URBAN GROWTH AND AEROSOL EFECTS ON CONVECTION OVER HOUSTON Gustavo G. Carrió, William R. Cotton, William Y. Cheng, and Steve M. Saleeby Colorado State University, Colorado State University, Dept. of Atmospheric Science Dept. of Atmospheric Science Fort Collins, Colorado Fort Collins, Colorado
34
Embed
URBAN GROWTH AND AEROSOL EFECTS ON CONVECTION OVER HOUSTON Gustavo G. Carrió, William R. Cotton, William Y. Cheng, and Steve M. Saleeby Colorado State.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
URBAN GROWTH AND AEROSOL EFECTS ON CONVECTION OVER
HOUSTONGustavo G. Carrió, William R. Cotton,
William Y. Cheng, and Steve M. Saleeby
Colorado State University, Colorado State University, Dept. of Atmospheric ScienceDept. of Atmospheric Science
Fort Collins, ColoradoFort Collins, Colorado
Houston is one of the fastest growing metropolitan areas in the United States during the past three decades.
Regional Atmospheric modeling system (RAMS@CSU) coupled to the Town Energy Budget (TEB) generalized canyon model
We focused on a convective storm triggered by the sea breeze circulation (Aug 24 2001)
RAMS@CSU microphysics
Experimental design
Simulation conditions
Brief summary of results
Overview
Cloud Droplet Nucleation
Number nucleated obtained from lookup table as a function of
CCN number concentration
Vertical velocity
Temperature
kappaLookup table generated previously (offline) from detailed parcel-bin model
Nc1=Nccn
Nc2=Ngccn ; Sw > 0.0
bwS
Ice Crystal NucleationIce nucleation follows the approach described by Meyers et al. (1992):
Ni = NIN exp [12.96 (Si - 1)]
T < -5oC; rv > rsi (supersaturation with respect to ice), and T < -2oC ; rv > rsl (supersaturation with respect to liquid).
Secondary ice particle production model in RAMS is based on Mossop (1976). In MKS units, the formula is:
where B increases linearly from 0 to 1 as ice temperature T increases from -8 C to -5 C, B decreases linearly from 1 to 0 as T increases from -5 C to -3 C, and B is zero at other ice temperatures. Ni is the number of ice particles produced per second, N24 is the number of cloud droplets larger than 24 m in diameter that are collected by ice each second, N13 is the number of cloud droplets smaller than 13m in diameter that are collected by ice each second.
.9324 13iN = 9.1e-10 B N N ( )
Features of RAMS bin-emulating microphysics
Cloud droplets (1Cloud droplets (1stst mode) and drizzle drops are mode) and drizzle drops are independently nucleated by the activation of CCN and independently nucleated by the activation of CCN and GCCN (prognostic variables)GCCN (prognostic variables)
Collection is simulated using stochastic collection solutions Collection is simulated using stochastic collection solutions using look-up tables.using look-up tables.Sedimentation of hydrometeors uses bin-approach(allows Sedimentation of hydrometeors uses bin-approach(allows size-sorting)size-sorting)This bin-emulating approach has been extended to all This bin-emulating approach has been extended to all hydrometeor interactions, including sedimentation,hydrometeor interactions, including sedimentation, auto-auto-conversion, ice particle riming, and all interactions among 3 conversion, ice particle riming, and all interactions among 3 liquid modes.liquid modes.
Uses generalized gamma Uses generalized gamma distribution basis distribution basis functions :functions :
RAMS Liquid HydrometeorsRAMS Liquid Hydrometeors
Simulation conditionsTwo-moment microphysics for 8 water species:
Cloud and drizzle droplets, Rain, Pristine ice, Snow, Aggregates, Graupel, and Hail
Initialized: August 24 00Z (~12h before convection started)
Initialized: August 24 2001 00Z (~12h before storm started) Simulation time= 24h
Experimental design over 100 runs
CAPE (Jkg-1)CCN [cm-3]
City source* Background
Gulf
600700800900
10001100120013001400
Aug24 ± 100,200,300,400 Jkg-1
clean(0)5001000150020002500300035004000
500 200
* Values multiplied by the sub-grid urban fraction are nudged at the first model level.
Model vs. observations
The model and the configuration used for these sensitivity experiments was validated in the previous study.
Simulated precip rates and spatial patterns compared well to radar-derived data.
Comparisons among runsQuantity to be compared
Each graph point represents an individual simulation.
The blue arrow denotes runs with no urban sources.
The green arrow denotes runs using the atmospheric conditions of the case study.
Maximum updraft altitude
Difference respect to clean city
[m]
For all runs, the peak updrafts were attained at higher attitudes (respect to clean city).
It takes higher [CCN] for more unstable environments.
As expected, the largest impact corresponds to low instability runs.
Total precipitated volumeDifference with respect to clean city
[%]
Differences in the downwind integral volume are very small (2% max).
However,
Maximum accumulated precip
[mm]
Differences up to 12% in downwind accumulated maxima.
For each level of instability,
when [CCN]↑, the maximum accum. precip downwind first increases, and then decreases.
The “optimal” [CCN] is higher for runs with higher instability.
SC integral mass
[107x kg]
The initial increase in the precipitation is clearly linked to a greater amount of SC water.
When further enhancing [CCN], SC water mass does not vary much, therefore, another mechanism is suppressing precipitation.
Precipitation efficiency
[%]
Ratio between the total precipitated volume and the overall vapor flux (at cloud base levels).
Left of the curve is linked to a greater amounts of SC water.
Again, the “optimal” [CCN] is higher for run with higher instability.
This decrease beyond that point is independent on the intensity of the convection.
SC droplet concentrations
[# /cc]
Precip accumulations are highest when SC cloud droplet concentration are between 14 and 240/cc (~4-5m ).
Smaller SC droplet are less efficient to form large precip. Particles.
Conclusions Pollution can significantly intensify downwind convective cells (+12% in max accumulations), however the effect on integral precipitation values is less important.
In agreement with previous studies, [CCN] ↑ reduce the size of the droplets and the collision efficiencies increasing the amount of SC liquid content and enhancing latent heat release.
But, the effect of pollution is not monotonic.
Further enhancing [CCN] generates smaller SC droplets less efficient to form large precipitation particles, and more likely to be transported aloft as pristine ice crystals.
For a given level of pollution, precipitation is more likely to be enhanced in events characterized by higher instability.