# Presentation: 054641

## The seasonal cycle of tropospheric influence in the lowermost stratosphere derived from HIRDLS O3 observations

**Susan Strahan, *****University of Maryland, Baltimore County***
***	****** ******and NASA GSFC (Atmo Chem & Dynamics Branch)***
***	******in collaboration with Mark Olsen, Jose Rodriguez, ***
***	******and Anne Douglass***

## HIRDLS O3 and the seasonally varying composition of the lowermost stratosphere

_**HIRDLS O**__**3**__** and the seasonally varying composition of the lowermost stratosphere**_

**Because of HIRDLS’ ability to make high vertical resolution measurements of the lowermost stratosphere (~330-380K), its O3 data is well-suited for the study of seasonally varying composition and the processes that affect it. Several groups********* are interested in the lamina that have been observed:**
** ****how often?**
** ****where and when?**
** ****can transport by lamina be quantified?**
_***This study uses HIRDLS O***__***3***__*** observations of 150 ppb or less to identify air in the lowermost stratosphere (LMS) with recent tropospheric origin. ***_**Two questions are posed:**
** ****What fraction of the LMS is recently tropospheric and what is ****	****the seasonal variation of this fraction?**
** ****What does HIRDLS tell us about **_***how ***_**tropospheric air enters the stratosphere **_***(i.e., what processes)***_** ? **

***Olsen et al. (GRL, 2008, in press), and Nardi et al. and Gille et al. (this meeting)**
***Also, see the Olsen et al. poster this meeting.***

## Tropospheric Air Fraction from HIRDLS O3

**HIRDLS O****3**** V4 observations used:**
** *****poleward of 30******o******N***
***levels 215 hPa and above***
***150 ppb or less***
***Potential temperature for each observation is calculated using HIRDLS temperature data.***
***For 5******o****** latitude bands and 10K theta bins, the number of obs having O******3****** ≤ 150 ppb are counted and their daily fraction of total O******3 ******observations is calculated. ***
***Because some mixing occurs, the calculated tropospheric fraction represents a lower limit.***

***Tropospheric Fraction 2005-2007***

_**The Results:**_** Interannual variability in the seasonally varying tropospheric fraction are small. The seasonal cycle is robust.**

**JAN-DEC**

**30-35N**
**360K**
**45-50N**
**340K**
**60-65N**
**350K**

**1.0**
**0.0**
**0.8**
**0.0**
**0.4**
**0.0**

**Notes:**
The tropospheric fraction is calculated using...Version 4 O3, only poleward of 30N,...

## Tropospheric Air Fraction from 340-370K

_**Above and Below:**_** HIRDLS data density is much lower below 330K. At 380K and above a different O****3**** criteria would be needed.**

**The LMS composition shows:**
*** ******minimum tropospheric influence in winter/spring, greatest in summer and fall***
*** ******greatest tropospheric influence at 340K, then decreasing with height***
*** ******tropospheric influence extends nearly to the pole in fall at the lowest levels***
*** ******consistency with other studies of LMS composition (e.g., Hoor et al. (2004), Ray et al. (1999), Strahan et al. (2007))***

**Notes:**
This figure summarizes the seasonal variation in tropospheric fraction as a function of latitude for 4 levels in the LMS. Yellow, orange, and red indicate 70% or greater fraction; purple indicates <10% tropo air. The dashed red line indicates the zonal monthly mean tropopause latitude.

## Transport Conclusions

***Question: Can HIRDLS tell us anything about where tropospheric air enters the LMS?***

**The seasonally varying influence of tropospheric air indicates:**
*** ******downward branch of the Brewer-Dobson circulation strongly influences composition in winter and spring (no surprise here)***
*** ******tropospheric influence originates (horizontally) from low latitudes and has a maximum in summer and fall; this may coincide with monsoon circulations***

**Notes:**
The transport conclusions that can be drawn from this analysis are:

## HIRDLS tropospheric O3 values shows strong longitudinal preference near the subtropics (30-45N)

_**HIRDLS tropospheric O**__**3**__** values shows strong longitudinal preference near the subtropics (30-45N)**_

**This figure shows how the longitudinal asymmetry shifts as a function of height.**
***  ******Black, purple, dark blue: low O******3****** almost never found ***
***  ******Green, yellow, red: low O******3****** frequently found***

**340-370K, 30-35N****	**

**Time  (Jan-Dec)**

**LONGITUDE**

_**340-350K:**_
**Tropospheric air is found in 2 longitude bands (Strat-trop exchange by Asian and Mexican monsoons?)**

_**360-370K:**_
**Tropospheric air only found 0-100E (Asian monsoon influence? Extends to higher altitudes?)**

**Notes:**
These figures show the spatial distribution of low O3 values in the subtropics over a one year cycle, as a function of height from 340-370K. The black and purple colors indicate longitudes where low O3 is almost never found; yellow and green indicate preferred longitudes. And even distribution would be sky blue everywhere.

340K, 30-35N is almost always in the troposphere, that is, it is equatorward of the tropopause.
The patterns shift gradually with height. 

## Longitudinal asymmetry in the subtropics (30-45N), but greater symmetry >45N.

_**Longitudinal asymmetry in the subtropics (30-45N), **__**but greater symmetry >45N.**_

**This figure shows contoured distributions of the longitudes where low O****3**** is found.**
***  ******Black, purple, dark blue: low O******3****** almost never found ***
***  ******Green, yellow, red: low O******3****** frequently found***

- 	             **360K**
**30-35N                35-40N                40-45N**

**Time  (Jan-Dec)**

***LONGITUDE***

**45-50N              50-55N               55-60N**

_**Latitudes 45-60N:**_
** ****Summer/fall has a wave 1 pattern.**
**Winter/spring is ‘splotchy’ – not a lot of tropospheric observations.**

_**Latitudes 30-45N:**_
- Zonal asymmetry is clear from May-Sept.

**Notes:**
In this figure we look at the 360K surface and examine how the spatial asymmetry varies with latitude.
In the last slide we saw... Here we see the same pattern at latitudes up to 45N.

The pattern is different above 45N. In winter and spring, there aren’t many observations of tropospheric air so the distributions look splotchy. In summer/fall, the pattern is more symmetric than at lower latitudes with some wave 1 character.
Tropospheric air found in the mid and high latitudes is better mixed than in the subtropics.

## Tropospheric Influence in the LMS: how do the GMI models compare? 

**Two GMI ‘Combo’ Models: GEOS4-GCM and GEOS4-DAS meteorology.**
**GMIDAS is a 2005-2006 average; GMIGCM is a 3-yr average.**** **

**Both have tropospheric and stratospheric chemistry, **
**2****o**** lat x 2.5****o**** lon x 42 levels (1 km resolution in the UT/LS)**

**Months (Jan-Dec)**

**380K            360K              340K**
**         ****Latitude (30-80N)**

**HIRDLS                GMI-DAS              GMI-GCM**

**In general, the models’ seasonal patterns are very good but are too tropospheric at low latitudes in summer/fall but their tropospheric influence does not extend poleward enough.**

**Notes:**
The HIRDLS tropospheric fraction analysis provide a good test of model UT/LS transport processes.

## Tropospheric Influence in the LMS: how do the GMI models compare? 

_**Tropospheric Influence in the LMS: how do the GMI models compare? **_

**Winter (Jan-Apr) and Summer (Jul-Oct) Seasonal Composition**

_**Winter:**_** Models largely agree with HIRDLS that the LMS has little tropospheric influence above 40N.**
**GMIGCM is too tropospheric 30-40N. **

**380K       360K      340K**
**       ****Latitude (30-80N)**

** ****HIRDLS            GMIDAS           GMIGCM**

_**Summer:**_** Models are clearly too tropospheric 30-50N. **
**Above 60N, models **_***may  have***_** insufficient tropospheric influence, especially the GMIGCM. **

**Notes:**
This figure summarized the seasonal behavior of HIRDLS and the two models. The panels are a latitude-height cross section of the tropospheric fraction for winter and summer. 

The DAS winds seem to do a better job of transporting tropospheric air poleward in the LMS.

## Longitudinal Distribution of Tropospheric Air in HIRDLS and the GMI models – 340K

_**Longitudinal Distribution of Tropospheric Air **__**in HIRDLS and the GMI models – **__**340K**_

***HIRDLS data suggests transport of stratospheric O******3****** into the tropical upper troposphere – ***_***there is no evidence for this in the simulations!***_
***HIRDLS and GMIDAS have the same tropopauses... so why does O3 look so different???***

**At 340K, the models do NOT reproduce the longitudinal asymmetry at all. **
**The zonal monthly mean tropopause is >340K from 30-35N all year for both models....so these observations are in the upper troposphere.**

**Notes:**
Now let’s look at the longitudinal distribution of low O3 air in the models to see if the same signatures appear.
The models’ lack of stratospheric O3 in the tropical UT is intriguing, but I do not have an explanation.

## Longitudinal Distribution of Tropospheric Air in HIRDLS and the GMI models – 360K

**LONGITUDE**

_**Longitudinal Distribution of Tropospheric Air **__**in HIRDLS and the GMI models **__**– 360K**_

**GMIDAS and GMIGCM suggest transport of tropospheric air into the LMS by the Asian monsoon circulation in late spring and summer, 350-370K.**

**Months (Jan-Dec)**

**Longitudinal patterns at 360K are in much better agreement with HIRDLS in all seasons for 35-40N (stratospheric).**
**This is also true for 30-45N, 350-370K.**

**360K, 35-40N**

## Longitudinal Distribution of Tropospheric Air in HIRDLS and the GMI models – Higher Latitudes

_**Summer/Fall**_**:**** At high latitudes, unlike lower latitudes, HIRLDS shows fairly uniform distributions. This suggests that tropospheric air is better mixed (zonally) by the time it arrives at high latitudes. The models have fairly uniform distributions too.**
_**Winter/Spring**_**: Models have almost no tropospheric influence, and less than seen by HIRDLS.**

## Conclusions

_**Conclusions**_

**The longitudinal distribution of low O****3**** shows significant zonal asymmetry 30-45N, May-Oct. At 340-350K, there are two preferred longitude bands (Asian and Mexican monsoons?) At 360-370K there is only one wide band.**

**HIRDLS ****low O****3**** observations show a robust seasonal cycle in the fraction of tropospheric air in the lowermost stratosphere.**

**The longitudinal distribution of low O****3**** above 45N in summer/fall is fairly symmetric, suggesting that low latitude (i.e., tropospheric) air is becoming mixed as it moves poleward.**

**These results provide a good basis for evaluating the lower stratospheric composition in models and model pathways for trop-to-strat transport through the subtropics. GMIDAS shows better agreement. **

## Spatial distributions of low O3 in the LMS: where is it found?

_**Spatial distributions of low O**__**3**__** in the LMS: where is it found?**_

**This figure shows contoured distributions of the longitudes where low O****3**** is found.**
***  ******Black, purple, dark blue: low O******3****** almost never found ***
***  ******Green, yellow, red: low O******3****** frequently found***

- 	             **350K**
**30-35N               35-40N                40-45N**

**Time  (Jan-Dec)**

***LONGITUDE***

**45-50N              50-55N               55-60N**

_**Latitudes 30-45N:**_
**Zonal asymmetry similar to 340K, but not as strong.**

_**Latitudes 45-60N:**_
**Late summer/fall has a wave 1 pattern.**
**Winter/spring is ‘splotchy’ – not a lot of tropospheric observations.**

## Spatial distributions of low O3 in the LMS, or, where is low O3 missing in the troposphere?

_**Spatial distributions of low O**__**3**__** in the LMS, or, **__**where is low O**__**3**__** missing in the troposphere?**_

**This figure shows contoured distributions of the longitudes where low O****3**** is found.**
***  ******Black, purple, dark blue: low O******3****** almost never found ***
***  ******Green, yellow, red: low O******3****** frequently found***

- 	       **340K**
**30-35N       35-40N        40-45N**

**Time  (Jan-Dec)**

***LONGITUDE***

**45-50N           50-55N           55-60N**

_**Latitudes 30-45N:**_
**IN SUMMER THESE LATITUDES ARE  TROPOSPHERIC!**
**Virtual absence of low O****3**** from ~10-140E from May-Oct. *****In other words, this looks like stratospheric air in the troposphere.***

_**Latitudes 45-60N:**_
**Late summer/fall has no asymmetry.**
**Winter/spring is ‘splotchy’ – not a lot of tropospheric observations.**