earth&atmospheric sciences, georgia tech modeling the impacts of convective transport and...

Earth&Atmospheric Sciences, Georgia Tech

Modeling the impacts of convective transport and lightning NOx production over North America: Dependence on cumulus parameterizations

Chun Zhao Advisor: Yuhang Wang


Introduction

Convection and associated lightning are two important meteorological processes affecting the production and distribution of tropospheric chemical tracers.


Subgrid cloud convection redistributes chemical tracers vertically and then horizontally. It also deplete the soluble tracers by scavenging process [Wang et al., 2001; Doherty et al., 2005; Choi et al., 2005; Folkins et al., 2006].

http://www.metoffice.gov.uk/research/nwp/numerical/physics/convection_image.html


Lightning NOx (NO2+NO) production is the main source of NOx in the upper troposphere. It affects other species (e.g. O3, PAN, and HNO4) by NOx in the free troposphere. [Hudman et al., 2006].

http://someonewhocares.org/photos/lightning/


Introduction

Convection and associated lightning are two important meteorological processes affecting the production and distribution of tropospheric chemical tracers.

The parameterizations of sub-grid scale convection and lightning NOx production result in large uncertainties in chemical transport models (CTMs) [Doherty et al., 2005; Choi et al., 2005].


Introduction

Two parameterizations of cloud convection (KF-eta and Grell) are implemented in a Regional chEmical trAnsport Model (REAM). Lightning NOx production is parameterized differently in each scheme.

MM5 with Grell convective scheme is used to provide meteorological inputs for REAM with Grell scheme (MM5-REAM) [Choi et al., 2005].

WRF with KF-eta convective scheme is used to provide meteorological inputs for REAM with KF-eta scheme (WRF-REAM) [this work].


Data description

Data used in the study: Intercontinental Chemical Transport Experiment – North America

(INTEX-NA) in summer 2004.

CO, NMHCs, NOx, O3, and HNO3……

SCIAMACHY satellite retrieved tropospheric columns of NO2.


Cloud convective parameterization

Ground surface

Downdraft End layer

Downdraft Original layer

Cloud bottom

Cloud top

Updraft flux

entrainment detrainment

entrainment detrainment

Downdraft flux

Grell Scheme

Cloud top

Cloud bottom

Ground surface

Downdraft fluxUpdraft flux

Downdraft original layer

KF-eta Scheme


Cloud convective parameterization

Cloud information affecting convective transport:

Mass flux Cloud top height Cloud bottom height Downdraft original level Entrainment and detrainment


Mean deep convective updraft mass fluxes from WRF and MM5 simulations during summer (June, July, and August) 2004.

Earth&Atmospheric Sciences, Georgia TechVertical profiles of mean deep convective mass fluxes from WRF and MM5 simulations, and entrainment detrainment rate from WRF simulation during summer 2004.


Cloud convective impact

Tracers sensitive to convective transport: CO, C4H10, and C3H8

Tracers scavenged by convective updrafts: Highly soluble tracer HNO3


Cloud convective vertical transport

Convection affects non-soluble tracers through the turnover of their

concentrations in the troposphere [Wang et al., 2001].

Convection lifts the non-soluble tracers with elevated concentrations

from the low altitude to the high altitude and subsides them with low

concentrations from the high altitude to the low altitude, then increasing

the concentrations in the upper troposphere and decreasing the

concentrations in the lower and middle troposphere. Entrainment and

detrainment provide a pathway for mixing convective updrafts and

downdrafts with background atmosphere in the middle troposphere.


Convective transport enhances CO, C4H10, and C3H8

concentrations by 15, 150, and 100% respectively in the upper troposphere.

Entrainment and detrainment drive the enhancement of the concentrations of tracers in the mid-troposphere (3-7km) in WRF-REAM simulation.

Vertical profiles of CO, C4H10, and C3H8 from aircraft measurements, REAM simulations with and without convection.

Earth&Atmospheric Sciences, Georgia TechSpatial distributions of the average convection driven relative changes of the concentrations of CO, C 4H10, and C3H8 from REAM simulation during summer 2004.


Cloud convective scavenging

Convection affects highly soluble tracers through the scavenging

process in convective updrafts [Folkings et al., 2006].

Entrainment and detrainment provide additional channels to diminish

the soluble tracers in the free troposphere (above 3 km) through

grabbing the soluble tracers in the background atmosphere into

convective updrafts and downdrafts.

Earth&Atmospheric Sciences, Georgia TechVertical profiles of HNO3 from aircraft measurements and REAM standard simulations.

Earth&Atmospheric Sciences, Georgia TechVertical profiles of the average convection driven relative changes of the concentrations of HNO3 from REAM simulations during summer 2004.


Cloud convective outflow

Convective transport was found to be an important pathway for inflow

and outflow of pollution over high-polluted continents such as North

America [Wang et al., 2001; Choi et al., 2005; Li et al., 2005; Liang et

al., 2007].

INTEX-NA aircraft campaign was conducted with a major goal to

understand the mechanisms by which pollutants are lofted and

transported during the warm season [Singh et al., 2006].

Earth&Atmospheric Sciences, Georgia TechConcentration of C3H8 along the aircraft tracks in July 28th and August 6th.

Flight tracks in INTEX-NA campaign over the Atlantic Ocean in July 28th and August 6th.

Earth&Atmospheric Sciences, Georgia TechC3H8 concentrations and winds at 300 hpa at 18:00 GMT on July 28 th and August 6th in 2004 from REAM simulation with and without convection.

Flight tracks in INTEX-NA campaign over the Atlantic Ocean in July 28th and August 6th.


Cloud convective outflow

The higher concentration of C3H8 at 300 hpa in WRF-REAM simulation

totally results from the sub-grid convective transport, on the contrary,

that in MM5-REAM simulation just partly result from the sub-grid

convective transport. So in these two cases, besides sub-grid convective

transport, MM5-REAM simulation should include some other vertical

transport mechanisms, possibly large-scale convection or convection

related warm conveyor belts (WCBs). So it is likely that the vertical

transport in MM5-REAM has roots in large-scale convection, which is too

large to be parameterized as a sub-grid convection.



Through the investigation of convective impact:

Vertical transport of non-soluble tracers:

The convective impacts on non-soluble tracers are significantly affected by

the cumulus cloud top height. The maximum convective impact on non-

soluble tracers appears at different altitudes related with cloud top height.

The entrainment and detrainment effects are shown in the comparison.

They are important for the mixing in the middle troposphere.

Convective scavenging of soluble tracers:

The model (WRF-REAM) with entrainment and detrainment in convective

parameterization simulates more effective scavenging process for highly

soluble pollutants (e.g. HNO3) in the free troposphere.



Through the investigation of convective impact:

Convective outflow

The two days outflow during INTEX-NA were driven by the vertical transport

originating from the convection. However, the convection system was

parameterized as a sub-grid convection in WRF with KF-eta scheme

simulation but was too large to be parameterized as a sub-grid convection

in MM5 with Grell scheme simulation, indicating that MM5 with Grell

scheme involves less sub-grid convections.


Acknowledgments

I would like to thank Dr. Wang for his advisements. I thank Dr. Tao Zeng, Dr. Yunsoo Choi and all my officemates for their helps and suggestions.


Thank you very much!


Lightning NOx production parameterization

Lightning NOx production is parameterized as a function of certain meteorological variables: convective updraft mass fluxes (UMF), convective available potential energy (CAPE), cumulus cloud top height, and precipitation [Price et al., 1993; Allen et al., 1999; Choi et al., 2005].

MM5-REAM UMF and CAPE from Grell scheme (Choi et al., 2005)

WRF-REAM UMF and CAPE from KF-eta scheme (this work)

Cloud-to-ground lightning flash rate is scaled to National Lightning Detection Network (NLDN) observation. Intra-cloud (IC) to cloud-to-ground (CG) flash ratio is calculated following Wang et al. [1998].


Mean deep convective cloud top pressures from the measurements of GOE-10 and GOE-12 satellites and the simulations of WRF and MM5 during July 2004.


Lightning NOx production impact

Lightning NOx (NO2+NO) production is a major source of NOx in the

upper troposphere and significantly enhances tropospheric NO2

columns over the ocean, where NO2 columns are more sensitive to the

lightning NOx [Labrador et al., 2004; Choi et al. 2005; Martin et al.,

2006; Hudman et al., 2006]. It provides the best explanation for the

highly elevated NOx in the upper troposphere, which is difficult to be

explained by deep convective injection of boundary layer pollution and

aircraft emissions [Hudmain et al., 2006].


Spatial distributions of average NO2 concentrations above 8 km, from measurements and REAM simulation.


NO2 columns from SCIAMACHY, WRF-REAM and MM5-REAM.

Lightning enhances tropospheric NO2 columns especially over the ocean by more than a factor of 2. Both WRF-REAM and MM5-REAM reproduce tropospheric NO2 columns well and highly correlate with the measurements by a coefficient 0.73 and 0.63 respectively.

MM5-REAM with higher cloud top height (up to 15 km) over-distributes lightning NOx to above 12 km, while WRF-REAM with lower cloud top height (10-12 km) performs better in the vertically distribution of the lightning NOx.


Spatial distributions of average O3 concentrations above 8 km from measurements and simulations with and without lightning.

O3 concentration is increased by up to 20 ppb due to the lightning in the upper troposphere. The upper tropospheric O3 concentration is more sensitive to the lightning over the ocean. Both WRF-REAM and MM5-REAM simulates larger lightning related enhancement of upper troposphere O3 concentration in the regions where lightning NOx production is larger.


Through the investigation of lightning impacts: Lightning effects are found to be important in simulations. Lightning enhances

the concentrations of NO2 by a factor of 2 in the upper troposphere. It also enhances O3 concentration by up to 20 ppb in the upper troposphere.

The lightning NOx production and distribution are sensitive to cumulus cloud top height in the parameterization following Wang et al. [1998] and Allen et al. [1999]. The WRF-REAM model with lower cloud top height (10-12 km) simulates much better lightning NOx production and distribution than the MM5-REAM model with higher cumulus cloud top height (up to 15 km).

Lightning NOx production parameterization


Summary

The cumulus cloud top height (CTH) is a critical factor in determining the convective impacts on chemical tracers.

The CTH is also a significant factor controlling lightning NOx production and distribution. WRF-REAM model with lower CTH (10-12 km) simulates better lightning NOx production and distribution than the MM5-REAM model with higher CTH (up to 15 km).

The entrainment and detrainment provide effective scavenging processes for soluble pollutants. WRF-REAM model with convective parameterization including entrainment and detrainment simulates better highly soluble pollutants (e.g. HNO3) in the free troposphere.

KF-eta scheme are found to provide better simulations for tropospheric trace gases than Grell scheme.


Acknowledgments

I would like to thank Dr. Wang for his advisements. I thank Dr. Tao Zeng, Dr. Yunsoo Choi and all my officemates for their helps and suggestions.


Thank you very much!

earth&atmospheric sciences, georgia tech modeling the impacts of convective transport and...

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