data. The analysis shows that over the shelf edge the low‐frequency currents are predominantly barotropic and polarized in the long‐shelf direction. There is little coherence between the local wind and the barotropic current, while baroclinic currents are moderately coherent with the local wind. In the interior of the shelf, the currents and the local wind are highly coherent. The difference in response to wind forcing over the shelf edge and in the shelf interior suggests that currents in the shelf interior are wind driven, while currents over the shelf edge are generated by mechanisms other than direct wind forcing. Possible generation mechanisms for the barotropic long‐shelf low‐frequency currents are discussed, including meanders and eddy formation in the Labrador Current and continental shelf waves generated by distant storms.

Comparison of Canadian daily ice charts with surface observations off Newfoundland, winter 1992

Abstract Forecast ice drift rates and thicknesses displayed on daily ice charts and forecast winds for the Canadian east coast are compared to on‐ice observations made during the second Canadian Atlantic Storm Program (CASP II) of March 1992. Observed and 24‐hour forecasts of daily ice drift rates were weakly correlated even though long‐term means closely matched observations. Daily drift rates have an RMS error of 13 cm s‐1 relative to a 15 cm s‐1 mean in addition to an RMS direction error of 50 degrees. Contributions towards daily drift uncertainties were: the estimation of winds, unmodelled physics of ocean and ice cover processes; and the inconsistency in the methods used by the ice forecaster. Correlation coefficients between forecast winds and on‐ice observed winds decreased from 0.8 at 0‐hour to 0.7 for the 30‐hour forecast. Similar results were found between ice drift rates from forecast winds. Histograms of ice thicknesses observed along narrow swaths using a helicopter‐towed electromagnetic sensor compared well with undeformed ice thicknesses representing large areas on ice charts, with differences mainly caused by difference in ice type representation and by co‐registering the two data sets.

CASPII and the Canadian cyclones during the 1989–92 cold seasons

Abstract The climatological characteristics of cyclogenesis and the associated storm tracks in the domain of 40–70°N and 50–140° W are studied for the four cold seasons of 1989–1992. It is found that surface cyclones affecting the Canadian territories originate mostly from the eastern slopes of the Rocky Mountains, the Atlantic coast of the United States (U.S.) and the eastern North Pacific basin. The explosive deepening of these systems is shown to occur preferentially along the Atlantic coast of both Canada and the U.S. It is also found that regions of most frequent occurrence of cyclones include Baffin Island, the Davis Strait, the northern Great Lakes and northern Newfoundland. A more detailed examination of the Canadian Atlantic Storms Program II (CASP II) season of January‐March 1992 shows an anomalously large number of rapid cyclogenesis events occurring in the Atlantic coastal region of Canada. This anomaly is found in association with a persistent large‐scale region of an unusually cold air mass.

The life cycle of the intense IOP‐14 storm during CASP II. Part II: Sensitivity experiments

Abstract In this paper, two consecutive (48‐ and 54‐h) predictions of the IOP‐14 storm during the Canadian Atlantic Storm Program II (CASP II) with a mesoscale version of the Canadian Regional Finite‐Element (RFE) model are used as control runs to investigate the sensitivity of the cyclone development to different initial conditions and various physical and dynamical processes. It is shown that obtaining appropriate initial conditions is crucial in helping to improve the operational predictions of initial cyclogenesis and its subsequent amplification, particularly for travelling disturbances that propagate from upstream data‐sparse regions. It is found that at the cyclone's mature stage i) dry dynamics account for more than 70% of the cyclone's total depth; ii) the RFE model predicts the weakest (+32 hPa) and deepest (–12 hPa) storms (with respect to the control‐predicted) in the absence of the oceanic surface characteristics and Greenland topography, respectively; iii) concurrent surface sensible and latent heat fluxes have relatively weak positive impacts (–5 hPa) on the explosive deepening of the storm; iv) all experimental storms except for the no‐ocean‐surface run satisfy the “oceanic bomb” criterion; and v) the observed and predicted non‐classical frontal structures, such as the cold frontal “fracture”, the “bent‐back” warm front, the “T‐bone” thermal pattern and warm core structure, fail to develop in the absence of the oceanic surface characteristics. The results reveal that i) the IOP‐14 storm is baroclinically driven in nature and it is only modulated by other physical processes and ii) the explosively deepening nature and the previously documented non‐classical frontal structures appear to result from weak surface drag over the ocean.

Forecasting of Supercooled Clouds

Abstract Using parameterizations of cloud microphysics, a technique to forecast supercooled cloud events is suggested. This technique can be coupled on the mesoscale with a prognostic equation for cloud water to improve aircraft icing forecasts. The procedure is validated using comparisons with airborne measurements from the Canadian Atlantic Storms Program. As an illustration of the application of this forecast technique, constant-pressure maps showing regions of cloud ice, supercooled cloud water, and cloud liquid water are presented for two particular cases.

The Response of Ocean Waves to Turning Winds

Abstract A model is developed for the response of waves to turning wind, which we denote as the fetch relaxation model. Comparison is presented with the standard 𝒷 relaxation model pioneered by Hasselmann et al. The fetch relaxation model is shown to surpass the standard 𝒷 model, with respect to observations collected during the Canadian Atlantic Storm Program (CASP) of 1986. Both models are based on the premise that the net growth of energy due to the wind is centered on the wind direction. The fetch relaxation model also conforms to relations for fetch-limited growing waves derived from the CASP observations. Moreover, the fetch relaxation model is shown to provide estimates for the role of the drag coefficient in turning wind situations.

Physical, radiative, and dynamical processes within a nighttime marine stratus cloud

Observations taken by aircraft and conventional platforms are used to investigate dynamical, physical, and radiative processes within a marine stratus cloud during the Canadian Atlantic Storms Program (CASP) II field project which took place over the east coast of Canada. Stratus which formed over the ocean on February 6, 1992 during the nighttime, is studied to analyze cloud top and base processes. The cloud was supercooled during the study period. Fluctuations and fluxes are calculated along constant flight altitude legs approximately 100 km long in space. The scales of structures larger than 5 km are removed from the analysis using a running average technique. Droplet spectra obtained by a forward scattering spectrometer probe (FSSP) were used in a 1-D radiative transfer model to calculate infrared (IR) fluxes and radiative heating rates. A heat conservation equation was used to estimate vertical air velocity (wa) within the cloud. The results showed that, because of a warmer ocean surface, significant moisture and heat were transferred from the ocean surface to the boundary layer. The cloud base was at about 400 m height and the top was at about 1.4 km.wa at the cloud base was estimated about 5 cm s−1. Strong IR cooling rate at the cloud top was calculated to be 75°C day−1 for a 100 m thick layer. Negative skewness inwa, suggesting narrow downdrafts, was likely due to radiative cooling at the cloud top. The entrainment velocity was found to be about 1.5 cm s−1 at cloud top. Mean moisture and heat fluxes within the cloud were estimated to be comparable to those from the ocean surface. Vertical air velocity at the cloud top due to radiative cooling was found to be about −40 cm s−1.

Some characteristics of the precipitation formed within winter storms over eastern Newfoundland

During the second Canadian Atlantic Storms Program (CASP 11) field experiment, detailed measurements and photographs were made of the surface precipitation. Observed precipitation characteristics include aggregates of ice pellets, needles coexisting with ice pellets, very large snowflakes, very rough ice pellets, sharp-edged crystals and mixed phase particles. The nature and formation of such precipitation characteristics are discussed and conceptual models of the development of these forms of winter precipitation are developed.

Weather conditions associated with the passage of precipitation type transition regions over eastern Newfoundland

Abstract The passage of a winter storm is accompanied by changes in many surface and near‐surface parameters including temperature, humidity, wind, pressure, precipitation rate and type, cloud base height, visibility and accretion. These parameters were measured in association with the passage of precipitation‐type transitions over Newfoundland during the Canadian Atlantic Storms Program II field experiment. Three simple summaries of the observed weather events were developed. These summaries depend on the observed large‐scale synoptic conditions, which include warm fronts, a cold front and a trough.

Relating Friction Velocity to Spectral Wave Parameters

Abstract Field experiments, in the years since the JONSWAP results, have established fetch- and duration-limited relations for wave parameters such as total energy E0, peak frequency fp, and the Phillips α coefficient. The Canadian Atlantic Storms Program (CASP) Experiment of 1986 also found that the wind speed profile appropriate for the fetch-limited wave relations is a function of fetch, due to the change in surface roughness at the shoreline. Moreover, the fetch-limited wave relations may be altered by assuming differing wind speed variations with fetch. Inverting accepted fetch-limited wave relation the authors inter a fetch-limited relation for wind speed (or friction velocity) in terms of spectral wave parameters such as wave age. These relations are verified from measurements collected during CASP.

Aircraft Icing Measurements in East Coast Winter Storms

Abstract Analysis of the aircraft icing environments of East Coast winter storms have been made from 3 1 flights duringthe second Canadian Atlantic Storms Program. Microphysical parameters have been summarized and are compared to common icing intensity envelopes and to other icing datasets. Cloud regions with supercooled liquid water had an average horizontal extent of 4.3 km, with average droplet concentrations of 130 μ, liquid water contents of 0.13 g m-3, and droplet median volume diameters of 18 pm. In general, the icing intensity observed was classified as light, although moderate to severe icing was observed in several common synoptic situationsand several cases are discussed. Freezing drizzle was observed on four flights, and represented the most severeicing environment encountered.

Investigating the marginal ice zone on the Newfoundland Shelf

From the ice and current data collected over the Newfoundland Shelf by the second Canadian Atlantic Storms Program (CASP II), it is evident that ice motion is affected by wind‐generated ocean current. This points to the importance of coupled ice‐ocean response to wind forcing in the study of shortterm ice motion and operational ice forecasting. The mutual influence of ice and the ocean can also be seen in the water properties.To study the mature stages of explosive cyclogenesis in east coast winter storms and to investigate their influence on the circulation and sea ice properties on the Newfoundland continental shelf and Grand Banks, CASP II was conducted by scientists from the Bedford Institute of Oceanography (BIO), the Atmospheric Environment Service (AES), the National Research Council (NRC), and many universities, private companies, and other government agencies.

Measurement of ocean wave spectra using narrow-beam HE radar

A data interpretation algorithm is developed to extract ocean wave information from HF radar backscatter observed by a narrow-beam antenna system. The basis of this measurement is the inversion of the integral equation representing the second-order radar cross section of the ocean surface. This equation is numerically inverted by approximating it as a matrix equation and pseudoinverting the kernel matrix using a singular value decomposition. As a test of this algorithm, comparisons are made between wave spectrum estimates obtained from a WAVEC buoy and a pair of 25.4-MHz ground wave radars, using data collected during the 1986 Canadian Atlantic Storms Program (CASP). Overall, the results of this experiment have been positive and have demonstrated both the basic feasibility of the inversion algorithm and the wave sensing capability of HF radar. For example, significant wave height estimates deduced by two radars differed from the buoy, in an absolute value sense, by only 0.12 m on average. When using only one radar, the mean difference of this important parameter from the buoy was a reasonable 0.33 m. >

A study of a CASP storm: Analysis of radar data

Abstract The morphology and time evolution of a winter storm is studied using radar data taken during the Canadian Atlantic Storms Program (CASP). The vertical motions that generate the snow are derived from reflectivity measurements. The study reveals a banded structure in the precipitation pattern with ascending and descending air associated with the bands. Vertical velocities averaged over the radar‐covered area reach values close to 1 m s‐1 . The region of large‐scale uplifting moves horizontally at about half the horizontal air velocity. A persistent precipitation pattern results from the continuously renewed air within the uplifting region.

Aircraft observations of the origin and growth of very large snowflakes

A new high‐volume precipitation probe, capable of optically imaging particles up to about 5 cm, was flown in a winter storm during the second Canadian Atlantic Storms Program (CASP II). Dendritic crystals originating aloft in a region of (weak) convective instability were tracked and observed to aggregate into 4–5 cm snowflakes within an isothermal layer near 0° C. The region containing the very large snowflakes was characterized by high radar reflectivities and rapid accumulation of snow at the surface.

Subtidal Response of Scotian Shelf Circulation to Local and Remote Forcing. Part I: Observations

Abstract Data collected during the Canadian Atlantic Storms Program (CASP) suggest that two primary current regimes exist on the Scotian Shelf during winter. Moorings inside the 100-m isobath feature currents that are parallel to the general bathymetry and have small eastward mean velocities. The time-varying part of the flow is nearly rectilinear alongshelf at typical maximum velocities of 0.2–0.3 m s−1 and oscillates on 2–5-day periods in response to meteorological forcing. The current at locations beyond the 100-m isobath exhibits a persistent westward flow of about 0.3 m s−1, denoting the dominance of the Nova Scotian Current, although smaller magnitude along- and cross-shelf fluctuations occur at synoptic frequencies. Flow in both regions is largely uniform with depth at this time of year. The velocity response to local and remote sources of forcing was examined using a frequency (ω)-dependent multiple regression model. Flow is dominated by nonlocal forcing at ω < 0.2 cpd; alongshelf wind stress is t...

Nonlinear Optimal Estimation of Temperature and Integrated Water Vapor and Liquid Using a Ground-based Microwave Radiometer in Coastal Winter Storms

Atmospheric temperature profiles and integrated water vapor and liquid are retrieved from ground-based microwave radiometric measurements using both nonlinear optimal estimation (NLOE) and statistical inversion (SI) methods. The results obtained from both methods are compared with collocated radiosonde observations during the Canadian Atlantic Storms Program field project in 1986. In general, the NLOE was superior to the SI method when clouds with high liquid water contents or when precipitation was present. Under these conditions, temperature profiles derived using NLOE had smaller root-mean-square differences from radiosonde observations than those retrieved using SI. Also, the overestimation of integrated vapor retrieved using the SI method was eliminated using the NLOE method. The radiometric observations were used in two case studies of winter cyclonic storms striking Atlantic Canada.

Mesoscale Fluctuations of Columnar Water Vapor

Abstract During a 7-week period in the winter of 1986, a six-channel ground-based microwave radiometer was operated at a site near Halifax, Nova Scotia, as part of the Canadian Atlantic Storms Program. One of the quantities measured by this instrument was the vertically integrated water-vapor content of the atmosphere, called the columnar vapor. Time histories of the columnar vapor show variability on many scales. This paper focuses on the fluctuations with frequencies between 0.2 and 2 cycles per hour (periods between 30 min and 5 h), which we denote as mesoscale fluctuations. Power spectra of the columnar vapor fluctuations were calculated for seven records of at least 30 h length. Three of these records were found to have well-defined spectral peaks in the mesoscale range; three had weaker peaks; and one had no significant peak in this range, the spectrum failing off smoothly with approximately an f−2.5 power law. In all cases analyzed, the weather was generally fair, with a stable layer extending appr...