# Presentation: 669704

## Impact of Tropical Easterly Waves during the North American Monsoon (NAM) using a Mesoscale Model

- Jennifer L. Adams
*CIMMS/University of Oklahoma*
- Dr. David Stensrud
*NOAA/National Severe Storms Laboratory*
*October 27, 2005*

## What is the NAM?

- Distinct shift in mid-level winds accompanied by an increase in rainfall
- Occurs over NW Mexico and SW United States
- Onset usually in July and decays in September
- Great deal of variability

## NAM Moisture Source

- Moisture source current consensus
- low-level moisture  Gulf of California (GoC)
- mid-level moisture  Gulf of Mexico (GoM)
- Transport of low-level moisture by gulf surges (one way)
- Induced by passage of tropical easterly waves (TEWs) over GoC and/or outflow boundaries/gust fronts

## Gulf surges

- Hales (1972) and Brenner (1974)
- Cooler temps, increased dewpoints, pressure rise, southerly wind
- Increase in convection
- Shallow vertical extent
- Loss of definition upon entering desert SW

**Adams and Comrie (1997)**

## Motivation

- NAM predictability very low
- TEWs influential to strength of NAM
- Inverse relationship between NAM and U.S. central plains rainfall

## Goals

- Explore impact of TEWs on the NAM
-  gulf surges
-  NAM region rainfall
- Control run of MM5 compared to simulation where TEWs are removed

## Model Description

- Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5)
- Model domain (350x180x23) at 25 km grid spacing

**Puerto Penasco**

## MM5 Parameterization Schemes

- Kain-Fritsch convective scheme (Kain and Fritsch 1990)
- MRF PBL scheme (Hong and Pan 1996)
- Simple water and ice microphysics (Dudhia 1989)
- Global terrain dataset – 10 minute resolution (25 USGS land use categories)
- Rapid Radiative Transfer Model for radiation
- 5-layer soil model (Dudhia 1996)
- _Model initialization_: NCEP/NCAR reanalysis data
- supply boundary conditions every 6-h
- GoC SSTs set to constant 29.0oC

## Methodology

- Four one-month periods
- July 1990, July 1992, August 1988, August 1986
- ECMWF reanalysis data and CPC precipitation analysis
- Varying number of TEWs and rainfall amounts

## ECMWF Hövmoller Diagrams (850 mb)

**July 1990**

**August 1986**

**July 1992**

**August 1988**

## Methodology

- Harmonic analysis to remove TEWs from boundary conditions
- Reed et al. (1977) – TEWs average wavelength 2500 km, propagation speed of 8 m/s, and average period of 3.5 days
- TEWs with periods of approx. 3.5-7.5 days identified – amplitudes replaced with value of zero south of 30oN
- T, q, u, v, ght, and slp

## Slide 12

## Harmonics

| Harmonic | Period (days) |
| --- | --- |
| A | 31 |
| B | 15.5 |
| C | 10.33 |
| D | 7.75 |
| E | 6.20 |
| F | 5.17 |
| G | 4.43 |
| H | 3.88 |
| I | 3.44 |
| J | 3.10 |

## Harmonic Amplitudes (~40 ̊W)

- TEW

- No-TEW

- August 1988

- Harmonic E

- Harmonic E

## MM5 Hövmoller Diagrams (700 mb)

- TEW

- No TEW

## Results

- 18 surges over 4 months examined
- 17 induced by TEW/tropical storm
- Varying degrees of strength and frequency

| Month | # of surges |
| --- | --- |
| August 1986 | 6 |
| August 1988 | 4 |
| July 1990 | 5 |
| July 1992 | 3 |

## Surge Criteria

- Used time-series data at Puerto Penasco, Mexico as a “first pass” to ID surge events
- Surges occur when:
- winds shift to southerly
- maximum daily dewpoint exceeding 65oF for at least 2 days
- peak wind speeds greater than 5 m/s
- decrease in daily max temp of greater than 5oF from the previous day

## August 1986 Time-series

## August 1986 Time-series

## August 1986 Results

- 6 surges in the control run
- all show up in the time-series data at Puerto Penasco
- 5 induced by TEWs and 1 initiated by a tropical storm (Howard?)
- 2 TEWs possibly contained in the model initial conditions

## TEW passage (18Z Aug 26)

**TEW**

**No TEW**

## Pre-surge (18Z Aug 26)

**TEW**

**No TEW**

## Surge onset (06Z Aug 27)

**TEW**

**No TEW**

## Surge (12Z Aug 28)

**TEW**

**No TEW**

## Post-surge (12Z Aug 29)

**TEW**

**No TEW**

## Surge summary

- TEW passage 12 hours prior to surge onset
- Entire GoC shifts to southerly winds
- 10 of 18 surges (most common)
- Surge virtually absent from no-TEW simulation

## TEWs and NAM rainfall

- Absence of TEWs has impact on precipitation amounts over the NAM region
- Many areas receive more rainfall when TEWs are present
- Influences overall extent of NAM precipitation

## August 1988 Rainfall Differences (TEW-no TEW)

## Central Plains Rainfall Differences (TEW-no TEW)

**August 1988**

## Meridional Moisture Flux

## Rainfall Differences (00Z Aug 17-06Z Aug 23)

**August 1988**

## Rainfall Differences -- 12Z Aug 19 - 00Z Aug 25

**August 1986**

## Meridional Moisture Flux

## Precipitable Water

## Mid-latitude forcing -- 12Z Aug 20, 1986

**TEW**

**No TEW**

## Adding TEWs

- July 1992 --> weak monsoon season
-    July 1990 --> strong monsoon season
- Removed waves from July 1992 boundary conditions
- Inserted July 1990 TEWs into July 1992 boundary conditions

## MM5 Hövmoller Diagrams

- Hybrid

- July 1992

## 12Z July 19 Hybrid run TEW passage

## Surge (00Z July 20)

**Hybrid**

**July 1992**

## Hybrid - July 1992 TEW Run

## Conclusions

- Harmonic analysis successfully removes TEWs from the model boundary conditions
- MM5 reproduces surges over the GoC
- full gulf, partial gulf, and SMO
- NAM shows great deal of interannual variability
- Surges impacted by absence of TEWs

## Conclusions

- Reduction of surge events in the no-TEW run reduced rainfall amounts over the NAM region
- Absence of TEWs increases precipitation over the central United States
- CAPE
- mid-latitude forcing
- Adding waves enhances NAM
- more distinct surge events
- increase in rainfall over core monsoon region

## Harmonic Analysis

- Since the model data used to create the boundary conditions are equally spaced in time and contain no missing values, the model data can be represented exactly as a series of n points in time by summing a series of n/2 harmonic functions....