Hawaiian Rainband Research Articles

A comprehensive two-moment warm microphysical bulk scheme. II: 2D experiments with a non-hydrostatic model

A new bulk microphysical scheme containing advanced parametrizations to predict both concentration and mixing ratio of cloud droplets and raindrops (see Part I) is implemented in a three-dimensional non-hydrostatic model. This paper presents results obtained with the complete scheme for two specific experiments of idealized precipitating clouds. In the first case, an orographic cloud is produced at the windward edge of a plateau. The study focuses on the sensitivity of the microphysical fields and precipitation patterns to the upwind cloud condensation nuclei (CCN) activation spectrum. In the second case, a heavily precipitating tropical rainband of the Hawaiian Rainband Project is simulated in a kinematic framework. Analysis of simulated radar-reflectivity time evolution indicates that a realistic development of large raindrops can be obtained as compared to either observations or to results produced by a bin-resolving model. The overall control of raindrop tail distribution by collisional break-up is re-emphasized in the light of earlier results. The rather simple tests lead to the conclusion that a complete bulk microphysical scheme can simulate warm rain processes with reasonable accuracy. The ability of the model to account for realistic CCN activation spectra with modest programming effort makes the scheme suitable for three-dimensional precipitation simulations that properly take into consideration aerosol dynamics at mesoscale.

The Impact of Trade Wind Strength on Precipitation over the Windward Side of the Island of Hawaii

Abstract The effects of trade wind strength and the diurnal heating cycle on the production of summer trade wind rainfall on the windward side of the island of Hawaii are examined from the data collected from the Hawaiian Rainband Project (HaRP) during 11 July–24 August 1990 and from National Weather Service Hydronet and National Climatic Data Center rain gauge data during 11 July–24 August for the years 1997–2000. For strong trades, the daily rainfall totals on the windward lowlands west of Hilo are higher with a nocturnal maximum there due to the convergence of the katabatic flow and the incoming decelerating trade wind flow, and orographic lifting aloft. The maximum rainfall axis shifts farther inland when trades are stronger. Except in the late afternoon hours, rainfall amounts on the windward side are higher when trades are stronger. For weak trades (≤5 m s−1), the rainfall distributions have a pronounced late afternoon maximum on the windward slopes due to the development of anabatic winds. The nocturnal rainfall over the windward lowlands and the early morning coastal rainfall are lower when trades are weaker.

Effects of Terrain Heights and Sizes on Island-Scale Circulations and Rainfall for the Island of Hawaii during HaRP

AbstractIslands in Hawaii have different sizes and terrain heights with notable differences in climate and weather. In this study, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) land surface model (LSM) is used to conduct numerical simulations for the island of Hawaii with different model terrain heights and sizes during the diurnal cycle for the Hawaiian Rainband Project (HaRP) period. In addition to island blocking and orographic lifting, terrain heights also affect the land surface thermal forcing throughout the diurnal cycle by the variations of orographic clouds during the day and by the variations of the longwave radiation heat loss at night. The simulated rainfall distributions and amounts throughout the diurnal cycle are closely related to rising motions caused by nonlinear interactions among island blocking, orographic lifting, and land surface processes. Besides the terrain/mountain height, island size is another factor that affects rainfall production and distribution in Hawaii, because island size affects orographic lifting, surface forcing, and the advection time scale for an air parcel to reach the mountaintop. The heavy rainfall maximum on the mountaintop of the island of Kauai is due to its suitable height and size. This study confirms that the evening rainfall maximum along the western Kona leeside coast is caused by the convergence between the westerly return flow and the offshore flow. For a lower model terrain, the westerly return flow is weaker; as a result, there are smaller evening rainfall amounts.

Numerical Simulations of the Island-Induced Circulations over the Island of Hawaii during HaRP

Abstract The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)/land surface model (LSM) is used to simulate the diurnal island-scale circulations over the island of Hawaii during the Hawaiian Rainband Project (HaRP, 11 July–24 August 1990). The model is initialized with the NCEP–NCAR reanalysis data. The diurnal variations of the land–sea thermal contrast at the land surface and the planetary boundary layer at Hilo, Hawaii, are well simulated. The main discrepancy occurs on the leeside areas of mountains or ridges below the trade wind inversion (2 km), where the simulated afternoon land–sea thermal contrast at the surface is 1°–3°C lower than observed mainly due to the misrepresentation of lava rocks by the bare ground category in the U.S. Geological Survey (USGS) data and stronger than observed simulated sea breezes bringing in relatively cool maritime air. The flow deceleration and splitting of the incoming trade wind flow and the evolution of the diurnal circulation cells on the windward side, the thermally driven diurnal winds, and the wake circulations on the lee side are well simulated. The simulated diurnal variations in rainfall are also in good agreement with observations. However, the simulated winds in areas well exposed to the trade wind flow are weaker (1–3 m s−1) than observed mainly due to the underestimation of trade wind flow in the NCEP–NCAR reanalysis. The simulated rainfall over windward lowlands at night is underestimated and the maximum rainfall axis shifts farther toward the coast as compared with observations, due to an underestimation of orographic lifting aloft and a relatively large horizontal extent of the simulated katabatic flow because of the weaker- than-observed trade wind flow in the initial conditions. In the afternoon hours on the windward side, the strongest winds (anabatic/sea breeze and trade wind flow) are simulated in low levels over land in response to the surface heating, with a westerly wind deviation beneath the mean trade wind inversion (2 km) and sinking motion over the adjacent oceans. The simulated low-level flow deceleration of the incoming trade wind flow is most significant in the early morning as a combination of island blocking and nocturnal cooling over land. At that time, the simulated upward motion representing the rising branch of the thermally direct circulation extends more than 40 km offshore. Sensitivity tests show that with better surface conditions in the model coupled with the Oregon State University (OSU) LSM, the simulated thermal forcing over land is improved. The improvements in simulated ground temperature, land–sea thermal contrast at the land surface, and mixing ratio lead to better simulation of the strength of land/sea breezes over the island.

A two-moment parameterization of aerosol nucleation and impaction scavenging for a warm cloud microphysics: description and results from a two-dimensional simulation

A new two-moment warm bulk scheme has been developed including explicitly nucleation and impaction scavenging of aerosol particles as well as all other microphysical processes. The scheme is built upon a quasispectral representation of the aerosol particle, cloud droplet and raindrop distributions. It predicts mixing ratios and number concentrations for each category. Each process is treated explicitly and independently to establish an analytic expression for each contribution for the time-dependant microphysical equations. The scheme has been tested in the dynamical framework of a two-dimensional kinematic model, developed for the Hawaiian Rainband Project (HaRP, 1990). In this frame, the scheme has performed reasonably well compared to the observations as well as to other similar parameterization schemes, and to the spectral model DESCAM. Sensitivity tests demonstrate the great sensitivity of the scheme to the initial aerosol spectrum characteristics. Moreover, they have also shown its capability to calculate nucleation and impaction scavenging and to follow the taken up particle mass in the cloud and raindrop spectra until the deposition on the ground by the rain. Therefore, the parameterization offers a possibility of treating the evolution of the liquid phase of the cloud together with the aerosol particle scavenging. However, due to the severe limitations of a two-dimensional kinematic model, the scheme needs to be further validated in a three-dimensional dynamical model.

Circulations and Rainfall on the Lee Side of the Island of Hawaii during HaRP*

Using aircraft data and portable automated mesonet (PAM) data during the Hawaiian Rainband Project (HaRP; 11 July‐24 August 1990), the circulation on the lee side of the island of Hawaii is analyzed. The largest temperature deviations (2‐3 K) and negative mixing ratio deviations (2 2t o2 4gk g 21) are found in the northwestern and southwestern leeside areas of the island because of the descending trade winds aloft in the lee of low (,2 km) mountain ridges and tops. Rainfall in these areas is much less compared to the central leeside coast (e.g., Kona coast). In these areas, the sea-breeze duration is short compared to the Kona coast because of the presence of opposing trade winds; the wind speed is greater because of larger land‐sea thermal contrast. During the stronger trade wind days, adiabatic descent in the lee is more significant and the LCL is higher. As a result, in the northern and southern leeside areas, localized trade wind rain showers driven by local circulations occur mainly under weak trades. In the Kona area, the mean surface winds are calm behind the massive mountains, with a daytime upslope (onshore) and nighttime downslope (offshore) flow. Along the axis of the return flow in the large wake off the Kona coast, the water vapor content is higher than the ocean upstream, possibly caused by the low-level convergence between the counterrotating eddies along the wake axis. Along the Kona coast, the rainfall in the afternoon hours continues in the early evening and reaches the daily maximum after the onset of the land breezes, possibly because of the lifting of stronger moist return flow by the land-breeze front. Under stronger trade winds, the return flow would be stronger because of stronger island blocking. More rainfall occurs there when trades are stronger.

Evolution of Downslope Flow under Strong Opposing Trade Winds and Frequent Trade-Wind Rainshowers over the Island of Hawaii

Abstract The evolution of downslope flow on the windward side of the island of Hawaii during 7–8 August 1990 is investigated. This period is characterized by atypical strong (∼11 m s−1) upstream trade winds and frequent nocturnal rainshowers. In the late afternoon, the onset of downslope flow first occurs on the upper slope, which is frequently dominated by orographic clouds. With mean weak surface winds on the windward slopes because of island blocking, the downslope flow onset occurs after the slope surface becomes negatively buoyant. Along the Hilo coastal areas that are well exposed to decelerating trade-wind flow, the offshore flow onset there occurs much later (∼2 h) than the Hawaiian Rainband Project (HaRP) mean as the drainage front moves slowly from the windward lowlands toward the coast against the atypical strong incoming trade-wind flow. The downslope flow onset along the coast occurs after a band of trade-wind rainshowers has moved over the coast. These rainshowers produce evaporatively coole...

Numerical Simulations of Airflow and Cloud Distributions over the Windward Side of the Island of Hawaii. Part I: The Effects of Trade Wind Inversion*

Abstract Initialized by the composite upstream sounding during the Hawaiian Rainband Project, high-resolution numerical experiments are conducted using the Pennsylvania State University–National Center for Atmospheric Research fifth generation Mesoscale Model to study the island-scale airflow and cloud distributions over the island of Hawaii without considering the diurnal heating cycle. The observed mean patterns of island airflow and weather under the summer trade wind conditions are well simulated. These features include orographic clouds on the windward slopes, flow deceleration and splitting on the windward side, leeside vortices, and cool and moist (warm and dry) conditions on the windward (lee) side. In addition to Fr (Froude number), the simulated island airflow and cloud distributions are sensitive to net diabatic heating associated with clouds and precipitation and rain evaporative cooling. The trade wind inversion height determines the depth of the moist trade wind layer, which also affects the...

The Effects of the Island of Hawaii on Offshore Rainband Evolution

The diurnal cycle of airflow on the island of Hawaii and the structure of the low-level flow separation line between the island-induced offshore flow and incoming trade winds are reasonably well understood. This study examines the formation and organization of nocturnal and early morning offshore Hawaiian rainbands, the flow patterns associated with these rainbands, and their relationship to the flow separation line. This investigation uses the complete radar and aircraft dataset from the 1990 Hawaiian Rainband Project. The formation and evolution of nocturnal and early morning rainfall on the windward side of the island were found to be related to four major factors: (a) the existence or absence of trade wind cloud patches arriving from the northeast Pacific, (b) the effect of island blocking on the trade wind flow as it approaches the island, (c) the low-level convergence and thermal and moisture stratification near the flow separation line, and (d) the position of the flow separation line, controlled by both the Froude number and diabatic processes (nocturnal radiative cooling and evaporative cooling) occurring over the island. If trade wind cloud patches are absent over the ocean upstream of the island, rainbands normally will not form upstream of the flow separation line. Under normal Froude number conditions, if the ratio of the low-level trade wind speed to the Brunt–Väisälä frequency, U/N, is greater than the height of the level of free convection, rainbands will form offshore over the flow separation line. Otherwise, rainbands will not form at this location. When the Froude number is elevated (typically, strong trade wind cases) rainbands will form over the windward shore independent of this ratio. If trade wind cloud patches are present, which is the typical case, light rain normally falls far from the island. These rain regions are enhanced by the weak convergence associated with island blocking and are organized into shore-parallel linear features by the deformation flow induced by the island. The enhancement generally occurs about 35–50 km from the island. Rainbands undergo a second enhancement over the flow separation line. Rainbands will also form over the flow separation line if U/N is greater than the level of free convection.

A Case Study of Nocturnal Rain Showers over the Windward Coastal Region of the Island of Hawaii

Abstract Nocturnal rain showers over the windward side of the island of Hawaii were investigated from the late afternoon of 2 August to the early morning of 3 August 1990 during the Hawaiian Rainband Project (HaRP). Three types of rainbands produce rainfall peaks over the lowland/coastal region during the evening, after midnight, and along the coast in the early morning. In the early evening, an inland rainband develops over the lower slopes as a result of orographic lifting and low-level forcing along the drainage front. As the drainage front progresses toward the coast, new rain cells continue to develop along the drainage front. These cells move westward with the trade winds aloft and dissipate. After the drainage front moves to the Hilo coast, new cell generation along the drainage front ceases. It appears that in the absence of orographic lifting, the shallow (∼0.2 km) katabatic flow offshore is not deep enough to lift the low-level air to saturation. Thus, the rainband weakens and dissipates. It las...

A double moment warm rain scheme: description and test within a kinematic framework

A two-moment warm rain parameterization that includes prognostic equations for mixing ratios and number concentrations is described. The number of activated condensation nuclei is also prognosed assuming a log–linear relationship between the total number of activated condensation nuclei and the supersaturation. Collective processes are evaluated using pre-calculated matrices of interaction based on numerical integrations of the corresponding stochastic collection equations. Mass transfer between cloud and raindrops due to autoconversion is evaluated using Berry's parameterization, its attendant raindrop number concentration change is calculated considering the corresponding rates of change for the first and second moments of the raindrop volume distribution. A Doppler kinematic reconstruction of a summertime Hawaiian rainband occurred on August 10, 1990 is used to evaluate the credibility of this scheme.

The Microphysical Structure and Evolution of Hawaiian Rainband Clouds. Part III: A Test of the Ultragiant Nuclei Hypothesis

Abstract A Lagrangian drop-growth trajectory model, applied within dual-Doppler-derived four-dimensional kinematic fields, is used to test the hypothesis that accretion of cloud water on giant and ultragiant cloud condensation nuclei (CCN) can explain the growth of raindrops in warm maritime convective clouds. Radar data collected within offshore rainbands during the 1990 Hawaiian Rainband Project are used to provide realistic timescales and magnitudes of convective updrafts and to capture the horizontal flow variations responsible for transporting drops into and out of these updrafts. The range of conditions under which cloud droplets can grow to large raindrops during simple up–down trajectories is determined. The model results show that accretion of cloud water on giant and ultragiant nuclei can account for the formation of rain in observed timescales. Raindrops with diameters of 1–4 mm formed for the entire range of tested conditions. Maximum drop sizes ranged from 3.5 to 8.5 mm. The general tendency ...

Evolution of Katabatic Flow on the Island of Hawaii on 10 August 1990

Abstract The data collected on 10 August 1990 from the Hawaiian Rainband Project were analyzed to study the evolution of katabatic flow. Results from this study show that for a relatively dry case thermal forcing is important to account for the onset, evolution, depth, and offshore extension of the katabatic flow on the windward side of the island of Hawaii where the mean winds are weak because of island blocking. During the evening transition, the initiation of katabatic flow on the windward slopes was mainly driven by the diurnal heating cycle. At the windward coast, the wind shift was caused by the arrival of the drainage front from the windward lowlands. After the arrival of the drainage front at Hilo, slopewise instability was generated because of rain showers along the coast and diminishing orographic clouds and showers on the slopes. Along the coast, because of the reduced radiative cooling due to cloud cover and vertical mixing associated with rains, a warm period was observed at Hilo. On the slop...

Simple two-dimensional kinematic framework designed to test warm rain microphysical models

This paper describes a two-dimensional kinematic framework designed to test warm rain microphysical models. The idealized flow field is based on a case study from the Hawaiian Rainband Project (1990). Analyses of radar and aircraft data collected in convective cells embedded in Hawaiian rainbands are included for model validation. A Cloud Condensation Nuclei (CCN) activation spectrum, a condensational growth equation including the CCN chemical composition, collection efficiencies, and terminal velocities of the growing drops, are all provided. A sample set of figures from the test run with bulk microphysical parameterization illustrates the desired format for comparisons between the detailed model results and for validation of the model output with observations. This simple yet realistic test formulation includes vertical and horizontal advection, and can be used to evaluate microphysical model performance without complexities resulting from dynamical–microphysical interactions in dynamic cloud models. Without such interactions, scientists can focus on key physical processes involved in the formation of warm rain. The accurate representation of these processes in detailed models is essential to successfully simulate the observed evolution of warm precipitating clouds.

A Case Study of Trade-Wind Rainbands and Their Interaction with the Island-Induced Airflow

Abstract A case study of trade-wind rainbands observed on 22 August 1990 during the Hawaiian Rainband Project is presented. It shows that the interaction between the morning rainbands and the island-induced airflow is important for the evolution of the rainbands. In the early morning of 22 August, there are two convective periods, 0400–0600 and 0700–0900 HST (Hawaii standard time), on the windward side of the island of Hawaii. For both periods, preexisting rain cells are observed in the trade-wind flow at least 40 km upstream of the island and move westward toward the island. At night and in the early morning, the offshore flow opposes the trade winds resulting in a convergent region over the area immediately upstream of the island. As the first group of rain cells (0400–0600 HST) moves toward the island, the low-level convergent airflow provides a favorable kinematic background for the enhancement of the coming rain cells. These rain cells merge in the convergent zone and become a well-defined rainband. ...

The Microphysical Structure and Evolution of Hawaiian Rainband Clouds. Part I: Radar Observations of Rainbands Containing High Reflectivity Cores

Abstract Radar reflectivity factors exceeding 60 dBZ are documented within shallow ( 0°C), summertime tropical rainbands offshore of the island of Hawaii. Dual-Doppler radar measurements from the Hawaiian Rainband Project are used to document the formation, evolution, and kinematic structure of the high reflectivity cores. The authors show that extremely high radar reflectivities (50–60 dBZ) can develop from echo free regions (−20 dBZ) within approximately 15 min and are preceded by 5–9 m s−1 peak updrafts. High reflectivities (>50 dBZ) typically first formed in the middle or upper part of the clouds. Over the next 10–15 min, the mature high reflectivity cores extended vertically through the cloud depth and then collapsed to the surface as the updrafts weakened. A near-upright orientation of most updrafts producing these high reflectivity cores is conceptually consistent with the idea that large raindrops grow in the highest liquid water content while falling through the updraft core. Strong...

Precipitation efficiency of trade wind clouds over the north central tropical Pacific Ocean

The magnitude and distribution of precipitation within trade wind clouds east of the Hawaiian Islands and the efficiency of these clouds in returning water evaporated from the ocean surface back to the ocean through precipitation are examined using highresolution visible and infrared DMSP satellite data and 5‐cm wavelength radar data collected during the 1990 Hawaiian Rainband Project. The analyses indicate that on average, precipitation rates greater than 0.1 mm h−1 occur over about 11% of the area occupied by trade wind clouds at any given time, while about 1.5% of the cloudy areas had rainfall rates which exceeded 1.0 mm h−1. None of the clouds in the sample had precipitation rates exceeding 10.0 mm h−1. The average precipitation rate was 1.1 mm d−1. An estimate of the precipitation efficiency of the trade wind clouds, made by comparing the latent heat input into the atmosphere due to precipitation with the ocean surface latent heat flux, suggests that clouds in the trade wind regime are no more than 20–30% efficient in returning evaporated water vapor to the ocean through precipitation processes.

Trade-Wind Clouds and Hawaiian Rainbands

Abstract Two case studies of rainband evolution off the windward coast of the Big Island of Hawaii are presented along with an overview of the complete radar and satellite dataset from the Hawaiian Rainband Project conducted in the summer of 1990. These studies reveal that radar-observed rainbands and cells offshore of the windward coastline are nearly always embedded within larger-scale stratocumulus cloud patches and/or cloud lines moving in from the northeastern Pacific Ocean in the easterly trade winds. These cloud patches develop in the trade-wind flow downstream of the large shallow stratocumulus cloud mass that covers much of the northeastern Pacific Ocean between Hawaii and California. Tropical cyclones originating in the intertropical convergence zone sometimes move north through this stratocumulus cloud mass, producing, in their wake, cloud-free regions that then advect toward the Hawaiian Islands. These regions contribute to the variability of the organization of cloud patterns and rainband occ...

Vertical Mass Flux Calculations in Hawaiian Trade Cumulus Clouds from Dual-Doppler Radar

Abstract Two ground-based Doppler radars and an instrumented aircraft provided a means for computing the vertical mass flux in trade wind cumulus clouds that formed east of the island of Hawaii dining the Hawaiian Rainband Project of 1990. This study compares the mass fluxes of small isolated cells with larger groups of clouds and rainbands. Because of excellent sensitivity, the 5.5-cm wavelength radars were capable of detecting Bragg backscatter, which extended the measurements to include some precipitation-free air within and surrounding the clouds. The shape of the vertical profile of vertical mass flux within shallow cumulus clouds and cloud groups varied considerably over the cloud's life cycle but was comparatively independent of cloud size. The early stages of convection displayed a mass flux profile that resembled those produced by buoyancy sorting and entraining plume models, but the mature and later stages were considerably more affected by precipitation-driven down-drafts and included mean down...

Characteristics of Near-Surface Winds and Thermal Profiles on the Windward Slopes of the Island of Hawaii

Abstract The near-surface winds and thermal profiles on the windward side of the island of Hawaii are studied using the dataset from the Hawaiian Rainband Project (HaRP), with special attention to the early morning and late afternoon transition periods that separate the daytime upslope and nighttime downslope flow regimes. The effects of rain showers on the development of daytime upslope and nighttime downslope flow are also discussed. Before sunrise (∼0600 HST), the temperature profiles are usually characterized by a nocturnal inversion about 50–150 m above the ground, with a strength of 1–4 K. The stable downslope flow layer usually extends above the nocturnal inversion with a nocturnal jet beneath the inversion. For rain cases, the nocturnal inversion and the nocturnal jet are weaker with a deeper stable downslope flow than clear cases because of rain evaporation aloft, reduced infrared radiation heat loss at the lowest levels, and vertical mixing associated with precipitation. After sunrise the noctur...