THE OLYMPIC MOUNTAINS EXPERIMENT (OLYMPEX) Houze, Robert A.; McMurdie, Lynn A.; Petersen, Walter A. ...
Bulletin of the American Meteorological Society,
10/2017, Letnik:
98, Številka:
10
Journal Article
Recenzirano
Odprti dostop
OLYMPEX is a comprehensive field campaign to study how precipitation in Pacific storms is modified by passage over coastal mountains.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The southeastern Atlantic (SEA) and its associated cloud deck, off
the west coast of central Africa, is an area where aerosol–cloud
interactions can have a strong radiative impact. Seasonally,
...extensive biomass burning (BB) aerosol plumes from southern Africa
reach this area. The NASA ObseRvations of Aerosols above CLouds and
their intEractionS (ORACLES) study focused on quantitatively
understanding these interactions and their importance. Here we
present measurements of cloud condensation nuclei (CCN)
concentration, aerosol size distribution, and characteristic
vertical updraft velocity (w∗) in and around the marine boundary
layer (MBL) collected by the NASA P-3B aircraft during the August
2017 ORACLES deployment. BB aerosol levels vary considerably but
systematically with time; high aerosol concentrations were observed
in the MBL (800–1000 cm−3) early on, decreasing
midcampaign to concentrations between
500 and 800 cm−3. By late August and early September,
relatively clean MBL conditions were sampled (<500 cm−3). These data then drive a state-of-the-art
droplet formation parameterization from which the predicted cloud
droplet number and its sensitivity to aerosol and dynamical
parameters are derived. Droplet closure was achieved to within
20 %. Droplet formation sensitivity to aerosol concentration,
w∗, and the hygroscopicity parameter, κ, vary and
contribute to the total droplet response in the MBL clouds. When
aerosol concentrations exceed ∼900 cm−3 and maximum
supersaturation approaches 0.1 %, droplet formation in the MBL
enters a velocity-limited droplet activation regime, where the cloud
droplet number responds weakly to CCN concentration increases. Below
∼500 cm−3, in a clean MBL, droplet formation is
much more sensitive to changes in aerosol concentration than to
changes in vertical updraft. In the competitive regime, where
the MBL has intermediate pollution (500–800 cm−3),
droplet formation becomes much more sensitive to hygroscopicity
(κ) variations than it does in clean and polluted
conditions. Higher concentrations increase the sensitivity to
vertical velocity by more than 10-fold. We also find that
characteristic vertical velocity plays a very important role in
driving droplet formation in a more polluted MBL regime, in which
even a small shift in w∗ may make a significant difference in
droplet concentrations. Identifying regimes where droplet number
variability is driven primarily by updraft velocity and not by aerosol
concentration is key for interpreting aerosol indirect effects,
especially with remote sensing. The droplet number responds
proportionally to changes in characteristic velocity, offering the
possibility of remote sensing of w∗ under velocity-limited
conditions.
Scattering models of precipitation‐size ice particles have shown that aggregates and spheroidal particles occupy distinct regions of the Ku‐Ka‐W‐band dual‐frequency ratio (DFR) plane. Furthermore, ...past ground‐based observations suggest that particle bulk density and characteristic size can be retrieved from the DFR plane. This study, for the first time, evaluates airborne DFR observations with coincident airborne microphysical measurements. Over 2 hr of microphysical data collected aboard the University of North Dakota Citation from the Olympic Mountains Experiment are matched with Airborne Precipitation and cloud Radar Third Generation triple‐frequency radar observations. Across all flights, 31% (63%) of collocated data points show nonspheroidal (spheroidal) particle scattering characteristics. DFR observations compared with in situ observations of effective density and particle characteristic size reveal relationships that could potentially be used to develop quantitative dual‐ and triple‐frequency DFR ice property retrievals.
Plain Language Summary
Currently, remote sensing retrievals of ice clouds require assumptions since particle shape and size vary greatly in the atmosphere. Additionally, particle shape and size constrain relationships of mass and fall velocity of ice within a cloud, which affect remote sensing retrievals. Modeling studies have shown that the scattering characteristics of complex ice particles (e.g., aggregates) have a distinct signature compared to spherical representations of the same particles when using three frequencies under the following conditions: (1) at least one radar with its wavelength close to the size of the particle and (2) particles have low effective densities. Thus, there is potential to retrieve information about particle shape using triple‐frequency radar observations to constrain the assumptions of particle shape in the ice cloud retrieval. This paper is the first study to use airborne triple‐frequency radar observations coincident with airborne in situ microphysical measurements to evaluate both the scattering signal discussed and retrievals of characteristic size and effective density. We found that 31% (63%) of the observations from the Olympic Mountains Experiment show nonspheroidal (spheroidal) scattering characteristics. Furthermore, the triple‐frequency observations confirm the relationships with observed particle size and effective density outlined in a previous study supporting future use of triple‐frequency missions.
Key Points
First airborne evaluation of snowfall property retrievals using triple‐frequency radar and microphysical measurements
A limited fraction of coincident points show nonspheroidal scattering behavior, challenging the value of differentiating particle type
Bulk statistics of effective density and characteristic size support the retrieval hypotheses in previous literature
Measurements of the ice nucleating ability of aerosol particles in air masses over Florida having sources from North Africa support the potential importance of dust aerosols for indirectly affecting ...cloud properties and climate. The concentrations of ice nuclei within dust layers at particle sizes below 1 pn exceeded 1/cu cm; the highest ever reported with our device at temperatures warmer than homogeneous freezing conditions. These measurements add to previous direct and indirect evidence of the ice nucleation efficiency of desert dust aerosols, but also confirm their contribution to ice nuclei populations at great distances from source regions.
Aerosol–cloud–precipitation interactions (ACIs) provide the greatest source of uncertainties in predicting changes in Earth's energy budget due to poor representation of marine stratocumulus and the ...associated ACIs in climate models. Using in situ data from 329 cloud profiles across 24 research flights from the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign in September 2016, August 2017, and October 2018, it is shown that contact between above-cloud biomass burning aerosols and marine stratocumulus over the Southeast Atlantic Ocean was associated with precipitation suppression and a decrease in the precipitation susceptibility (So) to aerosols. The 173 “contact” profiles with aerosol concentration (Na) greater than 500 cm−3 within 100 m above cloud tops had a 50 % lower precipitation rate (Rp) and a 20 % lower So, on average, compared to 156 “separated” profiles with Na less than 500 cm−3 up to at least 100 m above cloud tops. Contact and separated profiles had statistically significant differences in droplet concentration (Nc) and effective radius (Re) (95 % confidence intervals from a two-sample t test are reported). Contact profiles had 84 to 90 cm−3 higher Nc and 1.4 to 1.6 µm lower Re compared to separated profiles. In clean boundary layers (below-cloud Na less than 350 cm−3), contact profiles had 25 to 31 cm−3 higher Nc and 0.2 to 0.5 µm lower Re. In polluted boundary layers (below-cloud Na exceeding 350 cm−3), contact profiles had 98 to 108 cm−3 higher Nc and 1.6 to 1.8 µm lower Re. On the other hand, contact and separated profiles had statistically insignificant differences between the average liquid water path, cloud thickness, and meteorological parameters like surface temperature, lower tropospheric stability, and estimated inversion strength. These results suggest the changes in cloud microphysical properties were driven by ACIs rather than meteorological effects, and adjustments to existing relationships between Rp and Nc in model parameterizations should be considered to account for the role of ACIs.
In situ cloud probe data from the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign were used to estimate the effective radius (R.sub.e ), cloud optical ...thickness (Ï), and cloud droplet concentration (N.sub.c) for marine stratocumulus over the southeast Atlantic Ocean. The in situ R.sub.e, Ï, and N.sub.c were compared with co-located Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals of R.sub.e and Ï and MODIS-derived N.sub.c . For 145 cloud profiles, a MODIS retrieval was co-located with in situ data with a time gap of less than 1 h. On average, the MODIS R.sub.e and Ï (11.3 µm and 11.7) were 1.6 µm and 2.3 higher than the in situ R.sub.e and Ï with Pearson's correlation coefficients (R) of 0.77 and 0.73, respectively. The average MODIS N.sub.c (151.5 cm.sup.-3) was within 1 cm.sup.-3 of the average in situ N.sub.c with an R of 0.90.
The detailed microphysical processes and properties within the melting layer (ML)-the continued growth of the aggregates by the collection of the small particles, the breakup of these aggregates, the ...effects of relative humidity on particle melting-are largely unresolved. This study focuses on addressing these questions for in-cloud heights from just above to just below the ML. Observations from four field programs employing in situ measurements from above to below the ML are used to characterize the microphysics through this region. With increasing temperatures from about -4 degree to +1 degree C, and for saturated conditions, slope and intercept parameters of exponential fits to the particle size distributions (PSD) fitted to the data continue to decrease downward, the maximum particle size (largest particle sampled for each 5-s PSD) increases, and melting proceeds from the smallest to the largest particles. With increasing temperature from about -4 degree to +2 degree C for highly subsaturated conditions, the PSD slope and intercept continue to decrease downward, the maximum particle size increases, and there is relatively little melting, but all particles experience sublimation.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Advancing understanding of deep convection microphysics via mesoscale modeling studies of well-observed case studies requires observation-based aerosol inputs. Here, we derive hygroscopic aerosol ...size distribution input profiles from ground-based and airborne measurements for six convection case studies observed during the Midlatitude Continental Convective Cloud Experiment (MC3E) over Oklahoma. We demonstrate use of an input profile in simulations of the only well-observed case study that produced extensive stratiform outflow on 20 May 2011. At well-sampled elevations between −11 and −23 C over widespread stratiform rain, ice crystal number concentrations are consistently dominated by a single mode near approx. 400 microm in randomly oriented maximum dimension (Dmax). The ice mass at −23 C is primarily in a closely collocated mode, whereas a mass mode near Dmax approx. 1000 microns becomes dominant with decreasing elevation to the −11 C level, consistent with possible aggregation during sedimentation. However, simulations with and without observation-based aerosol inputs systematically overpredict mass peak Dmax by a factor of 3-5 and underpredict ice number concentration by a factor of 4-10. Previously reported simulations with both two-moment and size-resolved microphysics have shown biases of a similar nature. The observed ice properties are notably similar to those reported from recent tropical measurements. Based on several lines of evidence, we speculate that updraft microphysical pathways determining outflow properties in the 20 May case are similar to a tropical regime, likely associated with warm-temperature ice multiplication that is not well understood or well represented in models.
Marine stratocumulus cloud properties over the Southeast Atlantic Ocean are impacted by contact between above-cloud biomass burning aerosols and cloud tops. Different vertical separations (0 to 2000 ...m) between the aerosol layer and cloud tops were observed on six research flights in September 2016 during the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. There were 30 contact profiles, where an aerosol layer with aerosol concentration (Na) > 500 cm−3 was within 100 m of cloud tops, and 41 separated profiles, where the aerosol layer with Na > 500 cm−3 was located more than 100 m above cloud tops. For contact profiles, the average cloud droplet concentration (Nc) in the cloud layer was up to 68 cm−3 higher, the effective radius (Re) up to 1.3 µm lower, and the liquid water content (LWC) within 0.01 g m−3 compared to separated profiles. Free-tropospheric humidity was higher in the presence of biomass burning aerosols, and contact profiles had a smaller decrease in humidity (and positive buoyancy) across cloud tops with higher median above-cloud Na (895 cm−3) compared to separated profiles (30 cm−3). Due to droplet evaporation from entrainment mixing of warm, dry free-tropospheric air into the clouds, the median Nc and LWC for contact profiles decreased with height by 21 and 9 % in the top 20 % of the cloud layer. The impact of droplet evaporation was stronger during separated profiles as a greater decrease in humidity (and negative buoyancy) across cloud tops led to greater decreases in median Nc (30 %) and LWC (16 %) near cloud tops. Below-cloud Na was sampled during 61 profiles, and most contact profiles (20 out of 28) were within high-Na (> 350 cm−3) boundary layers, while most separated profiles (22 out of 33) were within low-Na (< 350 cm−3) boundary layers. Although the differences in below-cloud Na were statistically insignificant, contact profiles within low-Na boundary layers had up to 34.9 cm−3 higher Nc compared to separated profiles. This is consistent with a weaker impact of droplet evaporation in the presence of biomass burning aerosols within 100 m above cloud tops. For contact profiles within high-Na boundary layers, the presence of biomass burning aerosols led to higher below-cloud Na (up to 70.5 cm−3) and additional droplet nucleation above the cloud base along with weaker droplet evaporation. Consequently, the contact profiles in high-Na boundary layers had up to 88.4 cm−3 higher Nc compared to separated profiles. These results motivate investigations of aerosol–cloud–precipitation interactions over the Southeast Atlantic since the changes in Nc and Re induced by the presence of above-cloud biomass burning aerosols are likely to impact precipitation rates, liquid water path, and cloud fraction, and modulate closed-to-open-cell transitions.
Abstract
On 7 February 2020, precipitation within the comma-head region of an extratropical cyclone was sampled remotely and in situ by two research aircraft, providing a vertical cross section of ...microphysical observations and fine-scale radar measurements. The sampled region was stratified vertically by distinct temperature layers and horizontally into a stratiform region on the west side, and a region of elevated convection on the east side. In the stratiform region, precipitation formed near cloud top as side-plane, polycrystalline, and platelike particles. These habits occurred through cloud depth, implying that the cloud-top region was the primary source of particles. Almost no supercooled water was present. The ice water content within the stratiform region showed an overall increase with depth between the aircraft flight levels, while the total number concentration slightly decreased, consistent with growth by vapor deposition and aggregation. In the convective region, new particle habits were observed within each temperature-defined layer along with detectable amounts of supercooled water, implying that ice particle formation occurred in several layers. Total number concentration decreased from cloud top to the −8°C level, consistent with particle aggregation. At temperatures > −8°C, ice particle concentrations in some regions increased to >100 L
−1
, suggesting secondary ice production occurred at lower altitudes. WSR-88D reflectivity composites during the sampling period showed a weak, loosely organized banded feature. The band, evident on earlier flight legs, was consistent with enhanced vertical motion associated with frontogenesis, and at least partial melting of ice particles near the surface. A conceptual model of precipitation growth processes within the comma head is presented.
Significance Statement
Snowstorms over the northeast United States have major impacts on travel, power availability, and commerce. The processes by which snow forms in winter storms over this region are complex and their snowfall totals are hard to forecast accurately because of a poor understanding of the microphysical processes within the clouds composing the storms. This paper presents a case study from the NASA IMPACTS field campaign that involved two aircraft sampling the storm simultaneously with radars, and probes that measure the microphysical properties within the storm. The paper examines how variations in stability and frontal structure influence the microphysical evolution of ice particles as they fall from cloud top to the surface within the storm.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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