Radar observations of precipitation production in thunderstorms

Abstract

Precipitation production is investigated for 9 intense thunderstorms that developed over the Lowveld in South Africa. A C‐band radar is used to observe the 3‐dimensional reflectivity pattern. Using an empirical relation between reflectivity factor and precipitation content and integrating over the storm volume provides an estimate of the total precipitation content aloft. Likewise, an area integration of the instantaneous rain rate at cloud base yields an estimate of the rate of total outflow. At their maturing stage, the storms had precipitation contents of 0.2 to 5.0 Tg and rainfall rates of about 0.3 to 2.0 Gg s^?1. The total accumulation of rain at the ground ranged from 1 to 10 Tg. The characteristic storm updraft, defined as the ratio of the area‐averaged rainfall rate to the volume‐averaged precipitation content, was about 5 ms^?1 for all storms. The time evolution of integral storm parameters is also presented and related to the overall storm development. The precipitation production values observed in the Lowveld storms compares well with previous estimates reported for large thunderstorms observed in Alberta and New England.     

The circulation and hydrography of conception bay,Newfoundland

Abstract

The hydrography and circulation of Conception Bay (Newfoundland) are described based on hydrographic, current‐meter and drifter data collected over four years (1988–1991). The seasonal cycles of temperature (‐1.6 to 13–17°C) and salinity (31–32.5) in the bay closely follow those on the adjacent shelf. Exchange of bottom water was observed in April 1989. Deepwater exchange was observed from late fall to early winter of 1989–90. Tidal currents are weak, 1–2 cm s^‐1 for the M₂ and K₁ constituents. Observed Eulerian mean currents (<3 cm s^‐1) are smaller than the standard deviation (1–11 cm s^‐1); however, there is a persistent outflowing current of 10 to 20 cm s^‐1 within 2 km of the shoreline on the eastern side of the outer bay. The Lagrangian correlation length scale is from 4 to 10 km, in agreement with the weak coherence squared (≤0.4) found between the fixed current‐meter sites separated by greater than 4–5 km. The variable currents (up to 20 cm s^‐1) tend to be cyclonic. Cyclonic eddies were observed near the mouth on the eastern side of the bay, adjacent to the outflow. A simplified fractal dispersion model gives residence times of 42 d similar to those obtained from a scaling analysis (30–40 d) and a diagnostic numerical model (30 d).     

Global precipitation estimates from satellite: Using difference fields of outgoing long‐wave radiation

Abstract

Global precipitation estimates using satellite data are derived using difference fields of outgoing long‐wave radiation (OLR). The difference fields consist of clear OLR minus cloudy OLR, which is a measure of long‐wave cloud radiative forcing at the top of the earth‐atmosphere system; and clear daytime OLR minus clear night‐time OLR, which is a measure of the diurnal variation of surface heating. All geophysical parameters used to compute OLR are derived from an analysis of the HIRS2/MSU sounding data. The derived global precipitation estimates show good agreement with collocated raingauge data over land. The correlation coefficient between the precipitation estimates derived using difference fields of OLR and raingauge data over land is about 0.65 for the FGGEyear. The correlation coefficient between precipitation estimates derived using difference fields of OLR and the GOES Precipitation Index (GPI) fraction is about 0.914 from 30°S to 30°N for July 1983, and between the precipitation estimates derived using difference fields of OLR and the difference field of atmospheric reflectance is about 0.86.

Using one set of coefficients, global precipitation fields are derived for each 10‐day period and each month of the FGGE year (from December 1978 to November 1979). These fields contain rich information on seasonal variations.     

Airborne surveys of the atmospheric boundary layer above the marginal ice zone on the Newfoundland shelf

Abstract

Airborne measurements in the atmospheric boundary layer (ABL) above the marginal ice zone (MIZ) on the Newfoundland Shelf reveal strong lateral variations in mean wind, temperature and the vertical fluxes of heat and momentum under conditions of cold, off‐ice wind. Flux measurements in (and near) the surface layer indicate that the neutral 10‐m drag coefficient depends on ice concentration, ranging from 2 × 10^‐3 at 10% coverage to 5 × 10^‐3 at 90%. Furthermore, cross‐ice‐edge transects consistently show increasing wind speed, temperature and heat flux in the off‐ice direction, but the momentum flux may either increase or decrease, depending on the relative importance of surface buoyancy flux and roughness. For the conditions encountered in this experiment, it appears surface wave maturity does not have a significant influence on the drag coefficient in fetch‐limited regimes near the ice edge.     

The transport of water,heat and salt through the strait of Belle Isle

Abstract

We present an analysis of current‐meter, sea‐level and hydrographic data collected in the Strait of Belle Isle and the northeastern Gulf of St Lawrence. From an array of moorings in the Strait from July to October 1980, we calculate a net transport into the Gulf of 0.13 × 10⁶ m³ s^?1 and show that the mean and eddy fluxes of heat through the Strait represented a net loss of heat to the northeastern Gulf. The estimated rate of loss of heat is less than the long‐term mean computed by Bugden (1981) but becomes comparable if adjusted for interannual changes of transport and water temperature. Moreover, the 1980 data permit the permanent tide‐gauge stations in the Strait at West Ste Modeste and Savage Cove to be levelled relative to one another, thus allowing surface currents to be calculated from sea‐level alone. Hence the long‐term wintertime transport into the Gulf can be calculated after fractional effects on the vertical structure of the flow are considered. During an average winter it appears that advection through the Strait can account for about 35% of the Gulf Intermediate Layer. A multiple regression involving average Intermediate Layer temperatures over 9 years suggests that winter air temperature in the Gulf, representative of atmospheric cooling, and sea‐level difference across the Strait, representative of advection, are equally important variables and together account for 50% of the Layer's temperature variability. Analysis of current‐meter, sea‐level and hydrographic data collected in 1975 supports earlier hypotheses that the strongest inflow of water with ? < 0° C and salinity between 32 and 3 3 should occur in winter. It appears that during the 1975 field program the inflow was about 0.6 × 10⁶ m³ s^?1, which is about twice the long‐term average for January to May.     

Diagnostic simulations of the summer circulation in the Canadian arctic archipelago

Abstract

Gridded fields of potential temperature and salinity, interpolated to the time of minimal ice coverage, are constructed for the Canadian Arctic Archipelago based on archived data. In order to overcome the large variations in the horizontal coverage of the observations, the gridding is performed in an iterative procedure where the horizontal correlation scales depend on the data coverage as well as on the flow field. The mean flow corresponding to the temperature and salinity fields are calculated with a diagnostic numerical ocean model. The simulations show that the relative flow through the different straits depends on the elevation difference from the Arctic Ocean to Baffin Bay, and on the density distribution and baroclinic pressure gradients. A 5‐cm increase in the Arctic‐Baffin elevation difference can double the transport. Mean values of the summer flow are a total transport of 0.9 Sv, with 34% flowing through Barrow Strait, 20% through Jones Sound, and 46% through Nares Strait.     

Non‐linear response of current‐meter compasses: Research note

Abstract

A non‐linear response in current‐meter compasses is examined in terms of its effect on the measured residual current. It is shown that, even for a compass response within typical manufacturers’ specifications, the induced errors are important for regions where the residual speed is less than about 10% of the peak tidal current. In an M₂ current, the non‐linearity also induces an M₄ signal, but this is not sufficiently large to be easily detected by tidal analyses.     

Atmospheric contribution to hydrologic variations in the Arctic

Abstract

High‐latitude rawinsonde data for 18 years (1973–1990) are used to compute the atmospheric moisture flux convergence over two regions: the Arctic Ocean and the Mackenzie River drainage basin. The primary objectives are to assess the interannual variability and to compare the macroscale hydrologie regimes of the two regions. The moisture flux convergence is positive in all months over the Arctic Ocean, but is occasionally negative during summer over the Mackenzie Basin. The climatological seasonal cycle of the moisture convergence contains a late‐summer (August‐September) maximum over the Arctic Ocean but a late‐summer minimum over the Mackenzie Basin. Evaporation, deduced from the moisture inflow and independent data on precipitation, makes a much greater contribution to the atmospheric moisture budget of the Mackenzie domain, especially during summer. The respective equivalent area averages of the 18‐year annual mean moisture flux convergence, precipitation and derived evaporation are 17.3, 19.5 and 2.2 cm a^‐1 for the Arctic Ocean and 24.9, 33.6 and 8.7 cm a^‐1 for the Mackenzie domain. However, the range of interannual variations of the flux convergence is about ±50% of the annual means and more than twice the monthly means. The annual totals of the flux convergence are correlated with station‐derived precipitation over the Mackenzie domain and with yearly variations of the Mackenzie discharge. The moisture flux convergence over the Mackenzie domain suggests that station reports underestimate precipitation during the winter months by amounts equivalent to several centimetres per annum.     

Potential evaporation in the peace river region of British Columbia

Abstract

Micrometeorological data collected over pasture in the Peace River area of British Columbia during the wet summer of 1977 were used to test the Priestley and Taylor (1972) model for potential evaporation. The model performed very well. RMSE was less than 10% of the mean evaporation rate on a daytime basis using an alpha value of 1.26. Since the model is mainly dependent on net radiation, which is rarely measured in such remote areas, this quantity was also estimated from more readily available meteorological data and used to calculate evaporation. Results were encouraging; calculated values were generally within 20 and 10% of energy balance estimates on daily and 5‐day mean bases.     

On theoretical wind speed and temperature profiles over the sea with applications to data from sable island,Nova Scotia

Abstract

In October 1985, the Boundary‐Layer Research Division of the Atmospheric Environment Service conducted an experiment on Sable Island, Nova Scotia, where 10‐m wind measurements were made at a number of locations. Wind data were also collected at 4 levels on one of the 10‐m masts and at 6 levels on a 26‐m mast, both located on the South Beach. Other data used in the present study consisted of air temperature measured at 9 m and sea temperature measured at the beach.

The theory for wind speed and temperature profiles over the sea is reviewed. A method of deriving over‐sea profile parameters (u_*, θ_*, Z₀, L) from wind data at one level and the air‐sea temperature difference is described. The method is limited to applications either over homogeneous open ocean or, provided measurements are taken above the internal boundary layer generated by the change of roughness at the coastline, over a flat beach (without coastal orography). The heights at which the method is applied must be within the surface layer which must not have any discontinuities in wind speed or temperature in the vertical, such as are often associated with inversion layers. An application to data collected at beach sites in onshore flow during the October 1985 experiment is illustrated.

Once the above parameters are obtained, theoretical wind profiles may be computed and compared with observed profiles. In order to make a proper comparison it is essential to account for internal boundary layers generated at the shoreline by the step‐change in surface roughness. Only the data measured above the internal boundary layer are representative of over‐sea conditions and may, therefore, be used for verifying the theoretical profiles. The agreement between calculated and measured data is generally very good. One complication, however, is a slight upstream‐blockage effect due to a 7‐m high dune located about 140 m downwind of the 26‐m mast. Estimates of the magnitude of this effect partially account for small discrepancies in the results at the 26‐ and 10‐m mast locations.

An estimation of the most probable errors in the calculated parameters, based on assumed measurement errors, is included in the computer program. Results suggest that small measurement errors can explain the above discrepancies.     

Radar observations of snow formation in a warm pre‐frontal snowband

Abstract

Radar reflectivity measurements and sounding data were analyzed to investigate snowfall production in a long‐lasting snowband that formed in advance of a warm surface front moving across Alberta. The sounding data indicated that the band could have been forced by slantwise overturning during the release of moist symmetric instability combined with frontogenesis. The stability analysis presented here is novel in that it includes ice phase thermodynamics, neglected in previous studies of slantwise convection.

Radar reflectivity fields were analyzed to determine the total snow content and the mass outflow rate as factors of time. The peak value of total snow content was 17 kilotons per km of snowband, and the peak mass outflow rate was 10 tons s^‐1 km^‐1. The snowfall rate averaged across the cloud base was about 0.8 cm h^‐1, and the average snow content remained close to 0.2 g m^‐1. The characteristic time (defined as the ratio of total snow content over mass outflow rate) was about 30 minutes, which is approximately the time needed for the growth of snowflakes by aggregation in the observed temperature range. The precipitation efficiency of the snowband, defined as the ratio of snow mass outflow to water vapour inflow was estimated to be 14%. The precipitation production values observed in the Alberta snowband are compared with previous estimates reported for frontal rainbands and Alberta thunderstorms.     

A complementary depiction of the interannual variation of atmospheric circulation associated with ENSO events: Research note

Abstract

A simple diagnostic scheme, which combines a low‐pass temporal filter (with an 18‐month cutoff time) with a regular empirical orthogonal function (EOF) analysis, is used to delineate the synchronous evolution of El Nino‐Southern Oscillation‐related (ENSO‐related) modes in various variables of the ocean‐atmosphere system. Based on the causal relation chain of diabatic heating, divergent circulation and rotational flow, the diagnostic scheme extracts ENSO modes from the following data sources: the Pacific sea surface temperature (SST), the past 14‐years (1979–1992) of data generated by the Global Data Assimilation System of the National Meteorological Center, and a 10‐year (1979–1988) general circulation model climate simulation made at the Goddard Laboratory for Atmospheres. The analysis reveals the following: (a) the eigencoefficient time series of the first eigenmodes of selected filtered variables, which explain about 40–50% of their total variance, synchronize with the filtered SST averaged over Area NINO‐3; (b) the spatial structures of the first eigenmodes resemble the ensemble departures associated with ENSO events of these variables from their long term means; and (c) the results show that the proposed scheme can be easily applied to isolate and illustrate the time evolution of ENSO modes which exist in the long term observational database as well as in climate simulations.     

The significance of surface temperature patterns on the energy balance of a small lake in the Canadian Shield

Abstract

Temperature patterns of a small lake in the Canadian Shield are examined by means of thermal imagery. The effect of the different surface temperatures on the daytime energy balance is examined for two points over the lake at the time the thermal imagery was taken. For 14 June 1979, two distinct energy balance regimes are noted. At the lake centre, where the deeper water registers a relatively cool thermal signature, a boundary‐layer inversion is observed. The downward sensible heat flux augments the net radiation, and the latent heat flux is 105% of the radiant input. Along the lake margins, the shallows register warm thermal signatures and a lapse profile is observed. The sensible heat flux is an energy sink and the latent heat flux is diminished to 88% of the net radiation. This difference indicates that a single point estimate may introduce a bias if it is assumed to be representative of the lake average for the purposes of studying lake evaporation.

The calculation of the latent heat flux and evaporation is very sensitive to the value of the surface temperature. When the spatial patterns of surface temperature are considered in an estimate of the lake evaporation, the spatially integrated value differs by —6% from the estimate based upon a single point observation at the lake centre for a mid‐day in June and by +10% from the estimate based upon observations collected over the warm shallows.     

Wind Forcing of Ice Cover in the Labrador Shelf Marginal Ice Zone

Abstract

Anemometer‐measured winds for the period 5–13 March 1994 were used to study the coherence of observed and forecast coastal winds along the mid‐Labrador shelf. The reliability of these variables in predicting the response of the ocean and ice to wind forcing is an important issue for ice forecasting in this area. Two anemometer‐equipped 2‐m ice beacons were deployed on pack ice north of Wolf Island and a third beacon was deployed on Grady Island. The results indicate that due to the influence of local topography, 10‐m winds observed at the meteorological station in Cartwright, Labrador provide a poor estimate (r² = 0.2) of wind conditions over the offshore sea‐ice. In contrast, the σ = 1 level (～10 m) winds from the Canadian Meteorological Centre's Regional Finite Element (RFE) model provided a better correlation with anemometer beacon winds (0.90 for the 6‐hour forecast down to 0.45 at 36 hours). However, the RFE model overestimates the magnitude of the winds by 10–40%.

The response of the ocean and ice cover to wind forcing was measured by an ocean bottom‐mounted acoustic Doppler current proþler (ADCP). Relative to the 2‐m beacon winds, the ice moved at 2.5% of wind magnitude and turned 0.6° to the left of the wind. The ocean response decreased with depth until it reached a constant value of 0.9% of the wind speed. The turning angle increased from 0.3° to the right of the wind at 3.5 m to 50° at the lowest level measured by the ADCP (73 m depth). Approximately 57% of the variance in the ocean currents at 3 m below the surface can be attributed to the 2‐m winds; at 73 m the explained variance decreases to 27%.     

Verification of Toronto temperature and precipitation forecasts for the period 1960–1979

Abstract

The accuracy of temperature and precipitation forecasts for Toronto was studied for the 20‐year period 1960–1979. Since any archive of official forecasts extends for only a small part of this period, it was necessary to retrieve the forecasts from newspaper records. The possible errors involved in such a data source were examined through a comparison of newspaper reported observations and the official record. On only a few occasions were significant differences observed.

For temperature forecasts, the record indicates a significant loss of skill over the 20‐year periodin the prediction of maximum temperature for the first day. This was observed not only for the Bloor Street observing station for which the entire 20‐year record was analysed, but also for observing stations at Toronto Island, Downsview and Malton. The loss of skill over the years is greatest in winter when temperature is consistently predicted too low at all stations.

For the entire period under study, precipitation forecasts consisted only of words and no quantitative information (such as probability of precipitation forecasts) was issued. Word choice is intended to carry information on the duration and expected spatial coverage of precipitation, but substantial inconsistencies between word choice and subsequent precipitation occurrence were found. Consequently, the verification procedure for these forecasts was very simple and ignored any differences implied in word choice. With this technique precipitation forecasts were shown to have improved over the 20‐year period.     

A proposal for a new statistic and technique development for the design and evaluation of cloud seeding experiments

Abstract

The use of short‐term predictions of rain flux from isolated showers is discussed in the context of the design and evaluation of cloud‐seeding experiments. It is found that for a sample of 85 seeded convective clouds a seeding effect of 25% could be detected at the 10% significance level.     

Aggregation of small ice crystals in an electric field

Abstract

Small ice crystals with average diameter of about 30 μm are produced in a large cold room and allowed to fall in a settling chamber in the presence of a quasi‐uniform electric field. Aggregates (flakes) of ice crystals are collected by permanent replicas. Results show that an electric field above a threshold value of about 4 × 10⁴ V m^‐1 rapidly increases the growth of flakes by the capture of small ice crystals. The influence of the electric field upon the growth of ice aggregates is maximum at a field strength of about 1.5 × 10⁵ V m^‐1. Comparison of the results with Jiusto's mathematical model of the growth rate gives values of the collection efficiency at different field strengths. It is very likely that the electric field increases the adhesion (aggregation) efficiency rather than the collision (cross‐section) efficiency.     

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.     

The oil spill behaviour model of the Canadian atmospheric environment service Part I: Theory and model evaluation

Abstract

This paper describes a model to simulate the behaviour of oil spills in marine environments. The model includes parametrizations of various physical processes representing the movement and weathering of an oil slick. The movement of the slick is affected by wind‐driven, tidal and residual water currents. Turbulent dispersion is an important mechanism influencing the horizontal spreading of the slick for time periods greater than about a day.

The model is used to simulate successfully the movement of spill‐following buoys deployed in the Bay of Fundy, where some of the strongest tidal currents in the world occur. The ability of the model to simulate the horizontal spreading of an oil slick was evaluated with observed data from the Argo Merchant oil spill for a 10‐day period. It was found that the observed shape and extent of the spill could be fairly well described by the parametrization of turbulent dispersion effects.     

Wind forcing due to synoptic storm activity over the North Pacific Ocean

Abstract

Six‐hourly surface wind analyses over the North Pacific Ocean covering the 10‐year period 1969–78 are used to describe synoptic storm activity in terms of parameters that are directly related to the atmospheric forcing of the ocean. The cube of the atmospheric friction velocity, u³ _* and the curl of the surface wind stress, curl τ, are used because of their relationship to turbulent vertical mixing and Ekman pumping in the ocean, respectively. In an attempt to isolate synoptic disturbances from mean fields, the time series of surface wind components at each individual grid point are partitioned into “high‐pass” (periods shorter than 10 days) and “low‐pass” (periods longer than 10 days) components by means of conventional filtering procedures. The two quantities u³ _* and curl τ are then calculated from (a) the high‐pass filtered wind components only, (b) a combination of the filtered wind components that include the interaction between the high‐ and low‐pass fields, and (c) the unfiltered wind components. These quantities describe the atmospheric forcing of the ocean that is attributable to (a) synoptic storm activity by itself, (b) synoptic storm activity in the presence of the low‐pass (mean) flow, and (c) the total spectrum of wind forcing, respectively.

Maps of the long‐term (10‐year) monthly mean u³ _* calculated from (a) and (b) are coherent across the mid‐latitude North Pacific and appear to coincide with the normal seasonal evolution of synoptic storm activity in that region. In mid‐latitudes, the values of u³ _* calculated from (a) and (b) are 27 and 83%, respectively, of the value of u³ _* itself. Thus, a major fraction of the production of turbulent energy available for mixing in the upper layers of the ocean comes from synoptic disturbances with a period shorter than 10 days. Maps of the long‐term monthly mean wind stress curl are quite different in that the mean wind stress curl calculated from (a) is essentially negligible. However, the mean curl calculated from (b) closely resembles the pattern of total curl (c), but with a magnitude of only 41% of (c). Thus, synoptic disturbances with a period shorter than 10 days are also responsible for a significant fraction of the Ekman pumping of the ocean.

Future studies with these data will attempt to determine whether a relationship exists between synoptic storm activity, as measured by the parameters developed in this study, and large‐scale sea‐surface temperature anomalies.