Coastal Precipitation Research Articles

Contributions from California Coastal-Zone Surface Fluxes to Heavy Coastal Precipitation: A CALJET Case Study during the Strong El Niño of 1998

Abstract Analysis of the case of 3 February 1998, using an extensive observational system in the California Bight during an El Niño winter, has revealed that surface sensible and latent heat fluxes within 150 km of the shore contributed substantially to the destabilization of air that subsequently produced strong convection and flooding along the coast. Aircraft, dropsonde, and satellite observations gathered offshore documented the sea surface temperatures (SSTs), surface fluxes, stratification, and frontal structures. These were used to extrapolate the effects of the fluxes on the warm-sector, boundary layer air ahead of a secondary cold front as this air moved toward the coast. The extrapolated structure was then validated in detail with nearshore aircraft, wind profiler, sounding, and buoy observations of the frontal convection along the coast, and the trajectory transformations were confirmed with a model simulation. The results show that the surface fluxes increased CAPE by about 26% such that the nearshore boundary layer values of 491 J kg−1 were near the upper end of those observed for cool-season California thunderstorms. The increased CAPE due to upward sensible and latent heat fluxes was a result of the anomalously warm coastal SSTs (+1°–3°C) typical of strong El Niño events. Applications of the extrapolation method using a surface flux parameterization scheme and different SSTs suggested that convective destabilization due to nearshore surface fluxes may only occur during El Niño years when positive coastal SST anomalies are present. The fluxes may have no effect or a stabilizing effect during non–El Niño years, characterized by zero or negative coastal SST anomalies. In short, during strong El Niños, it appears that the associated coastal SST anomalies serve to further intensify the already anomalously strong storms in southern California, thus contributing to the increased flooding. This modulating effect by El Niño–Southern Oscillation (ENSO) of a mesoscale process has not been considered before in attempts at assessing the impacts of ENSO on U.S. west coast precipitation.

A model study of the seasonal variability and formation mechanisms of the barrier layer in the eastern equatorial Indian Ocean

By modifying mixed layer thickness, the barrier layer (BL) may have a significant impact on air–sea interactions. In the tropical Indian Ocean, thermohaline observations are lacking over most of the basin, despite a few BL observations in the Bay of Bengal and off Sumatra. A 15‐yearlong model simulation is used to investigate the seasonal variations and formation mechanisms of the BL. In the eastern equatorial area, the model thermohaline structure is in close agreement with observations and simulates a realistic seasonal variability. A robust and thick BL (of more than 40 m) is observed seasonally with a maximum horizontal extent (of about 1500 km) in November. This BL corresponds to the merging of two BLs that undergo different formation mechanisms. The first is formed as the Wyrtki Jet propagates eastward, deepening the thermocline while zonally advecting a salinity subsurface maximum. At the same time, northward wind stresses induce a northward advection of a thin fresh water layer toward the equator. This combination decouples salinity from temperature, and a BL is formed. Later, as the jet penetrates further toward the fresh pool, a second BL appears off Sumatra. This one is controlled by two different mechanisms. A westward surface current trapped in the thin mixed layer off Sumatra advects fresh water from the coastal precipitation area. Then, significant upwelling associated with the westward current further raises the halocline. In situ observations confirm the realism of these simulated formation mechanisms. From year to year, the timing, thickness, and shape of the BLs follow the strong interannual variability of the Wyrtki Jet. Despite these variations, the formation mechanisms remain unchanged.

Numerical Simulations of a Landfalling Cold Front Observed during COAST: Rapid Evolution and Responsible Mechanisms*

This paper identifies mechanisms that led to the observed rapid evolution of a landfalling weak cold front along the steep mountainous northern California coast on 1 December 1995. This event was simulated down to 3-km horizontal grid spacing using the Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5). The MM5 simulation reproduced the basic features such as the timing, location, and orientation of the cold front and associated precipitation evolution, as well as the tendency for enhanced precipitation to extend ∼50–100 km upwind of the coastal barrier, with the heaviest amounts occurring over the windward slopes (0–20 km inland); locally, however, the model underestimated the magnitude of the prefrontal terrain-enhanced flow by as much as 30% since the simulated low-level static stability was weaker than observed. The MM5 simulations illustrate the complex thermal, wind, and precipitation structures in the coastal zone. Upstream flow blocking by the steep coastal terrain led to the development of a mesoscale pressure ridge and prefrontal terrain-enhanced winds exceeding 25 m s−1. Because of the irregular coastline and highly three-dimensional terrain, the low-level winds were not uniform along the coast. Rather, prefrontal southerly flow was significantly reduced downwind of the major capes (viz. Mendocino and Blanco), while there were localized downgradient accelerations adjacent to regions of higher topography along uninterrupted stretches of coastline. Terrain–front interactions resulted in a slowing of the front as the system made landfall, and blocking contributed to a “tipped forward” baroclinic structure below 800 mb. The MM5 was used to investigate some of the reasons for the rapid intensification of the frontal temperature gradient and banded precipitation in the coastal zone. During this event the large-scale vertical motions increased in an environment favorable for moist convection, and a simulation without coastal topography illustrated rapid development of coastal precipitation even in the absence of local terrain influences. The coastal topography helped to further enhance and collapse the thermal gradient and associated cold-frontal rainband through enhanced deformation frontogenesis associated with the prefrontal terrain-enhanced flow. Diabatic effects from precipitation are also shown to have been important in organizing the precipitation in the coastal zone and further enhancing the frontal temperature gradient.

Dominant Factors Responsible for Interannual Variability of the Summer Monsoon in the Southwestern United States

Interannual variability of the summer monsoon in the southwestern United States is controlled by various ocean- and land-based conditions (e.g., SST, soil moisture, and snow cover) that provide sources of memory of antecedent climate anomalies such as ENSO. It is hypothesized that this interannual variability is also modulated by decade-scale fluctuations in the North Pacific SSTs. The following observations have been made in support of this hypothesis. First, the summer precipitation regime is dominated by a continental-scale precipitation pattern characterized by an out-of-phase relationship between precipitation in the southwestern United States and that in the Great Plains of the United States. Second, interannual fluctuations in the onset date of the monsoon in the southwestern United States are significantly correlated with interannual fluctuations in the intensity of summer rainfall in this region such that early monsoons are often very wet and late monsoons tend to be dry. Third, wet (dry) monsoons in the southwestern United States often follow winters characterized by dry (wet) conditions in the southwestern United States and wet (dry) conditions in the northwestern United States. Finally, interannual variability of the summer monsoon in the southwestern United States is modulated by long-term (decade scale) fluctuations in the North Pacific SSTs. The mechanism relating the North Pacific SST pattern to interannual variability in the summer monsoon appears to be via the impact of variations in the Pacific jet on West Coast precipitation regimes during the preceding winter. Multiyear fluctuations in the North Pacific SST pattern are consistent with multiyear fluctuations in the atmospheric circulation and in the West Coast precipitation regimes during Northern Hemisphere (NH) winter, hence with multiyear variability in the summer monsoon state. Influences on the summer monsoon during the preceding winter and spring are tied together using appropriate SST indices that capture decade-scale variability in the North Pacific during NH winter and interannual variability in the eastern tropical Pacific during NH spring. The results suggest that decadal variability in the North Pacific SSTs may be an important factor in determining long-term periods of summertime drought or rainy conditions both in the southwestern United States and in the Great Plains of the United States.

Nonhydrostatic, Mesobeta-Scale Model Simulations of Cloud Ceiling and Visibility for an East Coast Winter Precipitation Event

Experiments are described that provide an example of the baseline skill level for the numerical prediction of cloud ceiling and visibility, where application to aviation-system safety and efficiency is emphasized. Model simulations of a light, mixed-phase, East Coast precipitation event are employed to assess ceiling and visibility predictive skill, and its sensitivity to the use of data assimilation and the use of simple versus complex microphysics schemes. To obtain ceiling and visibility from the model-simulated, state-of-the-atmosphere variables, a translation algorithm was developed based on empirical and theoretical relationships between hydrometeor characteristics and light extinction. The model-simulated ceilings were generally excessively high; however, the visibility simulations were reasonably accurate and comparable to the existing operational terminal forecasts. The benefit of data assimilation for such very short-range forecasts was demonstrated, as was the desirability of employing a reasonably sophisticated microphysics scheme.

Coastal precipitation regimes in kenya

Kenya is under the influence of the seasonal reversal of the Indian ocean monsoons. However, its coastal belt, up to about 50 km inland, exhibits original climatic features. Hierarchical clustering of mean monthly rainfall for a large number of stations, particularly in the south‐eastern region, strongly differentiate a maritime climate characterised by enhanced and delayed “long rains” and no dry season in the northern summer. Most of these rains fall at night or in the morning. Using daily rainfall data and twice‐daily surface and upper‐air wind observations, monsoon‐breeze interactions and their role on April–August coastal precipitation are assessed. Rain spells common to the whole Kenya coast are associated to a slight weakening of the sea breeze, a strong easterly or south‐easterly wind anomaly over most of Kenya at around 850–700 hPa, and sea surface temperatures greater than air temperatures.

Coastal Precipitation Regimes in Kenya

Coastal Precipitation Regimes in Kenya

Interdecadal Variation in U.S. Pacific Coast Precipitation over the Past Four Decades

Abstract The interdecadal variation of precipitation along the U.S. Pacific coast over the past 45 winters (1950–94) was examined with station observations. An interdecadal decreasing (minor increasing) trend appeared north (south) of 36°N. This interdecadal trend in precipitation is related to the development of an anomalous dipole structure (a major anomalous high centered over the Pacific coast at British Columbia and a minor anomalous low centered over northern Baja California) throughout the past several decades. The correlation coefficient patterns between the Pacific coast precipitation and upper-air geopotential heights, and the difference charts of upper-air geopotential heights between the first (1950–59) and last (1985–94) 10 winters revealed that the interdecadal variation of the western U.S. circulation is a part of that in the Northern Hemisphere wintertime circulation. In turn, the interdecadal variation of the Pacific coast precipitation is a consequence of the interdecadal variation of th...

The Influences of Inversion Height on Precipitation and Airflow over the Island of Hawaii

Abstract Both the daily rainfall on the windward side of the island of Hawaii and the early morning (0200 HST—Hawaiian standard time) trade-wind inversion height at Hilo varied substantially during the Hawaiian Rainband Project (HaRP). In general, the inversion was higher during the passage of a cold front to the north or during the approach of tropical disturbances from the east to the south. The daily rainfall on the windward side is highly correlated with the 0200 HST inversion height at Hilo with a maximum correlation greater than 0.7 in the Hilo area. The daily daytime (1100–1900 HST) rainfall on the windward slopes, nocturnal (1900–0300 RST) showers an the windward lowland, and coastal precipitation in the predawn and early morning hours (0300–1100 HST) are also highly correlated with the inversion height with a maximum correlation greater than 0.7, 0.7, and 0.5, respectively. For the low- (high-) inversion days, the median daily rainfall on the windward side is about one-half (more than twice) of t...

Coastal Frontogenesis and Precipitation during ERICA IOP 2

Abstract The inverted trough and coastal front that occurred during ERICA IOP 2 were studied in order to assess the ability of an operational mesoscale model, the Canadian Regional Finite Element Model, to provide credible forecast guidance concerning the regional distribution of precipitation associated with such events. The observed distribution of the heaviest precipitation was dependent on the orientation and inland penetration of the coastal front and its associated (geostrophic) frontogenetic forcing, although considerable local variability was evident. The observations indicate that the front penetrated inland along a line west of Boston through east-central Connecticut. The model, which performed exceptionally well in forecasting the eventual development of the main cyclone, correctly forecast the existence and general location of the inverted trough and coastal front and thereby gave indications of the development of heavy snowfall within the coastal region of New York–New England 18–24 h in adva...

Accumulation variation in eastern Kemp Land, Antarctica

The spatial pattern of accumulation rate for eastern Kemp Land in the elevation range 1850-2700 m is presented together with observations of the physical parameters of snow temperature, average microwave emissivity (19 GHz, H polarization), distribution of depth hoar and firn-crystal diameter. The broad accumulation pattern in the region was found to be significantly low when compared to other coastal areas of East Antarctica such as Wilkes Land. The low accumulation regime is attributed to low atmospheric moisture transport and low penetration of synoptic cyclonic systems on to the coastal slopes. In the absence of high coastal precipitation, the accumulation rate is determined predominantly by surface snow redistribution processes. Attempts to determine accumulation-rate time series using visible layer, δ18O isotope and electrical conductivity stratigraphies were unsuccessful due to the relatively low coastal accumulation rates (less than 280 kg m-2a-1) and the complex modification of precipitation by redistribution processes.δ18O variations of seemingly cyclic nature observed throughout the cores were interpreted as a product of the snow-dune building and erosion processes, together with general redistribution of snows by the surface wind field, under the influence of mesoscale topographic roughness

Accumulation variation in eastern Kemp Land, Antarctica

The spatial pattern of accumulation rate for eastern Kemp Land in the elevation range 1850-2700 m is presented together with observations of the physical parameters of snow temperature, average microwave emissivity (19 GHz, H polarization), distribution of depth hoar and firn-crystal diameter. The broad accumulation pattern in the region was found to be significantly low when compared to other coastal areas of East Antarctica such as Wilkes Land. The low accumulation regime is attributed to low atmospheric moisture transport and low penetration of synoptic cyclonic systems on to the coastal slopes. In the absence of high coastal precipitation, the accumulation rate is determined predominantly by surface snow redistribution processes. Attempts to determine accumulation-rate time series using visible layer, δ18O isotope and electrical conductivity stratigraphies were unsuccessful due to the relatively low coastal accumulation rates (less than 280 kg m-2a-1) and the complex modification of precipitation by redistribution processes.δ18O variations of seemingly cyclic nature observed throughout the cores were interpreted as a product of the snow-dune building and erosion processes, together with general redistribution of snows by the surface wind field, under the influence of mesoscale topographic roughness

Accumulation variation in eastern Kemp Land, Antarctica

The spatial pattern of accumulation rate for eastern Kemp Land in the elevation range 1850-2700 m is presented together with observations of the physical parameters of snow temperature, average microwave emissivity (19 GHz, H polarization), distribution of depth hoar and firn-crystal diameter. The broad accumulation pattern in the region was found to be significantly low when compared to other coastal areas of East Antarctica such as Wilkes Land. The low accumulation regime is attributed to low atmospheric moisture transport and low penetration of synoptic cyclonic systems on to the coastal slopes. In the absence of high coastal precipitation, the accumulation rate is determined predominantly by surface snow redistribution processes. Attempts to determine accumulation-rate time series using visible layer, δ18O isotope and electrical conductivity stratigraphies were unsuccessful due to the relatively low coastal accumulation rates (less than 280 kg m-2a-1) and the complex modification of precipitation by redistribution processes.δ18O variations of seemingly cyclic nature observed throughout the cores were interpreted as a product of the snow-dune building and erosion processes, together with general redistribution of snows by the surface wind field, under the influence of mesoscale topographic roughness

The Role of Convective Parameterization in the Simulation of a Gulf Coast Precipitation System

Three simulations were performed with the Limited Area Mesoscale Prediction System (LAMPS) model in order to examine the role of convective parameterization in the simulation of the Gulf Coast precipitation system of March 6-7, 1982. The first simulation neglected moist processes, the second used the representation for moist physics of Orlanksi and Ross (1984) and Rosenthal (1978, 1979), and the third simulation incorporated the LAMPS sequential plume cumulus cloud model as convective parameterization for the model. The effects of various representations of moist physics on the evolution of the model's precipitation, kinematic and diabatic heating, and mass and wind fields are investigated. It is determined that the representation of moist thermodynamics in the meso-alpha-scale model influences the manner in which the model describes the mesoscale structures.

Local Relationships between United States West Coast Precipitation and Monthly Mean Circulation Parameters

Abstract Monthly accumulations of area-averaged precipitation along the West Coast of the United States are related to estimates of local circulation parameters. The annual cycle as well as anomalous components of these quantities are compared. A strong annual cycle in most of the circulation parameters reflects the influence of the large-scale circulation on the annual variation of the precipitation field. For the anomalous monthly components, especially in winter, high correlations are found between precipitation and sea-level pressure or 70 kPa height. Other circulation parameters are also significantly correlated with the precipitation. These include the zonal and meridional wind components and the advection of relative vorticity.

The Problem of Artificial Control of Rainfall on the Globe: II. The Coastal Orographic Maxima of Precipitation in Autumn and Winter

The coastal maxima of rainfall during autumn and winter cannot be explained alone by the classical theory of orographic precipitation. Three different frictional effects , and one effect leading to orographically conditioned convergence at quasi-isophysic flow, are discussed and illustrated by Scandinavian and Dutch rainfall maps of remarkable special cases of maximum coastal precipitation. The rainfall maps over Holland even hint secondary maxima further inland, at zero-level and parallel to the coastal maximum; they might be explained in connection with standing waves in the sense of Queney and others. With all these effects a low condensation level and an efficient precipitation release , by ice-nuclei or in other ways, seem indispensable to cause intense orographic condensation and bring its results promptly down to the earth. As stated in article I such orographic cloud-systems, if lacking an efficient release, just represent the clouds where artificial seeding might materially increase precipitation, because (1) condensation may for a long time keep intense and stationary, and (2) the clouds may keep releasable but unreleased. This increase, if at all possible, would diminish precipitation inland where it is much more useful. — A better scheme would therefore be trying to overseed coastal cloud-systems in order to prevent their precipitation release, and thereby increasing precipitation inland, provided the overseeding could be avoided there. This overseeding process might also be used for dissipating inland Cunb, but presumably not for dissipating tropical hurricanes. The generally accepted idea of the increase of precipitation with height is often, e.g. in Norway, contradicted by individual cases with a marked decrease from the very coast and upward when moving inland. A rational attack on this problem was hitherto impossible because the complex nature of orographic precipitation (hinted above) was not recognized. - Outside the Tropics excessive precipitation needs low cloud formed by an intense updraft at high temperature (= maximum condensation and minimum evaporation) in continual contact with an upper cloud (part) surpassing the —10 p C-level, or seeded artificially (= efficient precipitation release). These conditions will be fulfilled in certain orographic clouds discussed above, and within some convective and frontal clouds. DOI: 10.1111/j.2153-3490.1949.tb01264.x